FloTHERM收斂控制課件

專業(yè)的電子設(shè)備散熱分析軟件FloTHERM收斂控制YourInitials,PresentationTitle,MonthYear1殘差曲線收斂發(fā)散高位震蕩低位震蕩高位穩(wěn)定低位穩(wěn)定<10>10>10<102小技巧:通常情況下,如果收斂慢就需要用戶終止計算進行調(diào)整模型檢查模型錯誤,比如在密閉系統(tǒng)中裝有離心風扇或進入系統(tǒng)的熱無法向外傳遞在殘差大的區(qū)域檢查網(wǎng)格–

網(wǎng)格不足無法捕獲詳細信息檢查本身不穩(wěn)定性-用監(jiān)控點來追蹤不穩(wěn)定區(qū)域用監(jiān)控點和殘差場來分析低位震蕩或低位穩(wěn)定-通常不需要再做修改3收斂曲線發(fā)散第一步檢查復(fù)查SanityCheck結(jié)果復(fù)查建模過程尋找明顯網(wǎng)格不足問題確定問題點采用MonitorPointsDeactivateObjects/Assemblies用于隔離問題點檢查問題點?網(wǎng)格是否足夠描述物理現(xiàn)象?是否有荒謬的類似熱源,粗糙度,表面屬性等設(shè)置?單位是否正確?切記:

如果曲線發(fā)散的很明顯,結(jié)果一定是不正確的.

對于發(fā)散問題重新求解時,一定要重新初始化4低位穩(wěn)定和震蕩打開殘差場存儲設(shè)置在ProjectManager中[Solve/OverallControl]設(shè)置殘差存保留儲每個單元格的殘差參數(shù)可以在FLOMOTION中檢查確定最大和最小值的位置同時打開網(wǎng)格顯示來判斷問題是否與網(wǎng)格有關(guān)聯(lián)如果發(fā)現(xiàn)殘差較大的位置不在所關(guān)注的范圍內(nèi),就不需要再對模型再做修改使其收斂如果監(jiān)控點已經(jīng)達到穩(wěn)態(tài)殘差?？梢詭椭l(fā)現(xiàn)劣質(zhì)網(wǎng)格問題5Hint:

Storingtheerrorfieldwillslowthingsdown.Onlyactivatetheerrorfieldstorageafterstoppingthesolution.Then,re-runthesolutionfor1iteration.Theerrorfieldcanonlybestoredforonevariableatatime.Ifallvelocitydirectionsorcontinuityareoscillating,activatetheerrorfieldstorageforpressure.Ifanindividualvelocityisoscillating,activatetheerrorfieldstorageforthatvelocity.Iftemperatureisoscillating,activatetheerrorfieldstoragefortemperature.Aftersolution,the‘Error’fieldcontainsthelocalresidualerrorsfortheselectedvariable.Byusingthiserrorfield,theusercanidentifyareaswheretheprogrammayhavedifficultyinconvergingtheequations.Theusercanthenfocusattentiononthegrid,projectset-upandassumptionsintheareasofdifficulty.Thequantitiesstoredintheerrorfieldarethecellresidualsdividedbytheterminationresidual.Itisthisnormalizederrordistributionwhichmaybedisplayed.Forthepressurevariable,thepressureresidualisthecontinuityerrorinagridcell,i.e.itisthedifferencebetweenthemassflowinginandthemassflowingoutofthecell.Displayofthecontinuityerrorfieldwillhelptoidentifylocationsofthelargestmassimbalances--itcansometimesbethecasethatalltheerrorislocatedinonegridcell.Again,whatisstoredintheerrorfieldandwhatisthereforedisplayedistheresidualerrorsnormalizedbytheterminationresidualforpressure.6低位穩(wěn)定和震蕩溫度場殘差可以使用自動收斂設(shè)置[Solve/OverallControl]

