计算流体力学CFD外文翻译(13页).doc

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1、-第 1 页计算流体力学计算流体力学CFD外文翻译外文翻译-第 2 页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 designefforts used tomake Volvo 60 round the world racers and Americas Cup yachts faster.Computational Fluid Dynamic

2、s or CFD actually covers a great many engineering specialtiesand is not the sole domain of boat and ship design.In this article we will review what types ofCFD products exist and hopefully provide some understanding of when and how CFDproducts are best suited to a project.Computational Fluid Dynamic

3、s is the application of computers to the modeling of fluidcharacteristics when either the fluid is in motion or when an object disturbs a fluid.A fewexamples of a fluid in motion are water or chemical flow in pipes,heating and ventilationsystems conducting cooling,heating or fresh air supplies to a

4、building.Fluids in motion alsoinclude flame and fire effects in combustion or jet engines.Surprised by these fields ofinterest?What about examples of an object disturbing a fluid?Examples include stirring paddlessubmerged in a tank of water and effluent in a waste treatment plant,aircraft of all kin

5、ds,carsand trucks at highway or racing speeds and even monohull sailboats,ship,multihull sailboatsto name but a few.Obviously,an open mind is important when considering what constitutes a fluid.Fluidscan exist in gaseous and liquid states and science has recently found that even some solids canexhib

6、it fluid like characteristics under right conditions.Scientists have found that some of themost spectacular and deadly landslides or rock falls behave as a fluid while the mass of stoneand soil or sand is in motion,only to return to a most decidedly solid form when the motionsubsides.The general fie

7、ld of fluid dynamics differs from the field of boat design in one criticalway.Only boat design deals with a vehicle passing through the two fluids of air and watersimultaneously.Our atmosphere is a compressible fluid,though not at yachting or evenhigh-powered boat racing speeds.Air can change in den

8、sity according to altitude,temperatureand humidity.Water is anincompressible fluid that can vary in viscosity according to itssalinity and temperature.For most of us,small effects such as variable salinity andtemperature are not of concern,but can make the difference between winning and loosing amaj

9、or international yacht race.How do CFD programs Work?CFD programs are based on the laws of physics,such as the law of conservation ofmomentum,and special“boundary conditions”.The law of conservation of momentum states-第 3 页that the total momentum of a system remains constant regardless of how the sy

10、stem maychange.A boundary condition limits how and where a fluid can travel.A simple example isthat motion of the fluid must emain tangent or parallel to the surface of an object passingthrough it.Another example is that pressure applied by the fluid against the object must beperpendicular to the su

11、rface at all points.These laws and conditions are critical to the development of a CFD program becausethey 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 equationsthat describe

12、 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 anddifferential calculus and require specialized computer techniques to solve them.Typically theprogrammer writes an algorithm that mak

13、es a series of estimates using algorithms thatiteratively solve the sets of equations by looking for“balance”in the system of equations.Afinal answer is obtained when the algorithm converges on a solution with an error that issufficiently small for the desired accuracy.Once an algorithm has been dev

14、eloped to implement the laws of momentum andboundary conditions,it cannot be applied to the entire surface of the hull and appendages atonce.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

15、 patch in turninfluences the fluid flow on the patch area of its neighbors and therefore the solution mustaccount for the conditions surrounding the patch currently being solved.As a result theprogram must solve and resolve the equations for all of the patches until the solution obeysthe physical la

16、ws and boundary conditions.Sometimes the complexities of the laws of physics are too difficult to implement all atthe same time.As a result the aerodynamicists choose to write programs that make certainlimiting assumptions that permit the programming to become more practical and still result inreaso

17、nable results.A specific example arises in the case of what actually happens to fluidsvery near the surface of an object.The boundary layer as it is called experiences shear forcesin the objects direction of travel that result in viscous drag.These shear forces are described ina special set of equat

18、ions called the Navier Stokes relationships.The Navier Stokes equationsaresufficientlycomplexthemselvesthatattempting toincludethemwithineveryaerodynamics or hydrodynamics program would make the solutions nearly impossible.As aresult there are Navier Stokes based programs that specifically address v

19、iscous drag and Panelmethod programs that compute lift,wave drag and induced drag.A complete estimate of thedrag encountered by a boat requires the data supplied by both programs.-第 4 页What do CFD programs Calculate?The most obvious calculation that would be of interest in boat design is thedetermin

20、ation 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 thesedrag areas.The second general area of calculation is lift.The term lift arises from itsapplication to aircraft and becomes

21、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 andthe driving force of the sails themselves.It also applies to the turning forces of a rudder,andthe supportive force acting on“foils”to elevate a

22、 hydrofoil sail or powerboat above the watersurface.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 awing of infinite length.Here the term“foil”is used to define the shape of a

23、 keel or rudderalong the chord from the leading to the trailing edge.Foil shapes are best known by thealphanumerical names given to them such as NACA 63A012.So a 2D fluids program wouldcompute the lift,drag,velocity distribution,turbulence onset and the generation of bubblessimilar to cavitation for

24、 a 2D shape such as a wing or keel foil,and would not include any 3Dinformation such as keel span or thickness distribution or the presence of a bulb.A 3D fluidsprogram would compute wave and induced drag from a hull,keel and rudder,including theeffects of a bulbed keel carrying winglets.CFD codes a

25、re critical for more than optimizing the performance of a top-notchAmericas Cup class racing yacht.These codes can be of great value to determine loadsplaced on boat structures of all types and are invaluable when applied to unique marinestructures such as oil platforms that are frequently subjected

26、 to the worlds worst storms.Lift and drag effects translate directly into loads that must be sustained by the boat or oilderrick if it is to remain intact in its intended operational conditions.For example,severalyears ago when the race was known as the Whitbread Round The World Race,many boatsdevel

