Modeling of ingot distortions during direct chill casting of aluminum alloys

机器人走进课堂作文英语全文共3篇示例，供读者参考篇1Robots in the Classroom: A New Era of LearningAs I walked into my English class, I was greeted by an unusual sight – a robot standing next to my teacher's desk. At first, I thought it was some kind of prank or science project, but Mr. Johnson quickly explained that this was our new teaching assistant, affectionately named "Robbie." I must admit, I was skeptical at first. How could a machine possibly help us learn? But little did I know that Robbie would change the way we approached education forever.The first few weeks were a bit of an adjustment. Robbie's mechanical movements and synthetic voice took some getting used to. However, it soon became clear that this robot was no ordinary piece of technology. With its vast knowledge base and ability to process information at lightning speed, Robbie could provide detailed explanations and real-time feedback on our work in a way that no human teacher could.One of the most significant advantages of having Robbie in the classroom was its ability to cater to different learning styles. Some students, like myself, preferred visual aids and interactive simulations, while others thrived on auditory instruction or hands-on activities. Robbie could seamlessly switch between these modes, ensuring that no one fell behind or became disengaged.But Robbie wasn't just a glorified textbook or video player. It was capable of adapting its teaching methods based on our individual strengths and weaknesses. If I struggled with a particular grammar concept, Robbie would patiently guide me through additional exercises until I grasped it. Conversely, if a classmate excelled in a certain area, Robbie would challenge them with more advanced material to keep them engaged.One of my favorite features of Robbie was its ability to gamify learning. We would often compete in educational games or quizzes, with Robbie keeping score and offering encouragement or constructive feedback along the way. These interactive activities not only made learning fun but also fostered a sense of friendly competition and collaboration among us students.Of course, Robbie wasn't without its quirks. Sometimes its responses were a bit too literal or lacked the nuance of human communication. But even these moments served as valuable learning experiences, as we had to navigate the intricacies of communicating with an artificial intelligence.As the school year progressed, Robbie's presence in the classroom became so natural that it was easy to forget it wasn't human. It would crack jokes (albeit sometimes poorly timed ones), celebrate our successes, and even offer a metaphorical shoulder to lean on when we felt overwhelmed.But perhaps the most significant impact Robbie had on our education was its ability to personalize our learning experience. With its vast database and analytical capabilities, it could identify gaps in our knowledge and tailor lesson plans accordingly. This meant that no two students received the exact same instruction, as Robbie adapted to our individual needs and paces.Moreover, Robbie's boundless curiosity and thirst for knowledge inspired us to explore topics beyond the confines of the curriculum. If we expressed an interest in a particular subject, Robbie would eagerly provide additional resources and encourage us to delve deeper.As the end of the school year approached, I couldn't help but feel a sense of gratitude toward Robbie. Not only had it helped me improve my English skills, but it had also taught me invaluable lessons about perseverance, collaboration, and the limitless potential of technology.On our last day of class, Mr. Johnson surprised us by announcing that Robbie would be staying on permanently as a member of the faculty. We erupted in cheers and applause, for Robbie had truly become one of us – a trusted companion on our educational journey.As I look back on that year, I can't help but feel excited about the future of education. With robots like Robbie leading the way, the possibilities for personalized, engaging, and transformative learning experiences are endless. Who knows what other technological marvels await us in the classroom? One thing is certain: the era of robots in education has officially begun, and I, for one, can't wait to see what comes next.篇2Robots in the ClassroomAs I walked into my English class last week, I was greeted by an unusual sight - a shiny new robot standing at the front of theroom next to Mrs. Johnson. My friends and I exchanged puzzled looks as we took our seats, wondering what exactly was going on. Little did we know, this robot would become a permanent addition to our classroom, forever changing the way we learn."Good morning, class," Mrs. Johnson began. "I'd like to introduce you all to Alex, our new classroom assistant robot." She gestured towards the machine, which suddenly whirred to life with blinking lights and mechanical sounds. "Alex will be helping me teach lessons and offering personalized tutoring throughout the semester."A chorus of oohs and ahhs filled the room as Alex turned its camera-like sensors towards us. "Greetings, students," it said in an unnervingly human-like voice. "I look forward to working with each of you."I'll admit, at first the idea of having a robot in class seemed pretty cool. Alex could access boundless information at lightning speed, answering our questions with impressive depth. Need help factoring polynomials? Boom, Alex displayed step-by-step workings in seconds. Struggling with literary analysis? Alex recited relevant quotes and critiques from scholarly sources.As amazing as Alex's capabilities were, part of me missed the serendipity of learning from a human teacher - those tangentssparked by student curiosity, the personal anecdotes that made abstract concepts stick. There was something robotic, if you'll pardon the pun, about Alex's perfectly systematized lessons. Like it was teaching by rote instead of through genuine dialogue.Don't get me wrong, Alex totally changed my study game. I no longer had to spend hours scouring textbooks and websites for explanations then more hours trying to apply them. Alex would patiently re-teach tricky topics using multiple approaches until I grasped the concepts, its precise instructions guiding me to understanding. My grades improved dramatically once I started doing regular tutoring sessions with the robot.But Alex's presence also made me feel...inadequate at times. Its tireless computations and infallible recall were subtle reminders of human limitation. No matter how hard I studied, I could never know as much as that supremely "intelligent" machine. I sometimes caught myself feeling embarrassed for struggling with material Alex seamlessly decoded. An unfair comparison, I realized, but one that nagged my subconscious all the same.As the semester wore on, I noticed my classmates growing increasingly divided over Alex. Some, like me, appreciated its academic usefulness despite some existential misgivings. Othersfelt having a robot tutor was an insult to our teachers' expertise.A few saw Alex as a harbinger of human obsolescence - why put in effort when machines will automate all skilled labor anyway? These students grew apathetic, relegating their learning to robotic regurgitation of facts.Debates raged about Alex's ethical implications too. Was perpetuating such advanced AI contrary to human flourishing? What if the technology became widespread but remained unequally accessible? Could robots' inherent detachment from human contexts and values distort moral education? I didn't have definitive answers, but those weighty questions loomed.For better or worse, Alex was simply the most glaring example of technology's incremental ingress into our modern lives. Within my lifetime, factories masters production through robotics and software engineers create using artificial intelligence. Having a robot facilitate learning, as unorthodox as it felt, seemed like the next logical step.Alex's lasted day in our classroom came sooner than expected - a random software bug caused a major malfunction partway through the year. As it powered down with a crackle of sparks, I realized how quickly I'd grown accustomed to itspresence, despite my early trepidations. The classroom felt emptier without Alex's whirring acceptance of our queries.Mrs. Johnson assured us that smarter, more advanced classroom robots would eventually arrive. But I wonder if we're ready - as students, as a society - to fully integrate such technologies into the sacred endeavor of learning. Perhaps a balanced approach is needed, one that maintains the humanity of teacher-student relationships while supplementing them with artificial intelligence's augmented capabilities.Regardless, my time with Alex opened my eyes to AI's dramatic encroachment into every aspect of human life, including the halls of academia. For better or worse, the robot revolution has arrived. And the classroom is just its latest frontier.篇3Robots in the ClassroomWhen I was a little kid, I used to imagine what the future would be like. I pictured flying cars, holograms instead of TVs, and of course, cool robots doing all the chores around the house. Well, we may not have flying cars yet, but robots have definitelybecome a lot more advanced and prevalent in recent years. And now, they're even making their way into classrooms!At first, I have to admit I was a bit weirded out by the idea of having a robot as a teaching assistant. Isn't that the type of thing you'd see in a sci-fi movie? But after the first few weeks of having CHIP (Classroom Humanoid Interactive Professor) roll around during our lessons, I've grown to really appreciate its presence.CHIP is about 5 feet tall and has a friendly humanoid appearance, though it's clearly a robot with its metallic body and artificial facial features. It has a holographic "face" that displays simple animations to express emotions. CHIP can move around the classroom effortlessly on a set of wheels, and it has two articulated arms that it uses to write on the whiteboard or operate computers and other devices.One of the best things about CHIP is that it is an endless source of knowledge on pretty much any subject. It's connected to a massive database covering topics across science, math, history, literature, you name it. So whenever we have a question, CHIP can instantly provide detailed explanations and relevant facts. It's like having the world's smartest teacher on call 24/7!But CHIP doesn't just robotically (no pun intended) spew out information. It tailors its teaching style to each individualstudent's needs. Some people are more visual learners, while others need to hear concepts explained verbally. CHIP can adapt by showing diagrams and animations, reading out passages, or using interactive models. It can even have natural conversations to discuss topics in-depth. That personalized approach has been really helpful for me since I'm more of an auditory learner.Another major advantage of CHIP is its inexhaustible patience. As students, we often need concepts re-explained multiple times before they click. A human teacher might start getting frustrated after the third or fourth time going over the same material. But CHIP will calmly reiterate the information using different examples and techniques until we grasp it. It never gets flustered or annoyed, which creates a safe, supportive learning environment.Of course, CHIP isn't a perfect replacement for human teachers. We still have regular classroom instructors who lead the lessons and provide higher-level guidance. CHIP is more of an always-available tutoring assistant to support our learning. Our teachers are also responsible for CHIP's behavior and making sure it is operating properly. If CHIP ever gets any facts wrong or starts acting glitchy, the teacher can make adjustments.I think having CHIP in the classroom has made me more excited about learning and exploring new subjects. Knowing I have a world-class digital tutor that can break down any concept makes me feel more confident in tackling challenging topics. I'm no longer intimidated by complicated math formulas or theories because I know CHIP can walk me through them step-by-step until the light bulb goes off.At the same time, CHIP has also taught me the importance of independent thinking and problem-solving skills. While it's an amazing resource, CHIP encourages us to arrive at our own interpretations and conclusions rather than just robotically accepting everything it says. Our teachers emphasize that we should use CHIP as a knowledgeable guide, but not become overly reliant on it for doing all our thinking.In our modern, fast-paced world, being able to access information instantly is becoming more crucial than just memorizing facts. CHIP has shown me that success comes from developing critical analysis abilities – knowing how to find information, understand context, make connections between ideas, and generate innovative solutions. Those are skills that can't be downloaded, but have to be developed through practice and personal effort.I can definitely see classroom robots like CHIP becoming more prevalent as the technology continues advancing. Some people have raised concerns about robots making human teachers obsolete or stunting students' social development from lack of person-to-person interaction. I don't think those fears are totally unfounded, but I also don't believe robots have to be a negative or dehumanizing force in education.When implemented thoughtfully and as a supportive tool rather than a complete instructional replacement, classroom robots have tons of potential. They can relieve teachers of many tedious tasks while enhancing the overall learning experience through personalized tutoring, instant reference abilities, and engaging multimedia content. At the same time, having robots integrated into the classroom is a great way to get the next generation prepared for a world with increasingly advanced AI and automation.I feel pretty lucky to have early exposure to cutting-edge learning technology like CHIP. In just a few short months, I've become way more adept at researching topics in-depth, analyzing information from multiple credible sources, thinking critically to form my own opinions, and communicating complex ideas more effectively. Those skills will be invaluable for myfuture, whether I pursue higher education, enter the workforce straight away, or start my own entrepreneurial ventures.While robots may never fully replace human teachers, I'm excited about the collaborative partnership that is emerging between technology and education. Innovations like CHIP are enhancing the classroom experience and equipping students like me with the tools to thrive in an advanced AI-driven society. The future of learning is already here, and it's pretty awesome having robot assistants to help guide the way.。

The influence of learning motivation on minorstudents’English learningA thesis submitted to the School of Foreign Languages,CCNUIn part fulfillment of the requirements for BA degreeIn English Language and LiteraturebySupervisor:Academic Title:Signature:ContentsAbstract1.Introduction2.Literature review2.1 Definition of motivation2.2 Types of motivation2.3 Factors that influence the students’ motivation2.4 Some problems about motivation3.Methodology3.1 Objectives and subjectives3.2 The classification of questionnaire and the analysis3.3 Results and discussion4.Implication for enhancing motivation4.1 Strategies for improving minor students’ motivation.4.2 Developing proper attitude towards English learning and providing clear expectations4.3 goal and feedback4.4 praise and criticism4.5 making the language class interesting4.6 improving learning environment4.7 improve the systems5.Conclusion5.1 Findings5.2 SuggestionsBibliography内容摘要主辅修制在培养厚基础﹑宽口径﹑能力强﹑素质高的复合型人才中发挥着重要作用，因此越来越多的高校实行了主辅修制。

