project khoob
DESCRIPTION
project khoobTRANSCRIPT
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2011ANSYS,Inc. May1,20121 Release14.0
14.0Release
SolvingFSIApplicationsUsingANSYSMechanicalandANSYSFluent
Lecture6CosimulationviaSystemCouplingPartIV
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2011ANSYS,Inc. May1,20122 Release14.0
Outline
ConvergenceThislecturefocusesonconvergingSystemCouplingsimulations.Therearemanycontrolsthatcanimpactconvergence,sowellstartbyidentifyingthecontrolsavailable.
Obtaininggoodconvergencerequiresamethodicalworkflow.Welldiscuss: Usingseparatefluidandstructuralsimulationstofirstidentify
anynonFSIproblems CorrectinitializationforFSIcases Monitoringconvergence Convergencecontrolsandtheireffect IdentifyingunstableFSIcaseandhowtostabilizethesolution Generalconvergenceadviceanddebugtools
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2011ANSYS,Inc. May1,20123 Release14.0
Convergencerequiresanunderstandingofthesolutionprocessandthephysicsthatyoursetupdefines Thismaybedifferentthantherealworldphysics
E.g.yourinitialconditionsmaybedifferent
Whenconvergencedifficultiesareencountered,followthesolutionprocess:
Usemonitorpointsandbackupfilestoanswersthesequestions
Convergence
How does the flow field respond?
What is the force passed to the structure?
How does the structure respond?
What are the displacements passed back to Fluent?
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2011ANSYS,Inc. May1,20124 Release14.0
Thereareanumberofcontrolsthatplayaroleingoodconvergence,including: Initialization
TimeStepSize
StabilityControls
NumberofFluentiterationsperstep
NumberofCouplingIterations
InterfaceUnderRelaxationFactors
Itisimportanttounderstandwhyacasehasconvergenceproblemsbeforeadjustingthesecontrols
Convergence
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2011ANSYS,Inc. May1,20125 Release14.0
BeforestartinganFSIsimulation,youshouldrunseparatefluidsandstructuralsimulationsusingrepresentativeloads: Checksthatthebasicsetupiscorrectforeachside
UsesettingsthatareascloseaspossibletotheplannedFSIsimulation(sametimestepsize,etc)
StructuralsimulationusingafixedPressureboundary
AFluentsimulationwithafixedboundaryorasimplymovingboundary
Ifitsdifficulttoproviderepresentativefluidloads,couldrunaSystemCouplingsimulationwithloadspassed1wayfromFluenttothestructure
Convergence
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2011ANSYS,Inc. May1,20126 Release14.0
Next,considerhowtoinitializeyourFSIsimulation.Wellconsideratransient2wayFSIsimulationhere:
Option1:Noprevioussteadystateresultsprovided TheinitialFluentsolutioncomesfrompressureandvelocityvaluessetinFluent
Thestructureisnotprestressed(zeroinitialdisplacement)
Suitablewhentheinitialfluidpressurefieldisknownandcanbespecifiedaccurately(typicallyzero)
Noinitialforcepassedtothestructure MakeuseoftheOperatingPressuresettinginFluent
Nootherloadsonthestructurethatrequireprestressing
Convergence Initialization
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2011ANSYS,Inc. May1,20127 Release14.0
Convergence Initialization
Option2:SteadystateFluentresultsprovided TheinitialFluentsolutioncomesfromaprevioussteadystaterun Thestructureisnotprestressed(zeroinitialdisplacement)
SuitablewhentheinitialfluidforceontheFSIinterfacefromthesteadystateflowresultswillnotcausesuddendeformationofthestructureatthestartofthetransient,causingthesolverstofail
Nootherloadsonthestructurethatrequireprestressing
IftransientFluentresultsareprovidedasaninitialsolution,thetimemustberesettozerotobeconsistentwiththeinitialSystemCouplingtimeusing:(rpsetvar flow-time 0)
