下载文档原格式
流体力学专业名词续渐变流gradually varied flow 急变流rapidly varied flow临界流critical flow异重流density current, gravity flow堰流weir flow掺气流aerated flow含沙流sediment-laden stream降水曲线dropdown curve沉积物sediment, deposit沉[降堆]积sedimentation, deposition沉降速度settling velocity流动稳定性flow stability不稳定性instability奥尔-索末菲方程Orr-Sommerfeld equation涡量方程vorticity equation泊肃叶流Poiseuille flow奥辛流Oseen flow剪切流shear flow粘性流[动] viscous flow层流laminar flow分离流separated flow二次流secondary flow近场流near field flow远场流far field flow滞止流stagnation flow尾流wake [flow]回流back flow反流reverse flow射流jet自由射流free jet管流pipe flow, tube flow内流internal flow拟序结构coherent structure猝发过程bursting process表观粘度apparent viscosity运动粘性kinematic viscosity动力粘性dynamic viscosity泊poise厘泊centipoise厘沱centistoke剪切层shear layer次层sublayer流动分离flow separation层流分离laminar separation湍流分离turbulent separation分离点separation point附着点attachment point再附reattachment再层流化relaminarization起动涡starting vortex驻涡standing vortex涡旋破碎vortex breakdown涡旋脱落vortex shedding压[力]降pressure drop压差阻力pressure drag压力能pressure energy型阻profile drag滑移速度slip velocity无滑移条件non-slip condition壁剪应力skin friction, frictional drag壁剪切速度friction velocity磨擦损失friction loss磨擦因子friction factor耗散dissipation滞后lag相似性解similar solution局域相似local similarity气体润滑gas lubrication液体动力润滑hydrodynamic lubrication浆体slurry泰勒数Taylor number纳维-斯托克斯方程Navier-Stokes equation牛顿流体Newtonian fluid边界层理论boundary later theory边界层方程boundary layer equation边界层boundary layer附面层boundary layer层流边界层laminar boundary layer湍流边界层turbulent boundary layer温度边界层thermal boundary layer边界层转捩boundary layer transition边界层分离boundary layer separation 边界层厚度boundary layer thickness 位移厚度displacement thickness动量厚度momentum thickness能量厚度energy thickness焓厚度enthalpy thickness注入injection吸出suction泰勒涡Taylor vortex速度亏损律velocity defect law形状因子shape factor测速法anemometry粘度测定法visco[si] metry流动显示flow visualization油烟显示oil smoke visualization孔板流量计orifice meter频率响应frequency response油膜显示oil film visualization阴影法shadow method纹影法schlieren method烟丝法smoke wire method丝线法tuft method氢泡法nydrogen bubble method相似理论similarity theory相似律similarity law部分相似partial similarity定理pi theorem, Buckingham theorem 静[态]校准static calibration动态校准dynamic calibration风洞wind tunnel激波管shock tube激波管风洞shock tube wind tunnel水洞water tunnel拖曳水池towing tank旋臂水池rotating arm basin扩散段diffuser测压孔pressure tap皮托管pitot tube普雷斯顿管preston tube斯坦顿管Stanton tube文丘里管Venturi tubeU形管U-tube压强计manometer微压计micromanometer多管压强计multiple manometer静压管static [pressure]tube流速计anemometer风速管Pitot- static tube激光多普勒测速计laser Doppler anemometer, laser Doppler velocimeter 热线流速计hot-wire anemometer热膜流速计hot- film anemometer流量计flow meter粘度计visco[si] meter涡量计vorticity meter传感器transducer, sensor压强传感器pressure transducer热敏电阻thermistor示踪物tracer时间线time line脉线streak line尺度效应scale effect壁效应wall effect堵塞blockage堵寒效应blockage effect动态响应dynamic response响应频率response frequency底压base pressure菲克定律Fick law巴塞特力Basset force埃克特数Eckert number格拉斯霍夫数Grashof number努塞特数Nusselt number普朗特数prandtl number雷诺比拟Reynolds analogy施密特数schmidt number斯坦顿数Stanton number对流convection自由对流natural convection, free convec-tion强迫对流forced convection热对流heat convection质量传递mass transfer传质系数mass transfer coefficient热量传递heat transfer传热系数heat transfer coefficient对流传热convective heat transfer辐射传热radiative heat transfer动量交换momentum transfer能量传递energy transfer传导conduction热传导conductive heat transfer热交换heat exchange临界热通量critical heat flux浓度concentration扩散diffusion扩散性diffusivity扩散率diffusivity扩散速度diffusion velocity分子扩散molecular diffusion沸腾boiling蒸发evaporation气化gasification凝结condensation成核nucleation计算流体力学computational fluid mechanics多重尺度问题multiple scale problem伯格斯方程Burgers equation对流扩散方程convection diffusion equationKDU方程KDV equation修正微分方程modified differential equation拉克斯等价定理Lax equivalence theorem数值模拟numerical simulation大涡模拟large eddy simulation数值粘性numerical viscosity非线性不稳定性nonlinear instability希尔特稳定性分析Hirt stability analysis相容条件consistency conditionCFL条件Courant- Friedrichs- Lewy condition ,CFL condition 狄里克雷边界条件Dirichlet boundary condition熵条件entropy condition远场边界条件far field boundary condition流入边界条件inflow boundary condition无反射边界条件nonreflecting boundary condition数值边界条件numerical boundary condition流出边界条件outflow boundary condition冯.诺伊曼条件von Neumann condition近似因子分解法approximate factorization method人工压缩artificial compression人工粘性artificial viscosity边界元法boundary element method配置方法collocation method能量法energy method有限体积法finite volume method流体网格法fluid in cell method, FLIC method通量校正传输法flux-corrected transport method通量矢量分解法flux vector splitting method伽辽金法Galerkin method积分方法integral method标记网格法marker and cell method, MAC method特征线法method of characteristics直线法method of lines矩量法moment method多重网格法multi- grid method板块法panel method质点网格法particle in cell method, PIC method质点法particle method预估校正法predictor-corrector method投影法projection method准谱法pseudo-spectral method随机选取法random choice method激波捕捉法shock-capturing method激波拟合法shock-fitting method谱方法spectral method稀疏矩阵分解法split coefficient matrix method不定常法time-dependent method时间分步法time splitting method变分法variational method涡方法vortex method隐格式implicit scheme显格式explicit scheme交替方向隐格式alternating direction implicit scheme, ADI scheme 反扩散差分格式anti-diffusion difference scheme紧差分格式compact difference scheme守恒差分格式conservation difference scheme克兰克-尼科尔森格式Crank-Nicolson scheme杜福特-弗兰克尔格式Dufort-Frankel scheme指数格式exponential scheme戈本诺夫格式Godunov scheme高分辨率格式high resolution scheme拉克斯-温德罗夫格式Lax-Wendroff scheme蛙跳格式leap-frog scheme单调差分格式monotone difference scheme保单调差分格式monotonicity preserving diffe-rence scheme 穆曼-科尔格式Murman-Cole scheme半隐格式semi-implicit scheme斜迎风格式skew-upstream scheme全变差下降格式total variation decreasing scheme TVD scheme 迎风格式upstream scheme , upwind scheme计算区域computational domain物理区域physical domain影响域domain of influence依赖域domain of dependence区域分解domain decomposition维数分解dimensional split物理解physical solution弱解weak solution黎曼解算子Riemann solver守恒型conservation form弱守恒型weak conservation form强守恒型strong conservation form散度型divergence form贴体曲线坐标body- fitted curvilinear coordi-nates[自]适应网格[self-] adaptive mesh适应网格生成adaptive grid generation自动网格生成automatic grid generation数值网格生成numerical grid generation交错网格staggered mesh网格雷诺数cell Reynolds number数植扩散numerical diffusion数值耗散numerical dissipation数值色散numerical dispersion 数值通量numerical flux放大因子amplification factor 放大矩阵amplification matrix 阻尼误差damping error离散涡discrete vortex熵通量entropy flux熵函数entropy function分步法fractional step method。
非接触机械密封端面间流体膜流动状态临界雷诺数的讨论付朝波;宋鹏云【摘要】非接触式机械密封端面间流体流动是一种包含压差流和剪切流在内的复合流动,其流动状态受内外压差、密封间隙、转速等多种因素的影响.梳理压差流、剪切流、复合流的临界雷诺数相关文献,发现大多数文献认为压差流的临界雷诺数为2000,剪切流的临界雷诺数也接近2000.提出一种利用复合速度计算压差剪切复合流动雷诺数的方法,即将压差流与剪切流形成的速度的矢量和作为雷诺数的特征速度来计算雷诺数,简称复合速度雷诺数,并以复合速度雷诺数等于2000作临界雷诺数判据,来判断流体膜的压差剪切复合流动是处于层流状态还是处于湍流状态.对于压差流和剪切流相互垂直的复合流动,用复合速度法确定的复合速度雷诺数与分别用压差流雷诺数和剪切流雷诺数复合的复合雷诺数等价.以GABRIEL经典论文数据为例,利用复合速度方法得到的最大复合雷诺数为100,小于复合流动临界雷诺数,处于层流状态.【期刊名称】《润滑与密封》【年(卷),期】2019(044)007【总页数】7页(P63-68,77)【关键词】机械密封;流体膜;压差流;剪切流;临界雷诺数【作者】付朝波;宋鹏云【作者单位】昆明理工大学化学工程学院云南昆明650500;昆明理工大学化学工程学院云南昆明650500【正文语种】中文【中图分类】TH136非接触机械密封一般是指气体润滑的气膜密封(干气密封)和液体润滑的上游泵送机械密封(液膜密封)，它们已在工业上获得广泛应用[1-4]。
在该类机械密封的设计、研究和应用过程中，一般认为密封端面间的流体膜流动是处于层流状态[5]；但在转速较高、流体膜厚度较大或被密封流体的压力较高时，端面间流体膜的流动状态可能进入湍流状态。