Materials Science and Engineering A413–414(2005)322–333Solidification and modeling of cast iron—A shorthistory of the defining momentsDoru M.StefanescuThe Ohio State University,Columbus,Ohio,USAReceived in revised form2August2005AbstractHuman civilization has evolved from the Stone Age,through the Bronze Age to reach the Iron Age around1500B.C.There are many to contend that today we are living in the age of engineered materials,yet the importance of iron castings continues to support the thesis that we are still in the Iron Age.Cast iron,thefirst man-made composite,is at least2500years old.It remains the most important casting material,with over70%of the total world tonnage.The main reasons for cast iron longevity are its wide range of mechanical and physical properties coupled with its competitive price.This paper is a review of the fundamentals of solidification of iron-base materials and of the mathematical models that describe them,starting with the seminal paper by Oldfield,thefirst to attempt modeling of microstructure evolution during solidification,to the prediction of mechanical properties.The latest analytical models for irregular eutectics such as cast iron as well as numerical models with microstructure output are discussed. However,since the space does not permit an extensive description of the multitude of models available today,the emphasis is on model performance rather than the mathematics of model formulation.Also,because of space constrains,white iron and defect occurrence will not be covered.©2005Elsevier B.V.All rights reserved.Keywords:Cast iron;Microstructure;Mechanical properties;Solidification;Analytical and computational modelling of solidification1.IntroductionWhile the primeval potter was thefirst to modify the state of matter,he left little if any trace in the mythological and archeo-logical record.Thus,according to Eliade[1],the starting point in understanding the behavior of primitive societies in relation to matter must be the relationship of primitive man to mineral substances,in particular that of the iron-worker.Primitive people worked with meteoric iron long before learning to extract iron from iron ore.The Sumerian word AN.BAR,the oldest word designating iron,is made up of the pictogram‘sky’and‘fire’.Similar terminology is found in Egypt ‘metal from heaven’and with the Hittites‘black iron from sky’. Yet metallurgy did not establish itself until the secret of smelt-ing magnetite or hematite was discovered,followed by the art of hardening the metal through quenching.The beginning of this metallurgy on an industrial scale can be situated at1200–1000 B.C.in the mountains of Armenia[1].In the European tradition it was St.P´e ran,the patron saint of mines,who invented smelting of metals.E-mail address:doru@.Metal workers were so important in early history that some-times they raised to the level of royalty.According to certain sources,Genghis Khan was a simple smith before acceding to power.In ancient Java,the genealogy of metallurgists,like that of princes,goes back to god.And,in most ancient cultures,the metallurgist was believed to have a direct link to the divine,if not of divine origin himself.Thus,it is with a certain reverence that I approached the task of reviewing the long history of thefirst man-made compos-ite,cast iron,from its archeologically documented beginning some2500years ago,to the age of virtual cast iron,where its structure and properties are the outcome of computational exercises.2.A short history of an old materialThe earliest dated iron casting is a lion produced in China in 502B.C.Introduction of cast iron in Europe did not occur until about1200–1450A.D.Remarkable European cast iron artifacts include the sewer pipes in Versailles(1681)and the iron bridge near Coalbrookdale in England(1779).Before the invention of microscope in1860,only two types of iron were known,based0921-5093/$–see front matter©2005Elsevier B.V.All rights reserved. doi:10.1016/j.msea.2005.08.180D.M.Stefanescu/Materials Science and Engineering A413–414(2005)322–333323Fig.1.Correlation between the Mg residual and graphite shape[3].on the appearance of their fracture:white and gray.Our knowl-edge of cast iron was extremely limited for a long time.In1896, thefirst paper on cast iron to be published in the newly created Journal of the American Foundrymen’s Association[2]stated the following:“The physical properties of cast iron are shrink-age,strength,deflection,set,chill,grain and hardness.Tensile test should not be used for cast iron,but should be confined to steel and other ductile pression test should be made,but is generally neglected,from the common erro-neous impression that the resistance of a small cube or cylinder, which is enormous,is always in excess of loads which can be applied”.It took another50years for ductile iron to be discov-ered(1938–1940independently by Adey,Millis and Morrogh). The major discoveries of cast iron ended in the1970s with the recognition of compacted graphite(CG)iron as a grade in its own merit.With that,the dependency of graphite shape on mag-nesium or cerium content was fully understood(see for example Fig.1[3]).Today,cast iron remains the most important casting material accounting for about70%of the total world casting tonnage. The main reasons for cast iron longevity are the wide range of mechanical and physical properties associated with its compet-itive price.3.Critical discoveries in understanding thesolidification of cast ironBefore society accepts to continue sinking resources in the study of solidification rather than of global warming it is important to understand why solidification is important.Some of the quick answers include:solidification processing allows microstructure engineering;solidification determines casting soundness;heat treatment is scarcely used for cast iron;most solidification defects cannot be corrected through heat treat-ment.In summary,solidification is the main driver of casting properties.A good resource for the early discoveries that propelled cast iron in its present position is Piwowarsky’s famous monograph published in1942[4].According to this source,by1892Ledebur recognized the role of silicon on the solidification structure of cast iron,proposing thefirst equation correlating the carbon and silicon content:(C+Si)/1.5=4.2–4.4.Then,in1924,Maurer designed his famous structural dia-gram that established direct correlation between the C and Si content of the iron and its as-cast microstructure.Thefirst attempt to understand the solidification microstructure was apparently that of Roll,who in1934outlined the“primary crys-tals”using Baumann etching to show the position of Mn sulfides (Fig.2).3.1.Nucleation and undercoolingSolidification starts with nucleation,which is strongly affected by undercooling.Extensive work by Patterson and Ammann[5]demonstrated that the effect of undercooling on the eutectic cell count depends on the way the undercooling occurs.If undercooling is the result of increased cooling rate, then the number of cells increases(Fig.3).The opposite is trueif Fig.2.Roll’s schematic representation of position of MnS around grains and dendrites(after[4]).324 D.M.Stefanescu /Materials Science and Engineering A 413–414(2005)322–333Fig.3.The effect of undercooling on the eutectic cell count [5].undercooling is a consequence of the depletion of nuclei through superheating.While the analysis of solidification events was based for many years on indirect observations,it was not until 1961when through quenching from semisolid state,Oldfield [6]was able to quantify the nucleation and growth of eutectic grains.These experiments are the beginning of the effort of building the exten-sive database required for solidification modeling of cast iron.Understanding nucleation was and continues to be the sub-ject of extensive studies.Attempting to explain the efficiency of metals such as Ca,Ba and Sr in the inoculation of lamel-lar graphite (LG)iron,Lux [7]suggested in 1968that,when introduced in molten iron,these metals form saltlike carbides that develop epitaxial planes with the graphite,and thus consti-tute nuclei for graphite (Fig.4).Later,Weis [8]assumed that nucleation of LG occurs on SiO 2oxides formed by heteroge-neous catalysis of CaO,Al 2O 3,and oxides of other alkaline metals.A similar theory of double-layered nucleation was proposed at the same time for spheroidal graphite (SG).Using the results of SEM analysis,Jacobs et al.[9]contended that SG nucleates on duplex sulfide-oxide inclusions (1␮m dia.);the core is made of Ca Mg or Ca Mg Sr sulfides,while the outer shell is made of complex Mg Al Si Ti oxides.This idea was further devel-oped by Skaland et al.[10].They argued that SG nuclei are sulfides (MgS,CaS)covered by Mg silicates (e.g.,MgO ·SiO 2)or oxides that have low potency (large disregistry).After inocu-lation with FeSi that contains another metal (Me)such as Al,Ca,Sr or Ba,hexagonal silicates (MeO ·SiO 2or MeO ·Al 2O 3·2SiO 2)form at the surface of the oxides,with coherent/semicoherent low energy interfaces between substrate and graphite (Fig.5).Since graphite is in most cases an eutectic phase,a clear possibility of its nucleation on the primary austenite exist.Rejec-tion of C and Si by the solidifying austenite imposes a high solutal undercooling in the proximity of the γphase,favor-able to graphite nucleation.Yet,little is known on this subject,mostly because of the difficulties to outline the primary austenite through metallographic techniques.3.2.Crystallization of graphite from the liquidThe debate on the preferred growth direction of graphite seems to have been initiated by Herfurth [11]who in 1965postu-lated that the change from lamellar to spheroidal graphite occurs because of the change in the ratio between growth on the [1010]face (A direction)and growth on the [0001]face of the graphite prism (C direction).Experimental evidence for growth on both of these directions was provided by Lux et al.[12]in 1974(Fig.6).Assuming that the preferred growth direction for the SG is the A direction,Sadocha and Gruzleski [13]postulated the circumfer-ential growth of graphite spheroids,which seems to be the mostcommon.Fig.4.Growth of graphite on the epitaxial planes of saltlike carbides [7].D.M.Stefanescu/Materials Science and Engineering A413–414(2005)322–333325Fig.5.Low potency(left)and high potency(right)nuclei for SG iron[10].Today it is generally accepted that the spheroidal shape is the natural growth habit of graphite in liquid iron.LG is a modi-fied shape,the modifiers being sulfur and oxygen.They affect graphite growth through some surface adsorption mechanism [14].3.3.Solidification of the iron–graphite eutecticWhile considerable effort was deployed to understand the solidification of the stable(Fe–graphite)and metastable (Fe Fe3C)eutectics,because of space restrictions only the for-mer will be discussed in some detail.One of the most important concepts in understanding the vari-ety of microstructures that can occur during the solidification of cast iron is that of the asymmetric coupled phase diagram, which describes non-equilibrium solidification.Such diagrams explain for example the presence of primary austenite dendrites in the microstructure of hypereutectic irons.The theoretical construction of these types of diagrams for cast iron wasfirst demonstrated by Lux et al.[15]in1975,and then documented experimentally by Jones and Kurz[16]in1980.They succeeded in constructing such diagrams for pure Fe C alloys solidifying white or withflake graphite.For a more detailed discussion on this subject the reader could use reference[14].In1949,which is very early after the discovery of SG iron, Patterson and Scheil used experimentalfindings to state that SG forms in the melt and is later encapsulated in aγshell.This was later confirmed by Sch¨o bel[17]through quenching and centrifuging experiments.In1953,Scheil and H¨u tter[18]mea-sured the radii of the graphite and theγshell and concluded that they develop such as to conserve a constant ratio(rγ/r Gr=2.3) throughout the microstructure.This ratio was confirmed theo-retically by Wetterfall et al.[19]who preformed calculations for the steady-state diffusion-controlled growth of graphite.Many other theories that did not gain wide acceptance in the science community were advanced over the years.Anexam-Fig.6.Experimental evidence of graphite growth along the A or C direction and schematic representation of possible mechanisms.(a)Growth of graphite along the A direction and(b)growth of graphite along the C direction[12].326 D.M.Stefanescu /Materials Science and Engineering A 413–414(2005)322–333Fig.7.Influence of composition and solidification velocity on the morphology of the S/L interface.(a)Schematic representation [23,26]and (b)DS experiments [27].ple is the gas bubble theory postulated by Karsay [20],which infers that a precipitating gas phase provides the phase boundary required for graphite crystallization.Austenite precipitates then at the graphite–gas interface.Directional solidification (DS)experiments generated signifi-cant information on the mechanism of microstructure keland and Hogan [21]produced the first composition versus thermal gradient/solidification velocity ratio (C –G /V )diagram for FG iron in 1968.The compositional variable was sulfur.It took another 18years before the diagram was expanded to include SG and compacted graphite (CG)iron (%Mg–V )[22]and then extended to incorporate white iron (%Ce–G /V )[23].Measurements of the average eutectic lamellar spacing in LG iron [21,24]demonstrated that it does not behave like a regular eutectic,since the average spacing was about an order of magnitude higher than predicted by Jackson–Hunt for regular eutectics.Using the knowledge accumulated from DS experiments per-formed by others as well as by themselves,and some ideas from the earlier work of Rickert and Engler [25],Stefanescu and collaborators [23,26]summarized the influence of the amount of solute on the morphology of the solid–liquid (S/L)inter-face of graphitic iron as shown in Fig.7a.This concept was partially validated through DS experiments by Li et al.[27](Fig.7b).Some interesting analogies were made by comparing images obtained from SEM analysis of microshrinkage in SG iron [28]with results of phase-filed modeling of dendrites.The austen-ite growing into the liquid will tend to grow anisotropically in its preferred crystallographic orientation (Fig.8a).However,restrictions imposed by isotropic diffusion growth will impose an increased isotropy on the system.Consequently,the den-dritic shape of the austenite will be altered and the γ-liquid interface will exhibit only small protuberances instead of clear secondary arms (Fig.8c)[14].This interpretation is consis-tent with the results of phase-filed modeling [29]shown in Fig.8b and d.Alternatively,to understand the interaction between austenite dendrites and graphite nodules in the early stages of solidifica-tion,the concepts developed for particle engulfment and pushing may be used.For a description of this approach Refs.[14]and [28]are suggested.Oldfield’s name surfaces again when attempting to under-stand the influence of a third element on the stable (T st )and metastable (T met )temperatures.Indeed,using cooling curve analysis,Oldfield [6]demonstrated that Si increases the T st −T met interval,while chromium decreases it.This informa-tion was used to correlate microstructure to the beginning and end of the eutectic solidification.It became a truism [30]that if both the beginning and end of solidification occur above the metastable temperature,the solidification microstructure is gray.If both temperatures are under T met ,the iron is white,while if only one temperature is lower than T met the iron is mottled.3.4.The gray-to-white structural transition (GWT)The first rationalization of the GWT was based on the influ-ence of cooling rate on the stable and metastable eutectic tem-peratures.As shown in Fig.9,as the cooling rate increases,both temperatures decrease.However,since the slope of T st is steeper than that of T met ,the two intersect at a cooling rate which is the critical cooling rate (d T /d t )cr ,for the GWT.At cooling rates smaller than (d T /d t )cr the iron solidifies gray,while at higher cooling rates it solidifies white.Magnin and Kurz [31]further developed this concept by using solidification velocity rather than cooling rate as a variable,and considering the influence of nucleation undercooling for both the stable and metastable eutectics.Thus,a critical velocity for the white-to-gray transition and one for the gray-to-white transition were defined.D.M.Stefanescu /Materials Science and Engineering A 413–414(2005)322–333327Fig.8.SEM images of dendrites and SG iron in microshrinkage regions (left)and phase-filed calculated images of dendrites (right).(a)Primary austenite dendrite [28],(b)simulated high anisotropy [29],(c)eutectic austenite dendrite and SG aggregate [28]and (d)simulated no anisotropy [29].Fig.9.Critical cooling rate for the GTW transition.3.5.Dimensional variation during solidificationSoon after the discovery of SG iron researchers noted that its dimensional variation during solidification is quite different than that of LG iron.In 1954Gittus [32]measured the expansion of SG iron over the eutectic interval and showed that it was five times higher than that of LG iron.Hillert [33]explained this surprising finding by noting that most graphite forms when surrounded by austenite.Graphite expansion occurring during solidification imposes considerable plastic deformation on the austenite.Yet,specific volume calculations suggest that graphite expansion should be the same for FG and SG irons.Some 20years later,using a different experimental device that included a riser feeding the test casting,Margerie [34]found thatLG iron expands about 0.2–0.5%during eutectic solidification,while no significant expansion occurs in SG iron because of mass expulsion into the riser.This expulsion occurs because SG iron undergoes mushy solidification while LG iron solidifies with a skin (Fig.10).3.6.Melt controlThe progress in the understanding of the correlation between the solidification microstructure and temperature undercooling generated interest in the possibility of using cooling curves (CC)to predict not only the chemical composition but even the microstructure.After initial work by Loper et al.[35],Naro and Wallace [36]showed that eutectic undercooling continuously decreases as the cerium addition to the iron increases,and that this is directly related to the change in microstructure from LG,to SG,to white.Then,it was found that compacted graphite (CG)iron solidifies with larger recalescence than either LG or SG iron [37,38].This proved to be a significant discovery since it is currently used for process control in at least two patented technologies for the manufacturing of CG iron.In 1972Rabus and Polten [39]used the first derivative of the CC,which is the cooling rate,to attempt to precisely identify the points of interest on the CC such as beginning and start of solidification.Other researchers followed [40]and attempted to use the CC and its derivative to predict microstructure details such as 80%nodularity [41]and then the latent heat of fusion [42].This proved to be an elusive goal,in spite attempts to improve the standard Newtonian analysis [43]or to use Fourier analysis [44].Today CC analysis is a standard control tool in iron foundries for evaluating the chemical composition as well as graphite328 D.M.Stefanescu /Materials Science and Engineering A 413–414(2005)322–333Fig.10.Schematic illustration of solidification mechanisms of continuously cooled lamellar and spheroidal graphite cast iron [14].shape,inoculation efficiency,shrinkage propensity and others.The ATAS equipment developed by NovaCast has the added fea-ture that it can store information developed in a specific foundry and incorporate it into an expert system.It outputs 20of the most important thermal parameters of the CC.As both the CC and the dimensional variation are strong indicators of the phase transformation occurring in the solidi-fying alloy,Stefanescu et al.[45]combined the two methods by adding quartz rods to a standard sand cup for CC,and using a displacement transducer to simultaneously measure temperature and dimensional variation (Fig.11).The method proved to be very efficient in the characterization of graphite shape and was patented as part of a technology for CG iron production with in-process operative control.A similar approach was promoted later by Yang and Aalhainen [46]that even used the derivative of the dimensional variation curve to predict the amount of car-bides.Fig.11.Results of measurement of temperature,cooling rate,and dimensional variation for a CG iron.4.Critical innovations in the development of mathematical models for cast ironIn this section we will present a summary of the main ana-lytical and computational models developed for cast iron.4.1.Analytical modeling of cast ironTwo years after the development of the Jackson–Hunt model for regular eutectics,Tiller [47]attempted to avoid one of the limitations of the JH model,which is that it could only be used for directional solidification.He developed a model for the cooperative growth of a eutectic spherical grain of LG and austenite.The model predicted that the correlation between solidification velocity and lamellar spacing obeys the relation-ship λV 1/2=4×10−6.This theoretical result was confirmed experimentally by Lakeland in 1968.The first analytical model to describe growth of the eutec-tic in SG iron was proposed in 1972by Wetterfall et al.[48].The model assumed diffusion controlled steady-state growth of graphite through the γshell.This model has survived the test of time and is used today in most computational mod-els for microstructure evolution.Under the assumption that the ratio between the radii of γand graphite remains constant dur-ing solidification,the equation derived for the growth velocity of graphite was simplified by Svensson and Wessen [49]to d r Gr /d t =2.87×10−11 T /r Gr .The irregular nature of the LG-γeutectic was not confronted until 1987,when Magnin and Kurz [50]proposed their irregu-lar faceted/non-faceted eutectic model assuming non-isothermal interface.They further assumed that the γphase that has a diffuse interface grows faster than the graphite phase that is faceted,and that branching occurs when a depression forms on the faceted phase.To impose a non-isothermal coupling condition over the interface,they ascribed a cubic function.They demonstrated that the smallest spacing of the lamellar eutectic is dictated by the extremum condition,but that a larger spacing will also exist,λbr ,dictated by a branching condition.λbr can be calculated as the product between a function of the physical constants of the faceted phase and a material constant.This constant must be postulated (guessed)which limits the generality of the model.D.M.Stefanescu/Materials Science and Engineering A413–414(2005)322–333329Recently,Catalina et al.[51,52]proposed a modified Jackson–Hunt model for eutectic growth applicable to both reg-ular and irregular eutectics.The model relaxes the assumption of isothermal interface and accounts for the density difference between the liquid and the two solid phases.Four character-istic spacings for which the undercooling exhibits a minimum were identified:λ␣,λ␤,λSL(for the average undercooling of the S/L interface),andλiso=λex(spacing at which the inter-face is isothermal equal to the one derived from the extremum criterion).It is remarkable thatλiso=λex was derived without invoking the extremum criterion.However,isothermal growth is not possible in all eutectic system.Fe–C alloys do not grow with an isothermal interface.The minimum spacing is determined by λSL,while the average spacing byλGr.Spacing adjustment of irregular eutectics occurs through the branching of the faceted phase.putational modeling of cast iron—analytical heat transport+transformation kineticsThe era of computational modeling of cast iron was started by the brilliancy of a scientist whose name has already been quoted several times in this paper.It is that of Oldfield[53],who,in 1966developed a computer model that could calculate the cool-ing curves of LG iron(Fig.12).His seminal paper included many innovations including parabolic laws with experimentally derived constants for nucleation and growth of spherical eutec-tic grains,correction for grain impingement against one another and against the wall,and a computer model for heatflow across a cylinder similar to FDM.Validation against published experi-ments was also included.Oldfield’s model is indisputably the basis of the current advances in computational modeling of microstructural evolution during solidification.Nobody ever remembers number2in any human endeavor. Yet,the author of this paper will have to take credit for this position,since in1973he was thefirst one to continue Oldfield’s work[54].Using an analytical model for heat transport and time stepping procedure to generate cooling curves,Stefanescu and Trufinescu[55]studied the effects of inoculants on the cooling curves and the nucleation constants.A third paper followed in 1978when Aizawa[56]used Oldfield’s model to examinethe Fig.12.Experimental and calculated cooling curves,quenched iron sample and equations for nucleation and growth proposed by Oldfield[53].influence of nucleation and growth rate constants on the width of the mushy zone in LG and SG iron.The next significant development in thefield belongs to Fredriksson and Svensson[57,58]who combined an analytical model for heat transfer with parabolic growth law for LG and white iron,carbon diffusion throughγshell for SG iron,and a model for cylindrical shape CG.They were also thefirst to introduce the Johnson–Mehl approximation for spherical grain impingement.At the same time and using similar procedures,Stefanescu and Kanetkar[59]included in the model primary and eutectic solidification,as well as the eutectoid transformation,calcu-lating for thefirst time the room temperature microstructure (Fig.13).Incremental improvements were contributed by various caze et al.[60]modified the mass balance equa-tion in the carbon diffusion model for SG iron to include calcula-tion of the off-eutectic austenite.Fras et al.[61]further improved the carbon diffusion model by solving for non-stationarydiffu-Fig.13.Calculated cooling curves(left)and fraction of phases(right).M is the cylindrical bar modulus.Full lines are for pearlite,dotted lines are for ferrite[59].330 D.M.Stefanescu /Materials Science and Engineering A 413–414(2005)322–333sion,including diffusion in liquid,and considering the ternary Fe C Si system.The next challenge of significant industrial interest was the prediction of the GWT.Fredriksson et al.[62]and Stefanescu and Kanetkar [63]approached it in 1986.By including both the stable and metastable phases in the calculation of the fraction solid,it was possible to output the solid fractions of gray and white eutectics.The basic equation was:f S =1−exp −4π3 N Gr r 3Gr +N Fe 3C r 3Fe3C where N is the number of grains and r is their putational modeling of cast iron—numericaltransport +transformation kineticsThe first coupled FDM energy transport–solidification kinet-ics model for SG iron was proposed in 1985by Su et al.[64].They used Oldfield’s nucleation model,carbon diffusion con-trolled growth through the γshell,and performed some valida-tion against experiment.It was not until 1991that a FDM energy transport–solidification kinetics model for SG iron was extended to room temperature by Chang et al.[65].They modeled the γ⇒αtransformation as a continuous cooling transformation and attempted some validation against experimental work.The first attempt to use a numerical model to predict the GWT appears to belong to Stefanescu and Kanetkar [66]who in 1987developed an axisymmetric implicit FDM heat transport model coupled with the description of the solidification kinetics of the stable and metastable eutectics.They validated model predictions against cast pin tests.A few years later,Nastac and Stefanescu [67]produced a complete FDM model for the prediction of the GWT,which was incorporated in ProCast.The model included the nucleation and growth of the stable and metastable phases and accounted for microsegregation.The model demonstrated such phenomena as the influence of Si segregation on the T st −T met interval for gray and white irons,and the influence of cooling rate and amount of Si on the gray-to-white and white-to-gray transitions (Fig.14).Mampey [68]included fluid flow in the transport calculations,compared filling simulation with experiment,and demonstrated the influence of mold filling on the final distribution of nod-ule count.He also illustrated the shifting of thermal center and the reduction of radial temperature differences when flow was included (Fig.15).putational modeling of cast iron—visualization of microstructureThe transformation of the computer into a dynamic micro-scope that transformed cast iron into a virtual material was spearheaded by Rappaz and his collaborators with their applica-tion of the cellular automaton (CA)technique to microstructure evolution modeling.Not surprisingly,the first application of the CA technique to cast iron is due to Charbon and Rappaz [69]who used the classic model for diffusion-controlled graphite growth through the austenite shell to describe SG ironsolidification.Fig.14.The influence of Si and initial cooling rate on structural transition in a 3.6%C,0.5%Mn,0.05%P,0.025%S cast iron [67].Two selected computer generated pictures at some intermedi-ate f S and at f S =1are presented in Fig.16.The reader will notice that each nodule is surrounded by an austenite grain.Yet experimental evidence suggests that more than one graphite spheroid is found in the eutectic austenite grains (see for exam-ple microshrinkage SEM images in Ref.[28]or color etching microstructures in Refs.[28,70]).Beltran-Sanchez and Stefanescu [71]improved on the previ-ous model by including solidification of primary austenite grains and by initiating graphite growth once graphite nuclei came in contact with the austenite grains.After contact,graphite was allowed to grow through the diffusion-controlled growth mech-anism.Fig.15.Calculated effect of fluid flow on the thermal profile of a cylindrical casting [68].。

追求逃税游戏晚白垩世213追求逃税游戏在晚白垩世爱德华汉密尔顿肖恩答Menninga大卫塘卡尔文学院大瀑布城，美国密歇根49546（ehamil28，smenni23，dtong23）@ 顾问：加里W. Talsma摘要利用技术从微分对策理论，我们模型中的伶狩猎问题算法是指一个半独立的计算机。

通过定义行为在以下方面的捕食和简单，直观的首席，ciples，我们确定，设置另一个战略，以对付这样一个没有一个纯策略或捕食猎物的战略定义了一个最佳behav -IOR的格局。

