fluent12lecture04boundar教学讲解课件

Chapter4

BoundaryConditionsIntroductoryFLUENTTraining.Chapter4

BoundaryConditionsDefiningBoundaryConditionsTodefineaproblemthatresultsinauniquesolution,youmustspecifyinformationontheflowvariablesatboundaries.Specifyfluxesofmass,momentum,energy,othedomain.Definingboundaryconditionsinvolves:IdentifyingtheboundarylocationsSupplyinginformationattheboundariesThedatarequiredataboundarydependsupontheboundaryconditiontypeandthephysicalmodelsemployed.YoumustbeawareoftheinformationthatisrequiredoftheboundaryconditionandlocatetheboundarieswheretheinformationontheflowvariablesareknownorcanbereasonablyapproximatedPoorlydefinedboundaryconditionscanhaveasignificantimpactonyoursolution.DefiningBoundaryConditionsToCellZones–FluidAfluidcellzoneisagroupofcellsforwhichallactiveequationsaresolved.Fluidmaterialselectionisrequired.Formultiplespeciesormultiphase

flows,thematerialisnotshown.

Instead,thefluidzoneconsistsofthe

mixtureofthephases.OptionalinputsPorousregionSourcetermsLaminarregionFixedValuesRadiation.CellZones–FluidAfluidcellPorousMediaAporouszoneisaspecialtypeoffluidzone.EnablePorousZoneoptionintheFluidpanel.Pressurelossinflowdeterminedviauserinputs

ofresistancecoefficientstolumpedparameter

modelUsedtomodelflowthroughporous

mediaandotheruniformlydistributed

flowresistances.PackedbedsFilterpapersPerforatedplatesFlowdistributorsTubebanksInputsaredirectionalviscousand

inertialresistancecoefficients..PorousMediaAporouszoneisaCellZones–SolidAsolidzoneisagroupofcellsforwhichonlyheatconductionproblemsolved.Flowequationsarenotsolved.Onlyrequiredinputisthematerialname(definedintheMaterialspanel).Optionalinputsallowyoutoset

volumetricheatgenerationrate

(heatsource).Needtospecifyrotationaxisif

rotationallyperiodicboundaries

adjacenttosolidzone.Candefinemotionforasolidzone.CellZones–SolidAsolidzoneFuelAirCombustorWallManifoldbox1NozzleLocatingBoundaries–AnExampleThreepossibleapproachesinlocatinginletboundariesforthisexample:UpstreamofmanifoldCanuseuniformprofile.Properlyaccountsformixing.Non-premixedreactionmodelsRequiresmorecells.NozzleinletplaneNon-premixedreaction

models.Requiresaccurateinlet

profile.Flowisstillnon-premixed.3NozzleoutletplanePremixedreactionmodel.Requiresaccurateprofile.Notgenerallyrecommendedsince

inletBCsmaydrivetheinteriorsolution.123.FuelAirCombustorWallManifoldGeneralGuidelinesIfpossible,selectboundarylocationandshapesuchthatfloweithergoesinorout.Notnecessary,butwilltypicallyobservebetterconvergence.Shouldnotobservelargegradientsindirectionnormaltoboundary.Indicatesincorrectset-up.Minimizemeshskewnessneartheboundary.Otherwiseitwouldintroduceerrorearlyincalculation.21Upperpressureboundarymodifiedtoensurethatflowalwaysentersdomain..GeneralGuidelinesIfpossible,AvailableBoundaryConditionTypesExternalBoundariesGeneralPressureInletPressureOutletIncompressibleVelocityInletOutflow(notrecommended)CompressibleMassFlowInletPressureFarFieldOtherWallSymmetryAxisPeriodicSpecialInlet/OutletVentIntake/ExhaustFanInternalBoundariesFanInteriorPorousJumpRadiatorWallCell(Continuum)zonesFluidSolidPorousmediaorificeoutletinletplate

