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立项报告1、项目简介水下超空泡射弹是一种新型水下武器，在功能上与脱壳穿甲弹相似，依靠弹道末端高的动能存量打击目标。
超空泡射弹的工作介质是水，而物体在水中所受到的阻力约为在空气中的1000倍，用常规方法明显提高水下航行体速度要受到很大的限制。
一段时间以来，研究人员尝试了各种减阻的方法，如边界层抽吸减阻等，但减阻效果通常都不理想。
超空泡减阻技术是一种可以使水下高速运动航行体获得90%减阻量的革命性减阻方法，基于这种新概念、新原理研制的水下超空泡射弹，可以突破普通射弹水下运动极限，使水下射弹的速度提高到1000m/s的量级，大大增加射弹的行程和杀伤力，提高进攻和防御能力。
射弹在水中高速运动时，贴近其表面的液体压力就会降低，当射弹的速度增加到某一临界值时，流体的压力将等于其饱和蒸汽压力，此时流体就会发生相变，由液相变为汽相，这就是空化现象。
随着航行体速度的不断增加，空泡沿着航行体表面不断后移、扩大、发展进而形成超空泡。
它的形成使液体对物体表面的浸湿面积减少，从而大大降低了粘性阻力，达到减阻的效果。
本项目拟通过对已有的射弹结构进行动力特性和流体动力分析，并综合考虑阻力系数、升力系数等各种水动力系数以及应用情况来最终优化结构设计，进一步减小射弹运动时受到的阻力、提高其运动时的稳定性。
2、申请理由本试验小组四人，均来自航天学院飞行器设计与工程专业，在中学时都受过专门的数学竞赛培训，拥有扎实的数学功底和娴熟的研究技能。
通过一年的大学学习，我们熟练掌握了高等数学和线性代数这两项研究中基础性的工具以及必要的工程制图知识，学会了查阅文献的基本方法。
在导师魏英杰教授的指导下，我们查阅了相关的文献，对本课题已经具有了充分的了解和认识。
在与学长们的交流中，我们接触并自学了Matlab、AutoCAD等工具软件。
在接下来的暑假中，我们计划自学相关的专业知识如流体力学和结构动力学等，并自学相关的力学分析软件如Fluent，为接下来的设计优化工作打好基础。
螺旋桨瞬态流场数值模拟周文松【摘要】建立完善的螺旋桨绕流理论异常困难，这一方面是由于螺旋桨流场整体旋转性、非定常非线性、三维效应、气流粘性、涡桨干扰等原因；另一方面是螺旋桨流场的流场结构有待于进一步认知。
文章采用CFD方法对螺旋桨的瞬态流场结构进行了重点研究，详细绘制了自启动瞬间直至充分发展的螺旋桨瞬态流场结构图及所受气动力的变化图，分析了流场变化特性及其对升力的影响。
%The establishment of a perfect theory for the propeller flow field has been a hard job up to now. Firstly, the pro-peller flow field has many complicated characters such as the entire rotation, the unsteady and nonlinear characters, the 3-D effects, the interference between the vortex and the propeller, and so on. S econd, the structure of the flow field hasn ’t been well known. In this paper, the aerodynamic performance of the instantaneous propeller flow field was studied. The de-tailed flow field structures and the evolution of the aerodynamic forces were drawn. The characters of the flow field and its effects on lift and moment were analyzed.【期刊名称】《海军航空工程学院学报》【年(卷),期】2014(000)003【总页数】5页(P247-251)【关键词】螺旋桨;瞬态流场;数值模拟【作者】周文松【作者单位】海军航空工程学院训练部，山东烟台264001【正文语种】中文【中图分类】V211.44螺旋桨在军用和民用上有着广泛的应用。
英语推荐信范文及翻译英语推荐信英语推荐信英语推荐信英语推荐信篇 1 Dear Colleagues: MS. XXX requested a letter of reference from me to support her application for graduate studies at your university. As her research adviser when she was a graduate student in my school of science Beijing University of Chemical Technology, I am pleased to comply with her request. I have known Ms. Zhang since 2003, when she was admitted as a Master of Science candidate into the school. As her research adviser, I directed her research and found her a promising youth radiating with intelligence and creativity. During her first year, she got a very good record in major courses and earned good scores major and general GPA 3.3. She particularly enjoyed challenging areas of studies such as Quantum Chemistry and Theory of Electrochemistry. During the following two years, Ms. Zhang worked on a computational theory study of inorganic functional materials project for his degree thesis “Theoretical Study of Electronic Structures of Several Representative Metal Element in the Hydrotalcite Slabs”. To make the theory model meet the actual materials, she faced down many practical problems, such as building reasonable module of inorganic materials, and calculation techniques. Working hard and independently, she cracked the problems one by one, and came up with a thesis that was characterized by academic excellence. During her M.S. period, she had three papers published in international journals. Judging by her outstanding performance while she studied with me, I am convinced that Ms. Zhang has acquired the knowledge and intellectualsophistication on the basis of which she can undertake world-class training. I therefore lend her my enthusiastic support and would appreciate your favorable consideration of her application. From: x x x x Department of the ***** CLASS GIFTED for YOUTHS University of Science and Technology Hefei, Anhui *****, P.R. China July 1, 1998 英语推荐信篇2 Dear Sir or x x x:I take great pleasure in recommending Ren Ren, one of my favorite students, for admission into your distinguished graduate program. Mr. Ren was admitted in 1986 at 14 years of age into the ***** CLASS for the GIFTED YOUTHS, my university“s unique program that caters to the intellectual needs of unusually talented Chinese youngsters. It was a rare privilege he earned with his nearly impeccable academic performance through the years of his elementary and secondary school. He impressed me almost as he entered into my university, a major cradle of China“s scientific and technological talents. At the time, members of the Gifted Class all had to spend half a month studying by themselves the principles of calculus and then take an exam so that we could evaluate their self-study capability. Mr. Ren scored the highest grade in that exam. He also exhibited a keenly whetted mind during class discussions. To my regret at the time, his English was not as good as his mathematics or physics. But I noticed he made a point of working especially hard in improving his English during his five undergraduate years with us. By now, he seems to be at least as proficient in English as most of his former classmates in the Gifted Class. In my experience with Mr. Ren, I was impressed with not only his extraordinary intelligence but also his ambitions and persistence. I am sure that Mr. Ren will be an outstanding student in any doctoral program that he may care to enroll in. So I would like to support him firmly in his quest recommendation into account when considering his application. Iwould greatly appreciate it you decide to accept him as he wishes. Yours sincerely x x x x Professor and Deputy Head 英语推荐信篇3 Dear Admissions Committee, As Director of the State Key Laboratory of Electrical Insulation at ABC University in BLANK, China, it is my sincere and distinct pleasure to recommend Ms. Xiao Xiao for your graduate program in BLANK. I have known Ms. Xiao since 2001, when she was one of eighty students who completed their undergraduate thesis projects in my BLANK laboratory. Ms. Juan’s