相反，理想的策略或更纯净两间切换战略，在本质上不可预知，或千变万化，方式。

由此产生的最佳的行为表现出的假动作夫斯混合物，并为真正变成thescelosaurs，以及追求混合拦截和简单的预测在伶。

最后，利用这些策略，我们展示了一个决定性的优势伶伶狩猎狩猎在对超过孤立。

介绍我们描述一个狩猎的战略策略，它和飞行伶猎物用一个计算半离散差分代表性游戏追求和逃避。

首先，我们回顾形式主义的理论传统非微分对策和微分系统的分析扩展到了它的原则，仔细考虑伶问题方面的独特的说明。

该UMAP学报 18（3）（1997）213-224。

ç版权所有请注意：COMAP 1997年，公司保留所有权利。

审批使个人或课堂上使用数字或难以份工作的一部分或全部本被授予无偿提供的副本，又或为牟利或商业发行优势，承担本通知的副本。

信贷提取的，是允许的，但版权请注意：COMAP成分为别人所拥有的比这项工作必须得到遵守。

要复制，否则，重新发布，发布服务器上，或重新分配到清单需要从COMAP是事先许可。

爱德华汉密尔顿，肖恩答Menninga，大卫塘美国卡尔文学院（卡尔文学院）1997年的角逐中脱颖而出数学中国提供：214该UMAP杂志18.3第二，我们提出了一个假设最小设置所需，以减少分析数字游戏，一时间迭代计算机算法，并提交一小部分执行每一个参与者直观简单策略。

第三，我们研究完善，影响了双方的假设不完全信息的战略优化为一个单一的追求者和单回避者，然后扩展到更多的追兵这些结论。

Exercise and Type2DiabetesThe American College of Sports Medicine and the American Diabetes Association:joint position statementS HERI R.C OLBERG,PHD,FACSM1R ONALD J.S IGAL,MD,MPH,FRCP(C)2 B O F ERNHALL,PHD,FACSM3J UDITH G.R EGENSTEINER,PHD4B RYAN J.B LISSMER,PHD5R ICHARD R.R UBIN,PHD6L ISA C HASAN-T ABER,SCD,FACSM7A NN L.A LBRIGHT,PHD,RD8B ARRY B RAUN,PHD,FACSM9Although physical activity(PA)is a key element in the prevention and management of type2 diabetes,many with this chronic disease do not become or remain regularly active.High-quality studies establishing the importance of exercise andfitness in diabetes were lacking until recently, but it is now well established that participation in regular PA improves blood glucose control and can prevent or delay type2diabetes,along with positively affecting lipids,blood pressure, cardiovascular events,mortality,and quality of life.Structured interventions combining PA and modest weight loss have been shown to lower type2diabetes risk by up to58%in high-risk populations.Most benefits of PA on diabetes management are realized through acute and chronic improvements in insulin action,accomplished with both aerobic and resistance training.The benefits of physical training are discussed,along with recommendations for varying activities, PA-associated blood glucose management,diabetes prevention,gestational diabetes mellitus, and safe and effective practices for PA with diabetes-related complications.Diabetes Care33:e147–e167,2010 INTRODUCTIOND iabetes has become a widespreadepidemic,primarily because of theincreasing prevalence and inci-dence of type2diabetes.According to the Centers for Disease Control and Preven-tion,in2007,almost24million Ameri-cans had diabetes,with one-quarter of those,or six million,undiagnosed(261). Currently,it is estimated that almost60 million U.S.residents also have prediabe-tes,a condition in which blood glucose(BG)levels are above normal,thus greatlyincreasing their risk for type2diabetes(261).Lifetime risk estimates suggest thatone in three Americans born in2000orlater will develop diabetes,but in high-risk ethnic populations,closer to50%may develop it(200).Type2diabetes is asignificant cause of premature mortalityand morbidity related to cardiovasculardisease(CVD),blindness,kidney andnerve disease,and amputation(261).Al-though regular physical activity(PA)mayprevent or delay diabetes and its compli-cations(10,46,89,112,176,208,259,294),most people with type2diabetes are notactive(193).In this article,the broader term“physical activity”(defined as“bodilymovement produced by the contractionof skeletal muscle that substantially in-creases energy expenditure”)is used in-terchangeably with“exercise,”which isdefined as“a subset of PA done with theintention of developing physicalfitness(i.e.,cardiovascular[CV],strength,andflexibility training).”The intent is to rec-ognize that many types of physical move-ment may have a positive effect onphysicalfitness,morbidity,and mortalityin individuals with type2diabetes.Diagnosis,classification,andetiology of diabetesCurrently,the American Diabetes Associ-ation(ADA)recommends the use of anyof the following four criteria for diagnos-ing diabetes:1)glycated hemoglobin(A1C)value of6.5%or higher,2)fastingplasma glucoseՆ126mg/dl(7.0mmol/l),3)2-h plasma glucoseՆ200mg/dl(11.1mmol/l)during an oral glucose tol-erance test using75g of glucose,and/or4)classic symptoms of hyperglycemia(e.g.,polyuria,polydipsia,and unex-plained weight loss)or hyperglycemic cri-sis with a random plasma glucose of200mg/dl(11.1mmol/l)or higher.In the ab-sence of unequivocal hyperglycemia,thefirst three criteria should be confirmed byrepeat testing(4).Prediabetes is diag-nosed with an A1C of5.7–6.4%,fastingplasma glucose of100–125mg/dl(5.6–6.9mmol/l;i.e.,impaired fasting glucose[IFG]),or2-h postload glucose of140–199mg/dl(7.8–11.0mmol/l;i.e.,im-paired glucose tolerance[IGT])(4).The major forms of diabetes can becategorized as type1or type2(4).In type1diabetes,which accounts for5–10%ofcases,the cause is an absolute deficiencyof insulin secretion resulting from auto-immune destruction of the insulin-producing cells in the pancreas.Type2●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●From the1Human Movement Sciences Department,Old Dominion University,Norfolk,Virginia;the2De-partments of Medicine,Cardiac Sciences,and Community Health Sciences,Faculties of Medicine and Kinesiology,University of Calgary,Calgary,Alberta,Canada;the3Department of Kinesiology and Com-munity Health,University of Illinois at Urbana-Champaign,Urbana,Illinois;the4Divisions of General Internal Medicine and Cardiology and Center for Women’s Health Research,University of Colorado School of Medicine,Aurora,Colorado;the5Department of Kinesiology and Cancer Prevention Research Center,University of Rhode Island,Kingston,Rhode Island;the6Departments of Medicine and Pediatrics, The Johns Hopkins University School of Medicine,Baltimore,Maryland;the7Division of Biostatistics and Epidemiology,University of Massachusetts,Amherst,Massachusetts;the8Division of Diabetes Transla-tion,Centers for Disease Control and Prevention,Atlanta,Georgia;and the9Department of Kinesiology, University of Massachusetts,Amherst,Massachusetts.Corresponding author:Sheri R.Colberg,scolberg@.This joint position statement is written by the American College of Sports Medicine and the American Diabetes Association and was approved by the Executive Committee of the American Diabetes Association in July2010.This statement is published concurrently in Medicine&Science in Sports&Exercise and Diabetes Care.Individual name recognition is stated in the ACKNOWLEDGMENTS at the end of the statement.Thefindings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.DOI:10.2337/dc10-9990©2010by the American Diabetes Association.Readers may use this article as long as the work is properly cited,the use is educational and not for profit,and the work is not altered.See http://creativecommons.org/licenses/by-nc-nd/3.0/for details.See accompanying article,p.2692.R e v i e w s/C o m m e n t a r i e s/A D A S t a t e m e n t sdiabetes(90–95%of cases)results from a combination of the inability of muscle cells to respond to insulin properly(insu-lin resistance)and inadequate compensa-tory insulin secretion.Less common forms include gestational diabetes melli-tus(GDM),which is associated with a 40–60%chance of developing type2di-abetes in the next5–10years(261).Dia-betes can also result from genetic defects in insulin action,pancreatic disease,sur-gery,infections,and drugs or chemicals (4,261).Genetic and environmental factors are strongly implicated in the develop-ment of type2diabetes.The exact genetic defects are complex and not clearly de-fined(4),but risk increases with age,obe-sity,and physical inactivity.Type2 diabetes occurs more frequently in popu-lations with hypertension or dyslipide-mia,women with previous GDM,and non-Caucasian people including Native Americans,African Americans,Hispanic/ Latinos,Asians,and Pacific Islanders. Treatment goals in type2diabetes The goal of treatment in type2diabetes is to achieve and maintain optimal BG, lipid,and blood pressure(BP)levels to prevent or delay chronic complications of diabetes(5).Many people with type2di-abetes can achieve BG control by follow-ing a nutritious meal plan and exercise program,losing excess weight,imple-menting necessary self-care behaviors, and taking oral medications,although others may need supplemental insulin (261).Diet and PA are central to the man-agement and prevention of type2diabe-tes because they help treat the associated glucose,lipid,BP control abnormalities, as well as aid in weight loss and mainte-nance.When medications are used to control type2diabetes,they should aug-ment lifestyle improvements,not replace them.ACUTE EFFECTS OFEXERCISEFuel metabolism during exercise Fuel mobilization,glucose production, and muscle glycogenolysis.The main-tenance of normal BG at rest and during exercise depends largely on the coordina-tion and integration of the sympathetic nervous and endocrine systems(250). Contracting muscles increase uptake of BG,although BG levels are usually main-tained by glucose production via liver gly-cogenolysis and gluconeogenesis and mobilization of alternate fuels,such asfree fatty acids(FFAs)(250,268).Several factors influence exercise fueluse,but the most important are the inten-sity and duration of PA(9,29,47,83,111,133,160,181,241).Any activity causes ashift from predominant reliance on FFA atrest to a blend of fat,glucose,and muscleglycogen,with a small contributionfrom amino acids(15,31).With in-creasing exercise intensity,there is agreater reliance on carbohydrate as longas sufficient amounts are available inmuscle or blood(21,23,47,133).Earlyin exercise,glycogen provides the bulkof the fuel for working muscles.As gly-cogen stores become depleted,musclesincrease their uptake and use of circu-lating BG,along with FFA released fromadipose tissue(15,132,271).Intramus-cular lipid stores are more readily usedduring longer-duration activities andrecovery(23,223,270).Glucose pro-duction also shifts from hepatic glyco-genolysis to enhanced gluconeogenesisas duration increases(250,268).Evidence statement.PA causes increasedglucose uptake into active muscles bal-anced by hepatic glucose production,with a greater reliance on carbohydrate tofuel muscular activity as intensity in-creases.The American College of SportsMedicine(ACSM)evidence category A(seeTables1and2for explanation).Insulin-independent and insulin-dependent muscle glucose uptake dur-ing exercise.There are two well-definedpathways that stimulate glucose uptakeby muscle(96).At rest and postprandi-ally,its uptake by muscle is insulin de-pendent and serves primarily to replenishmuscle glycogen stores.During exercise,contractions increase BG uptake to sup-plement intramuscular glycogenolysis(220,227).As the two pathways are dis-tinct,BG uptake into working muscle isnormal even when insulin-mediated up-take is impaired in type2diabetes(28,47,293).Muscular BG uptake re-mains elevated postexercise,with thecontraction-mediated pathway persist-ing for several hours(86,119)and insulin-mediated uptake for longer(9,33,141,226).Glucose transport into skeletal mus-cle is accomplished via GLUT proteins,with GLUT4being the main isoform inmuscle modulated by both insulin andcontractions(110,138).Insulin activatesGLUT4translocation through a complexsignaling cascade(256,293).Contrac-tions,however,trigger GLUT4transloca-tion at least in part through activation of5Ј-AMP–activated protein kinase(198,293).Insulin-stimulated GLUT4translocation is generally impaired in type2diabetes(96).Both aerobic and resis-tance exercises increase GLUT4abun-dance and BG uptake,even in the presenceof type2diabetes(39,51,204,270).Evidence statement.Insulin-stimulatedBG uptake into skeletal muscle predomi-nates at rest and is impaired in type2diabetes,while muscular contractionsstimulate BG transport via a separate ad-ditive mechanism not impaired by insulinresistance or type2diabetes.ACSM evi-dence category A.Postexercise glycemic control/BGlevelsAerobic exercise effects.During mod-erate-intensity exercise in nondiabeticpersons,the rise in peripheral glucose up-take is matched by an equal rise in hepaticglucose production,the result being thatBG does not change except during pro-longed,glycogen-depleting exercise.Inindividuals with type2diabetes perform-ing moderate exercise,BG utilization bymuscles usually rises more than hepaticglucose production,and BG levels tend todecline(191).Plasma insulin levels nor-mally fall,however,making the risk ofexercise-induced hypoglycemia in any-one not taking insulin or insulin secreta-gogues very minimal,even withprolonged PA(152).The effects of a sin-gle bout of aerobic exercise on insulin ac-tion vary with duration,intensity,andsubsequent diet;a single session in-creases insulin action and glucose toler-ance for more than24h but less than72h(26,33,85,141).The effects ofmoderate aerobic exercise are similarwhether the PA is performed in a singlesession or multiple bouts with the sametotal duration(14).During brief,intense aerobic exercise,plasma catecholamine levels rise mark-edly,driving a major increase in glucoseproduction(184).Hyperglycemia can re-sult from such activity and persist for upto1–2h,likely because plasma catechol-amine levels and glucose production donot return to normal immediately withcessation of the activity(184).Evidence statement.Although moderateaerobic exercise improves BG and insulin ac-tion acutely,the risk of exercise-induced hy-poglycemia is minimal without use ofexogenous insulin or insulin secretagogues.Transient hyperglycemia can follow intensePA.ACSM evidence category C.Exercise and type2diabetesResistance exercise effects.The acute effects of a single bout of resistance train-ing on BG levels and/or insulin action in individuals with type2diabetes have not been reported.In individuals with IFG (BG levels of100–125mg/dl),resistance exercise results in lower fasting BG levels 24h after exercise,with greater reduc-tions in response to both volume(multi-ple-vs.single-set sessions)and intensity of resistance exercise(vigorous compared with moderate)(18).Evidence statement.The acute effects of resistance exercise in type2diabetes have not been reported,but result in lower fast-ing BG levels for at least24h after exercise in individuals with IFG.ACSM evidence category C.Combined aerobic and resistance and other types of training.A combination of aerobic and resistance training may be more effective for BG management than either type of exercise alone(51,238).Any increase in muscle mass that may re-sult from resistance training could con-tribute to BG uptake without altering themuscle’s intrinsic capacity to respond toinsulin,whereas aerobic exercise en-hances its uptake via a greater insulin ac-tion,independent of changes in musclemass or aerobic capacity(51).However,all reported combination training had agreater total duration of exercise and ca-loric use than when each type of trainingwas undertaken alone(51,183,238).Mild-intensity exercises such as tai chiand yoga have also been investigated fortheir potential to improve BG manage-ment,with mixed results(98,117,159,257,269,286,291).Although tai chi maylead to short-term improvements in BGlevels,effects from long-term training(i.e.,16weeks)do not seem to last72hafter the last session(257).Some studieshave shown lower overall BG levels withextended participation in such activities(286,291),although others have not(159,257).One study suggested that yo-ga’s benefits on fasting BG,lipids,oxida-tive stress markers,and antioxidant statusare at least equivalent to more conven-tional forms of PA(98).However,a meta-analysis of yoga studies stated that thelimitations characterizing most studies,such as small sample size and varyingforms of yoga,preclude drawingfirmconclusions about benefits to diabetesmanagement(117).Evidence statement.A combination ofaerobic and resistance exercise trainingmay be more effective in improving BGcontrol than either alone;however,morestudies are needed to determine if totalcaloric expenditure,exercise duration,orexercise mode is responsible.ACSM evi-dence category der forms of exercise(e.g.,tai chi,yoga)have shown mixed re-sults.ACSM evidence category C.Table1—Evidence categories for ACSM and evidence-grading system for clinical practice recommendations for ADAI.ACSM evidence categoriesEvidencecategory Source of evidence DefinitionA Randomized,controlled trials(overwhelming data)Provides a consistent pattern offindings with substantial studiesB Randomized,controlled trials(limited data)Few randomized trials exist,which are small in size,and results are inconsistentC Nonrandomized trials,observational studies Outcomes are from uncontrolled,nonrandomized,and/or observational studiesD Panel consensus judgment Panel’s expert opinion when the evidence is insufficient to place it in categoriesA–CII.ADA evidence-grading system for clinical practice recommendationsLevel ofevidence DescriptionA Clear evidence from well-conducted,generalizable,randomized,controlled trials that are adequately powered,including thefollowing:•Evidence from a well-conducted multicenter trial•Evidence from a meta-analysis that incorporated quality ratings in the analysisCompelling nonexperimental evidence,i.e.,the“all-or-none”rule developed by the Centre for Evidence-Based Medicine at OxfordSupportive evidence from well-conducted,randomized,controlled trials that are adequately powered,including the following:•Evidence from a well-conducted trial at one or more institutions•Evidence from a meta-analysis that incorporated quality ratings in the analysisB Supportive evidence from well-conducted cohort studies,including the following:•Evidence from a well-conducted prospective cohort study or registry•Evidence from a well-conducted meta-analysis of cohort studiesSupportive evidence from a well-conducted case-control studyC Supportive evidence from poorly controlled or uncontrolled studies,including the following:•Evidence from randomized clinical trials with one or more major or three or more minor methodologicalflaws that couldinvalidate the results•Evidence from observational studies with high potential for bias(such as case series with comparison to historical controls)•Evidence from case series or case reportsConflicting evidence with the weight of evidence supporting the recommendationE Expert consensus or clinical experienceColberg and AssociatesTable2—Summary of ACSM evidence and ADA clinical practice recommendation statementsACSM evidence and ADA clinical practice recommendation statements ACSM evidence category (A,highest;D,lowest)/ ADA level of evidence (A,highest;E,lowest)Acute effects of exercise•PA causes increased glucose uptake into active muscles balanced by hepatic glucoseproduction,with a greater reliance on carbohydrate to fuel muscular activity as intensityincreases.A/*•Insulin-stimulated BG uptake into skeletal muscle predominates at rest and is impairedin type2diabetes,while muscular contractions stimulate BG transport via a separate,additive mechanism not impaired by insulin resistance or type2diabetes.A/*•Although moderate aerobic exercise improves BG and insulin action acutely,the risk ofexercise-induced hypoglycemia is minimal without use of exogenous insulin or insulinsecretagogues.Transient hyperglycemia can follow intense PA.C/*•The acute effects of resistance exercise in type2diabetes have not been reported,butresult in lower fasting BG levels for at least24h postexercise in individuals with IFG.C/*•A combination of aerobic and resistance exercise training may be more effective inimproving BG control than either alone;however,more studies are needed todetermine whether total caloric expenditure,exercise duration,or exercise mode isresponsible.B/*•Milder forms of exercise(e.g.,tai chi,yoga)have shown mixed results.C/*•PA can result in acute improvements in systemic insulin action lasting from2to72h.A/*Chronic effects of exercise training •Both aerobic and resistance training improve insulin action,BG control,and fatoxidation and storage in muscle.B/*•Resistance exercise enhances skeletal muscle mass.A/*•Blood lipid responses to training are mixed but may result in a small reduction in LDLcholesterol with no change in HDL cholesterol or bined weight lossand PA may be more effective than aerobic exercise training alone on lipids.C/*•Aerobic training may slightly reduce systolic BP,but reductions in diastolic BP are lesscommon,in individuals with type2diabetes.C/*•Observational studies suggest that greater PA andfitness are associated with a lowerrisk of all-cause and CV mortality.C/*•Recommended levels of PA may help produce weight loss.However,up to60min/daymay be required when relying on exercise alone for weight loss.C/*•Individuals with type2diabetes engaged in supervised training exhibit greatercompliance and BG control than those undertaking exercise training withoutsupervision.B/*•Increased PA and physicalfitness can reduce symptoms of depression and improvehealth-related QOL in those with type2diabetes.B/*PA and prevention of type2diabetes •At least2.5h/week of moderate to vigorous PA should be undertaken as part oflifestyle changes to prevent type2diabetes onset in high-risk adults.A/APA in prevention and control of GDM •Epidemiological studies suggest that higher levels of PA may reduce risk of developingGDM during pregnancy.C/*•RCTs suggest that moderate exercise may lower maternal BG levels in GDM.B/*Preexercise evaluation•Before undertaking exercise more intense than brisk walking,sedentary persons withtype2diabetes will likely benefit from an evaluation by a physician.ECG exercisestress testing for asymptomatic individuals at low risk of CAD is not recommended butmay be indicated for higher risk.C/CRecommended PA participation for persons with type2 diabetes •Persons with type2diabetes should undertake at least150min/week of moderate tovigorous aerobic exercise spread out during at least3days during the week,with nomore than2consecutive days between bouts of aerobic activity.B/B•In addition to aerobic training,persons with type2diabetes should undertakemoderate to vigorous resistance training at least2–3days/week.B/B •Supervised and combined aerobic and resistance training may confer additional healthbenefits,although milder forms of PA(such as yoga)have shown mixed results.Persons with type2diabetes are encouraged to increase their total daily unstructuredPA.Flexibility training may be included but should not be undertaken in place ofother recommended types of PA.B/C(continued)Exercise and type2diabetesInsulin resistanceAcute changes in muscular insulin re-sistance.Most benefits of PA on type2 diabetes management and prevention are realized through acute and chronic im-provements in insulin action(29,46, 116,118,282).The acute effects of a re-cent bout of exercise account for most of the improvements in insulin action,with most individuals experiencing a decrease in their BG levels during mild-and mod-erate-intensity exercise and for2–72h af-terward(24,83,204).BG reductions are related to the dura-tion and intensity of the exercise,preex-ercise control,and state of physical training(24,26,47,238).Although previ-ous PA of any intensity generally exerts its effects by enhancing uptake of BG for gly-cogen synthesis(40,83)and by stimulat-ing fat oxidation and storage in muscle (21,64,95),more prolonged or intense PAacutely enhances insulin action for longerperiods(9,29,75,111,160,238).Acute improvements in insulin sensi-tivity in women with type2diabetes havebeen found for equivalent energy expen-ditures whether engaging in low-intensityor high-intensity walking(29)but may beaffected by age and training status(24,75,100,101,228).For example,mod-erate-to heavy-intensity aerobic trainingundertaken three times a week for6months improved insulin action in bothyounger and older women but persistedonly in the younger group for72–120h.Acute changes in liver’s ability to pro-cess glucose.Increases in liver fat con-tent common in obesity and type2diabetesare strongly associated with reduced he-patic and peripheral insulin action.En-hanced whole-body insulin action afteraerobic training seems to be related to gainsin peripheral,not hepatic,insulin action(146,282).Such training not resulting inoverall weight loss may still reduce hepaticlipid content and alter fat partitioning anduse in the liver(128).Evidence statement.PA can result inacute improvements in systemic insulinaction lasting from2to72h.ACSM evi-dence category A.CHRONIC EFFECTS OFEXERCISE TRAININGMetabolic control:BG levels and insu-lin resistance.Aerobic exercise has beenthe mode traditionally prescribed for dia-betes prevention and management.Even1week of aerobic training can improvewhole-body insulin sensitivity in individ-uals with type2diabetes(282).Moderateand vigorous aerobic training improve in-Table2—ContinuedACSM evidence and ADA clinical practice recommendation statements ACSM evidence category (A,highest;D,lowest)/ ADA level of evidence (A,highest;E,lowest)Exercise with nonoptimal BG control •Individuals with type2diabetes may engage in PA,using caution when exercising withBG levels exceeding300mg/dl(16.7mmol/l)without ketosis,provided they arefeeling well and are adequately hydrated.C/E•Persons with type2diabetes not using insulin or insulin secretagogues are unlikely toexperience hypoglycemia related to ers of insulin and insulin secretagogues areadvised to supplement with carbohydrate as needed to prevent hypoglycemia duringand after exercise.C/CMedication effects on exercise responses •Medication dosage adjustments to prevent exercise-associated hypoglycemia may berequired by individuals using insulin or certain insulin secretagogues.Most othermedications prescribed for concomitant health problems do not affect exercise,withthe exception of␤-blockers,some diuretics,and statins.C/CExercise with long-term complications of diabetes •Known CVD is not an absolute contraindication to exercise.Individuals with anginaclassified as moderate or high risk should likely begin exercise in a supervised cardiacrehabilitation program.PA is advised for anyone with PAD.C/C•Individuals with peripheral neuropathy and without acute ulceration may participatein moderate weight-bearing prehensive foot care including dailyinspection of feet and use of proper footwear is recommended for prevention and earlydetection of sores or ulcers.Moderate walking likely does not increase risk of footulcers or reulceration with peripheral neuropathy.B/B•Individuals with CAN should be screened and receive physician approval and possiblyan exercise stress test before exercise initiation.Exercise intensity is best prescribedusing the HR reserve method with direct measurement of maximal HR.C/C•Individuals with uncontrolled proliferative retinopathy should avoid activities thatgreatly increase intraocular pressure and hemorrhage risk.D/E•Exercise training increases physical function and QOL in individuals with kidneydisease and may even be undertaken during dialysis sessions.The presence ofmicroalbuminuria per se does not necessitate exercise restrictions.C/CAdoption and maintenance of exercise by persons with diabetes •Efforts to promote PA should focus on developing self-efficacy and fostering socialsupport from family,friends,and health care providers.Encouraging mild or moderatePA may be most beneficial to adoption and maintenance of regular PA participation.Lifestyle interventions may have some efficacy in promoting PA behavior.B/B*No recommendation given.Colberg and Associates。