27、oped life threatening hull delamination when subjected to the continuous pounding ofhigh speed downwind surfing and upwind beating.While delamination of a boat at sea isdefinitely related to structural design errors,those errors were caused by a lack of detailedinformation about the fluid forces exp

28、erienced by the boats.Knowledge of these forces wouldhave enabled designers to prevent the hull damage in the first place.Therefore,the potential application of CFD to your design project should depend onwhether or not the design regime that your vessel will operate in has well understoodengineering

29、 data available to prevent hull damage in addition to overall performance of the-第 5 页vessel.For example,the last few years have seen the development of high-speed hydrofoilsailboats for the consumer market.These top performance boats experience not onlysignificant speeds and loads,but the potential

30、 for unstable characteristics could make it highlydangerous to ride in one.However,the judicious use of field-testing and computer analysishas 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 particip

31、ating in atrans-Atlantic race.When the centerboards were raised in the outer hulls of the trimaran,theskipper was shocked to learn that the boards had been sheered off just below hull depth and hehad not had their use for some indeterminate time during the later portion of the race.Clearly,the struc

32、turaldesign of the boards had not taken into account the true forces of lift,drag orperhaps cavitation that would be experienced at sea.CFD programs do not calculate how fast a boat of any type will pass through the water orpredict the time to complete a course around the buoys.Predicting speed on a

33、 racecourse isthe domain of another class of programs called Velocity Prediction Programs or VPP.TheVPP makes use of lift and drag numbers calculated in a CFD program to estimate the speedabout will sail a course given the sail drive forces and the stability or righting moment of thehull.The VPP is

34、a closed loop simulation continuously varies estimated speed and resultinglift,drag and righting forces until retarding and driving forces are balanced and a stable speedresults.A CFD program on the other hand is an open loop simulation that simply states thatgiven an angle of heel and speed for a s

35、pecified hull and appendage configuration,here arethe forces that will result for that instant in time.No consideration is given to how the vesselachieved that speed or sailing condition.In summary then,CFD programs not only calculate lift and drag forces of a hull withappendages,they can also be us

36、ed to compute pressure loads due to waves and wave impact atspeed.The forces of lift,drag and pressure can be translated into structural requirements andprovide the means to optimize a hull working in concert with its appendages to produce lift inthe most efficient manner possible while satisfying t

37、he needs for stability.Predictions of liftand drag at various speeds can be used to develop a mathematical model needed to accuratelyclose 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 me

38、ans ofestimating the forces involved are available.In the case of a typical sailboat design,the forcesgenerated by the keel and rudder can be easily estimated if the keel lacks a bulb and if the keeland rudder shape are essentially straight leading and trailing edges.It is possible to make useof ana

39、lytical methods that are easily implemented on personal computers.A simple example is-第 6 页the program I wrote called LOFT that makes use of analytical methods developed by NASAand the US Air Force for initial performance prediction of wing designs.However,whilesimple programs like LOFT can adequate

40、ly address typical bulb-less keels and rudders theycannot analyze the performance of an Americas Cup racing keel with bulb and winglets.Only3D CFD programs can address that complex task.Who can operate a CFD program?While CFD programs can be of tremendous value,getting accurate and meaningfulresults

41、 is not typically within the reach of amateur and many professional boat designers.Adegreed Naval Architect or a fluids dynamicist is required to generate the key initial input to aCFD program called a mesh.The mesh is a mathematical description of the hull andappendages that are to be analyzed.It i

42、s not sufficient or even possible to use standard stations,waterlines and buttocks as inputs to a CFD program.The detailed shapes of the hull andappendages must be defined by a mesh of square patches that adjoin one another and whosedimensions are chosen according to the local curvature of the hull

43、or appendage or by theoccurrence of the intersection of the hull and a keel,rudder or lifting strut of a hydrofoil.Thegeneration of a mesh is a science unto itself and can require iterations by the analysts runningsuccessive trials to be sure that the mesh is sufficiently dense in critical areas.Som

44、e meshingcan be done automatically and then refined by hand.Typically the developers run complex 3DCFD programs or those trained in their use and as result are not really meant for use by therest of us.However,2D fluids programs designed for the analysis and development of 2D airor hydrofoil shapes(

45、recall the 63A010 example)are sufficiently easy to use for a designerwith basic mathematics skills and general knowledge of airfoil characteristics.Analyticalprograms such as Vacanti Yacht Designs FOIL program can aero/hydrofoil lift,drag,turbulence onset and bubble formation characteristics for any

46、one with basic computer skillsand a working knowledge of basic foil design.What CFD Programs Exist?Panel Method and Navier Stokes programs are two general classes of CFD programs thatapply to the issues of boat design.The most commonly used and most available are PanelMethod programs.Panel methods a

47、llow the prediction of wave drag,free surface effects andinduced 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 thehull within the fluid.However,due to viscosity,the fluid

48、 is subject to forces in shear more orless parallel to the hull surface that causes turbulence.Therefore the panel programs arereferred to as“inviscid”analysis methods.As a result they compute wave and induced dragbut not the effects of viscous drag.Viscous drag computations are computed by speciali

49、zed-第 7 页codes known as Navier Stokes programs.These programs are difficult to use and apply and arebest left to a professional skilled in their use.When a designer has a task that justifies the useof CFD programs,he should be using design tools that that can export true 3D surface shapesin the form

50、 of common Computer Aided Design(CAD)file formats.Designing in a typicalCAD program such as AutoCAD using lines and polylines,even though in 3D are notsufficient for use with CFD programs.True surface definitions such as Non-Uniform RationalB-spline(NURB)surfaces are required.Most professional versi