enthusiasm for and diligence in learning helped her become one of the mostoutstanding students in the entire group, making a strong, positive impression upon me. Ms. Xiao is a very driven woman and student who know what she wants to accomplish and does not falter in her chosen path. Her intellectual curiosity and desire to acquire new acknowledge continuously have led her to perform much study on her own outside the scope of her classes and research work;oftentimes, I witnessed her consulting scientific books and journals to help her discover how she could improve her knowledge and her work in the laboratory. Countless times, Ms. Xiao remained in the laboratory for more than twelve hours in a row to complete her work and make sure there were no errors. With such devotion and fortitude, Ms. Xiao did not surprise me with her superior performance on her thesis paper. In terms of her character, Ms. Xiao is an amiable, courteous, and helpful young woman who has many friends amongst our laboratory personnel. Ms. Xiao is ready to move on to a more rigorous program of graduate study. Her academic performance, research work, and upstanding personal character have won her my highest recommendation, and I hope you give her application strong consideration. Thank you. Sincerely, NAME Director State Key Laboratory of Electrical Insulation, ABC University 英语推荐信篇4 ToWhom It May Concern: I take great pleasure in recommending Longgang Wu, one of my favorite students, for admission into your distinguished graduate program in Machinery Manufacturing. I have known Mr. Wu 5 years since his freshman year at Harbin Engineering University. At that time, he took the course “Advanced Manufacturing Technology”, which I taught mainly for junior students Majoring in Mechanical Engineering. To my surprise, I found he was among the few students in his class who could keep up with the pace of my lectures. Moreover, he often posed thought-provoking questions during the course. Thus it was natural that I liked discussing with this young man and became familiar with him. The more I knew him, the more I found he was talented at Mechanical Engineering. As you can see, he got excellent scores on the course. In fact, he ranked No.1 on this 27 hours course.After the course, he was admitted as a Master of Mechanical Engineering. As his research adviser, I direct his research and found his a promising youth radiating with intelligence and creativity. During the following two years, Mr. Wu worked on a computational theory study of Drag Reduction Characteristics of Bionic Jet Surface. To make the theory model meet the actual materials, he faced down many practical problems, such as building reasonable module of inorganic materials, and calculation techniques. Working hard and independently, he cracked the problems one by one, and came up with a thesis that was characterized by academic excellence. In my experience with Mr. Wu, I was impressed with not only his extraordinary intelligence but also his ambitions and persistence. I am sure that Mr. Wu will be an outstanding student in any doctoral program that he may care to enroll in. So I would like to support him firmly in his quest recommendation into account when considering his application. I would greatly appreciate it you decide toaccept him as he wishes. Yours sincerely Gang Zhao Intelligent Manufacturing System Lab Harbin Engineering University 英语推荐信篇5 To whom it may Concern, I am writing this letter to attest to Paul’s skills in language and public relations. In the four years I have known him, I have been consistently impressed with his ability not only to negotiate complex ideas in other languages, but also to relate these things in a personable, conscientious fashion. His manner in these cases is both professional and personal, two qualities which I find particularly valuable in a professional setting. He has personally helped me in professional negotiation for everything from train tickets to contract information, and I have always been able to count on him.I first met Paul in school, where he was a student at the university at which I taught. He was well-known to most of the westerners in town, who could call from different universities to ask for his help. Sometimes this help involved translation of professional documents, and sometimes it involved personal help in making phone calls. Many of these westerners continue to call him today, even though they live quite far away from him, because they have come to trust him very much. From this foundation in public relations Paul has found positions in various professional capacities and has been highly-valued in each place. He is generally the sort of employee a company finds most valuable in its dealings with both foreign and domestic clients. He puts people at their ease with hislanguage ability and manner, both of which communicate to people that they can relax and simply communicate. I would highly recommend Paul as an employee. His experience and manner are rare and very valuable. Robert Moore Mentor。
英语论文写作论文结论部分（Conclusion）写作特点总结ConclusionConclusion是作者对所研究课题进行的总体性讨论，具有严密的科学性和客观性，反映本研究课题的价值，同时对以后的研究具有指导意义。
Conclusion与Introduction遥相呼应，因为Introduction部分介绍了本课题的研究目的，那么Conclusion要告诉读者这些目的是否达到，在研究中做了哪些工作，取得了什么结果，这些结果说明了什么问题，有何价值和意义，研究过程中存在或发现了哪些问题，原因是什么，建议如何解决等。
Conclusion的具体内容通常包含以下几个部分：(1) 概括说明本课题的研究内容、结果及其意义与价值。
(2) 比较具体地说明本研究证明了什么假设或理论，得出了什么结论，研究结果有何实用价值，有何创造性成果或见解，解决了什么实际问题，有何应用前景等。
(3) 与他人的相关研究进行比较。
(4) 本课题的局限性、不足之处，还有哪些尚待解决的问题。
(5)展望前景，或指出进一步研究的方向。
Conclusion通常使用现在时态Result和Conclusion本次选取5篇文章，第一篇，论文中的主要Result已在第2部分和第三部分中叙述，在Conclusion又重新总结了一下。
第二篇，论文中的主要Result写在Conclusion中。
第三篇，论文中的主要Result写在第3部分（3.CASE STUDIES AND RESULTS）中，Result和Conclusion是分开的。
第四篇，论文中的主要Result已第4部分的（IV. Results and Discussion）中进行叙述，Result和Conclusion是分开的。
第五篇，论文中的主要Result已第4部分的（4. Results and discussion）中进行叙述，Result 和Conclusion是分开的。