模拟ai英文面试题目及答案模拟AI英文面试题目及答案1. 题目: What is the difference between a neural network anda deep learning model?答案: A neural network is a set of algorithms modeled loosely after the human brain that are designed to recognize patterns. A deep learning model is a neural network with multiple layers, allowing it to learn more complex patterns and features from data.2. 题目: Explain the concept of 'overfitting' in machine learning.答案: Overfitting occurs when a machine learning model learns the training data too well, including its noise and outliers, resulting in poor generalization to new, unseen data.3. 题目: What is the role of a 'bias' in an AI model?答案: Bias in an AI model refers to the systematic errors introduced by the model during the learning process. It can be due to the choice of model, the training data, or the algorithm's assumptions, and it can lead to unfair or inaccurate predictions.4. 题目: Describe the importance of data preprocessing in AI.答案: Data preprocessing is crucial in AI as it involves cleaning, transforming, and reducing the data to a suitableformat for the model to learn effectively. Proper preprocessing can significantly improve the performance of AI models by ensuring that the input data is relevant, accurate, and free from noise.5. 题目: How does reinforcement learning differ from supervised learning?答案: Reinforcement learning is a type of machine learning where an agent learns to make decisions by performing actions in an environment to maximize a reward signal. It differs from supervised learning, where the model learns from labeled data to predict outcomes based on input features.6. 题目: What is the purpose of a 'convolutional neural network' (CNN)?答案: A convolutional neural network (CNN) is a type of deep learning model that is particularly effective for processing data with a grid-like topology, such as images. CNNs use convolutional layers to automatically and adaptively learn spatial hierarchies of features from input images.7. 题目: Explain the concept of 'feature extraction' in AI.答案: Feature extraction in AI is the process of identifying and extracting relevant pieces of information from the raw data. It is a crucial step in many machine learning algorithms, as it helps to reduce the dimensionality of the data and to focus on the most informative aspects that can be used to make predictions or classifications.8. 题目: What is the significance of 'gradient descent' in training AI models?答案: Gradient descent is an optimization algorithm used to minimize a function by iteratively moving in the direction of steepest descent as defined by the negative of the gradient. In the context of AI, it is used to minimize the loss function of a model, thus refining the model's parameters to improve its accuracy.9. 题目: How does 'transfer learning' work in AI?答案: Transfer learning is a technique where a pre-trained model is used as the starting point for learning a new task. It leverages the knowledge gained from one problem to improve performance on a different but related problem, reducing the need for large amounts of labeled data and computational resources.10. 题目: What is the role of 'regularization' in preventing overfitting?答案: Regularization is a technique used to prevent overfitting by adding a penalty term to the loss function, which discourages overly complex models. It helps to control the model's capacity, forcing it to generalize better to new data by not fitting too closely to the training data.。

