Ansys12.0CFX官方教程15A.ppt

Chapter15ReviewandTips,IntroductiontoCFX,DomainInterfaces,DomainInterfacescanbeusedaspartofameshingstrategyaswellasforconnectingdifferentdomainsorreferenceframestogetherBoundaryconditionsarecreatedineachdomainwhenaDomainInterfaceiscreated;generallyyoushouldnoteditthesedirectlyWhenthemeshisdifferentoneachsideoftheinterfaceaGGI(GeneralGridInterface)isusedThiswillusemorememoryintheSolverthanacontinuousmeshAccuracyacrossaGGIinterfaceisusuallynotaconcernaslongasthemeshlengthscalesoneachsidearesimilarAutomaticDomainInterfacesarecreatedbyCFX-PreinsomecasesAlwayschecktheseanddontassumethatalltherequiredDomainInterfaceshavebeencreated,Sources,SourcesareusedtoaccountforphysicsorprocessesthathavenotbeendirectlyresolvedinthesimulationMomentumsourcescanbeusedtocreateapressuredrop(e.g.ascreen,aporousmaterial)orapressurerise(e.g.afan)Energysourcescanaccountforheatadded/removedfromthesimulationWhensourcesarefunctionsofthesolvedvariable(e.g.momentumsourcesthatarefunctionsofvelocity,energysourcesthatarefunctionsoftemperature)theSourceCoefficientshouldbesetTheSourceCoefficientmustbenegativeotherwisethesolverwilldivergeMayneedtore-writetheSourcesothatishasanegativederivative,TransientSimulations,InatransientanalysisthetimestepshouldbesmallenoughtocapturethetransientbehaviourofinterestBoundaryconditionscanbefunctionsoftimeConvergenceshouldbemonitoredsothateachtimestepisconvergedItisgenerallybettertoreducetheoveralltimestepsizetoimproveconvergenceratherthanincreasingthenumberofcoefficientloopsRemembertocreatetheTransientResultsobjectbeforerunning,Turbulence,EstimatetheflowReynoldsNumbertodetermineiftheflowislaminarorturbulentChecky+valuestomakesurethenear-wallmeshissuitabley+enableShowAdvancedControlsSolver/Partitioner/Interpolatortab)“Notenoughfreememoryiscurrentlyavailableonthesystem”Asystemlimitationhasbeenreached!“Memory”referstoRAMPossiblesolutions:RuninparallelorincreasethenumberofpartitionstodistributethememoryloadReducethememoryrequirementsforthecaseSmallermeshFewerorsmallerGGIinterfaces,WhyDoesMyCaseFailintheSolver?,“Floatingpointexception:Overflow”ThesolverhasdivergedOftensomeoftheequationswillshow“F”insteadof“OK”beforetheerrormessageWhenthiserroroccursinthefirstfewiterationsperformsomebasicchecks:Aretheboundaryconditionsphysical?WhatstheReferencePressure?Whatpressureissetattheboundaries?Whatstheinitialpressure?Whatdirectionwouldyouexpecttheflowtogogiventhespecifiedpressures?Reducethetimescale,particularlyifthesolverfailslaterintherun,WhyDoesMyCaseFailintheSolver?,“Floatingpointexception:Overflow”Writeoutbackupfilesbeforethefailureandexaminethesolutionfields(Pressure,Velocity,)Lookforthemax/minvalues,theywillusuallybeveryhigh/lowCansettheexpertparameter“backupfileatzero”towriteoutafilebeforethefirstiteration,showingtheinitialguessLookforthefirst“F”ifU,V,WorPfailedinthe10thiteration,butTurbulencefailedinthe9thiteration,thenchecktheturbulencefield,WhyDoesMyCaseNotConverge?,WallsplacedatoutletsIfthewarningmessageshowntotherightappearsduringthesolutionitmeansthatflowistryingtocomebackinthroughanoutletboundaryNotaproblemifthemessagethengoesawayOtherwisetheoutletmaybelocatedinarecirculationzone,-COEFFICIENTLOOPITERATION=6CPUSECONDS=5.754E+05-|Equation|Rate|RMSRes|MaxRes|LinearSolution|+-+-+-+-+-+|U-Mom|0.82|3.3E-06|3.3E-04|4.1E-02OK|V-Mom|0.82|2.2E-06|5.6E-04|6.4E-02OK|W-Mom|0.64|2.3E-06|9.2E-05|1.6E-02OK|P-Mass|0.66|2.3E-07|6.9E-06|21.61.7E-01ok|+-+-+-+-+-+-+|*Notice*|Awallhasbeenplacedatportion(s)ofanOUTLET|boundarycondition(at83.8%ofthefaces,89.9%ofthearea)|topreventfluidfromflowingintothedomain.|Theboundaryconditionnameis:PV33.|Thefluidnameis:D2O.|Ifthissituationpersists,considerswitching|toanOpeningtypeboundaryconditioninstead.|+-+|K-TurbKE|0.45|1.4E-05|5.9E-04|5.92.7E-07OK|E-Diss.K|0.45|4.5E-05|2.8E-03|7.36.5E-06OK|+-+-+-+-+-+,MovetheoutletoruseanOpeningboundaryOr,iftheareafractionthathasbeen“walledoff”is100%,thenthelocalfluidpressureislikelylessthanthespecifiedboundarypressure,WhyDoesMyCaseNotConverge?,ChangingthetimescalecanhelpconvergenceSlowsteadyconvergencemaybeacceleratedthroughalargertimescaleBouncyconvergenceorsolverfailuremaybefixedwithasmallertimescaleSometimessimulationswhichareruninsteadystatemodewillnotconvergeevenwithgoodmeshqualityandawellselectedtimescaleIfasteadystaterunshowsoscillatorybehavioroftheresidualplots,theflowmaybetransientRunthecaseintransientmodeandobserveiftheresidualsreduceIfconvergencehasstalledtryrunningindouble-precisionWriteouttheresidualfields(OutputControlResultsOutputEquationResiduals)anduseIsosurfacestolookforthelocationswithhighresiduals,SettingExpertParameters,ExpertParametercanbesetinCFX-Pre,orbyeditingtheCCLInCFX-Pre:InsetSolverExpertParameterMost,butnotallExpertParametersareshowninCFX-Pre,SettingExpertParameters,InCCLaddtheEXPERTPARAMETER:objectundertheFLOW:objectandtypeintheparameterYoucanusetheCommandEditorinCFX-Pre(ToolsCommandEditor)totypeinCCL,MeshRefinementStudies,Errorsinaconvergedsolutionarisefrom:NumericalErrorsE.g.round-offerrors,convergence(lack-of)errorsModelErrorsE.g.accuracyofboundaryconditions,physicalmodelsDiscretizationErrorsErrorsarisingfromconvertingthecontinuousgoverningequationsintoadiscreteformthatcanbesolvedonacomputerDiscretizationerrorsreducewithmeshspacingMeshrefinementstudiesareusedtoestimatethesignificanceofdiscretizationerrorsonyoursolutionMeshrefinementstudiesarerecommendedforeachnewtypeofsimulationyouperform,MeshRefinementStudies,Ameshrefinementstudyconsistofsolvingthesamecaseonprogres