第1篇题目：An overview of NACA6-digit airfoil series characteristics with reference to airfoils forlarge wind turbine bladesIV. ConclusionsThe two-dimensional aerodynamics characteristics of the NACA 63 and 64 six-digit series of airfoils measured in the NASA LTPT have been investigated, with a view to verify RFOIL calculations at high Reynolds numbers. The following conclusions can be drawn: - The zero-lift angle of the NACA 64-618 airfoil needs to be adjusted with -0.4 degrees.- The zero-lift angle of The NACA 63-615 needs to be corrected with -0.87 degrees in the smooth case and with +1 degree in case of wrap around roughness.-The maximum lift coefficients predicted with RFOIL match the LTPT data well at Re=3x106, but under predict the Cl,max at Re=6x106 by 3.5 % , up to 6.5% at Re=9x106.-It is uncertain if the established differences in lift between experiment and calculations are caused by a constant bias in the measurements or by the fact that the RFOIL code fails to predict the right level of maximum lift.-RFOIL consistently under predicts the drag coefficient. The difference is about 9% for a wide range of airfoils and Reynolds numbers-NACA standard roughness causes a reduction in the lift coefficient of 18% to 20% for most airfoils from the NACA 64 series-The zero-lift angle of airfoil NACA 64-418 with wrap-around roughness needs a correction of +0.54 degrees.-Wind tunnel experiments and side-by-side tests in the field with one clean rotor need to be done to be able to better predict the effects of roughness.写作特点：内容：第1句，概括了文章的的主要研究内容。
文章编号：1000-4750(2021)06-0151-12倒角化处理对于矩形高层建筑风荷载特性的影响机理研究董 欣1,2，丁洁民1，邹云峰3，左太辉3(1. 同济大学建筑设计研究院(集团)有限公司， 上海 200092；2. 上海防灾救灾研究所， 上海 200092；3. 中南大学土木工程学院，湖南，长沙 410075)摘 要：通过风洞测压试验，对比不同风向下、不同倒角半径的矩形高层建筑表面风压分布、整体风力及斯托罗哈数St ；采用PIV 试验，给出建筑的近尾流流动特性，并从流场作用角度，揭示倒角化处理对于矩形高层建筑风荷载特性的影响机理。
研究表明：临界风向下，在建筑一侧分离的剪切层发生流动再附，形成分离泡；此时，建筑的阻力达谷值，升力和St 达最大值。
相比而言，倒角化矩形高层建筑的临界风向小于无气动措施的工况。
St 主要受到横风向投影宽度和尾流涡对间距的影响，在一定的风向范围内，当倒角半径达一定数值，St 将有所增大。
在建筑的整体阻力方面，倒角化处理将使得建筑尾流涡对尺寸减小；涡对横向流速增大，涡量掺混运动加剧，旋涡强度减弱。
在此作用下，建筑整体阻力降低。
在建筑的整体升力方面，采用倒角化处理后，旋涡脱落的不规则性和随机性增大，脱落强度减弱，这促使建筑整体升力减小；但倒角化处理对于升力的减小效应并非见于所有风向。
关键词：倒角化；矩形高层建筑；PIV ；分离泡；尾流涡对中图分类号：TU973+.213 文献标志码：A doi: 10.6052/j.issn.1000-4750.2020.07.0451EFFECT OF ROUNDED CORNERS ON WIND LOAD CHARACTERISTICSOF RECTANGULAR TALL BUILDINGSDONG Xin 1,2, DING Jie-min 1, ZOU Yun-feng 3, ZUO Tai-hui3(1. Tongji Architectural Design (Group) Co. Ltd, Shanghai 200092, China;2. Shanghai Institute of Disaster Prevention and Relief, Shanghai 200092, China;3. School of Civil Engineering, Central South University, Changsha, Hu’nan 410075, China)Abstract: Through pressure measurement in a wind tunnel, the wind pressure distribution, total forces and Strouhal numbers of rectangular tall buildings with different corner radius under various wind angles were investigated. The near wake characteristics of the rectangular tall buildings with and without rounded corners were observed by PIV experiment, through which the influence mechanism of rounded corners on the wind load characteristics of the buildings was revealed from the perspective of flow field. Results indicated that under critical wind angle, the separated shear layer reattaches on one side face to form a separation bubble. Therefore,the drag force attains a valley value, and the lift forces and Strouhal number reach peak values. Compared with the building without rounded corner, the critical wind angles of those with rounded corners are smaller. The parameters which control the Strouhal number are the transverse projected width and the distance between vortex pairs in the wake. For the rounded-corner buildings of specific rounded radius, the Strouhal number is increased within some wind angles. The drag force is closely related with vortex pairs in the wake. After adopting the收稿日期：2020-07-08；修改日期：2020-11-16基金项目：国家自然科学基金青年基金项目(51408353)；上海市自然科学基金项目(19ZR1421000)；上海市青年科技启明星计划项目(15QB1404800)；国家自然科学基金面上项目(52078504)通讯作者：邹云峰(1984−)，男，湖南隆回人，副教授，工学博士，主要从事结构风工程研究(E-mail: ******************.cn ).作者简介：董 欣(1982−)，女，江苏扬州人，正高级工程师，工学博士，主要从事结构风工程研究(E-mail: ******************);丁洁民(1957−)，男，上海人，研究员，工学博士，主要从事结构工程研究(E-mail: ***********);左太辉(1987−)，男，湖南衡南人，博士生，主要从事结构风工程研究(E-mail: *****************).第 38 卷第 6 期Vol.38 No.6工 程 力 学2021年6 月June2021ENGINEERING MECHANICS151rounded corner, the dimensions of the vortex pairs decrease, while the transverse velocity in the wake increases which implies the intense mixing motion of the fluid and weakened vortex pairs. It gives rise to the decreasing of drag forces. In addition, the irregularity and randomness of vortex shedding are enhanced, while its strength is attenuated by the rounded corner. Then the lift forces are caused to be decreased. However, this decrease tendency is not found for all wind angles.Key words: rounded corner; rectangular tall building; PIV; separation bubble; vortex pair in the wake高层建筑的外形是影响其风荷载特性的重要因素。
绕流线型回转体通气两相流动的非定常特性黄磊;王生捷;彭雪明;何春涛;段磊【摘要】为深入了解通气两相流的流场结构及水动力特性,在循环水洞中利用高速全流场显示技术及六分力天平测量技术,对绕流线型回转体通气两相流动的非定常特性进行实验研究.结果表明:弹身区域流场呈均匀分布的水气两相混合状态,流动稳定,非定常特性不明显;尾部区域流场较为紊乱,非定常特性明显,尾部空泡涡的周期性脱落引起模型阻力系数出现周期性脉动现象.绕流线型回转体通气两相流动的非定常特性与雷诺数及通气率有关.通气率的增加可降低由于尾部空泡涡的脱落而引起的阻力系数的波动幅度,而对尾部涡的脱落频率无明显影响;随着雷诺数的增加,由于尾部空泡涡的脱落而引起的阻力系数的波动幅度减小,尾部涡的脱落频率增加.%To explore the unsteady characteristics of two-phase flows and hydrodynamic force, the experiment with micro-hole array on a streamlined cylinder had been conducted by applying the high speed camera system and six-component balance system. Reynolds numbers and air entrainment coefficient are defined. Some conclusions can be drawn as follows: Elastic body area flow field is evenly distributed water and gas two-phase hybrid state, flow stability, non steadiness is not obvious; Tail areas flow field is more disordered, the unsteady characteristic is obvious, and the tail periodic shedding causes a cyclical fluctuation phenomenon of vortex cavitation resistance coefficient. The unsteady characteristic of ventilated two-phase flow around a streamlined cylinder relates to the Reynolds number and air entrainment coefficient. The fluctuating range of drag coefficient decreases and the vortex shedding frequency remainsunchanged as the air entrainment coefficient increases, and they all increase as the Reynolds numbers increases.