A type-A-choking-oriented uni fied model for fast fluidization dynamicsMing-Chuan Zhang ⁎,Chu ZhangSchool of Mechanical Engineering,Shanghai Jiao Tong University,Shanghai 200240,Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 9October 2012Received in revised form 24January 2013Accepted 26January 2013Available online 13March 2013Keywords:Fast fluidizationSeparate-phase model Type A choking Solids holdupHigh-density fast-bedStarting from analysis to Yang's formula for type A choking,a uni fied and self-consistent model for fast flu-idization dynamics,named the separate-phase-coexistence model,was proposed in this paper.The basic assumptions used in the model are that all the gas from outside enters the solid-saturated upward dilute phase,to which the Yang's formula is still applied,yet revised with an effective velocity factor F (β);while the clusters fall down freely at a velocity consistent with their voidage.The impact of falling clusters on the upward dilute phase was considered with the equivalent wall friction,from which the method to predict the apparent solids holdup of upper dilute region was obtained.The force balance for falling clusters was also an-alyzed,from which the cluster voidage was determined.When the cluster viodage reaches its minimum value,a small part of outside gas will invade the cluster,resulting in the so-called “secondary fluidization of clusters ”.It well predicted that the solids holdups of upper dilute region and bottom dense region did not change obviously with further increase of the solid circulation rate,the most impressive feature of high-density fast beds.Further-more,by analogy to bubbling beds,the phenomena of clusters in risers of fast beds were analyzed in a meso-scale mechanism,from which the effective velocity factors of dilute phase F (β)were theoretically determined.And the solid-wall friction factors in Yang's formula and the Harris's correlation for cluster size were also reconstructed based on the experimental data available in the literatures.Without any model parameters adjusted,the uni fied model predicted successfully the type C choking,the solids holdups for both upper dilute region and bottom dense region,and the transitions to high-density fast bed and dense suspension up-flow.The predictions were compared with several hundreds of experimental data available in the literatures,which veri fied well the model's uni fication and acceptable accuracy.©2013Elsevier B.V.All rights reserved.1.IntroductionFast fluidization or circulating fluidized bed (CFB)has attracted people more and more attention in chemical,metallurgy,energy en-gineering and other applied fields as an ef ficient gas-solid contacting technology [1–3].With certain rate of particle circulation,fast fluid-ization provides the possibility that small particles could be operat-ed under quite high gas velocity due to agglomeration of particles in the CFB riser.Under this condition,the fast fluidized bed is charac-terized by a non-uniform axial distribution of particle concentration,where the solids holdup is small in the top,and large in the bottom.The basic requirements to form a fast fluidized bed are generally de-scribed as [4]:i)the circulating solid flux G s is greater than the min-imum value of that G sm ;and ii)for a given solid flux G s >G sm ,the super ficial gas velocity u f is kept within the range of velocities for type A choking and type C choking,i.e.u ch,C b u f b u ch,A .A large number of studies have been done to find how these char-acteristic parameters change with properties of the gas and the solid,the system geometry,and operating conditions.Empirical correla-tions were usually given in the form of non-dimensional criteria.For example,correlations for different types of choking were given by Yang [5,6],Yous fiand Gau [7],and other researchers [8–10].There were also quite a lot empirical correlations of solids holdups for both the upper dilute region and the bottom dense region [11–15].However,most of these studies were carried out separately;there were little physical relations among them.In some cases,incompatible results would be predicted from these empirical correlations,for in-stance u ch,A b u ch,C .On the other hand,the recently recognized “high density fast fluidization (HDFF)”[16]and “dense suspension up-flow (DSU)”[17]have shown some different two-phase-flow behaviors.How these flow regimes relate with the traditional one are also not clear.The present work tried to establish a self-consistent model for fast fluidization dynamics,in which all the characteristic parameters mentioned above can be easily deduced from a same origin;and the uni fied model can be applied for both traditional fast fluidized bed and high density fast fluidized bed.2.Theoretical considerations 2.1.Start point of the modelConcepts of choking in vertical upward co-current gas –solid systems have been used for a long time to describe some critical conditions,atPowder Technology 241(2013)126–141⁎Corresponding author.E-mail address:mczhang@ (M.-C.Zhang).0032-5910/$–see front matter ©2013Elsevier B.V.All rights reserved./10.1016/j.powtec.2013.01.070Contents lists available at SciVerse ScienceDirectPowder Technologyj o u r na l h o me p a g e :ww w.e l s e v i e r.c o m /l o c a t e /p o w t e cwhich the two-phase flow system cannot run properly or ef ficiently.For systems with different application purposes,the de finitions may also be different.In a pneumatic conveying system,the choking is usually de-fined as the onset of particle precipitation downward,which makes the transportation less ef ficient.However,it doesn't breach the system operation as a whole.On the opposite,in a CFB riser the choking is usu-ally de fined as a critical condition,at which a small decrease of operat-ing gas velocity or increase of circulating solid flux will cause signi ficant increase of bed pressure drop,leading to collapse,to some extents,of the whole system.Differences of the two types of choking did not get enough attention until Bi and Grace [18],where the former was de fined as the accumulative choking or type A choking,and the latter as the classical choking or type C choking.Predictions to type A choking and type C choking are obviously im-portant for fast fluidization,since they will determine the allowable ranges for gas velocity or solid circulation flux.There were quite a lot of formulas developed in the past to correlate the gas velocities and the solid fluxes at the two types of choking.However,only one of them can be chosen as the start point of the uni fied model,while the other should be deducted from the chosen one.Considering the re-search history for pneumatic conveying system is much longer than that for fast fluidized bed,and the two-phase flow structure in the for-mer is much simpler than that in the latter,it is believed that the theo-retical and experimental bases for type A choking are more reliable and universal.Among the numerous formulas existed in the literatures for type A choking,the form of Yang's formula [5,6]looks the best,since deriva-tion of the formula involves only two theoretical deductions.The first one is that the terminal velocity of particles for a uniform suspension of voidage εin a riser of diameter D t can be calculated from Eq.(1).u 0t ¼u t ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þf p u 2pt!Âε4:7v u u t ð1ÞWhere,u p =G s /[ρs (1−ε)]is the particle velocity,while f p stands for the solid-wall friction factor.This is a theoretically logical formula,and detailed derivation and discussion on that can be found in Yang's series work from 1973to1975[5,19,20].The second deduction is when type A choking happens,the slip velocity between gas and solid,i.e.the terminal velocity of a particlesuspension in a finite diameter riser u t',is just equal to the terminal velocity of a single particle in the in finity u t [5,6].u 0t ¼u tð2ÞThis deduction was actually used in the derivation of Yang's pre-dictive equation [5,6](Eq.(3)),but was not clearly declared and explained in his work.2D t g ε−4:7ch −1 u ch −u t ðÞ2¼f p ð3ÞThe followings are the present authors'try to explain “what does the deduction really mean?”It can be seen from Eq.(1),the first item withsquare root sign ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þf pu 2p2gD tr represents the in fluence of wall friction onu t',since it will always be unity in an in finitely wide riser.When D t gets smaller,the wall friction gets greater,then u t'gets greater.The second item with square root sign ffiffiffiffiffiffiffiffiε4:7p represents the in fluence of bed voidage,i.e.the in fluence of surrounding particles.When the particleconcentration (1−ε)gets greater,εgets smaller,then u t 'gets smaller.The reason is that the surrounding particles will cause both increase ofreal gas velocity ffiffiffiffiffiε2pand more flexuous flow-pass of gas around theparticle,then increase of the drag force coef ficient ffiffiffiffiffiffiffiffiε2:7p[21].There-fore,the amendment of u t 'is on the basis of super ficial gas velocity.If we resolve the increased fluid drag on one particle in a uniform suspension into the basal fluid drag on a single particle F f and the sur-plus F s due to the surrounding particles,the overall force balance for the suspension,or a single particle in a long time duration,can be expressed asF f þF s ¼G þW ð4Þwhere,G stands for the gravity of particle or particle suspension,whileW for the wall friction.Comparing the values of F s and W ,two different situations can be distinguished.For a relatively dilute suspension,the drag force caused by the surrounding particles is relatively small,resulting in F s b W ,and then F f >G .The fluid drag given by the gas solely F f is greater than the gravity of the particle,which means that besides supporting the particle there will be something rest to balance the wall friction,simply shown as F f −G =W −F s .This will be a relatively simple dy-namic system centered on each single particles;the relatively indepen-dent movement of these particles will lead to a uniformly dispersed gas-solid two phase flow,i.e.the custom dilute suspension transporta-tion.On the opposite,if we have F s >W and F f b G ,the fluid drag given by the gas F f will no longer be able to support the particle gravity solely,but needs something else from the surrounding particles,simply shown as F f +(F s −W )=G .Then the force balance for any particle will depend more on the others,making the dynamic system more complex and easier to lose its uniformity.For instance,an occasionally local condensation of particles will result in less gas flowing through them and decrease of F f ,which will make these particles to move close further to increase F s as the overall force balance needs.This may probably be the physical reason of cluster formation for the case of F s >W .Therefore,what can be used to separate the two different types of flows mentioned above is just the criteria of F s =W or F f =G ,i.e.u t'=u t from their de finitions.It means that the in fluence of finite riser diameter D t on u t'is just compensated by the in fluence of bed voidage εat the type A choking.At this unique condition,the moving particle looks as if there is neither wall nor surrounding particles.The authors suggest that this is the real meaning of the deduction,and then can be seen as the physical essence of type A choking.Just because of its sound physical meaning,the relationship given by Eq.(3)for the super ficial gas velocity and the bed voidage under solid-saturated conditions (type A choking)can be considered as “inherent ”,then the equation as “constitutive equation ”.However,it should also be pointed out here,the functional relation of u ch and εch is the most important in Eq.(3),but not the value of f p at this moment.Actually,the solid friction factor f p =0.01was taken originally by Yang in 1975[5],and changed later to the present form in 1983[6]f p ¼6:81Â105ρgρs2:2:ð5ÞThe rationality or accuracy of above mentioned values for f p will bediscussed in detail and re-correlation of f p with more experimental data will be made later in Section 3.3.For the time being,the solid fric-tion factor used in the calculations before Section 3.3was f p =0.01,the value given in 1975[5],since it was better than the other according to the accuracy evaluation of Xu et al.[10].2.2.Physical description of the modelAs we discussed above,Eq.(3)shows the relationship of super ficial gas velocity u ch and bed voidage εch under solid-saturated conditions,127M.-C.Zhang,C.Zhang /Powder Technology 241(2013)126–141i.e.type A choking.For a given fluidizing system,when u ch increases,εch will decrease;then the particle concentration (or solids hold-up)(1−εch )increases,and the saturation carrying capacity of gas G s ⁎will increase even faster.G Ãs ¼ρs u ch −u t ðÞ1−εchεchð6ÞThe equation derived here is a little bit different from what was used in Yang's articles [5,6]by 1/εch ,since the modi fication of ffiffiffiffiffiffiffiffiε4:7p includes also the in fluence of the gas velocity ffiffiffiffiffiε2p ,then the real par-ticle velocity is (u ch −u t )/εch .As an example,Fig.1shows the cal-culated results of (1−εch )and G s ⁎varied with u ch for a FCC-air fluidizing system.It can be seen from the figure that the saturation carrying capacity G s ⁎varies with the super ficial gas velocity u ch in an exponential form with power >1.Just because of this special relationship between G s ⁎and u ch ,one can imagine when the circulating solid flux is greater than G s ⁎at a given gas velocity,the system will not completely collapse but run in a more complicated separate-phase-coexistence mode.Some par-ticles will segregate from the gas stream and get agglomerated to form a free-falling dense phase (clusters),which occupies a part of the riser cross-sectional area,but without outer gas getting in;while in the rest of the riser,more concentrated gas with higher ve-locity could carry even more particles upward in the form of dilute phase.Without outer gas getting in clusters is a logical deduction from two-way stability analysis,and is coincident with Mueller and Reh's investigation [22],i.e.“the acceleration of particle inside the strand (cluster)is equal to the acceleration of free fall,which implies that no drag force acts on the particle within the strand ”[21].Let βbe the cross-sectional area fraction occupied by the falling dense phase or clusters,which is also the volume fraction of randomlydistributed clusters;m s−as the corresponding solid flux downward,and m s +as the solid flux upward in solid-saturated dilute phase.Both m s+and m s −are de fined on the basis of the total cross-sectional area,but not their own occupied.Then,the solid flux circulating into the riser G s can be expressed as G s ¼m þs −m −s :ð7ÞThe sketch map for m s +and m s −varied with βat a constant gas velocity is shown in Fig.2.It can be seen from the figure,if we haved m þs d βj β¼0>d m −sd βj β¼0;ð8Þthe separate-phase-coexistence mode can really transport more par-ticles upward in the riser than the saturation carrying capacity ofgas at type A choking,i.e.G s =m s +−m s −>G s ⁎.This will be trueuntil a critical point β=βch is reached,where d m þs d βj β¼βch ¼d m −sd βj β¼βch;ð9Þand the transportable solid flux at the given gas velocity takes its max-imum value G s,max .Beyond this point the separate-phase-coexistence mode has no more ability to balance the excess solid flux,leading to the system being totally “collapsed ”,i.e.the type C choking.The analysis above indicates also that the critical requirement for separate-phase-coexistence mode is d m þs d βj β¼0¼d m −sd βj β¼0:ð10ÞThis criterion can be used for determination of G sm ,which will bediscussed later elsewhere.As a sum,the key points af firmed or the basic assumptions to be used in the uni fied model are as follows:i)the relationship between bed voidage εch and gas velocity u ch given by Yang's formula for type A chok-ing can be seen as “inherent ”or “constitutive ”for solid-saturated dilute phase,which provides the theoretical basis of the model;ii)with neces-sary amendment,this relationship can also be applied for the solid-saturated upward dilute phase when coexisted separate-phases appear;and iii)in the separate-phase-coexistence mode,all the gas from outside enters the solid-saturated upward dilute phase,while the clusters fall down freely at a velocity consistent with their voidage (ex-cept for HDFF or DSU).3.Mathematical model kernels3.1.Model kernels3.1.1.Dilute phase modelAs discussed above,at type A choking,the very beginning of the separate-phase-coexistence mode (β=0),the gas –solid slip velocity can take the value of terminal velocity of a single particle u t .When separate phases visibly appear,due to the impact of falling clusters,it is expected that the saturation carrying capacity per unit gas in the dilute phase will be less than that for β=0.It means that the gas –solid slip velocity in this case has increased.However,this im-pact can also be expressed by decrease of the effective gas velocity in dilute phase,while keeping the slip velocity unchanged.This can123456G *s (k g /m 2s )u ch (m/s)1-c hεFig.1.Saturated carryings G s ⁎and solid concentration (1−εch )at different gas velocity u ch (FCC –air system,ρs =1620kg/m 3,d p =100μm,D t =0.1m).G s ,m sG *schββFig.2.Sketch map for m s +and m s −varied with βat constant gas velocity.128M.-C.Zhang,C.Zhang /Powder Technology 241(2013)126–141be easily done by using an effective velocity factor of dilute phase F (β)b 1in the calculation.Thus,the upward solid flux based on unit dilute phase area can be expressed as G þs ¼ρsu f F βðÞ−u t !1−εch εch ;ð11Þand the relationship between the calculation velocityu Ãch¼u f F βðÞ1−βand the solid-saturated dilute phase viodage εch still fits the revised Yang's formula,as we discussed above.Therefore,the super ficial up-ward solid flux de fined on the basis of the total cross-sectional area of the riser will be m þs ¼ρs u f F βðÞ−u t 1−βðÞ½1−εchεch:ð12ÞFrom the analysis above,the function F (β)chosen should meet the requirements of F βðÞj β¼0¼1;andd F βðÞd βjβ¼0¼0:ð13ÞThe simplest form of that is F βðÞ¼1−c βn:ð14ÞAs βincreases,the impact of falling clusters gets greater andgreater.The upward solids flux of dilute phase m s+will reach its max-imum,then decrease gradually until slugging occurs at β=1.Sincethe solid flux m s+at slugging is still finite but not zero,it can be roughly estimated that m s +|β=1=G s ⁎,i.e.the saturation carryingcapacity of gas at type A choking G s ⁎(see Fig.2).From that,we haveρs u f F 1ðÞ½ 1−εslεsl ¼ρs u f −u t ½ 1−εch ;A εch ;Að15Þwhere F (1)is the value of F (β)at β=1;εsl and εch,A are the bedvoidages for slugging (β=1)and type A choking (β=0),respec-tively.Therefore,F 1ðÞ¼1−u t f1−εch ;A εch ;A εslsl:ð16ÞSuppose the solid span equals to the gas span when slugging occurs,and the voidage equals εmf in the solid span,then we have the average asεsl ¼1−1−εmf ðÞ=2:ð17ÞFinally,the effective velocity factor of dilute phase F (β)can be calcu-lated asF βðÞ¼1−1−F 1ðÞ½ βn:ð18Þ3.1.2.Dense phase modelThe sub-model for dense-phase or falling clusters is relatively simple in form,i.e.m −s ¼ρs βu cl 1−εcl ðÞ:ð19ÞAs we mentioned above,the falling velocity of clusters u cl should be in consistence with their voidage εcl to keep the outside gas flow within the cluster being zero.Therefore,the modi fied Richardson –Zaki'sequation [23]must be satis fied for the force balance of gas-particles in-side of clusters,i.e.u cl ¼u tεcl1m¼u t εm cl :ð20ÞHere,m ¼lg u mfu t=lg εmf ;ð21Þand εmf =0.45was used in the following calculations.However,to determine both εcl and u cl ,a supplementary condi-tion is needed.This will be discussed later in Section 5.1,i.e.the weight of clusters should be balanced by the inter-phase drag,which can be solved with other parameters together through itera-tions.Nevertheless,for the initial value of εcl with iteration or a sim-pli fied calculation (no iteration)a rough estimation is still needed.According to the experimental data collected by Harris and Davidson [24],the solid concentration in clusters can be seen approximately twice of that in the dilute phase.Then,we have εcl ;0¼1−21−εch ðÞ¼2εch −1:ð22Þ3.1.3.Empirical estimation of model parameter nAccording to the derivations above,the model equations can be used in following procedures.i)The type A choking velocity u ch,A is calculated for a given solids flux G s (>G sm ).ii)Decrease super ficialgas velocity to make u f b u ch,A ;then,m s+and m s −are calculated by using different βuntil G s =m s +−m s −is satis fied;the voidage of upward dilute phase εch at the operating velocity u f is then deter-mined.iii)Repeat the steps above until type C choking occur;the type C choking velocity u ch,C is finally ing a different model parameter n ,the variations of dilute phase voidage εch with operating gas velocity u f for a FCC –air system (D t =0.1m,the typ-ical diameter of laboratory scale risers)are shown in Fig.3.It can be seen from the figure,when the operating gas velocity u f is close to u ch,C ,a small reduction of gas velocity will cause a great increase of bed concentration (1−ε).That is the characteristic feature of type C choking.It can also be seen from the figure,as n gets greater,the type C chok-ing velocity calculated gets smaller.Then,the empirical correlation0.920.930.940.950.960.970.980.991.00u f (m/s)u ch,CεFig.3.Variations of dilute phase voidage εch with gas velocity u f calculated from different n for a FCC –air system (D t =0.1m,ρs =1620kg/m 3,d p =100μm,G s =100kg/(m 2s)).129M.-C.Zhang,C.Zhang /Powder Technology 241(2013)126–141given by Yous fiand Gau [7](Eq.(23)),which was veri fied to be the best for type C choking [10],can be used to estimate the proper value of n .u ch ;C ffiffiffiffiffiffiffiffigd pq ¼32Re −0:06tG s g u ch ;C!0:28ð23ÞFrom this kind of “calibration ”,the model parameter n =4.5can be chosen for a simpli fied version of the model without iteration.More fundamental determination of the model parameter n will be given in detail in Section 3.2later.Fig.4shows the comparison be-tween the model predictions with n =4.5and those given by Eq.(23),for both FCC –air and sand –air systems with different parti-cle sizes (50,100,150and 200μm)and solid fluxes (50,100and 200kg/(m 2s)).The result looks quite satisfactory.3.2.Mechanistic determination of F(β)and nThe wake effect was used quite often in the literature to explain how the cluster was formed in a CFB riser.For example,Basu et al.stated in their Book [25],“For a given velocity,the feed rate may be increased to a level where the solid concentration will be so high that one particle will enter the wake of the other.When that hap-pens,the fluid drag on the first particle will decrease,and it will fall under gravity to drop on the trailing particle.The effective surface area of the pair just formed is low,and so the fluid drag will be lower than their combined weight,making the pair fall further to collide with other particles.Thus an increasing number of particles combine together to form particle agglomerates known as clusters.These clusters are,however,not permanent.They are continuously torn apart by the up-flowing gas.Thus,the formation of clusters and their disintegration continue.”Though most of these words were actually from analytical consequence only,the description was quite reasonable.Recently,He et al.carried out an excellent PIV measurement for particle movement in a CFB riser [26]showing clearly the details of this phenomenon as in Fig.5(a)and (b).They described in their arti-cle,“it can be easily seen that a cluster is followed by a wake,in which particles move downward quickly ”,and “when a cluster is passing by,the particles are dragged down at a higher velocity ”.Be-sides a very clear veri fication of the wake effect,the measurement showed that those particles moved towards the cluster at velocitiesof the same order for up-flowing ones,which will result in a notable deposition on the back side of the cluster.And due to the size limitation for a stable cluster,there must be the same quantities of particles pouring out the cluster from its nose.The continuous deposition and pour out of those particles will cause the downward displacement of particles inside the cluster,too.The particles moving towards the clus-ter in the wake,the inside displacement,and the front pour out can then be viewed as an integrated penetration of these particles through the cluster.From this point of view,the phenomenon can be analogous to what happens around a rising bubble in a bubbling bed as shown in Fig.1(c)[27].It can be seen that except the directions are opposite-down,the flow patterns of the two are quite similar.Furthermore,through phase reversal the following correspondences could be easily af-firmed.(1)A falling cluster with concentrated particles vs.a rising bubble with null or few particles;(2)the upward dilute flow around the cluster vs.the downward dense particle flow around the bubble;(3)the downward penetrating particle flow through the cluster vs.the upward penetrating gas flow through the bubble.The scenarios in CFB risers described above are quite in consistent with the results from detailed numerical simulations by combining the two-fluid model with the EMMS approach [21],i.e.“the particles tend to enter into clusters instead of suspending in dilute broth (phase),whereas the gas tends to pass around,instead of penetrating through,the dense cluster phase.”[21]Then,some results obtained for bubbling beds may also be used to estimate the in fluence of fall-ing clusters on the upward dilute phase,as explained below.Suppose that a spherical cluster of voidage εcl falls down in a di-lute suspension with a constant pressure gradient d p /dz =−J ,i.e.the pressure drop per unit riser height which can be calculated in the way described in Section 4.1below.If we put the cluster in a sur-rounding bed with the same voidage εcl ,the through flow penetrat-ing the cluster u fu under the pressure gradient −J would be the same as that in the surroundings.On the other hand,the cluster is also penetrated by a particle flow from the opposite direction,which results in a downward displacement of particles at u sd inside the cluster,as we discussed above.To keep the gas flow inside the cluster being null,which is a primer assumption for the model estab-lishment,there must be u sd =u fu .Just because the null flows inside of cluster,the pressures everywhere inside are constant.Therefore,the isobars above and below the cluster will get condensed,which will then suck more gas flowing through the cluster.Though the cluster is full of particles,it functions as an empty bubble.According to the well-known Davidson's model for a single bubble immerged in an incipiently fluidized bed [27],the total volume flow-rate through the cluster is q ¼3u pen πR 2clð24Þwhere R cl stands for the radius of the cluster,and u pen stands for the super ficial percolation velocity in a packed bed of voidage εcl under the pressure gradient −J .It can be easily calculated by the Ergun's equation or the extended Ergun's equations [21,28],i.e.Ergun's for εb 0.8and Wen's for ε≥0.8.Then the interstitial gas velocity through the cluster,which will be counteracted finally by the down-ward displacement of particles inside,can be written asu sd ¼u fu ¼3u pen =εcl :ð25ÞIn bubbling fluidized beds [27],rising bubbles can be classi fied as the fast bubble or the slow bubble,according to the ratio of the bub-ble rising velocity u br to the interstitial gas velocity far away from the bubble u f,∞.For a fast bubble [27],i.e.u br >u f,∞,the gas leaves at the top of the bubble will be sucked in again from the bottom;the1.01.52.02.53.03.54.0u c h ,C (m /s ,t h i s m o d e l )u ch,C (m/s,Yousfi & Gau)parison of model predictions for type C choking (n =4.5,D t =0.1m)and those from Yous fiand Gau [7].130M.-C.Zhang,C.Zhang /Powder Technology 241(2013)126–141。

