Ansys12.0CFX官方教程9A课件.ppt

Chapter9Turbulence,IntroductiontoCFX,WhatisTurbulence?,Unsteady,irregular(non-periodic)motioninwhichtransportedquantities(mass,momentum,scalarspecies)fluctuateintimeandspaceIdentifiableswirlingpatternscharacterizeturbulenteddiesEnhancedmixing(matter,momentum,energy,etc.)resultsFluidpropertiesandvelocityexhibitrandomvariationsStatisticalaveragingresultsinaccountable,turbulencerelatedtransportmechanismsThischaracteristicallowsforturbulencemodelingContainsawiderangeofturbulenteddysizes(scalesspectrum)Thesize/velocityoflargeeddiesisontheorderofthemeanflowLargeeddiesderiveenergyfromthemeanflowEnergyistransferredfromlargereddiestosmallereddiesInthesmallesteddies,turbulentenergyisconvertedtointernalenergybyviscousdissipation,IstheFlowTurbulent?,ExternalFlows,InternalFlows,NaturalConvection,alongasurface,aroundanobstacle,where,where,Otherfactorssuchasfree-streamturbulence,surfaceconditions,anddisturbancesmaycausetransitiontoturbulenceatlowerReynoldsnumbers,istheRayleighnumber,isthePrandtlnumber,FlowscanbecharacterizedbytheReynoldsNumber,Re,ObservationbyO.Reynolds,Laminar(LowReynoldsNumber),Transition(IncreasingReynoldsNumber),Turbulent(HigherReynoldsNumber),TurbulentFlowStructures,EnergyCascadeRichardson(1922),GoverningEquations,ConservationEquations,Continuity,Momentum,Energy,where,NotethatthereisnoturbulenceequationinthegoverningNavier-Stokesequations!,OverviewofComputationalApproaches,DirectNumericalSimulation(DNS)Theoretically,allturbulent(andlaminar/transition)flowscanbesimulatedbynumericallysolvingthefullNavier-StokesequationsResolvesthewholespectrumofscales.NomodelingisrequiredButthecostistooprohibitive!NotpracticalforindustrialflowsLargeEddySimulation(LES)typemodelsSolvesthespatiallyaveragedN-SequationsLargeeddiesaredirectlyresolved,buteddiessmallerthanthemesharemodeledLessexpensivethanDNS,buttheamountofcomputationalresourcesandeffortsarestilltoolargeformostpracticalapplicationsReynolds-AveragedNavier-Stokes(RANS)modelsSolvetime-averagedNavier-StokesequationsAllturbulentlengthscalesaremodeledinRANSVariousdifferentmodelsareavailableThisisthemostwidelyusedapproachforcalculatingindustrialflowsThereisnotyetasingle,practicalturbulencemodelthatcanreliablypredictallturbulentflowswithsufficientaccuracy,RANSModelingTimeAveraging,Ensemble(time)averagingmaybeusedtoextractthemeanflowpropertiesfromtheinstantaneousonesTheinstantaneousvelocity,ui,issplitintoaverageandfluctuatingcomponentsTheReynolds-averagedmomentumequationsareasfollowsTheReynoldsstressesareadditionalunknownsintroducedbytheaveragingprocedure,hencetheymustbemodeled(relatedtotheaveragedflowquantities)inordertoclosethesystemofgoverningequations,Fluctuatingcomponent,Time-averagecomponent,Example:Fully-DevelopedTurbulentPipeFlowVelocityProfile,Instantaneouscomponent,(Reynoldsstresstensor),RANSModelingTheClosureProblem,Closureproblem:RelatetheunknownReynoldsStressestotheknownmeanflowvariablesthroughnewequationsThenewequationsaretheturbulencemodelEquationscanbe:AlgebraicTransportequationsAllturbulencemodelscontainempiricismEquationscannotbederivedfromfundamentalprinciplesSomecalibratingtoobservedsolutionsand“intelligentguessing”iscontainedinthemodels,RANSModelingTheClosureProblem,TheRANSmodelscanbeclosedinoneofthefollowingways(1)EddyViscosityModels(viatheBoussinesqhypothesis)BoussinesqhypothesisReynoldsstressesaremodeledusinganeddy(orturbulent)viscosity,T.Thehypothesisisreasonableforsimpleturbulentshearflows:boundarylayers,roundjets,mixinglayers,channelflows,etc.