plate-shadowwall.AvailableBoundaryConditionTChangingBoundaryConditionTypesZonesandzonetypesareinitiallydefinedinthepreprocessingphase.Tochangetheboundaryconditiontypeforazone:ChoosethezonenameintheZonelist.SelectthetypeyouwishtochangeittointheTypepull-downlist..ChangingBoundaryConditionTySettingBoundaryConditionDataExplicitlyassigndatainBCpanels.Tosetboundaryconditionsforparticularzone:SelectBoundaryConditionsintheprojecttree.ChoosetheboundarynameintheZonelist.ClicktheEdit…button.Boundaryconditiondatacanbecopiedfromonezonetoanother.BoundaryconditionscanalsobedefinedbyUDFsandprofiles.Profilescanbegeneratedby:WritingaprofilefromanotherCFDsimulationCreatinganappropriatelyformattedtextfile

withboundaryconditiondata..SettingBoundaryConditionDatVelocityInletVelocitySpecificationMethodMagnitude,NormaltoBoundaryComponentsMagnitudeandDirectionAppliesauniformvelocityprofile

attheboundary,unlessUDFor

profileisused.Velocityinletsareintendedfor

useinincompressibleflowsand

arenotrecommendedforcompressibleflows.VelocityMagnitudeinputcanbenegative,implyingthatyoucanprescribetheexitvelocity..VelocityInletVelocitySpecifiPressureInletPressureinletsaresuitableforbothcompressibleandincompressible

flows.Pressureinletboundaryistreatedas

aloss-freetransitionfromstagnation

toinletconditions.FLUENTcalculatesstaticpressureand

velocityatinletMassfluxthroughboundaryvaries

dependingontheinteriorsolutionand

specifiedflowdirection.RequiredinputsGaugeTotalPressureSupersonic/InitialGaugePressureInletflowdirectionTurbulencequantities

(ifapplicable)Totaltemperature(ifheattransfer

and/orcompressible).Incompressible:Compressible:.PressureInletPressureinletsMassFlowInletMassflowinletsareintendedforcompressibleflows;however,theycanbeusedforincompressibleflows.Totalpressureadjuststo

accommodatemassflowinputs.Moredifficulttoconvergethan

pressureinlet.RequiredinformationMassFlowRateorMassFluxSupersonic/InitialGaugePressureStaticpressurewhereflowis

locallysupersonic;ignoredifsubsonicWillbeusedifflowfieldis

initializedfromthisboundary.TotalTemperature(onThermaltab)Usedasstatictemperaturefor

incompressibleflow.DirectionSpecificationMethod.MassFlowInletMassflowinletPressureOutletSuitableforcompressibleandincompressibleflows.Specifiedpressureisignoredifflowislocallysupersonicattheoutlet.Canbeusedasa“free”boundaryinanexternalorunconfinedflow.RequiredinformationGaugePressure(static)–static

pressureoftheenvironmentinto

whichtheflowexits.Backflowquantities–Usedasinlet

conditionsif/whenbackflowoccurs

(outletactslikeaninlet).Foridealgas(compressible)flow,

non-reflectingoutletboundary

conditions(NRBC)areavailable..PressureOutletSuitableforcoWallBoundariesInviscousflows,no-slipconditionsareappliedatwalls.Shearstresscanbeapplied.ThermalboundaryconditionsSeveraltypesofthermalBCsareavailable.Wallmaterialandthicknesscanbedefinedfor1Dorshellconductionheattransfercalculations(detailswillbediscussedintheHeatTransferlecture).Wallroughnesscanbedefined

forturbulentflows.Wallshearstressandheat

transferbasedonlocalflow

field.Translationalorrotational

velocitycanbeassignedtowall

boundaries..WallBoundariesInviscousflowSymmetryandAxisBoundariesSymmetryBoundaryNoinputsarerequired.Flowfieldandgeometrymustbesymmetric:ZeronormalvelocityatsymmetryplaneZeronormalgradientsofallvariablesatsymmetryplaneMusttakecaretocorrectlydefinesymmetryboundarylocations.AxisBoundaryUsedatthecenterlineforaxisymmetricproblems.Nouserinputsrequired.Mustcoincidewiththe