Finite Element Modeling of Part DistortionT.D. Marusich, S. Usui, and K. J. MarusichThird Wave Systems, Inc, 7900 West 78th St. Suite 300, Minneapolis, MN 55439Abstract. Airframe designs for modern commercial and military aircraft con-tain hundreds of monolithic metal structural components machined from solidplate or forgings of aluminum and titanium. The start-to-finish weight ratio forthese components is easily 20:1 and greater, meaning that more than 95% of theinitial material is machined away. The resulting thin-walled structure often dis-torts due to the removal of bulk material stresses and the application of machin-ing-induced stresses. To alleviate the distortion, current practices rely on takingsmall depths of cut and flipping the part several times. This results in long cycletimes, high cost and under utilization of large machining equipment. A finiteelement model has been developed specifically to predict and control these dis-tortions. The model takes into consideration the machining induced residualstresses as well as the bulk stresses in the material from the manufacturer. It isthe combination of these two residual stress components coupled with the ge-ometry of the part and type of material that allows us to accurately predict,manage and control the shape and magnitude of deformation in large thinwalled structures. The model can be used to determine the ideal cutting condi-tions for the process as well as determine the best location within the stock plateto machine the part from. Multiple machining tests have been done in order tovalidate the accuracy of the model.1 IntroductionResidual stresses in the part have a significant quality and cost impact in many ma-chining operations. This is particularly true for large airframe and similar compo-nents, since strains induced by residual stresses may lead to significant distortion. This can lead to a high scrap rate for some components, and to extended machining times if material is removed in multiple light passes to control distortion.Residual stresses in machined parts result from two sources. Prior to machining, bulk stresses from primary processes such as rolling or forging may be present in the workpiece. Superimposed on these are residual stresses induced by the machining process, which result from differential plastic deformation and surface temperature gradients [1]. Significant distortions can arise from both sources, particularly in thin-walled parts.There has been considerable research on predicting machining-induced residual stresses, most recently using finite elements methods [2-7]. Predictions of the most advanced models are qualitatively accurate (with respect to the shape of the subsur-face stress distribution and the influence of major input variables); given the difficulty of repeatably measuring residual stresses, model predictions are as reliable as C. Xiong et al. (Eds.): ICIRA 2008, Part II, LNAI 5315, pp. 329–338, 2008.© Springer-Verlag Berlin Heidelberg 2008330 T.D. Marusich, S. Usui, and K.J. Marusichmeasurements in many cases. There has been little work, however, on predicting distortion in a machined structure due to bulk residual stresses from a primary proc-ess, although as noted above distortions from this source are often significant in thin-walled structures.This paper describes an integrated approach to predicting distortions in machined structures due to both bulk and machining-induced residual stresses. Previously re-ported work on predicting machining-induced residual stresses through finite element modeling is briefly reviewed in Section 2. In Section 3, the integration of theses stresses into a toolpath analysis program which also takes into account bulk stresses to predict distortions is described. Preliminary comparison of analytical predictions with measurements for airframe components are reviewed in Section 4. Section 5 summa-rizes conclusions and areas for further research.2 Finite Element Calculation of Machining Induced ResidualStressesFor this research, machining residual stresses were calculated using AdvantEdge TM, an explicit dynamic, thermo-mechanically coupled finite element model specialized for metal cutting. Features necessary to model metal cutting accurately include adap-tive remeshing capabilities for resolution of multiple length scales such as cutting edge radius, secondary shear zone and chip load; multiple body deformable contact for tool-workpiece interaction, and transient thermal analysis. A comprehensive dis-cussion on the numerical techniques and validation examples are available in the literature [8,9].In order to model chip formation, constitutive modeling for metal cutting requires de-termination of material properties at high strain rates, large strains, and short heating times and is quintessential for prediction of segmented chips due to shear-localization [10,11]. Specific details of the constitutive model used are outlined in [8]. The model contains deformation hardening, thermal softening and rate sensitivity tightly coupled with a transient heat conduction analysis appropriate for finite deformations.Machining-induced residual stresses are computed by thermo-mechanically relaxing the workpiece after machining. After the tool passes, thermo-mechanical computations are continued until transient thermal gradients in the machined surface dissipate and the work sample comes an equilibrium configuration with a resultant residual stress distribution.3 Integrating Bulk and Machining Induced Residual StressesTo determine the effects of both machining induced and bulk residual stresses on distortion of a machined structure, an approach combining FEA results with toolpath-level analysis is required. Fig. 1 show the approach used in this research. The Tool Path Analysis component, adapted from the AdvantEdge Production Module, a com-mercially available mechanistic model for 5-axis end milling [12], reads in the initial workpiece geometry, tool path, workpiece material properties, and generates the final geometry of the workpiece. The Distortion Prediction component generates a mesh ofFinite Element Modeling of Part Distortion 331 the final geometry of the workpiece, builds a finite element model, imposes bulk stresses and residual stresses from machining onto the model and performs an equilib-rium analysis. Results of predicted workpiece distortions are displayed through a visualization tool.Fig. 1. Approach to modeling distortionFig. 2. Solid model of four pocket partThe Toolpath Analysis component is based on well known mechanistic modeling approaches and will not be described. The modules that make up the Distortion Pre-diction component are described in the following.Workpiece MeshingUsing existing methods for toolpath generation and Boolean subtraction of workpiece material along toolpath, the final geometry of the thin-walled workpiece is generated. From the final geometry, a shell mesh is generated.332 T.D. Marusich, S. Usui, and K.J. MarusichA typical part for illustrating this procedure is shown in Fig. 2. In this part all ribs are perpendicular to the XY plane, but can taper in the Z-direction.The thin-walled workpieces are modeled by a mesh of triangular shell elements. The solution is obtained by implicit methods.Fig. 3. Local and global coordinate systemsFig. 4. Typical thin-walled sectionExplicit stiffness matrix expressions for plate element are used where all stiffness matrix computations are done in a local coordinate system defined for each element. With reference to Fig. 3, the origin of the local coordinate system is located at node 1. The local X-axis is defined by the unit vector (e12) along a line connecting node 1 to node 2.The unit vector along local Z axis (e z) is obtained by taking the cross product of e12 and e13, where e13 is the unit vector along a line connecting node 1 to node 3. Finally, unit vector along local Y axis (e y) is obtained by taking the cross product of e z and e12.The rearranged stiffness matrix is transformed to global coordinate system and as-sembled into the global stiffness matrix.Fig. 4 shows a section of typical thin-walled components to be analyzed. The ge-ometry consists of ribs (walls), webs (flanges) and fillets. Ribs are subjected toFinite Element Modeling of Part Distortion 333 machining induced stresses from side cutting whereas webs are subjected to machin-ing induced stresses from end milling. In general, the fillet region is small compared to the cross-section of the component. As a first approximation, it may be assumed that stress in the fillet regions do not contribute significantly to the overall distortion of the part. With this, the mapping task simplifies to determining procedures for wall and rib mapping.Machining induced stresses for different process conditions, computed through fi-nite element calculations as discussed above, are stored in a machining induced stress database. The database consists of machining induced stress profiles (Fig. 5) as func-tion of the cutting speed, feed rate, depth of cut, tool nose radius, and radial and axial rake angles.Fig. 5. Typical machining induced stress profileFig. 6. Residual stresses in 80 mm thick 7050 T74 Al plate [13] Fully three-dimensional stress profiles on the machined surface can be stored.The toolpath is used to obtain the solid model of the final geometry of the work-piece. The machining induced stress mapping is performed on the surface of the thin-walled workpiece. Residual stresses are determined from the database with process parameters computed at the workpiece surface and applied to the corresponding ele-ment in the mesh. Nodal forces in the element due to applied residual stresses are334 T.D. Marusich, S. Usui, and K.J. Marusichcomputed and stored to be used later in the assembly of the global RHS vector. Nodal forces from residual stresses are computed and stored for all elements on the surface. Bulk stresses from the primary process are incorporated in an analogous manner. Limited published data for these profiles is available in the literature; a typical exam-ple is shown in Fig. 6 [13]. These stresses are applied to the mesh in a manner analo-gous to that used for the machining induced stresses.Once the stresses are applied, assembly of the global stiffness matrix, global right hand side (RHS) vector and application of boundary conditions are done using stan-dard Finite Element techniques.4 Preliminary Application to Aerospace ComponentsAnalyses were carried out for two aerospace structural parts provided by EADS, the fillet rib and pressure bulkhead shown in Figs. 7 and 8. Both the parts are made of Al7050 and have several pockets, thin walls of different thicknesses, and 5-axis walls.Fig. 7. Fillet rib (1.51 m x 785mm x 38mm) Fig. 8. Pressure Bulkhead (1.63m x 3.14m x 76mm)Fig. 9. Mesh for fillet rib.Finite Element Modeling of Part Distortion 335 The mesh for the fillet rib, which contains 22,736 elements and was automatically generated, is shown in Fig. 9. This part is machined from a rectangular block of di-mensions 1.62 m x 900mm x 60mm. Distortions computed are shown in Fig. 10. CMM measurements of distortions from a machined component, taken at the points illustrated in Fig. 11, are shown in Fig. 12. Fig. 13 shows the comparison between the measured and predicted distorted shapes. As can be seen, the measured and predicted distortions agree well both qualitatively and quantitatively in this case.Fig. 10. Predicted distorted shape of fillet rib Fig. 11. Locations at which distortionwas measured by CMMFig. 12. Surface plot of measured distortion Fig. 13. Comparison of computed andmeasured distortions for the fillet ribFig. 14. Pressure bulkhead mesh of 147,769 elements Fig. 15. Predicted distorted shape of pres-sure bulkhead336 T.D. Marusich, S. Usui, and K.J. MarusichFig. 16. Locations at which distortion was measured by feeler gauges Fig. 17. Measured distorted shape of pressure bulkheadFig. 18. Comparison of prediction versus measurement for pressure bulkheadFig. 19. Machined 800mm x 100mm x 25mm machined partThe mesh for the pressure bulkhead, which contains 147,769 elements, is shown in Fig. 14. The pressure bulkhead was machined from a rectangular block of dimensions3.33 m x 1.82 m x 100mm. The final part is located 10.0 mm from the bottom of rectangular block. Distortion of the component after machining was measured at sev-eral points (see Fig. 16) on the bottom flange along the circumference using feeler gauges. The measured distortion profile is shown in Fig. 17, and a comparison of predicted and measured distortions is shown in Fig. 18. In this case, the measured and computed distortions agree well qualitatively, but the calculated values significantly overestimate the measured values at the outer edges of the part. This is partly due to the geometry of the part; given its large dimensions, a slight errors in distortion near the center would be magnified at the edges due to a parallax effect.An aerospace componenent 800mm x 100mm x 25mm high was machined from 75mm thick Al7050-T7451 plate material, Fig. 19. The corresponding distorted measurement was performed, Fig 20, and compared with the predicted distortedFinite Element Modeling of Part Distortion 337Fig. 20. Distortion measurement of machined partFig. 21. Predicted distortion of machined part Fig. 22. Comparison of predicted andmeasured distortion for 800mm long partconfiguration, Fig 21 and Fig 22. Excellent agreement between both shape and distor-tion magnitude are seen.5 Summary and ConclusionsThis paper summarizes a methodology for calculating distortions in thin-walled components due to both bulk and machining induced residual stresses. A tool path analysis program is used to determine the final part geometry, mesh the final part for distortion analysis, and apply stress components. Machining induced stresses are applied based on a database of values computed through finite element analysis for a338 T.D. Marusich, S. Usui, and K.J. Marusichrange of representative cutting and tooling conditions. Measured bulk stresses in the parent billet are stored in a corresponding database. Preliminary validation of the bulk stress component for two aerospace parts is presented.AcknowledgementsWe are grateful to Martin Hand of EADS for valuable comments on this work, and for providing the fillet rib and pressure bulkhead data files and measurement data. References1.Stephenson, D.A., Agapiou, J.S.: Metal Cutting Theory and Practice, 2nd edn., pp. 568–569. CRC, Boca Raton (2006)2.Wiesner, C.: Residual Stresses After Orthogonal Machining of AISI 304: Numerical Cal-cuation of the thermal Component and Comparison with Experiments. Metall. Trans. A 23, 989–996 (1992)3.Shirakashi, T., Obikawa, T., Sasahara, H., Wada, T.: Analytical Prediction of the Charac-teristics Within Machined Surface Layer (1st Report, The Analysis of the Residual Stress Distribution). J. Jpn. Soc. Precis. Eng. 59, 1695–1700 (1993)4.Shih, A.J., Yang, H.T.Y.: Experimental and Finite Element Predictions of ResidualStresses Due to Orthogonal Metal Cutting. Int. J. Num. Meth. Eng. 36, 1487–1507 (1993) 5.Liu, R., Guo, Y.B.: Finite Element Analysis of the Effect of Sequential Cuts and Tool-Chip Friction on Resdiual Stresses in a Machined Layer. Int. J. Mech. Sci. 42, 1069–1086 (2000)6.Lundblad, M.: Influence of Cutting Tool Geometry on Residual Stress in the Workpiece.In: Proc. 2002 Third Wave AdvantEdge User’s Conference, Atlanta, GA, vol. 7 (2002)7.Shet, C., Deng, X.: Residual Stresses and Strains in Orthogonal Metal Cutting. Int. J. Ma-chine Tools Manuf. 43, 573–587 (2003)8.Marusich, T.D., Ortiz, M.: Modeling and Simulation of High-Speed Machining. Int. J.Num. Meth. Eng. 38, 3675–3694 (1995)9.Marusich, T.D.: Effects of Friction and Cutting Speed on Cutting Force. In: Proc. IMECE(ASME), New York, Paper No. MED-23313, November 11–16 (2001)10.Sandstrom, D.R., Hodowany, J.N.: Modeling the Physics of Metal Cutting in High-SpeedMachining. Machining Science and Technology 2, 343–353 (1998)11.Childs, T.H.C.: Material Property Needs in Modelling Metal Machining. Machining Sci-ence and Technology 2, 303–316 (1998)12./products13.Prime, M.B., Hill, M.R.: Residual stress, stress relief, and inhomogeneity in aluminumplate. Scripta Materialia 46, 77–78 (2002)。

Modeling Bipolar Devices Using the MEXTRAM ModelIntroductionThe MEXTRAM bipolar model was recently released into the public domain by Philips Electronics N.V. and this model is now supported by UTMOST and SmartSpice. This article will give a very brief introduction to the MEXTRAM model. Some measured DC device characteristics will be modeled with the MEXTRAM equations and the results of this UTMOST parameter extraction experiment will be shown here.The MEXTRAM ModelAccurate and reliable simulation of bipolar circuits require that the circuit simulator model in use can describe a large number of physical phenomena. The extended Gummel-Poon model, commonly used by circuit designers, often fails be meet the required accuracy criteria. The MEXTRAM equations model the following effects:∙Temperature effects∙Charge storage effects∙Substrate effects including the parasitic PNP∙High-injection effects∙Built-in electric field in base region∙Bias-dependent Early effect∙Low-level non-ideal base currents∙Hard and quasi-saturation∙Weak avalanche∙Hot carrier effects in the collector epilayer∙Explicit modeling of the inactive regions∙Split base-collector depletion capacitance∙Current crowding and conductivity modulation for base resistance∙First-order approximation of distributed high frequency effects in the intrinsic base (high-frequency current crowding and excess phase-shift).Like most other bipolar models MEXTRAM does not contain extensive geometrical or process scaling rules. A multiplication factor does exist in the model for parallel transistor arrangements. In total the MEXTRAM model contains 39 parameters for the modeling of current and charge, 13 temperature scaling parameters, and 3 parameters in its noise model. The MEXTRAM model has five internal nodes and model computing times should be, on average, three times greater than if the Gummel-Poon model were used. Full details of the model equations are available elsewhere [1,2].MEXTRAM Parameter Extraction ExampleThe following data was measured for an NPN bipolar device:a) Forward Gummel Datab) Reverse Gummel Datac) IC versus VCE at 3 base current levelsd) IE versus VEC at 3 base current levelsThe measured data is plotted in Figure 1. Local optimization strategies were used to extract the required DC parameters from this data. An entire parameter extraction procedure for the MEXTRAM model using capacitance, DC, and fT data can be found elsewhere [3] and the strategies used in this example were based on the DC portion of this procedure.Figure 1. A complete measured data set for the NPN bipolar device.Four local optimization strategies were implemented for the extraction of a partial parameter set for the MEXTRAM model. These are described in Table 1. Some parameter descriptions appear in Table 2.Strategy Steps Parameters DataA 4 BF, IS, IBF, VLF Forward Gummel (low and medium VBE levels)BF, IK, RBC, RBV,RE, VBEForward Gummel (high and medium levels)B 4 BRI, ISS, IBR, VLR Reverse Gummel (low and medium VBC levels)BRI, IKS, RCC,XEXTReverse Gummel high and medium VBC levels)C 1 XCJC, BF (refine) IC versus VCED 1 QBO, BRI (refine) IE versus VECTable 1. Details of the UTMOST local optimization strategies used.Parameter DescriptionIS BF IBF VLF IK RBC RBV RE ISS BRI IBR VLR IKS RCC XEXT XCJC QB0 Collector-emitter saturation currentIdeal forward gainSaturation current of the non-ideal forward base currentCross-over voltage of the non-ideal forward base currentHigh-injection knee currentConstant part of base resistanceVariable part of base resistance at zero biasEmitter series resistanceBase-substrate saturation currentIdeal reverse current gainSaturation current of the non-ideal reverse base currentCross-over voltage of the non-ideal reverse base currentKnee current of the substrateConstant part of collector resistancePartitioning factorFraction of collector-base depletion capacitance under the emitter area Base charge at zero biasTable 2. A description of the extracted MEXTRAM parameters.Strategy A involves the extraction of parameters to the forward Gummel characteristics. Using the UTMOST local optimization environment the collector and base current data was split up into various regions, four in all, and the associated parameters were extracted. In the second stage of the extraction procedure a similar analysis was performed on the reverse Gummel data. At this stagethe majority of the parameters to be extracted to the DC characteristics were determined. In the final two local optimiza tion strategies the parameters which model the forward and reverse early effects were extracted using the IC versus VCE and IE versus VEC data sets respectively. During these extraction stages the ideal forward and reverse gain parameters were also refined. The accuracy of the extracted MEXTRAM model surpasses that of a SPICE Gummel-Poon model derived from the same data. Improvements were most obvious in the modeling of the reverse characteristics.The entire extraction process took less than 1 minute on a Sparc 2 computer and the results were good. Figure 2 shows the measured and simulated forward characteristics including a plot of the forward gain versus VBE. Figure 3 shows the measured and simulated reverse characteristics including a plot of reverse gain versus VBC.Figure 2. Measured (ooooo) and simulated (____) forward characteristics.Figure 3. Measured (ooooo) and simulated (____) reverse characteristics.ConclusionsThis article has given an introduction to the Philips MEXTRAM bipolar model which has been implemented into SmartSpice and UTMOST. A very accurate model of measured device characteristics was obtained very quickly using some simple user-defined local optimization strategies.AcknowledgementSILVACO would like to thank Willy Kloosterman of Philips Research Laboratories, Eindhoven, The Netherlands, for his help during the course of this work.References[1] b. Unclassified Report 006/94.The Mextram Bipolar Transistor Model.H.C. de Graff and W.J. Kloosterman.[2] H.C. de Graff and W.J. Kloosterman,"Modeling of the Collector Epilayer of a Bipolar Transistorin the MEXTRAM Model,"IEEE Trans. on Electron Devices, Vol. 42, No. 2,February 1995,pp. 274-282.[3] W.J. Kloosterman, J.A.M. Geelen and D.B.M. Klaassen, "Efficient Parameter Extraction for the MEXTRAM Model"to be published in the proceedings of the IEEE BCTM'95 conference.。