【期刊名称】《哈尔滨工业大学学报》【年(卷),期】2017(049)007【总页数】5页(P178-182)【关键词】通气两相流;回转体;通气率;脉动;脱落【作者】黄磊;王生捷;彭雪明;何春涛;段磊【作者单位】北京机械设备研究所,北京100039;北京机械设备研究所,北京100039;北京机械设备研究所,北京100039;北京机械设备研究所,北京100039;北京机械设备研究所,北京100039【正文语种】中文【中图分类】TV131.2微气泡减阻具有较好的减阻效果和较弱的环境响应[1]，其被认为是最有效的减阻方式之一. McCormick等[2]利用电解产生氢气泡的方法实现了减阻；Madvan等[3]采用多普勒测速仪研究了微气泡对湍流边界层的表面摩擦阻力的影响；Jinho等[4] 采用平板表面注气的方法实现减阻；Yoshiaki等[5]在循环水槽中针对不同平板进行了微气泡减阻实验；Kato等[6-7]进行了平板微气泡减阻实验，研究了气孔直径对减阻效果的影响特性，结果表明气孔直径对减阻效果影响极大；Sheng等[8]通过田口分析法研究了通气量、通气面积、气泡尺寸及来流速度对平板微气泡减阻效果的影响，结果表明通气量是主要影响因素；Deutsch等[9-10]采用通气缝出流方式，针对回转体模型研究了轴向压力梯度及气体成分对减阻效果的影响；Sanders等[11]提出了通过多孔质板向平板边界层内注入微气泡的方式，研究了不同速度下减阻效果随通气量的变化特性；Elbing等[12]改进了通气缝式出流方式，并对比了不同出流方式下的减阻效果. 董文才等[13]通过在回转体模型首部及中部开孔的方式，研究了不同区域组合通气对气泡减阻效果的影响；傅慧萍[14]研究了重力对平板气泡减阻的影响特性，认为重力对气泡减阻效率影响较大；李杰等[15]探索了尺寸效应及雷诺数对气泡减阻的影响特性. 由于通气两相流非定常过程的影响，模型阻力曲线会出现脉动现象，继而影响水中航行体的稳定性. 而目前针对气泡减阻的研究都是以阻力为研究对象进行的，并未对阻力时域曲线进行脉动分析. 本文针对微孔阵列式绕回转体模型，在封闭循环水洞中进行了通气两相流非定常特性实验研究，讨论了雷诺数Re及通气率Qv对流动非定常特性及模型水阻力脉动特性的影响.实验在循环水洞中进行，水洞具体参数参见文献[16]. 研究中采用六分力天平、水下绝压传感器、数据采集系统和高速摄像观察系统，分别对实验过程中的模型水阻力、通气量、流速、流场等进行记录.实验模型如图1所示，六分力天平安装于实验模型尾部，并与水洞尾支撑段固定连接. 天平测得的模型阻力数据通过动态测试分析系统进行采集，采集频率为1 024 Hz，采集时间为8 s，测量误差±0.1 N. 模型尾部距轴线16 mm处安装水下绝压传感器，压力传感器量程为0～100 kPa，线性精度为0.2%FS，采集频率为1 024 Hz，采集时间为8 s. 回转体模型长250 mm、直径40 mm，采用流线型头型以降低流场扰流对气泡减阻实验的影响. 模型表面微孔孔径为0.8 mm，沿轴向等距交错分布，共4排，排间距12 mm，首排孔距头部40 mm；每排共8个，且沿圆周方向均匀布置.本研究定义雷诺数Re、通气率Qv、压力系数Cp、阻力系数CD作为量纲一参数，且Re=VL/υ, Qv=Qin/(VS),Cp=p/(0.5ρV2), CD=R/(0.5ρV2S).式中：V为水洞工作段流速，L为实验模型长度，υ为水的动力黏度，Qin为通气量，S为回转体截面积，p为模型尾部压力，R为模型阻力.2.1 典型工况下的非定常特性分析本节通过分析模型水动力时域、频域曲线及流场形态，研究典型工况下的绕流线型回转体通气两相流动的非定常特性. 图2为雷诺数Re=2.03×106、通气率Qv=0.014下模型阻力系数的时域曲线. 由图2可知，测量时间内阻力系数平均值基本不变，且阻力系数存在周期性脉动现象.为了更好地研究典型工况下的实验模型阻力系数的脉动特性，分析脉动频率，采用FFT变换将实验模型阻力系数信号的时域特性转换为频域特性进行分析. 图3为雷诺数Re=2.03×106、通气率Qv=0.014下模型阻力系数的频域曲线.由图3可知，模型阻力系数变化的周期性较为明显，从功率谱密度较大可以看出，阻力系数变化的频率基本上为7.5 Hz.图4、5分别给出了雷诺数Re=2.03×106、通气率Qv=0.014下尾部压力系数的时域及频域曲线. 由图4、5可知，测量时间内尾部压力系数均值基本不变，同样存在周期性脉动现象，脉动频率与模型阻力系数脉动频率相同，为7.5 Hz. 模型阻力由压差阻力及摩擦阻力两部分组成，尾部压力的变化将引起模型阻力的变化，因此可认为尾部压力的周期性脉动是引起模型阻力周期性脉动的主要原因.图6为Re=2.03×106， Qv=0.014时通气两相流动形态随时间变化过程，图7为通气两相流形态图. 分析可知，气体经微孔喷出，在液相剪切流速的作用下形成气液两相流动，并沿下游发展. 在微孔近后方气泡表面光滑，紧贴壁面，呈细长透明形态；继续沿弹体表面向下游发展，细长透明气泡逐渐分裂成大量细碎气泡，流动区域内气泡与液相相互掺混，最终呈现均匀分布的水气两相混合状态. 各时刻弹身区域气液两相流流动较为稳定，非定常特性不明显. 进入尾部区域，气泡在尾部低压作用下形成回流，与液相相互掺混最终形成水气剧烈交换的云雾状模糊形态.回流区流场较为紊乱，水气混合边界呈收缩椭圆形，流动非定常特性明显，并伴有大尺度的云雾状空泡涡的断裂脱落现象，而且空泡涡的断裂脱落过程有明显的周期性. 在t 时刻空泡涡开始形成，并开始逐渐脱离尾部主流场的作用；在t+100 ms时刻，空泡涡完全脱落离尾部主流场，并开始沿来流方向运动. 尾部流场非定常变化的过程为空泡涡脱落的准周期运动过程，脱落周期约为100 ms. 进一步分析发现，尾部涡的脱落周期与尾部压力系数脉动频率相近，因此可认为，尾部涡的周期性脱落引起尾部压力系数产生周期性脉动现象，继而导致模型阻力系数出现周期性脉动.2.2 流动参数对水动力脉动特性影响图8给出了Re=1.49×106、2.03×106、2.48×106下，模型阻力系数的波动范围随通气率的变化过程.图中波动幅度为最大阻力系数及最小阻力系数之差. 由图8可知，3种雷诺数下阻力系数脉动变化规律一致，存在饱和通气率；随着通气率的增加，模型阻力系数平均值下降，波动范围减小，直至饱和通气率；此后，继续增加通气率，阻力系数均值及其波动范围变化不明显. 进一步分析发现，R e=1.49×106时，未通气下的阻力系数波动幅度及饱和通气率下的阻力系数波动幅度分别为0.053及0.040，Re=2.03×106时分别为0.028及0.019，Re=2.48×106时分别为0.026及0.016，由此可知雷诺数的增加将使阻力系数波动幅度降低. 综上分析，通气率及雷诺数的增加都将降低模型阻力系数的波动幅度. 进一步分析认为，通气率及雷诺数的增加使得两相流的不稳定性增加，模型尾部湍流掺混增强，从而引起尾部绕流分离点向下游移动，使得尾部回流区减弱，继而降低了阻力系数的脉动幅度，这与经典的流动分离理论相似.图9给出了Re=1.49×106、2.03×106、2.48×106下，模型阻力系数脉动频率随通气率的变化特性.1)描述了典型工况下通气两相流的流动过程，弹身区域流场呈均匀分布的水气两相混合状态，流动稳定，非定常特性不明显；尾部区域流场较为紊乱，非定常特性明显，尾部空泡涡的周期性脱落引起模型阻力系数出现周期性脉动现象.2) 绕流线型回转体通气两相流动的非定常特性与雷诺数及通气率有关. 通气率的增加可降低由于尾部涡的脱落而引起的阻力系数的波动幅度，而对尾部涡的脱落频率无明显影响；随着雷诺数的增加，由于尾部涡的脱落而引起的阻力系数的波动幅度减小，尾部涡的脱落频率增加.【相关文献】[1] WU S J, OUYANG K, SHIAH S W. 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DOI:10.1063/1.857837.[11]SANDERS W C, WINKEL E S, DOWLING D R, et al. Bubble friction drag reduction in a high-Reynolds-number flat-plate turbulent boundary layer [J]. Journal of Fluid Mechanics, 2006, 552:353-380.DOI:10.1017/S0022112006008688.[12]ELBINGL B R, WINKEL E S, LAY K S, et al. Bubble-induced skin-friction drag reduction and the abrupt transition to air-layer drag reduction[J]. Journal of Fluid Mechanics, 2008,612: 201-236. DOI:10.1017/S0022112008003029.[13] 董文才,周晨,张军. 回转体气层减阻降噪模型试验研究[J]. 海军工程大学学报, 2011,23(3):5-9. DOI:10.3969/j.issn.1009-3486.2011.03.002.DONG Wencai, ZHOU Chen, ZHANG Jun. Gyrodidal model experiment on resistance and noise reduction by bubble layer[J]. Journal of Naval University of Engineering,2011,23(3):5-9. DOI:10.3969/j.issn.1009-3486.2011.03.002.[14]傅慧萍. 平板微气泡减阻数值模拟及影响因素分析[J].哈尔滨工程大学学报，2015,36(10): 1297-1302.DOI:10.11990/jheu.201407002.FU Huiping. Numerical simulation of microbubble drag reduction in a plate and factors in fluencing its practicality process[J]. Journal of Harbin Engineering University, 2015,36(10): 1297-1302. DOI:10.11990/jheu.201407002.[15]傅慧萍, 李杰. 微气泡减阻的数值模拟方法及尺寸效应[J].上海交通大学学报，2016,50(2): 278-282.DOI:10.16183/ki.jsjtu.2016.02.020.FU Huiping, LI Jie. Numerical simulation of micro-bubbles drag reduction and scaleeffect[J]. Journal of Shanghai Jiao Tong University, 2016,50(2): 278-282.DOI:10.16183/ki.jsjtu.2016.02.020.[16]王威.通气参数对水下航行体流体动力影响[D]. 哈尔滨：哈尔滨工业大学,2013.WANG Wei. Experimental research on effect of ventilation parameters on hydrodynamic characteristic of under water vehicle [D]. Harbin: Harbin Institute of Technology,2013.。
排气三通管道分散流流动特性的数值模拟及分析李雄;刘伟军;唐飘;杜兴慧【摘要】为减小排气T型三通管道中的局部压力损失,利用FLUENT软件对其分散流动特性进行数值模拟,分析支管与总管流通截面积比、夹角、质量流量比及流体温度对管道总压损失系数的影响规律.结果表明:总管流速和气体温度对总压损失系数影响不大,支管与总管质量流量比却对总压损失系数影响显著;分支管与总管夹角、分支管与总管流通截面比对总管-通支管总压损失系数的影响不明显,但对总管-分支管总压损失系数的影响显著.通过数值模拟和分析建议T型三通管推荐结构为分支管与总管夹角α=45°,分支管与总管流通截面比A3/A1的适宜范围为0.8～1.数值模拟结果与前人研究及试验结果比对,趋势一致,计算精度较高,可为类似汽车排气分流技术开发提供依据.【期刊名称】《河北科技大学学报》【年(卷),期】2014(035)003【总页数】7页(P272-278)【关键词】三通管道;分散流;压力损失系数;数值模拟【作者】李雄;刘伟军;唐飘;杜兴慧【作者单位】上海工程技术大学汽车工程学院,上海 201620;上海工程技术大学机械工程学院,上海201620;上海工程技术大学汽车工程学院,上海 201620;上海工程技术大学汽车工程学院,上海 201620【正文语种】中文【中图分类】TK403在车用发动机中，受工作原理限制，燃料转变为有用功的部分占1/3左右，大部分的热量通过冷却介质和废气被带走[1-3]。