层流和湍流是2种完全不同的流动状态，由于密封端面间流体膜的流动是属于包含压差流和剪切流在内的一种复合流动，如何判断压差剪切复合流动从层流到湍流的转捩是一个尚未解决的问题。
Computational Fluid Dynamics – or How to Make a Good Boat FastDavid VacantiThe term CFD is showing up more often these days in articles describing the design efforts used to make Volvo 60 round the world racers and America’s Cup yachts faster. Computational Fluid Dynamics or CFD actually covers a great many engineering specialties and is not the sole domain of boat and ship design. In this article we will review what types of CFD products exist and hopefully provide some understanding of when and how CFD products are best suited to a project. Computational Fluid Dynamics is the application of computers to the modeling of fluid characteristics when either the fluid is in motion or when an object disturbs a fluid. A few examples of a fluid in motion are water or chemical flow in pipes, heating and ventilation systems conducting cooling, heating or fresh air supplies to a building. Fluids in motion also include flame and fire effects in combustion or jet engines. Surprised by these fields of interest?What about examples of an object disturbing a fluid? Examples include stirring paddles submerged in a tank of water and effluent in a waste treatment plant, aircraft of all kinds, cars and trucks at highway or racing speeds and even monohull sailboats, ship, multihull sailboats to name but a few. Obviously, an open mind is important when considering what constitutes a fluid. Fluids can exist in gaseous and liquid states and science has recently found that even some solids can exhibit fluid like characteristics under right conditions. Scientists have found that some of the most spectacular and deadly landslides or rock falls behave as a fluid while the mass of stone and soil or sand is in motion, only to return to a most decidedly solid form when the motion subsides.The general field of fluid dynamics differs from the field of boat design in one critical way. Onlyboat design deals with a vehicle passing through the two fluids of air and water simultaneously.Our atmosphere is a compressible fluid, though not at yachting or even high-powered boat racing speeds. Air can change in density according to altitude, temperature and humidity. Water is an incompressible fluid that can vary in viscosity according to its salinity and temperature.For most of us, small effects such as variable salinity and temperature are not of concern, but can make the difference between winning and loosing a major international yacht race.How do CFD programs Work?CFD programs are based on the laws of physics, such as the law of conservation of momentum, and special “boundary conditions”. The law of conservation of momentum states that the total momentum of a system remains constant regardless of how the system may change. A boundary condition limits how and where a fluid can travel. A simple example is that motion of the fluid must remain tangent or parallel to the surface of an object passing through it. Another example is that pressure applied by the fluid against the object must be perpendicular to the surface at all points. These laws and conditions are critical to the development of a CFD program because they allow an aerodynamicist to write equations that describe the system that is being studied. Without the physical laws and boundary conditions there would be no way to write equations that describe fluid motion. The complex equations that result take into account the viscosity, mass and other characteristics of the fluid. The equations are written using integral and differential calculus and require specialized computer techniques to solve them. Typically the programmer writes an algorithm that makes a series of estimates using algorithms that iteratively solve the sets of equations by looking for “balance” in the system of equations. A final answer is obtained when the algorithm converges on a solution with an error that is sufficiently small for the desired accuracy. Once an algorithm has been developed to implement the laws of momentum and boundary conditions, it cannot be applied to the entire surface of the hull and appendages at once. The surface area of the hull, keel and rudder are broken into thousands of small patches (collectively called a mesh) and the algorithm applied to each patch. Each patch in turn influences the fluid flow on the patch area of its neighbors and therefore the solution must account for the conditions surrounding the patch currently being solved. As a result the program must solve and resolve the equations for all of the patches until the solution obeys the physical laws and boundary conditions. Sometimes the complexities of the laws of physics are too difficult to implement all at the same time. As a result the aerodynamicists choose to write programs that make certain limiting assumptions that permit the programming to become more practical and still result in reasonable results. A specific example arises in the case of what actually happens to fluids very near the surface of an object. The boundary layer as it is called experiences shear forces in the objects direction of travel that result in viscous drag. These shear forces are described in a special set of equations called the Navier Stokes relationships. The Navier Stokes equations are sufficiently complex themselves that attempting to include them within every aerodynamics or hydrodynamics program would make the solutions nearly impossible. As a result there are Navier Stokes based programs that specifically address viscous drag and Panel method programs that compute lift, wave drag and induced drag. A complete estimate of the drag encountered by a boat requires the data supplied by both programs.What do CFD programs Calculate?The most obvious calculation that would be of interest in boat design is the determination of drag forces. But drag comes in several forms that can include, wave, viscous, and induced drag. Therefore, a designer must evaluate the effects of his design in each of these drag areas. The second general area of calculation is lift. The term lift arises from its application to aircraft and becomes a bit confusing when applied to the field of boat design. Lift applies to the forces generated by a keel or centerboard to resist the side force of sails and the driving force of the sails themselves. It also applies to the turning forces of a rudder, and the supportive force acting on “foils” to elevate a hydrofoil sail or powerboat above the water surface.There is also a distinction between 2 and 3 dimensional fluid dynamics analysis. Specifically, there are programs that predict the performance of foils as if they existed on a wing of infinite length. Here the term “foil” is used to define the shape of a keel or rudder along the chord from the leading to the trailing edge. Foil shapes are best known by the alphanumerical names given to them such as NACA 63A012. So a 2D fluids program would compute the lift, drag, velocity distribution, turbulence onset and the