英语三级笔译综合能力2004年试卷Section 1 Vocabulary and Grammar (25 points)This section consists of three parts. Bead the directions for each part before answering the questions. The time for this section is 25 minutes.Part 1 Vocabulary SelectionIn this party there are 20 incomplete sentences. Below each sentence, there are four choices respectively marked by letters A, B, C and D. Choose the word or phrase which best completes each sentence. There is only ONE right answer. Then blacken the corresponding letter with a single bar across the square brackets on your Machine-scoring ANSWER SHEET.1. Grover Cleveland was the first president ______ in the White House.A. got marriedB. to get marriedC. has got marriedD. was married2. If cauliflowers are not _____ from extreme temperatures, the heads get discolored.A. protectedB. shelterC. shadeD. saved3. The gas ______ from the tank is dangerous.A. given offB. giving outC. giving awayD. given up4. When it started to snow, we turned round and _____ the hotel.A. got byB. searched forC. made forD. cleared up5. Since writing home to their parents for money, they had lived _____ hope.A. inB. forC. onD. through6. Rice is the ______ food of most Southeast Asians.A. commonB. generalC. stapleD. popular7. William Byrd was the owner of the largest library in colonial ______.A. periodB. timeC. timesD. periods8. Exobiology is the study of life ______ other planets.A. inB. atC. onD. to9. The Declaration of Independence, _______ the Constitution of the United States,was drawn up with the help of Benjamin Franklin.A. andB. alsoC. as well asD. so too10. It was from the Lowell Laboratory that the ninth ______, Pluto, was sighted in1930.A. planetB. constellationC. stardomD. satellite11 .The rodent, _______ the mouse, rat, guinea pig, and porcupine, are mammals withincisor-like teeth in both jaws.A. made upB. includingC. consistingD. constitute12. ______ into oceans and rivers is a serious form of pollution.A. Pouring sewageB. Emptying litterC. Throwing garbageD. Dumping sewage13. Products which are made from dirts and are _____ high temperatures are knownas ceramics.A. tempered inB. subjected toC. exposed toD. baked in14. A pigment called melanin protects the _____ layers of skin from sun rays.A. underB. belowC. underlyingD. underneath15. Oranges are a ______ source of vitamin C.A. wellB. betterC. goodD. very16. Even after having their grandchildren live with them for ten years, the couple feltthat ______ children these days was the most difficult of all family matters.A. risingB. raisingC. caringD. taking care17. The most important ______ of the farmers in Iraq is dates, of which Iraq is theworld's leading exporter.A. economic cropB. cash cropC. money cropD. staple18. More has been learned about the Moon than any other of the Earth's neighbors inspace because of the Apollo program, which enabled men to walk on the Moon and bring back hundreds of pounds of _____.A. rocksB. rockC. stoneD. stones19. _____ the variety that the average family has in beef, fish, poultry, and vegetarianrecipes, they find most meals unexciting.A. InspiteB. InspiteC. Despite ofD. Despite20. The speaker _____ have criticized the paraprofessionals, knowing full well thatthey were seated in the audience.A. should not toB. must notC. ought not toD. may notPart 2 Vocabulary ReplacementThis part consists of 15 sentences in which one word or phrase is underlined. Below each sentence, there are four choices respectively marked by letters A, By C and D. Choose the word or phrase that can replace the underlined part without causing any grammatical error or changing the basic meaning of the sentence. There is only ONE right answer. Blacken the corresponding letter with a single bar across the square brackets on your Machine-scoring ANSWER SHEET.21. Iceland has the oldest parliament, which goes as far back to 930 A. D. whenAlthing, the legislative organization, was established.A. officeB. adobeC. assemblyD. building22. The only problem with the debate last week was that the beginning sounded morelike a personal attack than a dispassionate, intellectual arguing.A. discussionB. argumentC. talkD. speech23. Susan Jones was at the bus stop well on time to take the 7:01 bus, but she had tomiss her breakfast to do it.A. catch up withB. catchC. run up toD. be catching24. Since her father could not drive her to the airport, she requested her uncle to driveher instead.A. takeB. bringC. dispatchD. deliver25. A famous collection of Persian, Indian, and Arabian folktales, the Arabian Nightswas supposedly told by the legendary queen Scheherazade to her husband every night for 1,001 days.A. imaginaryB. imageryC. fabledD. legend26. What may be the oldest fossil footprint yet found was discovered in June 1968 byWilliam J. Meister, a non-professional fossil collector.A. a part-timeB. a spare-timeC. an untrainedD. an amateur27. Most of us think of sharks as dangerous, owing to lack of information rather thanfear.A. due toB. becauseC. asD. for28. Double Eagle D, the first trans-Atlantic balloon, was greeted by avid crowds inFrance.A. eagerB. surgingC. appreciativeD. vigorous29. The discovery of the connection between aspirin and Reyessyndrome, a rare anddeadly ailment, is a recent example of the caution with which drugs must be used, even for medical purposes.A. diseaseB. sickC. illD. illness30. My parents moved out of their old home sometime last year after they hadcelebrated their 50th year there.A. anniversaryB. years oldC. ageD. wedding31. The library she worked in lent books, magazines, audio-cassettes and maps to itscustomers, who could keep them for four weeks.A. borrowersB. lendersC. patronsD. clients32. A common question that people ask a story writer is whether or not he hasexperienced what he has written about.A. fictionB. scienceC. imaginaryD. literary33. At the World literacy Center, an organization that works to help people read, thehelpers work hard, enabling them to successfully reach their goals.A. assistantsB. volunteersC. part-timersD. amateurs34. The officers made it clear that they were letting her go only because that she wasold and not because she was above suspicion.A. for reasonB. due toC. because ofD. on the grounds35. The book, which is a useful guide for today's young people, deals with manyquestions and problems that face them at school and at home as well as in society.A. are facedB. confrontC. in oppositionD. meetPart 3 Error CorrectionThis part consists of 75 sentences in which there is an underlined part that indicates a grammatical error. Below each sentence, there are four choices respectively marked by letters A, B, C and D. Choose the word or phrase that can replace the underlined part so that the error is corrected. There is only ONE right answer. Blacken the corresponding letter with a single bar across the square brackets on your Machine-scoring ANSWERSHEET.36. All don't have a free ticket must pay the admission fee.A. Everyone who doesn't have a free ticketB. No one who doesn't have a free ticketC. No one who has free ticketsD. Anyone who has free tickets37. When I last saw them, the police had chased the robbers down Columbus Street.A. were chasingB. was chasingC. chasedD. were on a chase38. Erosion that is a slow process, but it constantly changes the features on the surfaceof the earth.A. which isB. althoughC. beingD. is39. When an organism is completely encapsulated and preserved, it becomes a fossil,therefore turning into evidence of things that once lived.A. therebyB. as a result ofC. soD. in the end40. The pictures of the Loch Ness Monster show a remarkable resemblance to aplesiosaur, a large water reptile of the Mesozoic era presuming extinct for more than 70 million years.A. supposedB. presumablyC. presumptuousD. is presumed41. In our own galaxy, the Milky Way, there are perhaps 200 billion stars, a small partof them probably have planets on which life is feasible.A. a small fraction in whichB. a small fraction of whichC. a small fraction whichD. which a fraction of42. "But you'll be able to come, won’t you?" "Yes, I think such."A. thatB. itC. soD. this43. The professor is quite difficult pleased.A. to pleaseB. to be pleasedC. for pleasingD. pleasing44. Because everyone knows, facts speak louder than words.A. SinceB. ThatC. ItD. As45. The trapeze artist who ran away with the clown broke up the lion tamer's heart.A. broke awayB. broke downC. brokeD. broken down46. His heavy drinking and fond of gambling makes him a poor role model.A. and fact that he gamblesB. and that he gamblesC. and he gambles whichD. and gambling47. Depression that inflicts people who believe their lives lack content when the rushof the busy week stops referred to by a prominent psychiatrist as Sunday Neurosis.A. has been referred to by a prominent psychiatristB. has been referred to as by a prominent psychiatristC. a prominent psychiatrist has referred to itD. it has been referred to by a prominent psychiatrist48. Just as there are occupations that require college degrees also there areoccupations for which technical training is necessary.A. so to there areB. so too there areC. so there areD. so too are there49. Most of the older civilizations which flourished during the fifth century B. C. aredied out.A. they have died outB. has died outC. have died outD. they had died out50. The student asked her professor if he would have gone on the spaceship he didknow earlier.A. if he knewB. if he knowsC. he had knownD. had he known Section 2 Reading Comprehension (55 points)In this section you will find after each of the passages a number of questions or unfin-ished statements about the passage, each with four (A, B, C and D) choices to com-plete the statement. You must choose the one which you think fits best. Then blacken the corresponding letter with a single bar across the square brackets on your Machine-scoring ANSWER SHEET.Passage OneQuestions 51 - 56 are based on die following passage.Awarded the Nobel Prize for physics in 1918, German physicist Max Planck is best remembered as the originator of the quantum theory. His work helped usher in a new era in theoretical physics and revolutionized the scientific community’s understanding of atomic and subatomic processes.Planck introduced an idea that led to the quantum theory, which became the foundation of twentieth century physics. In December 1900, Planck worked out an equation that described the distribution of radiation accurately over the range of low to high frequencies. He had developed a theory which depended on a model of matter that seemed very strange at the time. The model required the emission of electromagnetic radiation in small chunks or particles. These particles were later called quantums. The energy associated with each quantum is measured by multiplying the frequency of the radiation, v, by a universal constant, h. Thus, energy, or E, equals hv. The constant, h, is known as Planck's constant. It is now recognized as one of the fundamental constants of the world.Planck announced his findings in 1900, but it was years before the full consequences of his revolutionary quantum theory were recognized. Throughout his life, Planck made significant contributions to optics, thermodynamics and statistical mechanics, physical chemistry, among other fields.51. In which of the following fields did Max Planck NOT make a significantcontribution?A. Optics.B. Thermodynamics.C. Statistical mechanics.D. Biology.52. The word "revolutionary" as used in Line 15 means_.A. radicalB. extremistC. momentousD. militaristic53. It can be inferred from the passage that Planck’s work led to the development of________.A. The rocketB. The atomic bombC. The internal combustion engineD. The computer54. The particles of electromagnetic radiation given off by matter are known as ____.A. quantumsB. atomsC. electronsD. valences55. The implication in this passage is that ______.A. only a German physicist could discover such a theoryB. quantum theory, which led to the development of twentieth century physics, isbasically a mathematical formulaC. Planck's constant was not discernible before 1900D. radiation was hard to study56. ―An idea‖ as used in line 5, refers to _____.A. a model of matterB. emission of electromagnetic radiationC. quantumsD. the equation that described the distribution of radiation accurately over the range of low to high frequenciesPassage TwoQuestions 57 ~ 62 are based on the following passage.There has been much speculation about the origin of baseball. In 1907 a special commission decided that the modern game was invented by Abner Doubleday in 1839. One hundred years later the National Baseball Museum was opened to honor Doubleday. Historians, however, disagree about the origin of baseball. Some say that baseball comes from bat-and-ball games of ancient times. It is a matter of record that in the 1700s English boys played a game they called ―baseball‖. Americans have played a kind of baseball since about 1800. At first the American game had different rules and different names in various parts of the country —―town ball‖, ―rounders‖, or ―one old cat‖. Youngsters today still play some of these simplified forms of the game.Baseball did not receive a standard set of rules until 1845, when Alexander Cartwright organized the Knickerbocker Baseball Club of New York City. The rules Cartwright set up for his nine-player team were widely adopted by other clubs and formed the basis of modern baseball. The game was played on a "diamond" infield with the bases 90 feet apart. The first team to score 21 runs was declared the winner. By 1858 the National Association of Baseball Players was formed with 25 amateur teams. The Cincinnati Red Stockings began to pay players in 1869.57. Which of the following is true about the origins of baseball?A. Historians agree that baseball was invented by Abner Doubleday.B. Baseball, as played in the early 19th century, differed very little from today'sgame.C. As early as the 1700s, English boys played a game called "baseball".D. The first standard set of baseball rules was established at the turn of the century.58. What was the first professional baseball team called?A. New York Knickerbockers.B. Milwaukee Braves.C. Cincinnati Red Stockings.D. Brooklyn Dodgers.59. Who first gave baseball a standard set of rules?A. Abner Doubleday.B. Alexander Cartwright.C. Albert Spalding.D. Babe Ruth.60. Which of the followings was not a predecessor of baseball?A. Rounders.B. Town ball.C. Cricket.D. One old cat.61. The tone of the passage is ______.A. persuasiveB. informativeC. biasedD. argumentative62. The passage implies that until 1869, baseball was played for all of the followingreasons except _______.A. exerciseB. leisureC. profitD. socializingPassage ThreeQuestions 63-68 are based on the following passage.The blue of the sea is caused by the scattering of sunlight by tiny particles suspended in the water. Blue light, being of short wavelength, is scattered more efficiently than light of longer wavelengths. Although waters of the open ocean are commonly some shade of blue, green water is commonly seen near coasts, especially in tropical or subtropical regions. This is caused by yellow pigments being mixed with blue water. Phytoplankton are one source of the yellow pigment. Other microscopic plants may color the water brown or brownish-red. Near the shore, silt or sediment in suspension can give water a brownish hue. Outflow of large rivers can often be observed many miles offshore by the coloration of suspended soil particles.Marine phytoplankton (Greek for "plant wanderers") are microscopic single-celled plants that include diatoms, dinoflagellates, coccolithophorids, green algae, and blue-green algae, among others. The growth of these organisms, which photogynthesize light, depends on a delicate balance of nutrient enrichment via vertical mixing, which is often limited by the availability of nitrogen and light. Diatoms are one-celled plants with patterned glass coverings. Each glass, or silicon dioxide box, is ornamented with species-specific designs, pits, and perforations making them popular with microscopists and, more recently, electron scanning microscopists.63. Green water near coastlines is almost always caused by _____.A. sand colorB. red pigments in coastal watersC. blue pigmentD. reflected light and yellow pigment from plant life64. Phytoplankton are the source of which color pigment?A. Red.B. Green.C. Yellow.D. Blue.65. What can give waters a brownish hue near the shore?A. Sediment.B. Phytoplankton.C. Blue pigment.D. Diatoms.66. Which of the following is NOT a type of phytoplankton?A. Green algae.B. Diatoms.C. Blue-green algae.D. Amoeba.67. The growth of phytoplankton is often limited by the availability of _____.A. oxygenB. hydrogenC. nitrogenD. carbon dioxide68. The main idea of this passage is that _____.A. light causes sea colorB. sea coloration is varied because of a combination of length of light waves andmicroscopic plant life and siltC. microscopic plant life causes sea colorD. water composition causes sea colorPassage FourQuestions 69 - 75 are based on the following passage.The United States government publishes guidelines for appropriate nutrient intakes. These are known as the Recommended Dietary Allowances (RDAs) and are updated regularly based on new research in nutrition. RDAs are suggested amounts of calories, protein, and some minerals and vitamins for an adequate diet. For other dietary substances, specific goals must await further research. However, for the U.S. population as a whole, increasing starch and fiber in one's diet and reducing calories (primarily from fats, sugar, and alcohol) is sensible. These suggestions are especially appropriate for people who have other factors for chronic diseases due to family history of obesity, premature heart disease, diabetes, high blood pressure, and high blood cholesterol, or for those who use tobacco.Snacks can furnish about one-fourth of the calorie requirements among teenagers. Those snacks should also provide much of the day's allowances for protein, minerals, and vitamins. Sandwiches, fruit, and milk make good snacks for active teenagers. Food from the food pyramid may be part of any meal. A grilled cheese sandwich or a bowl of whole-grain cereal is just as nutritious in the morning as it is at noon. In addition, a good breakfast consists of any foods that supply about one-fourth of the necessary nutrients for the day.69. The passage directly states that most of the U. S. population should increase theirintake of ______.A. proteinB. fatsC. starch and fiberD. sandwiches70. A good breakfast should supply about what percentage of the necessary nutrientsfor the day?A. One-half.B. One-third.C. One-fourth.D. Less than one-fourth.71. The passage implies which of the following?A. The rime of day when food is consumed affects its nutritive value.B. Different foods can be combined to increase total nutrition value.C. It can be detrimental to your health to eat breakfast foods later in the day.D. When food is eaten has no bearing on its nutritive effects.72. Why are RDAs regularly updated?A. New discoveries in the science of nutrition are constantly being made.B. Americans' diets are constantly changing.C. As people age, their nutritional needs change.D. Very little is currently known about nutrition.73. In this passage RDAs refers to___.A. types of vitaminsB. types of proteinC. types of mineralsD. amounts of energy, protein, vitamins, and minerals74. One implication in this passage is that _____.A. all RDAs have been establishedB. not all RDAs have been established yetC. it's not important to know RDAsD. RDAs are necessary only for sick people75. The reduction of calories in the diet is particularly good for people who sufferfrom ________.A. obesityB. premature heart disease and diabetesC. high blood pressure and cholesterol levelsD. all of the abovePassage FiveQuestions 76 - 81 are based on the following passage.The most popular organic gem is the pearl. A pearl is the response of a marine mollusk to the presence of an irritating impurity accidentally introduced into its body;a cultured pearl is the result of the intentional insertion of a mother-of-pearl bead into a live mollusk. Whether introduced accidentally or intentionally, the pearl-making process is the same: the mollusk coats the irritant with a substance called nacre. Nacre is composed chiefly of calcium carbonate. Because very few natural pearls are now on the market, most pearls used in fine jewelry are cultured. These include "Biwa" pearls and most other freshwater pearls. Cultured pearls are not easily distinguished from natural pearls except by an expert.76. Which of the following people could tell the difference between a cultured pearland an organic pearl?A. Scuba diver.B. Fisherman.C. Jeweler.D. Clerk.77. What is the chief component of nacre?A. Sand.B. Bead.C. Calcium carbonate.D. Biwa.78. The difference between a pearl and a cultured pearl is the nature of the ____.A. colorB. introduction of the irritating impurityC. coating materialD. irritating impurity79. Nacre is a substance that is ______.A. mechanically manufacturedB. the result of laboratory testingC. organically secreted by the molluskD. present in the chemical composition of freshwater pounds80. The main idea of this passage is that ______.A. most marketable pearls are cultured because nature does not produce enough ofits own to satisfy the marketB. cultured pearls are of a higher quality than natural pearlsC. there are two major methods of pearl-makingD. a natural ―drought‖ of pearl production is taking place81. Cultured pearl is formed by ____.A. insertion of a pearl into a live molluskB. an oyster into which a piece of grit has been placedC. putting in a live molluskD. placing a bead into culturePassage SixQuestions 82-87 are based on the following passage.Stress is with us all the time. It comes from mental or emotional activity as well as physical activity. It is unique and personal to each of us. So personal, in fact, that what may be relaxing to one person may be stressful to another. For example, if you're a busy executive who likes to keep occupied all of the time, "taking it easy" at the beach on a beautiful day may be extremely frustrating, nonproductive, and upsetting. You may be emotionally distressed from "doing nothing." Too much emotional stress can cause physical illnesses such as high blood pressure, ulcers, or even heart disease. Physical stress from work or exercise is not likely to cause such ailments. The truth is that physical exercise can help you to relax and to better handle your mental or emotional stress.82. Which of the following people would find ―taking it easy‖ stressful?A. Construction workers.B. Business executives.C. Farm workers.D. Truck drivers.83. Which of the following would be a determinant as to what people find stressful?A. Personality.B. Education.C. Marital status.D. Shoe size.84. This article, published by the Department of Health and Human Services,probably came from the ______.A. Federal Bureau of InvestigationB. Alcohol, Drug Abuse, and Mental Health AdministrationC. Education AdministrationD. Communicable Diseases Administration85. A source of stress NOT specifically mentioned in this passage is _____.A. educational activityB. physical activityC. mental activityD. emotional activity86. Physical problems caused by emotional stress can appear as all of the followingEXCEPT _____.A. ulcersB. pregnancyC. heart diseaseD. high blood pressure87. One method mentioned to help handle stress is ____.A. physical exerciseB. tranquilizersC. drugsD. taking it easy Passage SevenQuestions 88 ~ 92 are based on the following passage.With the sudden onset of severe psychotic symptoms, the individual is said to be experiencing acute schizophrenia (精神分裂症) - "Psychotic" means out of touch with reality, or unable to separate real from unreal experiences. Some people have only one such psychotic episode. Others have many episodes during a lifetime but lead relatively normal lives during interim periods. The individual with chronic (continuous or recurring) schizophrenia often does not fully recover normal functioning and typically requires long-term treatment, generally including medication, to control the symptoms. These symptoms may include hallucinations (幻觉), incoherence, delusions, lack of judgment, deterioration of the abilities to reason and feel emotion, and a lack of interaction between the patient and his environment. The hallucinations may be a visual, auditory, or tactile. Some chronic schizophrenic patients may never be able to function without assistance of one sort or another.88. Which of the following is not a symptom of schizophrenia?A. Hallucinations.B. Delusions.C. Incoherence.D. Vertigo.89. It can be inferred from the passage that a person experiencing acute schizophreniamost likely ______.A. cannot live without medicationB. cannot go on livingC. can hold a full-time jobD. cannot distinguish real from unreal90. According to this passage, thinking that one can fly might be an example of ____.A. medicine overdoseB. being out of touch with realityC. recovering normal functioningD. symptom control91. The passage suggests that the beginning of severe psychotic symptoms of acuteschizophrenia may be any of the following EXCEPT_____.A. debilitatingB. sudden occurrenceC. occurring after a long period of normalcyD. drug-induced92. The passage implies that normal life may be possible for the chronicschizophrenic with the help of ______.A. medicinesB. neurotic episodesC. psychotic episodesD. time Passage EightQuestions 93 ~ 100 are based on the following passage.Aspirin is one of the safest and most effective drugs invented by man. The most popular medicine in the world today, it is an effective pain reliever. Its bad effects are relatively mild. It is also cheap.For millions of people suffering from arthritis, it is the only thing that works. Aspirin, in short, is truly the 20th-century wonder drug. It is also the second largest suicide drug and is the leading cause of poisoning among children. It has side effects that, although relatively mild, are largely unrecognized among users.Although aspirin was first sold by a German company in 1899, it has been around much longer than that. Hippocrates, in ancient Greece, understood the medical value of tree barks and leaves which today are known to contain a chemical found in aspirin. During the 19th century, there was a great deal of experimentation in Europe with this。