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2011ANSYS,Inc. May1,20128 Release14.0
Convergence Initialization
Option3:SteadystateFSIresultsprovided TheinitialFluentandstructuralsolutionscomefromaconvergedsteadystate2wayFSIsimulation
Thisprovidesthemostconsistentstartingpointforatransient
Suitablewhenyouneedtoprestress/deformthestructuretoproduceasmoothstarttothetransient
RequiresaSteady/StatictoTransientrestart,whichisnotcurrentlysupported
ButcanrunaninitialtransientSystemCouplingcasethatbehaveslikeaSteady/Staticcase
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2011ANSYS,Inc. May1,20129 Release14.0
TomimicaSteady/Staticcase: SetTimeIntegrationoffintheTransientStructuralsystem
EquivalenttoaStaticStructuralcase Fluentmustremainintransientmode
Canbehavelikeasteadystatesolverbyusingthepressurebasedcoupledsolverwith1iterationpertimestep
UnderSolutionControlssetaFlowCourantNumberof1e6orhigherandExplicitRelaxationFactorsof1forMomentumandPressure
SettheTimeStepSizeasapseudotransienttimestep
Solve,thenrestartafterturningonTimeIntegrationandadjustingtheFluentsettingstoresolvethetruetransient SeeRestartssection
Convergence Initialization
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2011ANSYS,Inc. May1,201210 Release14.0
Option2(steadystateflowsolution)isthemostcommonwaytoinitializeatransientFSIcase
Typicallyprovidesanonzeroforcetothestructureatthestartofthetransient ThismaybeOK,producingasmallbumpatthestartofthe
transientthatcanbeignoredwhenpostprocessingtheresults
ItmaynotbeOKwhen: Thefluidis(oralmostis)incompressible,particularlywhen
combinedwithasmalltimestep Evensmallbumpscancauselargepressurechanges
Convergence Initialization
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2011ANSYS,Inc. May1,201211 Release14.0
ItmaynotbeOKwhen: Thestructureisveryflexible
TheinitialforceproduceslargedeformationsthatcauseFluenttofailorproducelargepressurechangesthatarepassedbacktothestructure
Theinitialbumpcausesstructuralvibrationsthattakealongtimetodieout Stiffnessproportionaldamping(BETAdamping)canbeusedtodampouttheunwantedhighfrequencystructuralresponse
Forceanddisplacementmonitorpointshelpshowwhatshappeningatthestartoftherun Remembertotrackmonitordataeveryiterationsoyoucansee
howtheforce/displacementrespondateachCouplingIteration
Convergence Initialization
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2011ANSYS,Inc. May1,201212 Release14.0
Forstartupproblemsgobacktothequestionsoutlinedearlier: Whataretheinitialforcespassedtothestructure? Iftheresalargestepchangeintheforces,howwouldyouexpectthestructuretorespond,givenyourtimestep? Whatdisplacementscanyouexpect? Howwouldyouexpectthefluidsystemtorespondtothosedisplacements,givenyourtimestep?
Transientstartuprobustnessproblemsareoftenmadeworsebyreducingthetimestepsize Foragivendeformation,thefluidhaslesstimetorespond>largeracceleration,largerpressurechanges
Convergence Initialization
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2011ANSYS,Inc. May1,201213 Release14.0
ThedefaultDataTransferconvergencechartprovidesagoodoverviewoftheforce/displacementconvergenceattheFSIinterface(s)
Thesawtoothpatternisdesirable,showingconvergencewithineachtimestep,thentypicallyajumpatthestartofthenexttimestep
Bydefault,thenormalizedchangeinadatatransfermustbelessthan1%(i.e.2ontheLogscale)tobeconverged
MonitoringConvergence
Note:thischartshowsCouplingSteponthexaxis,soitisclearthatconvergenceisachievedwithineachstep.CouplingIterationisthedefault.