positivexdirection!SymmetryPlanesAxis.SymmetryandAxisBoundariesSyPeriodicBoundariesUsedtoreducetheoverallmeshsize.Flowfieldandgeometrymustcontain

eitherrotationalortranslational

periodicity.RotationalperiodicityΔP=0acrossperiodicplanes.Axisofrotationmustbedefinedinfluidzone.TranslationalperiodicityΔPcanbefiniteacrossperiodicplanes.Modelsfullydevelopedconditions.SpecifyeithermeanΔPperperiod

ornetmassflowrate.Periodicconditionscanbedefined

(ifnotdefinedinthemesh)usingthe

FLUENTTUI:

/mesh/modify-zones/make-periodicTranslationallyperiodicplanes2DTubeHeatExchangerFlowRotationally

periodic

planes.PeriodicBoundariesUsedtoredInternalFaceBoundariesDefinedonthecellfacesonly:ThicknessoftheseinternalfacesiszeroTheseinternalfacesprovidemeansofintroducingstepchangesinflowproperties.Usedtoimplementvariousphysicalmodelsincluding:FansRadiatorsPorous-jumpmodelsPreferableoverporousmediaforitsbetterconvergencebehavior.Interiorwalls.InternalFaceBoundariesDefineCaseSetupReplicationToreplicateacasesetup:Usetheread/writeboundaryconditionsfeatureviaTUIcommand:

/file/write-bc CreatesaBCfile

/file/read-bc ReadsanexistingBCfileYoucantransfersettingsfroma2Dcasetoa3Dcase!inlet-1inlet-2outlet-2outlet-1fluidinlet-1inlet-2outlet-2outlet-12DFlowDomain(approximation)Actual3DFlowDomain.CaseSetupReplicationTorepliSummaryBoundaryzonesareusedtocontrolthesolutionatexternalandinternalboundaries.Manydifferentboundarytypesexistforprescribingboundaryinformation.Cellzones(orcontinuumzones)areusedtoassignwhichfluid/solidmaterial(s)existinaregion.Optionsforporousmedia,laminarregion,fixedvalue,etc.Computationaleffortcanbereducedthroughuseofsymmetryandperiodicboundaries.Thereareseveralotherboundaryconditiontypeswhichwerenotpresented(seeappendixformoreinformationaboutthese).PressureFarFieldExhaustFan/OutletVentInletVent/IntakeFanOutflow.SummaryBoundaryzonesareusedAppendix.Appendix.OtherInlet/OutletBoundaryConditionsPressureFarFieldUsedtomodelfree-streamcompressibleflowatinfinity,withprescribedstaticconditionsandthefree-streamMachnumber.Availableonlywhendensityiscalculatedusingtheidealgaslaw.TargetMassFlowRateoptionforpressureoutlets(notavailableforthemultiphasemodels)Providestheabilitytofixthemassflowrateonapressureoutlet(eitherconstantorviaUDFhook)OptionstochooseiterationmethodinTUIExhaustFan/OutletVentModelsanexternalexhaustfanoroutletventwithspecifiedpressurerise/losscoefficientandambient(discharge)pressureandtemperature.InletVent/IntakeFanModelsaninletvent/externalintakefanwithspecifiedlosscoefficient/pressurerise,flowdirection,andambient(inlet)pressureandtemperature.Inletboundaryconditionsforlarge-eddy/detached-eddysimulationsarecoveredintheTurbulenceModelinglecture..OtherInlet/OutletBoundaryOutflowNopressureorvelocityinformationis

