2011ANSYS,Inc. May1,20121 Release14.0

14.0Release

SolvingFSIApplicationsUsingANSYSMechanicalandANSYSFluent

Lecture6CosimulationviaSystemCouplingPartIV

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Outline

ConvergenceThislecturefocusesonconvergingSystemCouplingsimulations.Therearemanycontrolsthatcanimpactconvergence,sowellstartbyidentifyingthecontrolsavailable.

Obtaininggoodconvergencerequiresamethodicalworkflow.Welldiscuss: Usingseparatefluidandstructuralsimulationstofirstidentify

anynonFSIproblems CorrectinitializationforFSIcases Monitoringconvergence Convergencecontrolsandtheireffect IdentifyingunstableFSIcaseandhowtostabilizethesolution Generalconvergenceadviceanddebugtools

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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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Thereareanumberofcontrolsthatplayaroleingoodconvergence,including: Initialization

TimeStepSize

StabilityControls

NumberofFluentiterationsperstep

NumberofCouplingIterations

InterfaceUnderRelaxationFactors

Itisimportanttounderstandwhyacasehasconvergenceproblemsbeforeadjustingthesecontrols

Convergence

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BeforestartinganFSIsimulation,youshouldrunseparatefluidsandstructuralsimulationsusingrepresentativeloads: Checksthatthebasicsetupiscorrectforeachside

UsesettingsthatareascloseaspossibletotheplannedFSIsimulation(sametimestepsize,etc)

StructuralsimulationusingafixedPressureboundary

AFluentsimulationwithafixedboundaryorasimplymovingboundary

Ifitsdifficulttoproviderepresentativefluidloads,couldrunaSystemCouplingsimulationwithloadspassed1wayfromFluenttothestructure

Convergence

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Next,considerhowtoinitializeyourFSIsimulation.Wellconsideratransient2wayFSIsimulationhere:

Option1:Noprevioussteadystateresultsprovided TheinitialFluentsolutioncomesfrompressureandvelocityvaluessetinFluent

Thestructureisnotprestressed(zeroinitialdisplacement)

Suitablewhentheinitialfluidpressurefieldisknownandcanbespecifiedaccurately(typicallyzero)

Noinitialforcepassedtothestructure MakeuseoftheOperatingPressuresettinginFluent

Nootherloadsonthestructurethatrequireprestressing

Convergence Initialization

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Convergence Initialization

Option2:SteadystateFluentresultsprovided TheinitialFluentsolutioncomesfromaprevioussteadystaterun Thestructureisnotprestressed(zeroinitialdisplacement)

SuitablewhentheinitialfluidforceontheFSIinterfacefromthesteadystateflowresultswillnotcausesuddendeformationofthestructureatthestartofthetransient,causingthesolverstofail

Nootherloadsonthestructurethatrequireprestressing

IftransientFluentresultsareprovidedasaninitialsolution,thetimemustberesettozerotobeconsistentwiththeinitialSystemCouplingtimeusing:(rpsetvar flow-time 0)

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Convergence Initialization

Option3:SteadystateFSIresultsprovided TheinitialFluentandstructuralsolutionscomefromaconvergedsteadystate2wayFSIsimulation

Thisprovidesthemostconsistentstartingpointforatransient

Suitablewhenyouneedtoprestress/deformthestructuretoproduceasmoothstarttothetransient

RequiresaSteady/StatictoTransientrestart,whichisnotcurrentlysupported

ButcanrunaninitialtransientSystemCouplingcasethatbehaveslikeaSteady/Staticcase

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TomimicaSteady/Staticcase: SetTimeIntegrationoffintheTransientStructuralsystem

EquivalenttoaStaticStructuralcase Fluentmustremainintransientmode

Canbehavelikeasteadystatesolverbyusingthepressurebasedcoupledsolverwith1iterationpertimestep

UnderSolutionControlssetaFlowCourantNumberof1e6orhigherandExplicitRelaxationFactorsof1forMomentumandPressure

SettheTimeStepSizeasapseudotransienttimestep

Solve,thenrestartafterturningonTimeIntegrationandadjustingtheFluentsettingstoresolvethetruetransient SeeRestartssection

Convergence Initialization

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Option2(steadystateflowsolution)isthemostcommonwaytoinitializeatransientFSIcase

Typicallyprovidesanonzeroforcetothestructureatthestartofthetransient ThismaybeOK,producingasmallbumpatthestartofthe

transientthatcanbeignoredwhenpostprocessingtheresults

ItmaynotbeOKwhen: Thefluidis(oralmostis)incompressible,particularlywhen

combinedwithasmalltimestep Evensmallbumpscancauselargepressurechanges

Convergence Initialization

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ItmaynotbeOKwhen: Thestructureisveryflexible

TheinitialforceproduceslargedeformationsthatcauseFluenttofailorproducelargepressurechangesthatarepassedbacktothestructure

Theinitialbumpcausesstructuralvibrationsthattakealongtimetodieout Stiffnessproportionaldamping(BETAdamping)canbeusedtodampouttheunwantedhighfrequencystructuralresponse

Forceanddisplacementmonitorpointshelpshowwhatshappeningatthestartoftherun Remembertotrackmonitordataeveryiterationsoyoucansee

howtheforce/displacementrespondateachCouplingIteration

Convergence Initialization

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Forstartupproblemsgobacktothequestionsoutlinedearlier: Whataretheinitialforcespassedtothestructure? Iftheresalargestepchangeintheforces,howwouldyouexpectthestructuretorespond,givenyourtimestep? Whatdisplacementscanyouexpect? Howwouldyouexpectthefluidsystemtorespondtothosedisplacements,givenyourtimestep?