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2011ANSYS,Inc. May1,201214 Release14.0
Asalreadydiscussed,alsomonitor:
TheFluentsolveroutputtoassessflowfieldconvergence
TheMechanicalsolveroutputtoassessstructuralconvergenceandthereceivedforces
Fluentmonitorpointsforforceanddisplacementateachiteration
MonitoringConvergence
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2011ANSYS,Inc. May1,201215 Release14.0
Thereareanumberofconvergencecontrols,soitseasytoheaddownthewrongpath
Ingeneralstartwith: Default#ofCouplingIterations:Min:1,Max5(fortransients)
DefaultDataTransferUnderRelaxationFactor:1
Perhaps5FluentiterationsperCouplingIteration(fortransients),butthisiscasedependent
ATimeStepSizebasedonthephysicsyouneedtoresolve 1/20th ofthehigheststructuralfrequencyofinterest FlowfieldCourantnumber
Fromthisstartingpointyoucanadjustthecontrolsbasedonthesolutionbehavior
ConvergenceControls
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2011ANSYS,Inc. May1,201216 Release14.0
AdjustthenumberofCouplingIterationswhen:
Acaseisconvergingwell,withforcesand/ordisplacementsmovingtowardsasteadyvaluewithinastepwithoutoscillating
YoujustneedafewmoreCouplingIterationstoreachtheDataTransferConvergenceTarget
Butconsiderreducingthetimestepfirstfortransientcases Steadystatecasestypicallyneedmorethan5iterations,assumingyouareusing1step Alternativelyusing1iterationandmanysteps
Usuallytheresnoneedtochangethiscontrolfortransientcases(donotmakethisthefirstthingyouchange)
ConvergenceControls CouplingIterations
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2011ANSYS,Inc. May1,201217 Release14.0
ConvergenceControls URF
AdjusttheDataTransferUnderRelaxationFactorwhen:
Youarerunningasteadystatecase Shouldmovegraduallytowardsasteadystatesolution AtypicalURFmightbe0.5
Fortransientcasesusethedefaultvalueof1ingeneral Thephysicalinertiaofthesystemprovidesstabilitythatisnotthereinsteadystatecases
Sometransientcasesareunstable,withforces/displacementsoscillatingwithinatimestep
IfyoucanachievestabilitybyreducingtheURFslightly,saynolowerthan0.75,thenthisisacceptable
Otherwiseusethestabilitycontrolsdiscussedlater
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2011ANSYS,Inc. May1,201218 Release14.0
KeepthefollowingformulainmindifyouadjusttheUnderRelaxationFactor:
% LoadReceived=(1 (1 URF)^Max.CouplingIters)*100%Forexample,givenaURFof0.25andamaximumof5CouplingIterations,wehave:
% LoadReceived=(1 (1 0.25)^5)*100%=76%Sobytheendofthestep,only76%ofthechangeinaloadwillhavebeenreceivedbythereceivingsolver
Assumingastepchangeintheload,whichthenremainsconstant
ConvergenceControls URF
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2011ANSYS,Inc. May1,201219 Release14.0
JudgingthenumberofFluentiterationstouseperCouplingIteration(notperTimeStepasshownintheGUI)requiresyoutowatchtheforcemonitorpointbehavior.
ConvergenceControls FluentIterations
ToomanyFluentiterationsperCouplingStep wastedCPU.AreasonableforceforthisCouplingIterationcouldbeobtainedinfeweriterations.
1Timestepshown
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2011ANSYS,Inc. May1,201220 Release14.0
ToofewFluentiterationsperCouplingIteration willneedtousemoreCouplingIterationunnecessarily
TheforceisincreasingsteadilywithineachCouplingIterationandfromoneCouplingIterationtothenext allowittoincreasefurtherbeforeexchangingdatawiththeMAPDLsolver
ConvergenceControls FluentIterations
NoticeitsnotobviouswhereeachCouplingIterationstarts/ends 5 CouplingIterationsperstepwereusedhere
Timestep1 Timestep2 Timestep3
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2011ANSYS,Inc. May1,201221 Release14.0
ReasonablenumberofFluentiterations
TheforceisjuststartingtoflattenoutwithineachCouplingIteration
ConvergenceControls FluentIterations
C.Iter 1 C.Iter 2 C.Iter 3 C.Iter 4 C.Iter 5
1TimeStep
TheresnopointinfullyconvergingwithineachCouplingIteration,sincetheforcewillchangeinthenextone MoreFluentiterationsinthefirstCouplingIterationwouldhavecaused
theforcetomovefurtherawayfromtheconvergedvalue
Less Fluentiterationscanhelpstability keepstightercouplingbetweenMAPDLandFluentbynotlettingtheforcemovetoofarbeforeupdatingthedisplacements
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2011ANSYS,Inc. May1,201222 Release14.0
TimeStepSizeguidelinesaregenerallythesameasfortheindividualsolvers;afewthingstowatchfor:
Makesuretheindividualsolversconvergewellusingtheselectedtimestep