required.Dataatexitplaneisextrapolatedfrom

interior.Massbalancecorrectionisappliedat

boundary.Flowexitingoutflowboundaryexhibitszeronormaldiffusivefluxforallflowvariables.Appropriatewheretheexitflowisfullydeveloped.Theoutflowboundaryisintendedforusewithincompressibleflows.Cannotbeusedwithapressureinletboundary(mustusevelocity-inlet).Combinationdoesnotuniquelysetpressuregradientoverwholedomain.Cannotbeusedforunsteadyflowswithvariabledensity.Poorrateofconvergencewhenbackflowoccursduringiterations.Cannotbeusedifbackflowisexpectedinthefinalsolution..OutflowNopressureorvelocityModelingMultipleExitsFlowswithmultipleexitscanbemodeledusingpressureoutletoroutflowboundaries,dependingontheinformationyouknow.Pressureoutlets–requiresknowledgeofdownstreampressures;FLUENTcalculatesthefractionoftotalflowthrougheachbranch.Outflow:MassflowratefractiondeterminedfromFlowRateWeighting(FRW)byStaticpressurevariesamongexitstoaccommodatetheprescribedflowdistribution.Velocityinlet(V,T0)ORPressureinlet(p0,T0)PressureoutletPressureoutletVelocityinlet(V,T0)Outflow(FRW2)Outflow(FRW1).ModelingMultipleExitsFlowswChapter4

BoundaryConditionsIntroductoryFLUENTTraining.Chapter4

BoundaryConditionsDefiningBoundaryConditionsTodefineaproblemthatresultsinauniquesolution,youmustspecifyinformationontheflowvariablesatboundaries.Specifyfluxesofmass,momentum,energy,othedomain.Definingboundaryconditionsinvolves:IdentifyingtheboundarylocationsSupplyinginformationattheboundariesThedatarequiredataboundarydependsupontheboundaryconditiontypeandthephysicalmodelsemployed.YoumustbeawareoftheinformationthatisrequiredoftheboundaryconditionandlocatetheboundarieswheretheinformationontheflowvariablesareknownorcanbereasonablyapproximatedPoorlydefinedboundaryconditionscanhaveasignificantimpactonyoursolution.DefiningBoundaryConditionsToCellZones–FluidAfluidcellzoneisagroupofcellsforwhichallactiveequationsaresolved.Fluidmaterialselectionisrequired.Formultiplespeciesormultiphase

flows,thematerialisnotshown.

Instead,thefluidzoneconsistsofthe

mixtureofthephases.OptionalinputsPorousregionSourcetermsLaminarregionFixedValuesRadiation.CellZones–FluidAfluidcellPorousMediaAporouszoneisaspecialtypeoffluidzone.EnablePorousZoneoptionintheFluidpanel.Pressurelossinflowdeterminedviauserinputs

ofresistancecoefficientstolumpedparameter

modelUsedtomodelflowthroughporous

mediaandotheruniformlydistributed

flowresistances.PackedbedsFilterpapersPerforatedplatesFlowdistributorsTubebanksInputsaredirectionalviscousand

inertialresistancecoefficients..PorousMediaAporouszoneisaCellZones–SolidAsolidzoneisagroupofcellsforwhichonlyheatconductionproblemsolved.Flowequationsarenotsolved.Onlyrequiredinputisthematerialname(definedintheMaterialspanel).Optionalinputsallowyoutoset

volumetricheatgenerationrate

(heatsource).Needtospecifyrotationaxisif

rotationallyperiodicboundaries

adjacenttosolidzone.Candefinemotionforasolidzone.CellZones–SolidAsolidzoneFuelAirCombustorWallManifoldbox1NozzleLocatingBoundaries–AnExampleThreepossibleapproachesinlocatinginletboundariesforthisexample:UpstreamofmanifoldCanuseuniformprofile.Properlyaccountsformixing.Non-premixedreactionmodelsRequiresmorecells.NozzleinletplaneNon-premixedreaction