generation of bubbles similar to cavitation for a 2D shape such as a wing or keel foil, and would not include any 3D information such as keel span or thickness distribution or the presence of a bulb. A 3D fluids program would compute wave and induced drag from a hull, keel and rudder, including the effects of a bulbed keel carrying winglets.CFD codes are critical for more than optimizing the performance of a top-notch America’s Cup class racing yacht. These codes can be of great value to determine loads placed on boat structures of all types and are invaluable when applied to unique marine structures such as oil platforms that are frequently subjected to the world’s worst storms.Lift and drag effects translate directly into loads that must be sustained by the boat or oil derrick if it is to remain intact in its intended operational conditions. For example, several years ago when the race was known as the Whitbread Round The World Race, many boats developed life threatening hull delamination when subjected to the continuous pounding of high speed downwind surfing and upwind beating. While delamination of a boat at sea is definitely related to structural design errors, those errors were caused by a lack of detailed information about the fluid forces experienced by the boats. Knowledge of these forces would have enabled designers to prevent the hull damage in the first place.Therefore, the potential application of CFD to your design project should depend on whether or not the design regime that your vessel will operate in has well understood engineering data available to prevent hull damage in addition to overall performance of the vessel. For example, the last few years have seen the development of high-speed hydrofoil sailboats for the consumer market. These top performance boats experience not only significant speeds and loads, but the potential for unstable characteristics could make it highly dangerous to ride in one. However, the judicious use of field-testing and computer analysis has produced a crop of very exciting hydrofoil sport boats that are a joy to fly in.Finally, several years ago a multihull sailboat arrived in port after participating in a trans-Atlantic race. When the centerboards were raised in the outer hulls of the trimaran, the skipper wasshocked to learn that the boards had been sheered off just below hull depth and he had not had their use for some indeterminate time during the later portion of the race. Clearly, the structural design of the boards had not taken into account the true forces of lift, drag or perhaps cavitation that would be experienced at sea.CFD programs do not calculate how fast a boat of any type will pass through the water or predict the time to complete a course around the buoys. Predicting speed on a racecourse is the domain of another class of programs called Velocity Prediction Programs or VPP. The VPP makes use of lift and drag numbers calculated in a CFD program to estimate the speed about will sail a course given the sail drive forces and the stability or righting moment of the hull. The VPP is a closed loop simulation continuously varies estimated speed and resulting lift, drag and righting forces until retarding and driving forces are balanced and a stable speed results. A CFD program on the other hand is an open loop simulation that simply states that given an angle of heel and speed for a specified hull and appendage configuration, here are the forces that will result for that instant in time. No consideration is given to how the vessel achieved that speed or sailing condition.In summary then, CFD programs not only calculate lift and drag forces of a hull with appendages, they can also be used to compute pressure loads due to waves and wave impact at speed. The forces of lift, drag and pressure can be translated into structural requirements and provide the means to optimize a hull working in concert with its appendages to produce lift in the most efficient manner possible while satisfying the needs for stability. Predictions of lift and drag at various speeds can be used to develop a mathematical model needed to accurately close the analysis loop of a velocity prediction program.When is a CFD Computer Program Required?CFD codes are not always required or justified however, when simpler means of estimating the forces involved are available. In the case of a typical sailboat design, the forces generated by the keel and rudder can be easily estimated if the keel lacks a bulb and if the keel and rudder shape are essentially straight leading and trailing edges. It is possible to make use of analytical methods that are easily implemented on personal computers. A simple example is the program I wrote called LOFT that makes use of analytical methods developed by NASA and the US Air Force for initial performance prediction of wing designs.However, while simple programs like LOFT can adequately address typical bulb-less keels and rudders they cannot analyze the performance of an America’s Cup racing keel with bulb and winglets. Only 3D CFD programs can address that complex task.Who can operate a CFD program?While CFD programs can be of tremendous value, getting accurate and meaningful results is not typically within the reach of amateur and many professional boat designers. A degreed Naval Architect or a fluids dynamicist is required to generate the key initial input to a CFD program called a mesh.The mesh is a mathematical description of the hull and appendages that are to be analyzed. It is not sufficient or even possible to use standard stations, waterlines and buttocks as inputs to a CFD program. The detailed shapes of the hull and appendages must be defined by a mesh of squarepatches that adjoin one another and whose dimensions are chosen according to the local curvature of the hull or appendage or by the occurrence of the intersection of the hull and a keel, rudder or lifting strut of a hydrofoil. The generation of a mesh is a science unto itself and can require iterations by the analysts running successive trials to be sure that the mesh is sufficiently dense in critical areas. Some meshing can be done automatically and then refined by hand.Typically the developers run complex 3D CFD programs or thosetrained in their use and as result are not really meant for use by therest of us. However, 2D fluids programs designed for the analysisand development of 2D air or hydrofoil shapes (recall the 63A010example) are sufficiently easy to use for a designer with basicmathematics skills and general knowledge of airfoil characteristics.Analytical programs such as Vacanti Yacht Design’s FOIL programcan aero / hydrofoil lift, drag, turbulence onset and bubbleformation characteristics for anyone with basic computer skills anda working knowledge of basic foil design.What CFD Programs Exist?Panel Method and Navier Stokes programs are two generalclasses of CFD programs that apply to the issues of boat design.The most commonly used and most available are Panel Method programs. Panel methods allow the