Asnotedearlier,ifyouhaveabumpatthestartofatransient,asmallertimestepcanmakethisworse,notbetter
Iftheforces/displacementsarechangingalotfromonetimesteptothenext,considerreducingthetimestep
buttoosmallatimestepmaystarttoresolvehighfrequencystructuralmodesthatarenotofinterest
ConvergenceControls TimeStepSize
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2011ANSYS,Inc. May1,201223 Release14.0
SomeFSIcasesareunstablenomatterwhatsettingareusedforthepreviousconvergencecontrols
Unstablecasestypicallyincludeoneormoreofthefollowing:
Thinstructures E.g.sheetmetal,diaphragms,membranes
StructuralmaterialswithalowYoungsModulus E.g.hyperelasticmaterials,rubber
Gasesmodeledasconstantdensity(incompressible)orliquids
ConvergenceStabilization
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2011ANSYS,Inc. May1,201224 Release14.0
Toidentifyanunstablecaselookattheforce/displacementmonitorpointsagain
Oscillationwithin atimestep,fromoneCouplingIterationtothenext,isindicativeofFSIinterfaceinstability
ConvergenceStabilization
1TimeStep
Mechanicalsolverfailed
Pressurevaluesveryhigh
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2011ANSYS,Inc. May1,201225 Release14.0
Theoscillationswilldevelopinthefirsttimestep ThecasemayfailwithininthefirstfewCoupling
Iterations,oritmaymakeittothesecondorthirdtimestep,withtheoscillationsgrowinginmagnitude
Itisimportanttodistinguishandunstablecasefromapoorlyinitializedcase Apoorlyinitializedcaseshowsasuddenjumpatthestart
oftherun Improvetheinitializationifthiscausesfailure
Anunstablecasestartswithsmalloscillationsthatgrow Butwithhighlyunstablecasesyoumayonlyreachthe2nd or3rd CouplingIteration,soitcanresembleasuddenjump
ConvergenceStabilization
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2011ANSYS,Inc. May1,201226 Release14.0
InstabilityMechanism:
ConvergenceStabilization
Fluent Mechanical
InitialpressureonFSIinterface
ForcespassedtoMechanical
Fluent Mechanical
P TheinterfacedeformsinMechanicalduefluidpressure
Fluent Mechanical
P
PressuredecreasesinFluentduetoincreaseinvolume
Fluent Mechanical
TheinterfacedeformsinMechanicalintheoppositedirection
Fluent Mechanical
P
PressureinFluentincreasesmoreduetoreducedvolume
DisplacementspassedtoFluent
ForcespassedtoMechanical
DisplacementspassedtoFluent
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2011ANSYS,Inc. May1,201227 Release14.0
Tostabilizethesetypesofcases,tworpvars areavailableintheFluentTUI:(rpsetvar dynamesh/sc-bc-compressibility-type )(rpsetvar dynamesh/sc-bc-compressibility )
Userpgetvar toquerythecurrentvalues:(rpgetvar dynamesh/sc-bc-compressibility-type)(rpgetvar dynamesh/sc-bc-compressibility)
ConvergenceStabilization
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2011ANSYS,Inc. May1,201228 Release14.0
Fromanumericalpointofview,theserpvars modifythecoefficientsintheFluentlinearsolutionmatrixforthecontinuityequationforcellsattachedtoanFSIinterface
Fromaphysicalpointofview,thisslowsdownthepressureresponseinFluentattheFSIinterface Thismeansthepressuredoesntincrease/decreaseasfastin
Fluentwhenanewdisplacementisreceived
Thisdampedpressureresponseallowsustomovetowardsaconvergedsolutioninacontrolledway,ratherthanoscillatingaround
Therpvars havenoeffectonconverged results
ConvergenceStabilization
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2011ANSYS,Inc. May1,201229 Release14.0
Thesc-bc-compressibility-type rpvar defaultsto0,whichmeansoff
Valuesof1and2arevalidandrepresentdifferentwaystoadjustthematrixcoefficients
Type2hasprovenmostreliableandisalwaysrecommended
Thesc-bc-compressibility rpvar isascalingfactorforthematrixcoefficientsanddefaultsto0,whichmeansnoadjustmentismade
Values>0willadddampingwhenthetypeissetto1or2 Thelargerthevaluethemoredamping Theappropriatevalueiscasespecific,butisgenerallyintherange0.01 500
ConvergenceStabilization
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2011ANSYS,Inc. May1,201230 Release14.0
Lookingathowthedampingvaluechangesthepressureresponse:
ConvergenceStabilization
(rpsetvar 'dynamesh/sc-bc-compressibility 0)
Baselinedivergentcase
(rpsetvar 'dynamesh/sc-bc-compressibility 1)
Stilldivergent,butcompletesmoreCouplingIterationsNoticethepressureresponseisnolongerastepchange
1TimeStep
1TimeStep
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2011ANSYS,Inc. May1,201231 Release14.0
ConvergenceStabilization
(rpsetvar 'dynamesh/sc-bc-compressibility 10)
Nowstable,butoverdamped.Notquitefullyconvergedwithinthetimestepafter7CouplingIterations.