发动机排气管中上游的废气为较高品位的热源，若能有效回收这部分能量，对提高发动机的燃油经济性具有重要的意义。
排气三通管道作为发动机尾气余热回收利用装置的重要组成部分，不仅对尾气能否正常利用起着关键作用，而且影响着发动机的整体性能。
三通是最常见的一种管道配件，三通处的流动十分复杂，流体的流速在此处发生突变，出现流体质点之间的撞击，产生漩涡，二次流以及流动的分离和再附壁等现象[4-5]。
摘要摘要表面活性剂是一大类有机化合物，它们的性质极具特色，应用极为灵活、广泛，有很大的应用价值和理论意义。
即能大大降低溶剂的表面张力，改变体系的界面结构与组成。
成为人们改变界面性质以适应各种要求的重要手段。
蠕虫状胶束是指表面活性剂球状胶束沿非轴方向一维增长形成的热力学平衡态。
尽管目前对化学减阻剂（这里指高分子聚合物减阻剂和表面活性剂减阻剂）已经有了相当规模的实际系统应用研究或工业应用，但由于对湍流本身的了解还不够全面，表面活性剂在湍流中产生减阻的原因至今尚不十分清楚。
围绕减阻机理，曾提出一些假设和模型，但没有一种理论可以圆满解释添加剂湍流减阻流动中的所有实验现象，减阻机理还有深入研究。
关键词：表面活性剂，蠕虫状胶束，减阻机理。
IABSTRACTABSTRACTSurfactant is a kinds of organic compounds ，Their distinctive character ，Extremely flexible application. Has great application value and theoretical significance. Which can greatly reduce the surface tension of the solvent. Change the system's interface structure and composition. As people change interface properties to meet the various requirements of the important means. Worm-like micelles is refers to the surfactant spherical micelles formed along the axial one-dimensional growth thermodynamic equilibrium state.Despite the current to the chemical drag reduction agent (here refers to the polymer reduction agent and surfactant drag reduction agent) has a considerable scale of actual system application research, or industrial applications. But due to the understanding of turbulence itself is not enough comprehensive, surfactant causes of drag reduction in turbulent flow is still not very clear. Around the drag reduction mechanism, has put forward some assumptions and models, but no one theory can explain satisfactorily additive turbulent drag reduction flow all of the experimental phenomena, the drag reduction mechanism and further research.Key words: The surfactant，Worm-like micelles，The drag reduction mechanismII目录1 引言 (1)2 表面活性剂简介 (2)2.1 表面活性剂的基本性质 (2)2.2 表面活性剂的分类 (2)2.3 阳离子表面活性剂的性质 (2)3 表面活性剂聚集体 (4)3.1 胶束的来源及定义 (4)3.2 胶束的结构 (4)3.3 蠕虫状胶束 (5)4 蠕虫状胶束 (6)4.1 概念 (6)4.2 发现和历史 (6)4.3 应用 (7)5 表面活性剂减阻性能的测试 (8)5.1 减阻的理论基础 (8)5.2 表面活性剂湍流减阻流动特性 (9)5.3 表面活性剂减阻流动特性的影响因素 (11)5.3.1 溶液浓度的影响 (11)5.3.2 流体温度的影响 (12)5.3.3 抗离子添加剂的影响 (12)5.3.4 金属离子和金属化合物的影响 (13)5.4 表面活性剂水溶液减阻机理假说 (13)5.4.1 剪切稀化假说 (14)5.4.2 弹性缓冲层减阻假说 (14)5.4.3 紊流强度抑制假说 (14)III5.4.4 解耦假说 (14)5.4.5 粘弹性假说 (14)6 未来发展前景 (16)参考文献 (17)致谢 ............................................... 错误！未定义书签。
Applied Surface Science 286 (2013) 206–211Contents lists available at ScienceDirectApplied SurfaceSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p s u scExperimental drag reduction study of super-hydrophobic surface with dual-scale structuresSungnam Lyu a ,Dang C.Nguyen b ,Dongseob Kim a ,Woonbong Hwang a ,Bumsang Yoon b ,∗a Department of Mechanical Engineering,POSTECH,Pohang 680-749,South KoreabSchool of Naval Architecture &Ocean Engineering,University of Ulsan,Ulsan 680-749,South Koreaa r t i c l ei n f oArticle history:Received 7May 2013Received in revised form 5September 2013Accepted 9September 2013Available online 16 September 2013Keywords:Dual-scale structure Friction drag reduction Super-hydrophobic Super-hydrophilic Slip lengtha b s t r a c tHydrophobic surfaces with micro-or nanoscale pillars have been attracting considerable interest from scientists.In nature,such surfaces can be found on lotus leaves or under the feet of pond skaters.One significant property of these surfaces is friction drag reduction (FDR).Many studies have been conducted to demonstrate this reduction in terms of laminar and turbulent flows.The slip-length hypothesis is often used to explain this phenomenon.In this study,processes with the advantages of simplicity and cost effec-tiveness were used to fabricate dual-scale structures.Durable super-hydrophilic and super-hydrophobic surfaces were easily obtained from these structures.FDR was measured on a super-hydrophobic sur-face and was compared to that on smooth and super-hydrophilic surfaces.The experimental results in a circulating water channel revealed the Reynolds number range within which substantial FDR can occur on a super-hydrophobic surface.The mechanism of FDR and the role of slip are discussed by comparing experimental results.© 2013 Elsevier B.V. All rights reserved.1.IntroductionThe friction drag reduction (FDR)property of hydrophobic sur-faces allows for their use in a wide range of applications such as car windows,ocean vessels,and building surfaces.There are two basic types of artificial hydrophobicity according to the surface struc-ture:pillar-type and channel-type.The common feature of these two types is the presence of micro-or nanoscale structures on their surfaces.These structures trap air and thus generate an air layer between the water and the body surface.Such a scenario allows for water slip,thus causing FDR.The air layer is thus considered a lubricant for the flow of water over the surface.Various experimental approaches have been used to create hydrophobic surfaces.Some of them include template methods [1],ion bombardment [2],lithography [3–5],chemical deposition [6],self-assembly of a monolayer [7]and photocatalysis [8].The com-mon characteristics of these methods are high cost,long fabrication times,and difficulties in covering a large surface area.To overcome these demerits,a super-hydrophobic surface using the so-called nanowire entanglement structures (NWES)was created by a dip-ping method based on an anodization process in oxalic acid [9].This∗Corresponding author.Tel.:+82522592160;fax:+82542592836.E-mail addresses:blueden@postech.ac.kr (S.Lyu),cuongshipdesign@ (D.C.Nguyen),yusae@postech.ac.kr (D.Kim),whwang@postech.ac.kr (W.Hwang),bsyoon@ulsan.ac.kr (B.Yoon).technique offers the advantages of low fabrication cost,simplicity,and easy coverage of a large surface area.Along with studies on the fabrication of artificial super-hydrophobic surfaces,much research has been carried out on the demonstration of FDR.FDR has been verified by theoretical,numer-ical,and