如果溫度監(jiān)控點在30步迭代保持在0.5度范圍內(nèi)波動和如果溫度收斂曲線降到10以下停止求解7改善收斂改變求解選項修正慢速溫度收斂松弛因子法內(nèi)部迭代Fan松弛控制線性松弛控制relaxation(欠松弛)Successiveover松弛控制(超松弛)調(diào)整FalseTimeStep松弛因子改進高位穩(wěn)定或震蕩非收斂問題8求解器選項求解器選擇在[Solve/Overall]中更改SegregatedConjugateResidual(默認狀態(tài))可以適用大多數(shù)分析狀況采用分離壓力區(qū)域改善收斂問題對于溫度收斂問題,可以使用塊校正法首先查找分析具有大溫度殘差的固體模型將其加入塊校正組在[Solve/Overall]中激活快校正求解9求解器選項MultiGrid求解器在質(zhì)量和動量求解收斂過程中和SegregatedConjugateResidual方法一樣采用multigrid技術(shù)加快求解溫度場線性方程可以很大的提高傳熱問題的收斂速度在多求解器和局域化中,MultiGrid求解器也可以使用10Notes:Therearetwosolveroptionsavailable:a)SegregatedConjugateResidual--generallysignificantlyfasterlinearequationsolver(thedefault)whichshouldnormallybeused.Fortemperatureconvergence,blockcorrectioncanbeactivated.Notethatblockcorrectionwillslowthesolverdownsomakesurethatyoudonotattachmorethan20blockcorrectiongroups.b)MultiGrid--isdesignedtochooseaniterationprocedurewhichusesmultigridaccelerationtosolvethelinearequationsfortemperature.Thisoptioneliminatesthenecessitytouseblockcorrectioninanyform.Forproblemswithconjugateheattransferitcanimproveconvergenceandsignificantlyreduceoverallcomputationtime.HowdoesMultiGridsolverspeedupconvergence?TheMultiGridsolverfortemperatureusesanalgebraicmultigridmethod.TheuserdefinesasingleblockstructuredCartesiangridasnormal.Forthetemperaturevariablethesolverconsiderscalculatingondifferentgrids,successivelycoarseningthegridbyafactorof2ineachdirectionateverylevel.Thesolverchoosesatwhichlevelitshouldsolvethecaseandthischangesdynamicallythroughoutthesolution.ThecycleprocessiscalledaFlexibleVcycle,whichmeansthatthesolverchoosesthesolvelevelbasedonconvergenceperformance,cyclingupanddownthroughthelevels.11松弛控制法

內(nèi)部迭代(在[Solve/VariableControl]中設(shè)置)對于壓力項可以采用大(等)于100步增加步數(shù)將會降低收斂速度線性松弛控制(在[Solve/SolverControl]中設(shè)置)通常設(shè)在0.5~0.9范圍內(nèi)0:凍結(jié)求解;1:無阻尼SuccessiveOver-relaxation(在[Control/Solver]中設(shè)置)很少使用非常容易導(dǎo)致不穩(wěn)定,慎用建議在1.0~1.5范圍用于純傳導(dǎo)問題和高壓系統(tǒng)12松弛控制法Fan松弛控制在[Solve/OverallControl]中設(shè)置有效值范圍是0.5~0.9有助于改善工作點在風扇PQ中梯度較小位置的收斂速度13FalseTimeStep通過變量求解控制欄來調(diào)節(jié)[Solve/VariableControl]自動設(shè)置的值是基于時間特征常數(shù),適用于大多數(shù)情況用戶可以針對每個變量設(shè)定不同的值variable使用滑標直接輸入值如果FalseTimeStep放大或縮小超過50倍容易導(dǎo)致發(fā)散切記要檢查監(jiān)控點以確認達到收斂14Notes:Thefalsetimestepsetsatransient-likeparameterwhichisusedto‘damp’or‘under-relax’thesolutionofanyvariableexceptpressure.Settocorrespondtotime-scaleofproblem,e.g.(typicallength)/(typicalvelocity).Largervaluesgivelightdamping,smallvaluesgiveheavydamping.AutomaticsetsFalseTime-stepsbasedonfansinforcedflowcasesandtheEFCVinnaturalconvectioncases.Press[CalculateAutomaticValues]toseetheautomaticvalue.Thesliderbarcanbeusedtoquicklychangetheautomaticsettingbyeitherdividingormultiplyingitbyafactorofupto20.Press[CalculateAutomaticValues]toseetheeffect.UserSpecifiedallowsyoutooverridetheautomaticsettingsmanually.CalculatingtheFalseTimeStepFalse-timestepunder-relaxationistheprincipalmeansthatyouwillusetocontrolconvergenceoftheiterativealgorithm.Foragivenvariable,thisunder-relaxationprovidesaninertialeffectwhichrestrainstheamountbywhichthevalueineachcellispermittedtochangeateachouteriteration.Theinertialeffectisproportionaltothemassinthecellandinverselyproportionaltothefalsetime-stepsetforthevariable.Consequently,thesmallerthefalsetime-stepthelargertheinertialeffect,andsothelessthevariableispermittedtochange.Bydefaulttheprogramcalculatesanautomaticfalsetime-stepwhichisusedforallvariables(theoneexceptionbeingpressuretowhichthemethodisnotapplied).15FalseTimeStep調(diào)節(jié)FalseTimeStep類似于調(diào)節(jié)阻尼falsetimestep較大值–減少阻尼falsetimestep較小值-增加阻尼

falsetimestep非常大值–無阻尼falsetimestep非常小值-無變化,無法求解FalseTimeStep舉例減少FalsetimeStep(提高阻尼)到10-50,目的在于改進高位震蕩增大FalseTimeStep(減少阻尼)到10-50,目的在于改進高位