prediction of wave drag, free surface effects and induced drag due to lift generated by a keel or rudder but they do not account for viscous drag. Programs using panel methods assume that there are only forces normal to the surface of the hull within the fluid. However, due to viscosity, the fluid is subject to forces in shear – more or less parallel to the hull surface that causes turbulence. Therefore the panel programs are referred to as “inviscid” analysis methods. As a result they compute wave and induced drag but not the effects of viscous drag. Viscous drag computations are computed by specialized codes known as Navier Stokes programs. These programs are difficult to use and apply and are best left to a professional skilled in their use. When a designer has a task that justifies the use of CFD programs, he should be using design tools that that can export true 3D surface shapes in the form of common Computer Aided Design (CAD) file formats. Designing in a typical CAD program such as AutoCAD using lines and polylines, even though in 3D are not sufficient for use with CFD programs. True surface definitions such as Non-Uniform Rational B-spline (NURB) surfaces are required. Most professional versions of the commonly known yacht design programs (AeroHydro, AutoShip, Maxsurf, New Wave, PROLINES) all provide this kind of file exchange.Licensing costs or consulting time is available from the companies or sources listed below.Company Name Program(s) Web AddressAerologic Cmarc,Postmarc/dwt.htmlAnalytical Methods Inc VSAERO,FSWAVEFluent FLUENT/solutions/marine/index.htm South BaySimulationsSPLASH /~brosenVacanti YachtDesignFOIL 97 Virginia Technical University Several Freesimple programs– Code Compilermay be required/aoe/faculty/marchman/softwareCFD Online Very extensivelinks to manysuppliers of CFDprograms ofevery possibletypeSpecialized consulting companies include:Bruce RosenSouth Bay Simulations44 Sumpwams AveBabylon, NY 11702 631 587 3770, brosen@Joe LaisoaFluid Motion Analysis Consulting, Inc.3062 Queensberry Dr.Huntingtown, MD 20639, 410 535 0307 X3351, laiosa@ConclusionCFD programs are best applied when there are either significant engineering unknown effects or load levels or where design optimization for a specific application in specific conditions are essential to the goal. For instance, there are many books of scantlings or building standards for typical sailboat or powerboat designs intended for inland cruising. But an attempt at the world record speed sailing at the “ditch” in France at speeds approaching 50 knots clearly calls for specialized analysis to prevent catastrophic failure that could risk lives or incur that last bit of drag that could prevent success in inching the speed record that much higher.Some CFD codes are only usable in the hands of a skilled practioner and others are designed and intended for use by those with reasonable technical skills and willingness to do a bit of reading or research to help them understand the results and limitations of their modeling efforts. CFD andanalytical programs are very important to the development of high performance vessels from the perspective of optimization for speed and safety. High speed sailing craft and those destined for offshore use can benefit the most from computer analysis methods. One final key point here is that we have only discussed vessels in displacement mode and have not referred to high performance planning powerboats. The prediction of planning vessel performance is an art unto itself and is the domain of yet another class of programs. I refer those of you who wish to know more about that subject area to research the Society of Naval Architects and Marine Engineers() web site.。
成矿流体动力学的原理、研究方法及应用池国祥;薛春纪【期刊名称】《地学前缘》【年(卷),期】2011(018)005【摘要】Fluid flow is an integral part of hydrothermal mineralization,and its analysis and characterization constitute an important part of a mineralization model.The hydrodynamic study of mineralization deals with analyzing the driving forces,fluid pressure regimes,fluid flow rate and direction,and their relationships with localization of mineralization.This paper reviews the principles and methods of hydrodynamic studies of mineralization,and discusses their significance and limitations for ore deposit studies and mineral exploration.The driving forces of fluid flow may be related to fluid overpressure,topographic relief,tectonic deformation,and fluid density change due to heating or salinity variation,depending on specific geologic environments and mineralization processes.The study methods may be classified into threetypes,megascopic（field） observations,microscopic analyses,and numerical modeling.Megascopic features indicative of significantly overpressured（especially lithostatic or supralithostatic） fluid systems include horizontal veins,sand injection dikes,and hydraulicbreccias.Microscopic studies,especially microthermometry of fluid inclusions and combined stress analysis and microthermometry of fluidinclusion planes（FIPs） can provide important information about fluid temperature,pressure,and fluid-structural relationships,thus constraining fluid flow models.Numerical modeling can be carried out to solve partial differential equations governing fluid flow,heat transfer,rock deformation and chemical reactions,in order to simulate the distribution of fluid pressure,temperature,fluid flow rate and direction,and mineral precipitation or dissolution in 2D or 3D space and through time.The results of hydrodynamic studies of mineralization can enhance our understanding of the formation processes of hydrothermal deposits,and can be used directly or indirectly in mineral exploration.%流体流动是热液成矿作用不可或缺的一部分,其研究是建立成矿模式的重要组成部分。
流体趋近流动的原因Fluid flow is a fascinating phenomenon that occurs in various natural and man-made systems. From the flow of blood in our bodies to the movement of water in rivers, understanding the reasons behind fluid flow is crucial in many fields of science and engineering. There are several factors that contribute to the tendency of fluids to flow, and these can be examined from multiple perspectives.One perspective to consider is the macroscopic view, which focuses on the overall behavior of fluids. One of the primary reasons for fluid flow is the presence of a pressure gradient. Fluids naturally flow from regions of high pressure to regions of low pressure. This pressure difference creates a driving force that propels the fluid through a conduit, such as a pipe or a blood vessel. For example, when we open a faucet, the water flows out due to the pressure difference between the water source and the atmosphere.Another perspective to explore is the microscopic view, which delves into the molecular nature of fluids. Fluid flow is influenced by the internal forces between the molecules that make up the fluid. These intermolecular forces, such as van der Waals forces or hydrogen bonding, determine the viscosity of the fluid. Viscosity is a measure of a fluid's resistance to flow. Fluids with low viscosity, like