(2)Reynolds-StressModels(viatransportequationsforReynoldsstresses)ModelingisstillrequiredformanytermsinthetransportequationsRSMismoreadvantageousincomplex3Dturbulentflowswithlargestreamlinecurvatureandswirl,butthemodelismorecomplex,computationallyintensive,moredifficulttoconvergethaneddyviscositymodels,AlargenumberofturbulencemodelsareavailableinCFX,somehaveveryspecificapplicationswhileotherscanbeappliedtoawiderclassofflowswithareasonabledegreeofconfidence,AvailableTurbulenceModels,Thevelocityprofilenearthewallisimportant:PressureDropSeparationShearEffectsRecirculationTurbulencemodelsaregenerallysuitedtomodeltheflowoutsidetheboundarylayerExaminationofexperimentaldatayieldsawidevarietyofresultsintheboundarylayer,Theabovegraphshowsnon-dimensionalvelocityversusnon-dimensionaldistancefromthewall.Differentflowsshowdifferentboundarylayerprofiles.,TurbulenceNeartheWall,Byscalingthevariablesnearthewallthevelocityprofiledatatakesonapredictableform(transitioningfromlineartologarithmicbehavior)Sincenearwallconditionsareoftenpredictable,functionscanbeusedtodeterminethenearwallprofilesratherthanusingafinemeshtoactuallyresolvetheprofileThesefunctionsarecalledwallfunctions,Linear,Logarithmic,Scalingthenon-dimensionalvelocityandnon-dimensionaldistancefromthewallresultsinapredictableboundarylayerprofileforawiderangeofflows,TurbulenceNeartheWall,Fewernodesareneedednormaltothewallwhenwallfunctionsareused,TurbulenceNeartheWall,2019/12/13,15,可编辑,TurbulenceNearTheWall,y+isthenon-dimensionaldistancefromthewallItisusedtomeasurethedistanceofthefirstnodeawayfromthewall,u,y,Boundarylayer,y+,Wallfunctionsareonlyvalidwithinspecificy+valuesIfy+istoohighthefirstnodeisoutsidetheboundarylayerandwallfunctionswillbeimposedtoofarintothedomainIfy+istoolowthefirstnodewilllieinthelaminar(viscous)partoftheboundarylayerwherewallfunctionsarenotvalid,Insomesituations,suchasboundarylayerseparation,wallfunctionsdonotcorrectlypredicttheboundarylayerprofileInthesecaseswallfunctionsshouldnotbeusedInstead,directlyresolvingtheboundarylayercanprovideaccurateresultsNotallturbulencemodelsallowthewallfunctionstobeturnedoff,Wallfunctionsapplicable,Wallfunctionsnotapplicable,TurbulenceNeartheWall,Standardk-ModelThe“industrialCFD”standardsinceitofferagoodcompromisebetweennumericaleffortandcomputationalaccuracyWallfunctionsarealwaysusedy+shouldtypicallybe300forthewallfunctionstobevalidThereisnolowerlimitony+CFXusesScalablewallfunctionsIfyourmeshresultsiny+valuesbelowthevalidrangeofthewallfunctions,thenodesnearestthewallareeffectivelyignoredThisensuresvalidresults,withinthemodellimitations,butisawasteofmeshKnownlimitations:SeparationgenerallyunderpredictedsincewallfunctionsareusedInaccuracieswithswirlingflowsandflowswithstrongstreamlinecurvature,k-epsilonModel,k-ModelOneoftheadvantagesofthek-formulationisthenearwalltreatmentforlow-ReynoldsnumbercomputationsHere“low-Reynolds”referstotheturbulentReynoldsnumber,whichislowintheviscoussub-layer,notthedeviceReynoldsnumberInotherwords“low-Reynoldsnumbercomputations”meansthenearwallmeshisfineenoughtoresolvethelaminar(viscous)partoftheboundarylayerwhichisveryclosetothewallAlow-Reynoldsnumberk-modelonlyrequiresy+=2Ifalow-Rek-emodelwereavailable,itwouldrequireamuchsmally+Inindustrialflows,eveny+2.Afinernearwallmeshisrequiredtoachievey+2.,ShearStressTransport(SST)ModelTheSSTmodelisbasedonthek-modelandhasthesameautomaticwalltreatmentItaccountsforthetransportoftheturbulentshearstressandgiveshighlyaccuratepredictionsoftheonsetandtheamountofflowseparationThisisagooddefaultchoice,SSTModel,y+fortheSSTandk-omegaModels,WhenusingtheSSTork-modelsy+shouldbeTurbulenceandNear-WallModelingModelingFlowNeartheWallGuidelinesforMeshGeneration,OtherTurbulenceModels,WhenRANSmodelsarenotadequate,EddySimulationmodelscanbeusedAsalreadymentioned,thesearemorecomputationallyexpensiveLargeEddySimulation(LES)Resolvesthelargeeddies,modelsthesmalleddiesProblem:Requiresaveryfineboundarylayermesh,makingitimpracticalformostflowsDetachedEddySimulation(DES)UsesaRANSmodelintheboundarylayer,switchesovertoLESinthebulkflowA“standard”boundarylayermeshcanbeusedProblem:theRANStoLESswitchdependsonthemesh,whichcangiveunphysicalresultsonthe“wrong”meshScale-AdaptiveSimulation(SAS)LikeDES,butwithoutthemeshdependencyproblems,InletTurbulenceConditions,Unlessturbulenceisbeingdirectlysimulated,itisaccountedforbymodelingthetransportofturbulenceproperties,forexamplekandSimilartomassandmomentum,turbulencevariablesrequireboundaryconditionspecificationsSeveraloptionsexistforthespecificationofturbulencequantitiesatinlets(detailsonnextslide)Unlessyouhaveabsolutelynoideaoftheturbulencelevelsinyoursimulation(inwhichcase,youcanusetheMedium(Intensity=5%)option),youshouldusewellchosenvaluesofturbulenceintensitiesandlengthscalesNominalturbulenceintensitiesrangefrom1%to5%butwilldependonyourspecificapplicationThedefaultturbulenceintensityvalueof0.037(thatis,3.7%)issufficientfornominalturbulencethroughacircularinlet,andisagoodestimateintheabsenceofexperimentaldata,InletTurbulenceConditions,DefaultIntensityandAutocomputeLengthScaleThedefaultturbulenceintensityof0.037(3.7%)isusedtogetherwithacomputedlengthscaletoapproximateinletvaluesofkand.Thelengthscaleiscalculatedtotakeintoaccountvaryinglevelsofturbulence.Ingeneral,theautocomputedlengthscaleisnotsuitableforexternalflowsIntensityandAutocomputeLengthScaleThisoptionallowsyoutospecifyavalueofturbulenceintensitybutthelengthscaleisstillautomaticallycomputed.Theallowablerangeofturbulenceintensitiesisrestrictedto0.1%-10.0%tocorrespondtoverylowandveryhighlevelsofturbulenceaccordingly.Ingeneral,theautocomputedlengthscaleisnotsuitableforexternalflowsIntensityandLengthScaleYoucanspecifytheturbulenceintensityandlengthscaledirectly,fromwhichvaluesofkandarecalculatedLow(Intensity=1%)Thisdefinesa1%intensityandaviscosityratioequalto1Medium(Intensity=5%)Thisdefinesa5%intensityandaviscosityratioequalto10ThisistherecommendedoptionifyoudonothaveanyinformationabouttheinletturbulenceHigh(Intensity=10%)This