experimental studies based on the slip hypothesis in terms of laminar and turbulent flows.Direct numerical simulation (DNS)[10]and theoretical prediction [11]of turbulent channel flow was performed to validate the effect of slip.Experimental results that demonstrated significant drag reduction for laminar [12–15]and turbulent [16–18]flows in microchannels and normal-sized chan-nels [19,20]were presented.Recent measurements of slip length on hydrophobic surfaces were not confined to channel flow.Flow slip between two rotating parallel disks [21]and a cone-plate [22]was observed.The factors that affect slip on a hydrophobic surface were determined experimentally.The role of nanoscale roughness [23]and the scaling effect [24]of surface geometry were investigated,and air layer restoration and retention [25]were shown.A typical example of super-hydrophobicity in nature is the lotus leaf.A water droplet on a lotus leaf shrinks to a sphere that has min-imum contact area with the surface.In addition,a water droplet easily runs off a lotus leaf even at a very small sliding angle (less than 1◦).Scanning electron microscope (SEM)images of structures on a lotus leaf are shown in Fig.1.The figure shows numerous microscale pillars.On each of these pillars,an extremely large number of nanoscale pillars are present.A structure that has a combination of micro-and nano-elements is called a dual-scale structure.0169-4332/$–see front matter © 2013 Elsevier B.V. All rights reserved./10.1016/j.apsusc.2013.09.048S.Lyu et al./Applied Surface Science286 (2013) 206–211207Fig.1.SEM images of a lotus leaf surface.Micropillars with a diameter of approximately7m are shown in(a),and an extremely large number of bristly nanopillars are shown in(b).Two factors are known to determine the contact angle withthe surface:the surface energy and the structure of the surface.According to the classical Young’s equationcosÂ= s v− lsl v(1)The surface energy of the solid material determines the con-tact angle of the material.Contact angle is calculated by the surface energy between solid/liquid( ls)and solid/vapor( lv).That means the material property itself determines hydrophilicity or hydropho-bicity of the surface.However,one more factor affects this property of the surface.From the equation of Cassie and Baxter,the rough-ness of the surface changes the surface’s wettability:cosÂCB=˛cosÂ−(1−˛)(2) where˛is the fraction of the contact area.If the roughness of the surface increases,the contact area increases.Thus,a hydrophilic surface changes to a super-hydrophilic surface and a hydrophobic surface changes to a super-hydrophobic surface.Therefore,if we change the surface structure,we can easily obtain super-hydrophilic and super-hydrophobic surfaces.In the Cassie state,the hydrophobic property prevents water from penetrating rough structures.Hydrophobic surfaces trap air and thus generate an air layer between the water and the body surface.Such a scenario allows for water slip,thus causing FDR.The air layer is thus considered a lubricant for the flow of water over the surface.The slip length hypothesis is considered a basis for explaining FDR owing to the slip of theflow over super-hydrophobic surfaces. Many techniques to measure slip length were developed to study flow on the surfaces[26].Navier’s hypothesis states that the velocity at a surface is pro-portional to the shear rate.This can be expressed asu(y=0)=ˇdudyy=0(3)whereˇ(shown in Fig.2)is the slip length and du/dy is the shear rate or velocity gradient in the direction normal to the surface.The velocity at the wall,u(y=0),is called the slip velocity.In the present work,considerable FDR was measured for smooth,super-hydrophilic,and super-hydrophobicflat surfaces. Simple and cost-effective processes were used to fabricate these three surfaces.Each experimental result was compared with the theoretical prediction,and FDR and the role of slip was explained by the comparison.A considerable FDR was observed for the super-hydrophobicflat plate in the Reynolds number (Re)region.The result indicated that a super-hydrophobic sur-face causes a turbulentflow to behave like a laminarflow in a small Re range,which is highly desirable forflat-plate friction reductionFig.2.Slip length over a surface.2.Experimental2.1.Fabrication of dual-scale super-hydrophobic andsuper-hydrophilic surfacesA pure(99.5%)industrial aluminum sheet(300×140×5mm) was used as the basic material.Maskant solution was painted on one side of the sheet to protect it during the anodization process. The aluminum was anodized in a0.3M oxalic acid solution at a constant voltage of40V for12h.During anodization,the oxalic acid solution was maintained at26◦C by a double-walled water jacket. Usually,a platinum plate is used as the cathode in the anodization process.However,it was not necessary to use platinum to achieve a regular pore arrangement,and thus,the same aluminum plate was used as a cathode.Aluminum reacts with the ionized acid solution because of the voltage applied during anodization.Aluminum surface becomes anodic oxidefilm and thisfilm grows as the reaction begins.Typi-cally,holes appear on the aluminum surface in a certain condition from the result of oxide growth.Fig.4(a)shows the surface of indus-trial aluminum anodized at40V and15◦C for12h.The anodization conditions(temperature,voltage,and time) affect the oxide growth and arrangement of these holes.High tem-perature accelerates the oxidefilm growth and irregular coral-likeFig.3.Schematic diagram of anodized surface structures:(a)low temperature (15◦C);(b)high temperature(26◦C).208S.Lyu et al./Applied Surface Science 286 (2013) 206–211Fig.4.SEM image of anodized surface:(a)porous surface anodized at 15◦C;(b)coral-like surface anodized at 26◦C.structures appear by the lateral repulsive forces caused by this rapid growth.Surface structures anodized in low temperature (15◦C)and high temperature (26◦C)were showed schematically in Fig.3.On the peaks of the microstructure,some nanosized structures that resemble corals are observed.Surface roughness resulting from high temperatures and long anodization times varies from nano to micro sizes.Fig.4(b)shows an SEM image of an industrial aluminum surface anodized at 40V and 26◦C for 12h.To achieve hydrophobicity on the surfaces,a heptadecafluoro-1,1,2,2-tetrahydrodecyl trimethoxysilane (HDFS)coating method with an N -hexane solution containing 0.1%HDFS was used.The anodized aluminum sheet with dual-scale roughness was dipped into the mixed solution for approximately 10min.After the HDFS coating step,the aluminum sheet was rinsed with a hexane solution for 20min.Finally,a very thin layer of HDFS was coated onto the dual-scale structure and hydrophobicity was imparted to one side of the aluminum plate.The overall fabrication process is briefly illustrated in Fig.5Fig.5.Fabrication process of super-hydrophobic dual-scale surface.The dual-scale structure maximizes the wetting property of the surface.This means that super-hydrophobic and super-hydrophilic surfaces have the same micro/nano surface structures.The only difference between the surfaces is their material properties,which change the interfacial energy.In other words,a thin HDFS layer decreases the surface energy and imparts hydrophobicity,whereas an anodized alumina oxide (AAO)layer obtained after anodization increases the surface energy,leading to hydrophilicity.The contact angle of a water droplet,as determined using a drop shape analysis system (DSA-100,Kruss Co.),is 0◦on the super-hydrophilic surface and 160◦on the super-hydrophobic surface (Fig.6).2.2.Description of the experimentIn the experiments,three 600×140×5mm test plates with either a super-hydrophobic,super-hydrophilic,or smooth alu-minum surface were used.The experiments were conducted in a circulating water chan-nel (CWC),as shown in Fig.7.The dimensions of the test tank were 3.0×1.0×0.8m.The maximum water velocity that the twin impellers could generate was 1.5m/s.As mentioned above,the test plates used in the experiments were made of aluminum and had dimensions of 600×140×5mm (two 300×140×5mm joined flat plates).The plates were attached to a 600×140×20mm robust acrylic panel and fixed by 12screws.Then the acrylic panel was in turn connected to a three-component load cell,and all equipment was firmly fixed to a robust structure.To eliminate form resistance that could be exerted on the model,a leading part with a length of 50mm was fabricated with a sharp edge.This part was located forward of the model and held by an independent structure.The clearance between the leading part and the model was sufficiently small to make the flow as smooth as pos-sible.Both the leading part and the model were adjustable in three directions (vertical by a screw shaft,horizontal,and transverse).An overview of the experimental setup is shown in Fig.8,andaFig.6.Contact angle of the (a)anodized aluminum surface (0◦);and (b)HDFS-coated surface (160◦).S.Lyu et al./Applied Surface Science286 (2013) 206–211209Fig.7.Schematic of the layout of the CWC.(a)supporting structure;(b)circulating water channel with twin impellers;(c)test tank;and(d)model with its holding structure.300×140×5mm super-hydrophobic plate model is displayed in Fig.9.The experimental setup for drag force measurement is shown in Fig.10.The drag force acting on the model was measured with a three-component load cell;only the x-direction component was used for the drag measurement.The data acquired were ultimately processed using Labview.At eachflow velocity,thousands of drag force samples were obtained.These values were then averaged to yield the friction drag force at that velocity.The velocity of theflow ranged from0.075m/s to0.503m/s(equivalent to a Reynolds number range of54,300to364,500).TheFig.8.Overview of the experimentalsetup.Fig.9.Super-hydrophobic plate(300×140×5mm).model was submerged to a depth of3mm and held parallel to thewater surface.The friction force was measured at every velocity value.The sys-tem was then turned off to stop theflow of the water.The velocitywas subsequently set to increase gradually.The measurement pro-cedure was then repeated,and after several loops,the desired datawere obtained.3.Results and discussionIn order to establish a basis from which to compare and jus-tify the attained results,a prediction should be made for cases oflaminar and turbulentflows.Hence,the Schoenherr and Blasiusformulas were applied.These formulas were analytically inferredfromflow problems on aflat plate in the above two cases.The friction drag force of aflat plate is calculated using the fol-lowing expression:F D=0.5C F v2A(4)where C F is the friction drag force coefficient, =1000kg/m3is thedensity of water,v(m/s)is theflow velocity,andA=0.084m2is the wetted surface area of theflat plate.C F is determined as follows:0.242C F=log10(Rn.C F)(5)for turbulentflow andC F=1.328Rn(6)for laminarflow,where Re=vL/ is the Reynolds number,L=0.6mis the model’s length, =8.97×10−7m2/s is the kinematic viscosityof water at25◦C.The estimated friction drag coefficient and the prediction linesare shown in Fig.11.The blue solid line is Schoenherr’s predic-tion for a smoothflat plate in the case of turbulentflow,while theblack solid line is Blasius’prediction for a smoothflat plate in theFig.10.Drag measurement system.210S.Lyu et al./Applied Surface Science 286 (2013) 206–211Fig.11.Friction drag versus the flow velocity and Reynolds number.case of laminar flow.Red dots denote the results obtained for the super-hydrophilic test plate,black stars represent the results for the super-hydrophobic test plate,and blue squares are the results for the smooth flat plate.It is evident that the friction drag of the super-hydrophilic and smooth surfaces was nearly identical over the entire