Transientstartuprobustnessproblemsareoftenmadeworsebyreducingthetimestepsize Foragivendeformation,thefluidhaslesstimetorespond>largeracceleration,largerpressurechanges

Convergence Initialization

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ThedefaultDataTransferconvergencechartprovidesagoodoverviewoftheforce/displacementconvergenceattheFSIinterface(s)

Thesawtoothpatternisdesirable,showingconvergencewithineachtimestep,thentypicallyajumpatthestartofthenexttimestep

Bydefault,thenormalizedchangeinadatatransfermustbelessthan1%(i.e.2ontheLogscale)tobeconverged

MonitoringConvergence

Note:thischartshowsCouplingSteponthexaxis,soitisclearthatconvergenceisachievedwithineachstep.CouplingIterationisthedefault.

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Asalreadydiscussed,alsomonitor:

TheFluentsolveroutputtoassessflowfieldconvergence

TheMechanicalsolveroutputtoassessstructuralconvergenceandthereceivedforces

Fluentmonitorpointsforforceanddisplacementateachiteration

MonitoringConvergence

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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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AdjustthenumberofCouplingIterationswhen:

Acaseisconvergingwell,withforcesand/ordisplacementsmovingtowardsasteadyvaluewithinastepwithoutoscillating

YoujustneedafewmoreCouplingIterationstoreachtheDataTransferConvergenceTarget

Butconsiderreducingthetimestepfirstfortransientcases Steadystatecasestypicallyneedmorethan5iterations,assumingyouareusing1step Alternativelyusing1iterationandmanysteps

Usuallytheresnoneedtochangethiscontrolfortransientcases(donotmakethisthefirstthingyouchange)

ConvergenceControls CouplingIterations

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ConvergenceControls URF

AdjusttheDataTransferUnderRelaxationFactorwhen:

Youarerunningasteadystatecase Shouldmovegraduallytowardsasteadystatesolution AtypicalURFmightbe0.5

Fortransientcasesusethedefaultvalueof1ingeneral Thephysicalinertiaofthesystemprovidesstabilitythatisnotthereinsteadystatecases

Sometransientcasesareunstable,withforces/displacementsoscillatingwithinatimestep

IfyoucanachievestabilitybyreducingtheURFslightly,saynolowerthan0.75,thenthisisacceptable

Otherwiseusethestabilitycontrolsdiscussedlater

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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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JudgingthenumberofFluentiterationstouseperCouplingIteration(notperTimeStepasshownintheGUI)requiresyoutowatchtheforcemonitorpointbehavior.

ConvergenceControls FluentIterations

ToomanyFluentiterationsperCouplingStep wastedCPU.AreasonableforceforthisCouplingIterationcouldbeobtainedinfeweriterations.

1Timestepshown

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ToofewFluentiterationsperCouplingIteration willneedtousemoreCouplingIterationunnecessarily

TheforceisincreasingsteadilywithineachCouplingIterationandfromoneCouplingIterationtothenext allowittoincreasefurtherbeforeexchangingdatawiththeMAPDLsolver

ConvergenceControls FluentIterations

NoticeitsnotobviouswhereeachCouplingIterationstarts/ends 5 CouplingIterationsperstepwereusedhere

Timestep1 Timestep2 Timestep3

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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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TimeStepSizeguidelinesaregenerallythesameasfortheindividualsolvers;afewthingstowatchfor:

Makesuretheindividualsolversconvergewellusingtheselectedtimestep

Asnotedearlier,ifyouhaveabumpatthestartofatransient,asmallertimestepcanmakethisworse,notbetter

Iftheforces/displacementsarechangingalotfromonetimesteptothenext,considerreducingthetimestep

buttoosmallatimestepmaystarttoresolvehighfrequencystructuralmodesthatarenotofinterest

ConvergenceControls TimeStepSize

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SomeFSIcasesareunstablenomatterwhatsettingareusedforthepreviousconvergencecontrols

Unstablecasestypicallyincludeoneormoreofthefollowing:

Thinstructures E.g.sheetmetal,diaphragms,membranes

StructuralmaterialswithalowYoungsModulus E.g.hyperelasticmaterials,rubber

Gasesmodeledasconstantdensity(incompressible)orliquids

ConvergenceStabilization

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Toidentifyanunstablecaselookattheforce/displacementmonitorpointsagain