models.Requiresaccurateinlet

profile.Flowisstillnon-premixed.3NozzleoutletplanePremixedreactionmodel.Requiresaccurateprofile.Notgenerallyrecommendedsince

inletBCsmaydrivetheinteriorsolution.123.FuelAirCombustorWallManifoldGeneralGuidelinesIfpossible,selectboundarylocationandshapesuchthatfloweithergoesinorout.Notnecessary,butwilltypicallyobservebetterconvergence.Shouldnotobservelargegradientsindirectionnormaltoboundary.Indicatesincorrectset-up.Minimizemeshskewnessneartheboundary.Otherwiseitwouldintroduceerrorearlyincalculation.21Upperpressureboundarymodifiedtoensurethatflowalwaysentersdomain..GeneralGuidelinesIfpossible,AvailableBoundaryConditionTypesExternalBoundariesGeneralPressureInletPressureOutletIncompressibleVelocityInletOutflow(notrecommended)CompressibleMassFlowInletPressureFarFieldOtherWallSymmetryAxisPeriodicSpecialInlet/OutletVentIntake/ExhaustFanInternalBoundariesFanInteriorPorousJumpRadiatorWallCell(Continuum)zonesFluidSolidPorousmediaorificeoutletinletplate

plate-shadowwall.AvailableBoundaryConditionTChangingBoundaryConditionTypesZonesandzonetypesareinitiallydefinedinthepreprocessingphase.Tochangetheboundaryconditiontypeforazone:ChoosethezonenameintheZonelist.SelectthetypeyouwishtochangeittointheTypepull-downlist..ChangingBoundaryConditionTySettingBoundaryConditionDataExplicitlyassigndatainBCpanels.Tosetboundaryconditionsforparticularzone:SelectBoundaryConditionsintheprojecttree.ChoosetheboundarynameintheZonelist.ClicktheEdit…button.Boundaryconditiondatacanbecopiedfromonezonetoanother.BoundaryconditionscanalsobedefinedbyUDFsandprofiles.Profilescanbegeneratedby:WritingaprofilefromanotherCFDsimulationCreatinganappropriatelyformattedtextfile

withboundaryconditiondata..SettingBoundaryConditionDatVelocityInletVelocitySpecificationMethodMagnitude,NormaltoBoundaryComponentsMagnitudeandDirectionAppliesauniformvelocityprofile

attheboundary,unlessUDFor

profileisused.Velocityinletsareintendedfor

useinincompressibleflowsand

arenotrecommendedforcompressibleflows.VelocityMagnitudeinputcanbenegative,implyingthatyoucanprescribetheexitvelocity..VelocityInletVelocitySpecifiPressureInletPressureinletsaresuitableforbothcompressibleandincompressible

flows.Pressureinletboundaryistreatedas

aloss-freetransitionfromstagnation

toinletconditions.FLUENTcalculatesstaticpressureand

velocityatinletMassfluxthroughboundaryvaries

dependingontheinteriorsolutionand

specifiedflowdirection.RequiredinputsGaugeTotalPressureSupersonic/InitialGaugePressureInletflowdirectionTurbulencequantities

(ifapplicable)Totaltemperature(ifheattransfer

and/orcompressible).Incompressible:Compressible:.PressureInletPressureinletsMassFlowInletMassflowinletsareintendedforcompressibleflows;however,theycanbeusedforincompressibleflows.Totalpressureadjuststo

accommodatemassflowinputs.Moredifficulttoconvergethan

pressureinlet.RequiredinformationMassFlowRateorMassFluxSupersonic/InitialGaugePressureStaticpressurewhereflowis

locallysupersonic;ignoredifsubsonicWillbeusedifflowfieldis

initializedfromthisboundary.TotalTemperature(onThermaltab)Usedasstatictemperaturefor