water, flow easily, while those with high viscosity, like honey, flow more sluggishly. The molecular interactions also play a role in determining the fluid's density, which affects its ability to flow.Furthermore, fluid flow can be understood from the perspective of conservation laws, specifically the principles of mass and energy conservation. According to the principle of mass conservation, the total mass of fluid entering a system must be equal to the total mass exiting the system. This principle ensures that there is a continuous flow of fluid through a conduit. The principle of energy conservation, on the other hand, dictates that the total energy of the fluid remains constant as it flows. This principle governs phenomena such as the Bernoulli'sprinciple, which explains the relationship between fluid velocity, pressure, and elevation.Additionally, fluid flow can be influenced by external factors such as gravity and external forces. Gravity playsa significant role in fluid flow, as it can create pressure differences and induce flow. For example, water flows downhill due to the gravitational force acting on it. External forces, such as those applied by pumps or fans,can also drive fluid flow. These forces overcome the resistance offered by the fluid's viscosity and other internal forces, facilitating the movement of the fluid.Finally, it is essential to consider the role of fluid properties and boundary conditions in determining fluid flow. Fluid properties, such as temperature and composition, can affect the flow behavior. For instance, changes in temperature can alter the fluid's viscosity, thereby influencing its flow characteristics. Boundary conditions, such as the shape and roughness of the conduit, also play a role in fluid flow. Smooth, streamlined surfaces promote laminar flow, while rough surfaces can induce turbulence.In conclusion, fluid flow is a complex phenomenon influenced by various factors. From a macroscopic perspective, pressure gradients drive fluid flow, whilefrom a microscopic view, intermolecular forces affect afluid's ability to flow. Conservation laws, external forces, fluid properties, and boundary conditions also contributeto fluid flow. Understanding the reasons behind fluid flowis crucial for a wide range of applications, from designing efficient transportation systems to studying the behaviorof biological fluids.。
INVESTIGATION OF CLOSED VALVE OPERATION USING COMPUTATIONAL FLUIDDYNAMICSG. Dyson CLYDEUNION Pumps Penistone, Sheffield, UKJ. Teixeira Cranfield University Cranfield, Bedford, UKABSTRACTPredicting the head of a centrifugal pump operating at closed valve remains a difficult task. The nature of the flow regime and the influence of geometric features on this flow is uncertain. In this paper both the flow regime and the influence on that regime by geometry is investigated using a commercially available CFD RANS code. A CFD methodology is presented that takes account of the difficult boundary conditions. This methodology is then used to present the flow regime in a volute pump against the background of available research.The theory of solid body rotation, as a major influence on the closed valve head, is shown by the CFD simulations to be analogous to but not representative of the actual flow regime. The nature of the flow is impulsive and unsteady with fluid interchange occurring between the pump collector and the impeller vane passages.At the inlet the pump impeller experiences a strong steady outflow from the impeller blade tip. This spiralling flow must be accommodated within the computational solution. The impeller outlet is filled with a vortex driven by the flow which cannot be accommodated within the stalled stator passageways. The annular gap between the impeller and collector is filled with a pulsating flow whose frequency is determined by the number of vanes within the impellerModel validation was carried out by reference to experimental papers and time averaged closed valve head values obtained under standard performance testing.Keywords: Centrifugal Pumps, CFD Analysis, Closed Valve HeadINTRODUCTIONThe process of designing a centrifugal pump with a predictable closed valve head is complex. Many factors can influence the value of the closed valve head and many researchers have attempted to define the deviation from the theoretical Euler maximum by application of empirically derived correction coefficients, Stepanoff (1957), Peck (1968), Thorne (1988). Although these correction coefficients generally approach the machines closed valve head their accuracy is suspect as the correction coefficients are generally derived by statistical analysis. The closed valve head predicted is seldom related to the actual geometric features of the hydraulic design One exception, and the most complete of the prediction methodologies was proposed by Stirling (1982)based on work by Levin and Poliokovsky (1965) and attempts to attribute proportions of the closed valve head to impeller, collector and suction design. Whist some geometry is linked to the closed valve head by the paper there are some noticeable omissions. Vane number and geometry in particular are not considered within the methodology. When applied to a wide range of the geometries the accuracy of the prediction is no better than the empirical formulas previously cited. A review of the prediction methods and their accuracy over a number of geometries is available in Dyson (2002). Conversely Computational Fluid Dynamics notionally contains all the geometric features of the design but in practicality some compromises must be made.Experimental studies at closed valve are not commonplace. The nature of the flow is unsteady and measurement is difficult. Early contributions from Ficher and Thoma (1932) and Acosta and Bowerman (1957) found significant cross channel flow between impeller passages and the existence of a relative eddy which rotated counter to the machine rotation at sub-synchronous speed. Worster (1963) also observed considerable flow interchange between impeller and volute and proposed this energy exchange was responsible for the closed valve head not reaching its theoretical Euler value. Notable contributions are available more recently form Kaupert and Thomas (1999) who presented the unsteady nature of closed valve flow and the non-linear nature of the pressure rise around a pump volute operating at closed valve. Frost and Nilsen (1991) also proposed a theory for the volute contribution to closed valve head based on the velocity profile across the throat, with theProceedings of the ASME 2009 Fluids Engineering Division Summer MeetingFEDSM2009August 2-6, 2009, Vail, Colorado USAFEDSM2009-78021impeller contribution based on solid body rotation. The potential for using CFD to predict off-design flow was investigated by Cooper and Graf (1994) and the science has advanced rapidly as computational power has increased. Work by Newton (1988) attempted to model the closed valve condition of a centrifugal fan and compare these experimental results