文档介绍：Chapter8 CFX流体瞬态分析CFX瞬态分析MotivationNearlyallflowsinnaturearetransient!Steady-stateassumptionispossibleifwe:IgnoreunsteadyfluctuationsEmployensemble/time-averagingtoremoveunsteadiness(thisiswhatisdoneinmodelingturbulence)InCFD,steady-putationalcostEasiertopostprocessandanalyzeManyapplicationsrequireresolutionoftransientflow:Aerodynamics(aircraft,landvehicles,etc.)–vortexsheddingRotatingMachinery–rotor/statorinteraction,stall,surgeMultiphaseFlows–freesurfaces,bubbledynamicsDeformingDomains–in-bustion,storeseparationUnsteadyHeatTransfer–transientheatingandcoolingManymoreOriginsofTransientFlowNaturalunsteadinessUnsteadyflowduetogrowthofinstabilitieswithinthefluidoranon-equilibriuminitialfluidstateExamples:naturalconvectionflows,turbulenteddiesofallscales,fluidwaves(gravitywaves,shockwaves)ForcedunsteadinessTime-dependentboundaryconditions,sourcetermsdrivetheunsteadyflowfieldExamples:pulsingflowinanozzle,rotor-statorinteractioninaturbinestageKelvin-HelmholtzCloudInstabilityRotor-pressorTransientCFDAnalysisSimulateatransientflowfieldoveraspecifiedtimeperiodSolutionmayapproach:Steady-statesolution–FlowvariablesstopchangingwithtimeTime-periodicsolution–()Time-averagedand/orRMSvaluesTime-relatedparameters(,residencetimeofapollutant)Spectraldata–fastFouriertransform(FFT)20Timestep=2sInitialTime=0sTotalTime=20sCoefficientLoops=524681012141618Time(seconds)putingasolutionformanydiscretepointsintimeAteachtimepointwemustiteratetothesolutionHowtoSolveaTransientCaseSimilarsetuptosteadystateThegeneralworkflowisSettheAnalysisTypetoTransientSpecifythetransienttimedurationtosolveandthetimestepsizeSetupphysicalmodelsandboundaryconditionsasusualBoundaryconditionsmaychangewithtimePrescribeinitialconditionsBesttouseaphysicallyrealisticinitialcondition,suc