incompressibleflow.DirectionSpecificationMethod.MassFlowInletMassflowinletPressureOutletSuitableforcompressibleandincompressibleflows.Specifiedpressureisignoredifflowislocallysupersonicattheoutlet.Canbeusedasa“free”boundaryinanexternalorunconfinedflow.RequiredinformationGaugePressure(static)–static

pressureoftheenvironmentinto

whichtheflowexits.Backflowquantities–Usedasinlet

conditionsif/whenbackflowoccurs

(outletactslikeaninlet).Foridealgas(compressible)flow,

non-reflectingoutletboundary

conditions(NRBC)areavailable..PressureOutletSuitableforcoWallBoundariesInviscousflows,no-slipconditionsareappliedatwalls.Shearstresscanbeapplied.ThermalboundaryconditionsSeveraltypesofthermalBCsareavailable.Wallmaterialandthicknesscanbedefinedfor1Dorshellconductionheattransfercalculations(detailswillbediscussedintheHeatTransferlecture).Wallroughnesscanbedefined

forturbulentflows.Wallshearstressandheat

transferbasedonlocalflow

field.Translationalorrotational

velocitycanbeassignedtowall

boundaries..WallBoundariesInviscousflowSymmetryandAxisBoundariesSymmetryBoundaryNoinputsarerequired.Flowfieldandgeometrymustbesymmetric:ZeronormalvelocityatsymmetryplaneZeronormalgradientsofallvariablesatsymmetryplaneMusttakecaretocorrectlydefinesymmetryboundarylocations.AxisBoundaryUsedatthecenterlineforaxisymmetricproblems.Nouserinputsrequired.Mustcoincidewiththe

positivexdirection!SymmetryPlanesAxis.SymmetryandAxisBoundariesSyPeriodicBoundariesUsedtoreducetheoverallmeshsize.Flowfieldandgeometrymustcontain

eitherrotationalortranslational

periodicity.RotationalperiodicityΔP=0acrossperiodicplanes.Axisofrotationmustbedefinedinfluidzone.TranslationalperiodicityΔPcanbefiniteacrossperiodicplanes.Modelsfullydevelopedconditions.SpecifyeithermeanΔPperperiod

ornetmassflowrate.Periodicconditionscanbedefined

(ifnotdefinedinthemesh)usingthe

FLUENTTUI:

/mesh/modify-zones/make-periodicTranslationallyperiodicplanes2DTubeHeatExchangerFlowRotationally

periodic

planes.PeriodicBoundariesUsedtoredInternalFaceBoundariesDefinedonthecellfacesonly:ThicknessoftheseinternalfacesiszeroTheseinternalfacesprovidemeansofintroducingstepchangesinflowproperties.Usedtoimplementvariousphysicalmodelsincluding:FansRadiatorsPorous-jumpmodelsPreferableoverporousmediaforitsbetterconvergencebehavior.Interiorwalls.InternalFaceBoundariesDefineCaseSetupReplicationToreplicateacasesetup:Usetheread/writeboundaryconditionsfeatureviaTUIcommand:

/file/write-bc CreatesaBCfile

/file/read-bc ReadsanexistingBCfileYoucantransfersettingsfroma2Dcasetoa3Dcase!inlet-1inlet-2outlet-2outlet-1fluidinlet-1inlet-2outlet-2outlet-12DFlowDomain(approximation)Actual3DFlowDomain.CaseSetupReplicationTorepliSummaryBoundaryzonesareusedtocontrolthesolutionatexternalandinternalboundaries.Manydifferentboundarytypesexistforprescribingboundaryinformation.Cellzones(orcontinuumzones)areusedtoassignwhichfluid/solidmaterial(s)existinaregion.Optionsforporousmedia,laminarregion,fixedvalue,etc.Computationaleffortcanbereducedthroughuseofsymmetryandperiodicboundaries.Thereareseveralotherboundaryconditiontypeswhichwerenotpresented(seeappendixformoreinformationaboutthese).PressureFarFieldExhaustFan/Ou