with CFD analysis. Newton’s work found an over prediction of the closed valve head by 160% associated with an over-prediction of the volute pressure rise. This was directly linked to the rotor/stator interaction prediction.Against this background the major flow features of a single stage centrifugal double volute pump of API OH2 style are explored using CFD. The pump in question has the following characteristicsSuction Branch Size 6”Discharge Branch Size 4”Impeller Outside Diameter D2 17”represent the inlet boundary condition elements of the system are required to accept the inlet backflow recirculation. This recirculation can extend 20 pipe diameters into the suction channel Palgrave (1985) & Frazer (1982). Newton’s (1988) work on a centrifugal fan did not model these system elements and this is one element that contributes to unreliability of the solution.It is important to model the time dependent nature of the flow when considering the closed valve solution and the value of the time-steps between solution iterations directly influences the solution accuracy. Figure 2 illustrates the changing time-dependent pressure for reducing time-steps. Subsequent reductions in time step size were carried out to ascertain any further difference in the unsteady flow regime. The solution was deemed to have converged at the 0.5degree time step size as differences on consecutive times step reductions were within 0.1% of the 0.5 degree time step value. Overall general recommendations cannot be given as the time step size must be appropriate for the impeller diameter and must take into consideration the scale of the model.of the art capability for desktop machines when this researchwas carried out they are now undoubtedly small by comparison to current capability and should be viewed from this perspective.A grid independency study is a prudent way to approach the question of necessary gird size. By consecutively increasing the grid refinement and comparing differences in important physical quantities, in this case differential pressure at the impeller outflow and relative velocities along the passage midstream line, independency is assumed to be reached when differences in these values are less the 0.1%.When considering grid refinement it is also prudent to view the simulation for a practical standpoint. It is difficult to give an overall recommendation for grid size without considering the overall scale of the machine. The grid refinement must be sufficient to capture all of the anticipated flow features and maintain the independency of the solution from the grid.A further consideration within any CFD simulation is the turbulence model. For this research the k-epsilon model with scalable wall function was used. This is the most widely used and validated turbulence model and has been shown to perform well where the Reynolds shear stresses are important and in confined flows. The turbulence model was found to be suitable for machines of high Reynolds number (Re>104), Chung (2002), where strong flow features dominate the regime.Whilst many models are available to predict turbulence a study of these found little difference in the overall prediction performance but found the k-epsilon model converged more quickly.VOLUTE OBSERVATIONSObservations from the computation model must align with current knowledge to add weight to the computational validity. Only after such reinforcement can further inference be made about the flow mechanisms at work.Kaupert and Thomas’ (1999) observation of the non linear pressure rise around the volute is reinforced by the computational solution and is illustrated in figure 3.Figure 3 Non-Linear pressure distribution around the volute The stalled nature of the volute passageways, proposed by Frost and Nilsen (1991) is inhibiting the impeller outlet flux. The liquid trapped within the upper proportion of the impeller and volute is subject to velocity variations from the rotating impeller blades approaching the volute lips. When the impeller vanes are congruent to the lip a finite volume of liquid is squeezed between this small gap causing an increase in velocity and a decrease in the instantaneous pressure. This is also supported by observations made by Goulas and Trouscott (1988).Much of the research mentioned comments on the frequency of the pressure pulsations within the volute and the dominance of vane pass frequency. The computational solutionFigure 4 Frequency spectra within V olute taken fromthe CFD solutionObservations of the nature of flow with the volute passageways also reinforce the mental image Frost proposed. Unlike Frost’s geometry the machine modeled in this instance is representative of conventional pump design and has a double volute. The stalled bound volume proposed is now separated by the central splitter altering the nature of the flow figure 5.Figure 5 Vector plot around casing splitterThe impulsive nature of the pressure development is linked to the position of the blade angle in relation to the volute lip. Figure 6 illustrates this instantaneous fluctuation in pressure with respect to the blade position. Within this figure the pressure is shown to develop as the blade approaches the volute lip and diminish as the blade passes the lip. This pulsating pressure was observed in the work by Kaupert and Thomas (1999).The pressure picture, viewed through a cruder breakdown of the fringe plot gives us further insight into the flow features evident in the machine. The strength of the wake is particularly noticeable as the blade approaches the volute lip. This strong feature diminishes when the impeller blade has passed the volute lip illustrated in figure 7.Figure 8 uses velocity profiles to illustrate the flow features. The diffusing passage ways are filled with a slow moving liquid. Pressure pulsations drive into the slow moving body of fluid at vane pass frequency.This again is in conflict with the perceived wisdom that the impeller acts as a solid disc (Nielsen 1989) driving the fluid with the volute by cylindrical viscous forces alone. Newton’s(1998) analysis of the solid body theory concluded that the impeller solid body rotation did not exist, but his experimental evaluation was flawed. He merely closed off the impeller outlets with a circumferential strip of metal and measured the generated pressure. This approach negates the rotor-stator blade interaction pressure fluctuations.Figure 9 also provides some insight in into the rotor stator interactions and is able to demonstrate why the vane pass frequency exists in the diffusing passageways. The velocity vectors are observed to be driven between the impeller blade and the casing cutwater as proposed by Kaupert and Thomas (1999). This provides a peristaltic effect at closed valve. The diffusing passages experience a forcing of the fluid into the fixed area chamber. They then are relieved as the impeller blade passes beyond the cutwater and the large A2BB area is in congruent with the casing cutwater. A2BB is designated as the area that extends from the