range of flow velocities.In Fig.12,at Re >125,000,it is clear that such results are in good agreement with Schoenherr’s prediction for a flat plate,in the case of turbulent flow.Thus,it is strongly asserted that the flow through the super-hydrophilic and smooth surfaces was com-pletely turbulent from an Re of about 125,000.The flow regime from the beginning to Re =125,000is the transition regime at which the flow may be either turbulent or laminar.Super-hydrophilicity is believed to induce an absolute non-slip flow at the surface.For this reason,the experiments showed that there was no slip at the smooth surface or the slip was not sufficiently large to change the friction drag.The friction drag of the super-hydrophobic surface at Re <100,000was very small when compared to that of the super-hydrophilic and smooth surfaces.A value of zero was attained at Re <90,000,which was similar to the results of Henoch’s experi-ments [19].At 100,000<Re <200,000,the amount of friction drag was about 50%.At Re >200,000,the friction drag began toincreaseFig.12.Friction drag coefficient versus Reynolds number.faster and finally reached a value close to that of the super-hydrophilic and smooth surfaces at Re ∼260,000.The mechanism of FDR can be explained with respect to the entrapment of air between the plate surface and the water.At a lower velocity,thee air layer covered almost the entire area of the wetted surface,and slip occurred everywhere.This caused a large reduction in friction drag.As the flow velocity increased,the air layer shrank,and the area of slip decreased.This caused the fric-tion drag to increase rapidly and become asymptotic to that of the smooth and super-hydrophilic surfaces.Finally,at a certain veloc-ity,the air layer was completely swept away by the flow.At this point,there was no slip,and no FDR occurred.At velocity v <0.15m/s (Re <110,000),when the air layer cov-ered almost the entire wetted surface,the rate of increase in the friction drag of the super-hydrophobic surface was not remarkable.At this range of velocities,the flows under the super-hydrophilic and smooth surfaces behaved like turbulence,while that under the super-hydrophobic surface behaved like a laminar flow.At velocity v >0.15m/s (Re >110,000),the air layer was observed to shrink,and the rate of increase in the friction drag increased further.Once the air layer vanished,no slip occurred,and the friction drag conformed to the laws of turbulent flow around non-slip surfaces.The experimental results showed that turbulent flow may have been retarded (the transition Re was increased)by super-hydrophobicity.In other words,the turbulent boundary layer seemed to transform into a laminar boundary layer in the low Reynolds number region because of super-hydrophobicity.Such a phenomenon caused a remarkable FDR in not only intensity but also in the rate of increase in that region.Furthermore,the fric-tion drag of the super-hydrophobic surface was even smaller than smooth surfaces.Such a result was due to flow slip caused by the hydrophobicity of the surface.It can be said that hydrophobicity is important in making the flow more ideal.In the experiments,the only difference between the super-hydrophilic and super-hydrophobic surfaces was their material properties (AAO versus HDFS).Other properties,such as the sur-face structures and size,were the same.As such,several questions have arisen regarding the differences in the behavior of the two sur-faces and the mechanism underlying the generation of an air layer between the super-hydrophobic surface and the water.With the same micro/nano structures,super-hydrophilicity did not allow air to be trapped.Instead,water rapidly occupied every micro hole and caused the contact angle of a water droplet to reach zero.It could be said that AAO (hydrophilic material)had such a high wettability that it allowed water molecules to access the holes easily.In con-trast,HDFS (hydrophobic material)had a low wettability,and this property protected the holes from the invasion of water.Thus,an air layer was generated because of the air trapped in the holes.A water layer contiguous to the air layer attained a certain velocity,and an FDR was achieved.The preservation of an air layer in the presence of a high velocity flow is a significant practical problem and may depend on the properties of the micro/nano structures (morphology,distribution,robustness,size).4.ConclusionsRobust super-hydrophobic and super-hydrophilic surfaces with the same surface structures were created with inexpensive mate-rials via a simple fabrication process.The material properties of the surfaces,not their micro/nano scale roughness,determined whether the surface was hydrophobic or hydrophilic.Roughness only served to increase the hydrophobicity or hydrophilicity of the surfaces.However,specific material properties and surface roughness were two necessary and sufficient conditions for super-hydrophilicity and super-hydrophobicity.S.Lyu et al./Applied Surface Science286 (2013) 206–211211A considerable FDR was observed for the super-hydrophobicflat plate for Re<200,000.The FDR increased as the Reynolds number decreased.However,an FDR was not obtained at a high Reynolds number(Re>260,000).The experiments demonstrated that an FDR is mainly caused by the existence of slip.Slip is in turn caused by an air layer between a surface and aflow.Furthermore,it is thought that theflow tran-sition is likely to be retarded by super-hydrophobicity,and hence, the friction drag is considerably smaller than that predicted in a very low Re region.That is,a super-hydrophobic surface makes a turbulentflow behave like a laminarflow in a small Re range,which is very desirable forflat plate friction.Further,the experiments revealed much of the overall behav-ior of hydrophobicity.Due to the ability to repel individual water 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