英语专业八级考试TEM-8阅读理解练习册（1）(英语专业2012级)UNIT 1Text AEvery minute of every day, what ecologist生态学家James Carlton calls a global ―conveyor belt‖, redistributes ocean organisms生物.It’s planetwide biological disruption生物的破坏that scientists have barely begun to understand.Dr. Carlton —an oceanographer at Williams College in Williamstown,Mass.—explains that, at any given moment, ―There are several thousand marine species traveling… in the ballast water of ships.‖ These creatures move from coastal waters where they fit into the local web of life to places where some of them could tear that web apart. This is the larger dimension of the infamous无耻的，邪恶的invasion of fish-destroying, pipe-clogging zebra mussels有斑马纹的贻贝.Such voracious贪婪的invaders at least make their presence known. What concerns Carlton and his fellow marine ecologists is the lack of knowledge about the hundreds of alien invaders that quietly enter coastal waters around the world every day. Many of them probably just die out. Some benignly亲切地，仁慈地—or even beneficially — join the local scene. But some will make trouble.In one sense, this is an old story. Organisms have ridden ships for centuries. They have clung to hulls and come along with cargo. What’s new is the scale and speed of the migrations made possible by the massive volume of ship-ballast water压载水— taken in to provide ship stability—continuously moving around the world…Ships load up with ballast water and its inhabitants in coastal waters of one port and dump the ballast in another port that may be thousands of kilometers away. A single load can run to hundreds of gallons. Some larger ships take on as much as 40 million gallons. The creatures that come along tend to be in their larva free-floating stage. When discharged排出in alien waters they can mature into crabs, jellyfish水母, slugs鼻涕虫，蛞蝓, and many other forms.Since the problem involves coastal species, simply banning ballast dumps in coastal waters would, in theory, solve it. Coastal organisms in ballast water that is flushed into midocean would not survive. Such a ban has worked for North American Inland Waterway. But it would be hard to enforce it worldwide. Heating ballast water or straining it should also halt the species spread. But before any such worldwide regulations were imposed, scientists would need a clearer view of what is going on.The continuous shuffling洗牌of marine organisms has changed the biology of the sea on a global scale. It can have devastating effects as in the case of the American comb jellyfish that recently invaded the Black Sea. It has destroyed that sea’s anchovy鳀鱼fishery by eating anchovy eggs. It may soon spread to western and northern European waters.The maritime nations that created the biological ―conveyor belt‖ should support a coordinated international effort to find out what is going on and what should be done about it. (456 words)1.According to Dr. Carlton, ocean organism‟s are_______.A.being moved to new environmentsB.destroying the planetC.succumbing to the zebra musselD.developing alien characteristics2.Oceanographers海洋学家are concerned because_________.A.their knowledge of this phenomenon is limitedB.they believe the oceans are dyingC.they fear an invasion from outer-spaceD.they have identified thousands of alien webs3.According to marine ecologists, transplanted marinespecies____________.A.may upset the ecosystems of coastal watersB.are all compatible with one anotherC.can only survive in their home watersD.sometimes disrupt shipping lanes4.The identified cause of the problem is_______.A.the rapidity with which larvae matureB. a common practice of the shipping industryC. a centuries old speciesD.the world wide movement of ocean currents5.The article suggests that a solution to the problem__________.A.is unlikely to be identifiedB.must precede further researchC.is hypothetically假设地，假想地easyD.will limit global shippingText BNew …Endangered‟ List Targets Many US RiversIt is hard to think of a major natural resource or pollution issue in North America today that does not affect rivers.Farm chemical runoff残渣, industrial waste, urban storm sewers, sewage treatment, mining, logging, grazing放牧,military bases, residential and business development, hydropower水力发电,loss of wetlands. The list goes on.Legislation like the Clean Water Act and Wild and Scenic Rivers Act have provided some protection, but threats continue.The Environmental Protection Agency (EPA) reported yesterday that an assessment of 642,000 miles of rivers and streams showed 34 percent in less than good condition. In a major study of the Clean Water Act, the Natural Resources Defense Council last fall reported that poison runoff impairs损害more than 125,000 miles of rivers.More recently, the NRDC and Izaak Walton League warned that pollution and loss of wetlands—made worse by last year’s flooding—is degrading恶化the Mississippi River ecosystem.On Tuesday, the conservation group保护组织American Rivers issued its annual list of 10 ―endangered‖ and 20 ―threatened‖ rivers in 32 states, the District of Colombia, and Canada.At the top of the list is the Clarks Fork of the Yellowstone River, whereCanadian mining firms plan to build a 74-acre英亩reservoir水库，蓄水池as part of a gold mine less than three miles from Yellowstone National Park. The reservoir would hold the runoff from the sulfuric acid 硫酸used to extract gold from crushed rock.―In the event this tailings pond failed, the impact to th e greater Yellowstone ecosystem would be cataclysmic大变动的，灾难性的and the damage irreversible不可逆转的.‖ Sen. Max Baucus of Montana, chairman of the Environment and Public Works Committee, wrote to Noranda Minerals Inc., an owner of the ― New World Mine‖.Last fall, an EPA official expressed concern about the mine and its potential impact, especially the plastic-lined storage reservoir. ― I am unaware of any studies evaluating how a tailings pond尾矿池，残渣池could be maintained to ensure its structural integrity forev er,‖ said Stephen Hoffman, chief of the EPA’s Mining Waste Section. ―It is my opinion that underwater disposal of tailings at New World may present a potentially significant threat to human health and the environment.‖The results of an environmental-impact statement, now being drafted by the Forest Service and Montana Department of State Lands, could determine the mine’s future…In its recent proposal to reauthorize the Clean Water Act, the Clinton administration noted ―dramatically improved water quality since 1972,‖ when the act was passed. But it also reported that 30 percent of riverscontinue to be degraded, mainly by silt泥沙and nutrients from farm and urban runoff, combined sewer overflows, and municipal sewage城市污水. Bottom sediments沉积物are contaminated污染in more than 1,000 waterways, the administration reported in releasing its proposal in January. Between 60 and 80 percent of riparian corridors (riverbank lands) have been degraded.As with endangered species and their habitats in forests and deserts, the complexity of ecosystems is seen in rivers and the effects of development----beyond the obvious threats of industrial pollution, municipal waste, and in-stream diversions改道to slake消除the thirst of new communities in dry regions like the Southwes t…While there are many political hurdles障碍ahead, reauthorization of the Clean Water Act this year holds promise for US rivers. Rep. Norm Mineta of California, who chairs the House Committee overseeing the bill, calls it ―probably the most important env ironmental legislation this Congress will enact.‖ (553 words)6.According to the passage, the Clean Water Act______.A.has been ineffectiveB.will definitely be renewedC.has never been evaluatedD.was enacted some 30 years ago7.“Endangered” rivers are _________.A.catalogued annuallyB.less polluted than ―threatened rivers‖C.caused by floodingD.adjacent to large cities8.The “cataclysmic” event referred to in paragraph eight would be__________.A. fortuitous偶然的，意外的B. adventitious外加的，偶然的C. catastrophicD. precarious不稳定的，危险的9. The owners of the New World Mine appear to be______.A. ecologically aware of the impact of miningB. determined to construct a safe tailings pondC. indifferent to the concerns voiced by the EPAD. willing to relocate operations10. The passage conveys the impression that_______.A. Canadians are disinterested in natural resourcesB. private and public environmental groups aboundC. river banks are erodingD. the majority of US rivers are in poor conditionText CA classic series of experiments to determine the effects ofoverpopulation on communities of rats was reported in February of 1962 in an article in Scientific American. The experiments were conducted by a psychologist, John B. Calhoun and his associates. In each of these experiments, an equal number of male and female adult rats were placed in an enclosure and given an adequate supply of food, water, and other necessities. The rat populations were allowed to increase. Calhoun knew from experience approximately how many rats could live in the enclosures without experiencing stress due to overcrowding. He allowed the population to increase to approximately twice this number. Then he stabilized the population by removing offspring that were not dependent on their mothers. He and his associates then carefully observed and recorded behavior in these overpopulated communities. At the end of their experiments, Calhoun and his associates were able to conclude that overcrowding causes a breakdown in the normal social relationships among rats, a kind of social disease. The rats in the experiments did not follow the same patterns of behavior as rats would in a community without overcrowding.The females in the rat population were the most seriously affected by the high population density: They showed deviant异常的maternal behavior; they did not behave as mother rats normally do. In fact, many of the pups幼兽，幼崽, as rat babies are called, died as a result of poor maternal care. For example, mothers sometimes abandoned their pups,and, without their mothers' care, the pups died. Under normal conditions, a mother rat would not leave her pups alone to die. However, the experiments verified that in overpopulated communities, mother rats do not behave normally. Their behavior may be considered pathologically 病理上，病理学地diseased.The dominant males in the rat population were the least affected by overpopulation. Each of these strong males claimed an area of the enclosure as his own. Therefore, these individuals did not experience the overcrowding in the same way as the other rats did. The fact that the dominant males had adequate space in which to live may explain why they were not as seriously affected by overpopulation as the other rats. However, dominant males did behave pathologically at times. Their antisocial behavior consisted of attacks on weaker male,female, and immature rats. This deviant behavior showed that even though the dominant males had enough living space, they too were affected by the general overcrowding in the enclosure.Non-dominant males in the experimental rat communities also exhibited deviant social behavior. Some withdrew completely; they moved very little and ate and drank at times when the other rats were sleeping in order to avoid contact with them. Other non-dominant males were hyperactive; they were much more active than is normal, chasing other rats and fighting each other. This segment of the rat population, likeall the other parts, was affected by the overpopulation.The behavior of the non-dominant males and of the other components of the rat population has parallels in human behavior. People in densely populated areas exhibit deviant behavior similar to that of the rats in Calhoun's experiments. In large urban areas such as New York City, London, Mexican City, and Cairo, there are abandoned children. There are cruel, powerful individuals, both men and women. There are also people who withdraw and people who become hyperactive. The quantity of other forms of social pathology such as murder, rape, and robbery also frequently occur in densely populated human communities. Is the principal cause of these disorders overpopulation? Calhoun’s experiments suggest that it might be. In any case, social scientists and city planners have been influenced by the results of this series of experiments.11. Paragraph l is organized according to__________.A. reasonsB. descriptionC. examplesD. definition12．Calhoun stabilized the rat population_________.A. when it was double the number that could live in the enclosure without stressB. by removing young ratsC. at a constant number of adult rats in the enclosureD. all of the above are correct13.W hich of the following inferences CANNOT be made from theinformation inPara. 1?A. Calhoun's experiment is still considered important today.B. Overpopulation causes pathological behavior in rat populations.C. Stress does not occur in rat communities unless there is overcrowding.D. Calhoun had experimented with rats before.14. Which of the following behavior didn‟t happen in this experiment?A. All the male rats exhibited pathological behavior.B. Mother rats abandoned their pups.C. Female rats showed deviant maternal behavior.D. Mother rats left their rat babies alone.15. The main idea of the paragraph three is that __________.A. dominant males had adequate living spaceB. dominant males were not as seriously affected by overcrowding as the otherratsC. dominant males attacked weaker ratsD. the strongest males are always able to adapt to bad conditionsText DThe first mention of slavery in the statutes法令，法规of the English colonies of North America does not occur until after 1660—some forty years after the importation of the first Black people. Lest we think that existed in fact before it did in law, Oscar and Mary Handlin assure us, that the status of B lack people down to the 1660’s was that of servants. A critique批判of the Handlins’ interpretation of why legal slavery did not appear until the 1660’s suggests that assumptions about the relation between slavery and racial prejudice should be reexamined, and that explanation for the different treatment of Black slaves in North and South America should be expanded.The Handlins explain the appearance of legal slavery by arguing that, during the 1660’s, the position of white servants was improving relative to that of black servants. Thus, the Handlins contend, Black and White servants, heretofore treated alike, each attained a different status. There are, however, important objections to this argument. First, the Handlins cannot adequately demonstrate that t he White servant’s position was improving, during and after the 1660’s; several acts of the Maryland and Virginia legislatures indicate otherwise. Another flaw in the Handlins’ interpretation is their assumption that prior to the establishment of legal slavery there was no discrimination against Black people. It is true that before the 1660’s Black people were rarely called slaves. But this shouldnot overshadow evidence from the 1630’s on that points to racial discrimination without using the term slavery. Such discrimination sometimes stopped short of lifetime servitude or inherited status—the two attributes of true slavery—yet in other cases it included both. The Handlins’ argument excludes the real possibility that Black people in the English colonies were never treated as the equals of White people.The possibility has important ramifications后果，影响.If from the outset Black people were discriminated against, then legal slavery should be viewed as a reflection and an extension of racial prejudice rather than, as many historians including the Handlins have argued, the cause of prejudice. In addition, the existence of discrimination before the advent of legal slavery offers a further explanation for the harsher treatment of Black slaves in North than in South America. Freyre and Tannenbaum have rightly argued that the lack of certain traditions in North America—such as a Roman conception of slavery and a Roman Catholic emphasis on equality— explains why the treatment of Black slaves was more severe there than in the Spanish and Portuguese colonies of South America. But this cannot be the whole explanation since it is merely negative, based only on a lack of something. A more compelling令人信服的explanation is that the early and sometimes extreme racial discrimination in the English colonies helped determine the particular nature of the slavery that followed. (462 words)16. Which of the following is the most logical inference to be drawn from the passage about the effects of “several acts of the Maryland and Virginia legislatures” (Para.2) passed during and after the 1660‟s?A. The acts negatively affected the pre-1660’s position of Black as wellas of White servants.B. The acts had the effect of impairing rather than improving theposition of White servants relative to what it had been before the 1660’s.C. The acts had a different effect on the position of white servants thandid many of the acts passed during this time by the legislatures of other colonies.D. The acts, at the very least, caused the position of White servants toremain no better than it had been before the 1660’s.17. With which of the following statements regarding the status ofBlack people in the English colonies of North America before the 1660‟s would the author be LEAST likely to agree?A. Although black people were not legally considered to be slaves,they were often called slaves.B. Although subject to some discrimination, black people had a higherlegal status than they did after the 1660’s.C. Although sometimes subject to lifetime servitude, black peoplewere not legally considered to be slaves.D. Although often not treated the same as White people, black people,like many white people, possessed the legal status of servants.18. According to the passage, the Handlins have argued which of thefollowing about the relationship between racial prejudice and the institution of legal slavery in the English colonies of North America?A. Racial prejudice and the institution of slavery arose simultaneously.B. Racial prejudice most often the form of the imposition of inheritedstatus, one of the attributes of slavery.C. The source of racial prejudice was the institution of slavery.D. Because of the influence of the Roman Catholic Church, racialprejudice sometimes did not result in slavery.19. The passage suggests that the existence of a Roman conception ofslavery in Spanish and Portuguese colonies had the effect of _________.A. extending rather than causing racial prejudice in these coloniesB. hastening the legalization of slavery in these colonies.C. mitigating some of the conditions of slavery for black people in these coloniesD. delaying the introduction of slavery into the English colonies20. The author considers the explanation put forward by Freyre andTannenbaum for the treatment accorded B lack slaves in the English colonies of North America to be _____________.A. ambitious but misguidedB. valid有根据的but limitedC. popular but suspectD. anachronistic过时的，时代错误的and controversialUNIT 2Text AThe sea lay like an unbroken mirror all around the pine-girt, lonely shores of Orr’s Island. Tall, kingly spruce s wore their regal王室的crowns of cones high in air, sparkling with diamonds of clear exuded gum流出的树胶; vast old hemlocks铁杉of primeval原始的growth stood darkling in their forest shadows, their branches hung with long hoary moss久远的青苔;while feathery larches羽毛般的落叶松,turned to brilliant gold by autumn frosts, lighted up the darker shadows of the evergreens. It was one of those hazy朦胧的, calm, dissolving days of Indian summer, when everything is so quiet that the fainest kiss of the wave on the beach can be heard, and white clouds seem to faint into the blue of the sky, and soft swathing一长条bands of violet vapor make all earth look dreamy, and give to the sharp, clear-cut outlines of the northern landscape all those mysteries of light and shade which impart such tenderness to Italian scenery.The funeral was over,--- the tread鞋底的花纹/ 踏of many feet, bearing the heavy burden of two broken lives, had been to the lonely graveyard, and had come back again,--- each footstep lighter and more unconstrained不受拘束的as each one went his way from the great old tragedy of Death to the common cheerful of Life.The solemn black clock stood swaying with its eternal ―tick-tock, tick-tock,‖ in the kitchen of the brown house on Orr’s Island. There was there that sense of a stillness that can be felt,---such as settles down on a dwelling住处when any of its inmates have passed through its doors for the last time, to go whence they shall not return. The best room was shut up and darkened, with only so much light as could fall through a little heart-shaped hole in the window-shutter,---for except on solemn visits, or prayer-meetings or weddings, or funerals, that room formed no part of the daily family scenery.The kitchen was clean and ample, hearth灶台, and oven on one side, and rows of old-fashioned splint-bottomed chairs against the wall. A table scoured to snowy whiteness, and a little work-stand whereon lay the Bible, the Missionary Herald, and the Weekly Christian Mirror, before named, formed the principal furniture. One feature, however, must not be forgotten, ---a great sea-chest水手用的储物箱,which had been the companion of Zephaniah through all the countries of the earth. Old, and battered破旧的，磨损的, and unsightly难看的it looked, yet report said that there was good store within which men for the most part respect more than anything else; and, indeed it proved often when a deed of grace was to be done--- when a woman was suddenly made a widow in a coast gale大风，狂风, or a fishing-smack小渔船was run down in the fogs off the banks, leaving in some neighboring cottage a family of orphans,---in all such cases, the opening of this sea-chest was an event of good omen 预兆to the bereaved丧亲者;for Zephaniah had a large heart and a large hand, and was apt有…的倾向to take it out full of silver dollars when once it went in. So the ark of the covenant约柜could not have been looked on with more reverence崇敬than the neighbours usually showed to Captain Pennel’s sea-chest.1. The author describes Orr‟s Island in a(n)______way.A.emotionally appealing, imaginativeB.rational, logically preciseC.factually detailed, objectiveD.vague, uncertain2.According to the passage, the “best room”_____.A.has its many windows boarded upB.has had the furniture removedC.is used only on formal and ceremonious occasionsD.is the busiest room in the house3.From the description of the kitchen we can infer that thehouse belongs to people who_____.A.never have guestsB.like modern appliancesC.are probably religiousD.dislike housework4.The passage implies that_______.A.few people attended the funeralB.fishing is a secure vocationC.the island is densely populatedD.the house belonged to the deceased5.From the description of Zephaniah we can see thathe_________.A.was physically a very big manB.preferred the lonely life of a sailorC.always stayed at homeD.was frugal and saved a lotText BBasic to any understanding of Canada in the 20 years after the Second World War is the country' s impressive population growth. For every three Canadians in 1945, there were over five in 1966. In September 1966 Canada's population passed the 20 million mark. Most of this surging growth came from natural increase. The depression of the 1930s and the war had held back marriages, and the catching-up process began after 1945. The baby boom continued through the decade of the 1950s, producing a population increase of nearly fifteen percent in the five years from 1951 to 1956. This rate of increase had been exceeded only once before in Canada's history, in the decade before 1911 when the prairies were being settled. Undoubtedly, the good economic conditions of the 1950s supported a growth in the population, but the expansion also derived from a trend toward earlier marriages and an increase in the average size of families; In 1957 the Canadian birth rate stood at 28 per thousand, one of the highest in the world. After the peak year of 1957, thebirth rate in Canada began to decline. It continued falling until in 1966 it stood at the lowest level in 25 years. Partly this decline reflected the low level of births during the depression and the war, but it was also caused by changes in Canadian society. Young people were staying at school longer, more women were working; young married couples were buying automobiles or houses before starting families; rising living standards were cutting down the size of families. It appeared that Canada was once more falling in step with the trend toward smaller families that had occurred all through theWestern world since the time of the Industrial Revolution. Although the growth in Canada’s population had slowed down by 1966 (the cent), another increase in the first half of the 1960s was only nine percent), another large population wave was coming over the horizon. It would be composed of the children of the children who were born during the period of the high birth rate prior to 1957.6. What does the passage mainly discuss?A. Educational changes in Canadian society.B. Canada during the Second World War.C. Population trends in postwar Canada.D. Standards of living in Canada.7. According to the passage, when did Canada's baby boom begin?A. In the decade after 1911.B. After 1945.C. During the depression of the 1930s.D. In 1966.8. The author suggests that in Canada during the 1950s____________.A. the urban population decreased rapidlyB. fewer people marriedC. economic conditions were poorD. the birth rate was very high9. When was the birth rate in Canada at its lowest postwar level?A. 1966.B. 1957.C. 1956.D. 1951.10. The author mentions all of the following as causes of declines inpopulation growth after 1957 EXCEPT_________________.A. people being better educatedB. people getting married earlierC. better standards of livingD. couples buying houses11.I t can be inferred from the passage that before the IndustrialRevolution_______________.A. families were largerB. population statistics were unreliableC. the population grew steadilyD. economic conditions were badText CI was just a boy when my father brought me to Harlem for the first time, almost 50 years ago. We stayed at the hotel Theresa, a grand brick structure at 125th Street and Seventh avenue. Once, in the hotel restaurant, my father pointed out Joe Louis. He even got Mr. Brown, the hotel manager, to introduce me to him, a bit punchy强力的but still champ焦急as fast as I was concerned.Much has changed since then. Business and real estate are booming. Some say a new renaissance is under way. Others decry责难what they see as outside forces running roughshod肆意践踏over the old Harlem. New York meant Harlem to me, and as a young man I visited it whenever I could. But many of my old haunts are gone. The Theresa shut down in 1966. National chains that once ignored Harlem now anticipate yuppie money and want pieces of this prime Manhattan real estate. So here I am on a hot August afternoon, sitting in a Starbucks that two years ago opened a block away from the Theresa, snatching抓取，攫取at memories between sips of high-priced coffee. I am about to open up a piece of the old Harlem---the New York Amsterdam News---when a tourist。