impeller vane tip at the outside diameter on the suction side of the vane to the pressure side of the subsequent vane.Figure 6 Pressure rise within volute with blade phase position Research papers, including Abraman and Howard (1988) and Fischer and Thoma (1952), pointed out the strong links between phase position and flow features. Pressure plots from the CFD presented in figure 7 denote the strong wake effects that propagate then diminish as the impeller blade passes the casing cutwaterFigure 8 is illustrative of different flow regimes that can be observed in different areas of the pump. The long and short diffusing passage ways are filled with slow moving liquid. All the dynamic rotor-stator interaction effects take place within the volute, around the impeller periphery.Figure 7 Fringe plots close to volute lip depicted forphase positionFigure 9 demonstrates the dynamic effect as the stalled nature of both impeller and volute drive the liquid trapped between these components into the annular gap between blade and lip.The nature of the impeller geometry causes the wake effect, streaming from the suction side of the blade, to be trapped by the liquid in the annular gap between the impeller and volute and, looking in the relative frame, to be turned inwards as it is bound by the higher velocity steam within this gap.The resulting vortex in the outer proportion of the impeller is as observed by Acosta and Bowermann (1957) who described this as “dead water”. Looking in the absolute frame this observationFigure 8 Overall image of pump throughwould hold true. Worsters (1953) observations where this exit relative eddy were first observed, also supports the CFD qualitative predictions.Acosta and Bowermann (1957) observed their eddy rotating counter to the machine rotation. The CFD predicts thatthe eddy is always apparent but its extent pulses with the impeller blade interaction. This pulsing phenomenon would appear to rotate counter to the machine rotation and would agree with the Acosta and Bowermann (1957) observations.Figure 9 Vector plots close to the volute lip IMPELLER CONSIDERATIONSThe mental model proposed by Simpson and Cinnamond (1964)for flow within the impeller defines the inner proportion of the impeller as filled with a standing vortex of a common radius of ½ of the impeller diameter. The proportion of liquid trapped above it drives this vortex, forcing fluid to re-circulate through the impeller eye and into the suction duct. The flow phenomenon from the CFD analysis generally conforms to the observations, but superimposed on this steady state picture must be the unsteady effects.The unsteady effects are evident when analyzing a 3 vane impeller within a double volute casing. The impeller blade passages experience a different flow field with respect to their position relative to the volute cut-water.Position A figure 10 & 11 – The upper proportion of the impeller passage, close to the discharge area of the impeller, is filled with a standing eddy. The viscous forces applied by the stalled collector passages drive this eddy as the fluid around the impeller periphery is squeezed between the volute lip and the approaching impeller blade.Position B figure 10 & 11 – This represents the interface between the discharge rotating eddy and the inlet backflow eddy. This suction eddy is characterized by a local separation from the pressure side of the blade, close to the hub. This local development is manifest as the volume flux is reduced, becoming more stable as the flux approaches zero. At closed valve this complex inlet discontinuity gives rise to a span wise flux from pressure to suction side.Position C figure 10 & 11 – At this position the flux stream has passed from the hub across the passage span to the blade suction side in the meridional plane. The region is associated with strong secondary flow phenomenon and three-dimensionality, as the inlet eddy is driven out from the impeller and into the suction channel.Position D figure 10 & 11 – The largest proportion of the streamline passage is characterized by strong flow re-circulation at mid position on the blade pressure side. The vortex, driven by the viscous interaction with the fluid trapped within the volute, experiences stochastic fluctuations that are characterized by the size of the discharge vortex with respect to the volute lip position. When a blade approaches the volute lip the relative volume flux is squeezed between the lip and the rotor-stator interface gap.Figure 10 Impeller Streamlines at Closed ValveFigure 11 Velocity Vectors Within at Closed ValveThis suppresses the vortex development by providing a firm viscous boundary. As the impeller blade moves past the volute lip the diffusive contribution of the volute decays the intensity of the trapped casing fluid, allowing the impeller discharge vortex to move forwards along the pressure side of the impeller blade. A snapshot of this phenomenon can be seen in figure10 by comparison of the flow regime in individual impeller passages. The arrows in figure 10 represent the position of the volute lips.SUCTION CONSIDERATIONSThe research by Levin and Poliokovsky (1965) was carried out to ascertain the suction re-circulation at shut-off conditions. Investigations took place on a radial un-shrouded machine. The investigation uncovered two distinct zones of fluid, which interact together within the suction channel. The mental model they proposed is somewhat simplified as it does not cover the three-dimensional interactions. Investigations using CFD yield the following observations for the two zones.High-energy liquid is expelled from the impeller eye. This expelled liquid dominates the suction passage, occupying the 2/3 of the flow area from the pipe outer diameter downwardstowards the channel centerline figure12.Figure 12 Helical Spiraling Inlet Backflow at Suction DuctPeripheryThe extent that the expelled liquid fills the suction channel remains unchanged with distance from the impeller. This flow spirals helically down the periphery of the suction pipe in a direction counter to the impeller rotation at a constant helical angle. Each impeller blade generates individual streams. The tangential velocity imposed on the regime by the impeller dominates the flow causing the helical spiral angle to be approximately equal to the blade inlet angle. This angle does not diminish with distance from the impeller implying that both the tangential and axial components of the velocity decay proportionally to maintain this angle. Independent streams can be viewed in figure13Figure 13 CFD Streamline of Backflow DischargeThe inner 1/3 of the suction channel area contains a spiraling core of slower moving fluid. Viscous effects transmit tangential energy from the high-energy peripheral flow and drive this core in a helical spiral. Flow is dominated by the axial component of the velocity and tangential forces exerted by the peripheral flow cause the inner helical flow angle to be approximately double the outer angle. Again the helical spiral angle