1TimeStep
(rpsetvar 'dynamesh/sc-bc-compressibility 2)
Nolongerdiverges,butstillunderdamped.Didnotfullyconvergeafter7CouplingIterations.
1TimeStep
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2011ANSYS,Inc. May1,201232 Release14.0
Forthecaseshownavalueof3wasthemagicnumber,butthiswillbedifferentforothercases
Alwaysstartwithsmallvaluesthenincrease Thefirstvalueyoutryshouldstilldiverge
Avoidsfallingintoatrapwheretheresponseissooverdampedthatitappearsflat(i.e.converged!)
ConvergenceStabilization
(rpsetvar 'dynamesh/sc-bc-compressibility 3)
Goodresponse.Wellconvergedafter5CouplingIterations.
1TimeStep
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2011ANSYS,Inc. May1,201233 Release14.0
Notethattheresponseisafunctionoftherpvar valueANDthenumberofFluentiterations Considerthecaseshown ifyouperformmoreFluent
iterationsperCouplingIterationeachoscillationwouldextendfurther(moreunstable)
Inpracticesetthenum.ofFluentiterationstoasmallnumberandthenadjusttherpvar togettheoptimalresponse TheresnopointinusinglotsofFluentiterations
ConvergenceStabilization
1TimeStep
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2011ANSYS,Inc. May1,201234 Release14.0
See2022119intheKnowledgeResourcesontheANSYSCustomerPortalforarecordeddemoofstabilizinganunstableFSIcase(justsearchFSI)
ConvergenceStabilization
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2011ANSYS,Inc. May1,201235 Release14.0
Anumberofredflagsthatmayindicateyouareonthewrongpathtoimprovingconvergence: UseofalargenumberofCouplingIterationspertimestep
Useofsmallinterfaceunderrelaxationfactorsintransientcases
Atimestepsizethatismuchsmallerthanthatrequiredbythefluidonlyorstructuralonlysimulations
AlargenumberofFluentiterationsperstep,whencomparedwithafluidonlysimulation
ConvergenceAdvice
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2011ANSYS,Inc. May1,201236 Release14.0
ConvergenceAdvice
Iftherunfails: Findoutwhichsolverfailedfirst lookatall theoutputfilestodeterminewheretheerrorstarted Checkthemapping IfMAPDL,werereasonableforcespassed? IfFluent,werereasonabledisplacementsreceived? Weretheindividualfieldsolversconverging? Ifarunfailsinthefirstfewtimesteps,rerunforalimitednumberoftimestepsand/orwithalimitednumberofCouplingIterationssothatyoucangenerateresultsfilesandexaminetheresults
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2011ANSYS,Inc. May1,201237 Release14.0
CanrequestadditiondebugoutputinSCunderExecutionControl
Usefulfordebugging:
Mappingproblems Toseetheforce/displacementvaluesthataretransferred
Simulationsthatcrash Findoutwhichsolverencounteredproblemsfirst,whattheerrorswere
DebugOutput
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2011ANSYS,Inc. May1,201238 Release14.0
Summary
Obtaininggoodconvergencerequiresagoodunderstandingofthedatatransferprocessbetweenparticipantsolversandhoweachsolverwillreacttothenewboundarydatareceived.
ThoughtneedstobegiventohowSystemCouplingcasesareinitialized,particularlywhentheinitialfluidfieldwillcausesignificantstructuraldeformations
Relyingonerrormessagesoradjustingcontrolswithoutunderstandingwhyasolverfailedisunlikelytoresolveproblems
Examiningmonitordataandbackupfilesareimportanttoolstoguideparameterchangestoimproveconvergence
MonitordataisalsousedtoidentifyunstableFSIcases.rpvarsinFluentcanbeusedtostabilizethesecases