Oscillationwithin atimestep,fromoneCouplingIterationtothenext,isindicativeofFSIinterfaceinstability

ConvergenceStabilization

1TimeStep

Mechanicalsolverfailed

Pressurevaluesveryhigh

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Theoscillationswilldevelopinthefirsttimestep ThecasemayfailwithininthefirstfewCoupling

Iterations,oritmaymakeittothesecondorthirdtimestep,withtheoscillationsgrowinginmagnitude

Itisimportanttodistinguishandunstablecasefromapoorlyinitializedcase Apoorlyinitializedcaseshowsasuddenjumpatthestart

oftherun Improvetheinitializationifthiscausesfailure

Anunstablecasestartswithsmalloscillationsthatgrow Butwithhighlyunstablecasesyoumayonlyreachthe2nd or3rd CouplingIteration,soitcanresembleasuddenjump

ConvergenceStabilization

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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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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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Fromanumericalpointofview,theserpvars modifythecoefficientsintheFluentlinearsolutionmatrixforthecontinuityequationforcellsattachedtoanFSIinterface

Fromaphysicalpointofview,thisslowsdownthepressureresponseinFluentattheFSIinterface Thismeansthepressuredoesntincrease/decreaseasfastin

Fluentwhenanewdisplacementisreceived

Thisdampedpressureresponseallowsustomovetowardsaconvergedsolutioninacontrolledway,ratherthanoscillatingaround

Therpvars havenoeffectonconverged results

ConvergenceStabilization

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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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Lookingathowthedampingvaluechangesthepressureresponse:

ConvergenceStabilization

(rpsetvar 'dynamesh/sc-bc-compressibility 0)

Baselinedivergentcase

(rpsetvar 'dynamesh/sc-bc-compressibility 1)

Stilldivergent,butcompletesmoreCouplingIterationsNoticethepressureresponseisnolongerastepchange

1TimeStep

1TimeStep

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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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Forthecaseshownavalueof3wasthemagicnumber,butthiswillbedifferentforothercases

Alwaysstartwithsmallvaluesthenincrease Thefirstvalueyoutryshouldstilldiverge

Avoidsfallingintoatrapwheretheresponseissooverdampedthatitappearsflat(i.e.converged!)

ConvergenceStabilization

(rpsetvar 'dynamesh/sc-bc-compressibility 3)

Goodresponse.Wellconvergedafter5CouplingIterations.

1TimeStep

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Notethattheresponseisafunctionoftherpvar valueANDthenumberofFluentiterations Considerthecaseshown ifyouperformmoreFluent

iterationsperCouplingIterationeachoscillationwouldextendfurther(moreunstable)

Inpracticesetthenum.ofFluentiterationstoasmallnumberandthenadjusttherpvar togettheoptimalresponse TheresnopointinusinglotsofFluentiterations

ConvergenceStabilization

1TimeStep

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See2022119intheKnowledgeResourcesontheANSYSCustomerPortalforarecordeddemoofstabilizinganunstableFSIcase(justsearchFSI)

ConvergenceStabilization

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Anumberofredflagsthatmayindicateyouareonthewrongpathtoimprovingconvergence: UseofalargenumberofCouplingIterationspertimestep

Useofsmallinterfaceunderrelaxationfactorsintransientcases

Atimestepsizethatismuchsmallerthanthatrequiredbythefluidonlyorstructuralonlysimulations

AlargenumberofFluentiterationsperstep,whencomparedwithafluidonlysimulation

ConvergenceAdvice

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ConvergenceAdvice

Iftherunfails: Findoutwhichsolverfailedfirst lookatall theoutputfilestodeterminewheretheerrorstarted Checkthemapping IfMAPDL,werereasonableforcespassed? IfFluent,werereasonabledisplacementsreceived? Weretheindividualfieldsolversconverging? Ifarunfailsinthefirstfewtimesteps,rerunforalimitednumberoftimestepsand/orwithalimitednumberofCouplingIterationssothatyoucangenerateresultsfilesandexaminetheresults

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CanrequestadditiondebugoutputinSCunderExecutionControl

Usefulfordebugging:

Mappingproblems Toseetheforce/displacementvaluesthataretransferred

Simulationsthatcrash Findoutwhichsolverencounteredproblemsfirst,whattheerrorswere

DebugOutput

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Summary

Obtaininggoodconvergencerequiresagoodunderstandingofthedatatransferprocessbetweenparticipantsolversandhoweachsolverwillreacttothenewboundarydatareceived.

ThoughtneedstobegiventohowSystemCouplingcasesareinitialized,particularlywhentheinitialfluidfieldwillcausesignificantstructuraldeformations

Relyingonerrormessagesoradjustingcontrolswithoutunderstandingwhyasolverfailedisunlikelytoresolveproblems

Examiningmonitordataandbackupfilesareimportanttoolstoguideparameterchangestoimproveconvergence

MonitordataisalsousedtoidentifyunstableFSIcases.rpvarsinFluentcanbeusedtostabilizethesecases