remains constant. figure14Figure 14 Internal Core of Spiraling Fluid in Suction Channel Visual experimental data is available to support the CFD solution. Figure 15 contains a photograph taken through a plexi-glass tube inserted at the suction of a centrifugal pump running at extreme low mass flux.Figure 15 Pump test to qualitatively illustrate the suction re-circulation effect at extreme part loadFiber tufts on the internal surface of the tube are used to illustrate the motion of the fluid as it exits the eye of the impeller and spirals down the suction pipe. The interface between the re-circulating flow is predicted, by the CFD model, to be 14 multiples of the eye diameter. The position of this interface occurs when the viscous and friction effects have slowed the net velocity within the channel to zero. This “plugs” the suction pipe, encouraging the outer flow to reverse and form the inner core figure12. The extent that the re-circulating flow pushes into the suction duct is linked to the peripheral speed of the impeller eye. Increasing rotational speed of the impeller or the eye diameter pushes the extent of the re-circulation further down the suction passage. Of particular interest with this analysis is the experimental FFT data provided in figure 16. The data is dominated by the vane pass rate of the frequency spectrum. The CFD data, in contrast, predicts a steady flow regime unaffected by the vane rate.There is an explanation for this apparent inconsistency. It is the practice for many pump designers to include within their machines a suction splitter in the suction annulus. This is used to eliminate suction pre-swirl into the impeller maximizing the generated head.The experimental FFT data was taken in such a machine. Although the regime may be steady when the outer spiraling core is unimpeded by the presence of a splitter, the opposite is true when a splitter is present. The spiraling fluid from each blade impacts on the splitter causing an unsteady interaction. The CFD solutions in this simulation were carried out without the presence of the splitter. This greatly simplifies the solution as only the unsteady interaction at the pump discharge is taken into consideration.Figure 16 Experimental pressure spectrum taken in suctionchannelThe time averaged closed valve head predicted by the CFD simulation conforms very closely to the predicted value of closed valve pressure from an experimental closed circuit test loop, conventionally used for production performance testing. Cooper P. and Graf E. (1994) “Computational Fluid Dynamical analysis of complex internal flows in centrifugal pumps”. Proceedings of the eleventh annual international pump users symposium. Pp83-94Dyson G (2002) A review of closed valve head prediction methods for centrifugal pumps. Proceedings of the Institution of Mechanical Engineers. Part A. Journal of power and energy. vol. 216, n o 4, pp. 329-337The predicted closed valve head for simulation 281m Experimental closed valve head 285mFicher K. and Thoma D.(1932) “Investigation of the Flow in A Centrifugal Pump” Presented at the meeting of ASME June27 1932The nature of testing a pump at closed valve is difficult, rapid heating of the pumped product and the unsteady pressure pulsations make accurate experimental results difficult to obtain, particularly in a production environment. Experimental tests carried out on production equipment are subject to inherent deviations attributable to manufacturing tolerances. Whilst design integrity is satisfied entirely by the simulation model dimensions the same cannot be said of the sand cast pump impellers and casings.Fraser WH (1982) Recirculation in Centrifugal Pumps. World Pumps 1982 vol188 p.227-235Frost T.H. and Nilsen E.(1991) “Shut-off head of centrifugal pumps and fans.” Proc. Instn. Mech. Engrs. V ol 205 pp 217-223Goulas A. and Truscott G. (1988) “The flow at the tip of an impeller at off-design conditions.”That being said the over prediction of pressure experienced in the previous Newton (1988) simulations is avoided under the presented methodology.C336/88 Proc. IMechE Conf. “Part-Load Pumping operation, control and behavior.”Kaupert and Thomas (1999) “The Unsteady Pressure Field in a High Specific Speed Centrifugal Pump Impeller Part1 – Influence of the V olute”. Journal of Fluids Engineering. V ol 121 pp621-626CONCLUDING REMARKSThe images and research presented within this paper represent a proportion of a 2005 PhD thesis whose aim was to point towards the viability of CFD in predicting the closed valve head of a centrifugal pump.Levin A.A. and Poliokovsky (1965) “To calculate the pressure characteristic of centrifugal pumps and fans at zero discharge.” Isvestya AN SSSR Energetika 1, Transport 2 pp 129-133As with much CFD research the quantitative results must be viewed with some skepticism unless they are backed by experimental analysis. That being said the case for CFD prediction of closed valve head is strong based on the observations of other researchers. Comparison of measured closed valve head from a standard pump performance test on a closed loop gave good agreement and was not subject to the over-prediction previous researchers had encountered.Newton T M (1988). “Rotor –Stator interaction in radial flow pumps and fans at shut-off conditions”. Pdh Thesis. Newcastle University.Palgrave. R(1985) –“Operating centrifugal pumps at partial capacity”, 9th BPMA Technical Conference, Paper 6, Warwick University, Coventry (April 1985)Peck, J.F. (1968) “Design of centrifugal pumps with computer aid.” Proc. Instn. Mech. Engrs. V ol. 183, Part 1, pp 321-352The rapid development is computing power allows more complex models to be attempted but problems still remain with post processing and handling data generated by the CFD tools. The accuracy of the analysis is not the only consideration. The CFD solution generates great volumes of information, finding what to analyze and how best to represent the data remains time consuming and challenging.Stepanoff A.J. (1957) “Centrifugal and axial flow pumps.” John Wiley, New York Chapman and Hall, LondonSimpson H.C. and Cinnamond (1964) “Studies of flow through centrifugal pump impellers.” IMechE Proc. 1963-64 V ol. 178, Pt. 3 1(ii) Paper 8Stirling T.E. (1982) “Analysis of the design of two pumps using NEL methods.” C183/82 Proc. ImechE Conf. “Centrifugal Pumps-Hydraulic Design.”REFERENCESThrone E.W. (1988) “Head and power at closed valve.” C331/88, Proc. IMechE Conf. Part-Load Pumping operation, control and behaviour.Abramian M. and Howard J.H.G(1998) “Experimental investigation of the steady and unsteady relative flow in a model centrifugal impeller passage” ASME Journal of Turbomachinery. V ol116 pp269-279Worster R.C.(1963) “The Flow in V olutes and its Effect on Centrifugal Pump Performance”. Proc. ImechE V ol177 (31)pp843-865Acosta A J and Bowerman RD(1957). “An experimental study of centrifugal pump impellers” Transactions of ASME Pp1821-1839Chung T J “Computational Fluid Dynianics” Cambridge University Press, UK, 2002。
