用动网格模拟超音速飞机投弹过程.pdf

用动网格模拟超音速飞机投弹过程.pdf

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Copyright2005FluentInc.JA212-Page1of4JOURNALARTICLESBYFLUENTSOFTWAREUSERSWorkingcloselywiththeU.S.AirForceMunitionsDirectorate,JacobsSverdrupengineershavedevelopedamethodtoaccuratelysimulatethebehaviorofstores（suchasmunitions）astheyarereleasedfromtransonicaircraft.Theseparationofstoresisachallengingproblemthatcostmanyaircraftduringtheperiodwhenflighttestingwastheonlywaytoevaluatealternativedesigns.Computationalfluiddynamics（CFD）simulationsofmunitionsseparationarechallengingbecausetherapidmovementrequirescontinualmodificationofthemesh,whichdrivesthedemandforcomputationalresources.JacobsSverdrupengineershaveovercomethisproblembyusinganunstructureddynamicmeshtechnique.Theabilitytodynamicallychangeanunstructuredmeshgreatlyincreasesthespeedofgridgenerationateachtimestepofthecalculation.ComparingtheCFDresultswithwindtunneltestingforonecasestudiedshowedthatthismethodwasabletoaccuratelypredictthelocationandvelocityofthecenterofgravity,theangularorientationanditsrateofchange,andsurfacepressureprofilesafterrelease.Theabilitytosimulateseparationeventswillsubstantiallyreducedesigntimeandcostandwillallowengineerstoevaluatemanymorepotentialdesigns,presentingtheopportunityforsubstantialperformanceimprovements.Theaerodynamicbehaviorofmunitionsorotherobjectsastheyarereleasedfromaircraftiscriticalbothtotheaccuratearrivalofthemunitionsatthetargetandthesafetyofthereleasingaircraft.Thechallengeisespeciallygreatforobjectsthataredynamicallyunstable,suchasfueltanksandsomemodern,agilemunitionsintheeventofcontrolfailure.Inthedistantpast,separationtestingwasaccomplishedsolelyusingflighttests.Thisapproachwasverytime-consuming,oftenrequiringyearstocertifyaweapon.Itwasalsoexpensiveandoccasionallyledtothelossofanaircraftduetounexpectedbehaviorofthestorebeingtested.Inthe1960s,methodsofpredictingstoreseparationinwindtunneltestsweredeveloped.Thesetestshaveprovensovaluablethattheyarenowtheprimarydesigntoolused.However,windtunneltestsarestillexpensive,havelongleadtimes,andsufferfromlimitedaccuracyincertainsituations,suchaswheninvestigatingstoresreleasedfromwithinweaponsbaysortheripplereleaseofmultiplemovingobjects.Inaddition,becauseverysmall-scalemodelsmustoftenbeused,scalingproblemscanreduceaccuracy.DynamicMeshingModelsStoreSeparationfromTransonicAircraftJA212Figure1:

Perspectiveviewofthewing/pylon/storegeometryByDerylO.SnyderAerospaceEngineer,JacobsSverdrup,Inc.EglinAirForceBase,FloridaCopyright2005FluentInc.JA212Page2of4TheadventofsimulationInrecentyears,engineershavebegunusingmodelingandsimulationtoreducecertificationcostandevaluateadditionaldesignsinordertoincreasethemarginofsafetyforflight-testing.ACFDsimulationprovidesfluidvelocity,pressure,temperature,andothervariables,asappropriate,throughoutthesolutiondomainforproblemswithcomplexgeometriesandboundaryconditions.Aspartoftheanalysis,anengineermaychangethegeometryofthesystemortheboundaryconditions,andobservetheeffectofthechangesonfluidflowpatternsordistributionsofothervariables.TheuseofCFDformodelingstoreseparationbeganbycombiningtraditionalsteady-statesolutionswithempiricalorsemi-empiricalapproaches.ThemethodologybecamemoreusefulwiththeinnovationofChimeraoverset（overlapping）gridapproachesthatmadeitpossibletoperformunsteady,fullfieldsimulationswithorwithoutviscouseffects.Untilrecently,CFDhasnotreacheditsfullpotentialformodelingtransonicseparationeventsbecauseofthelargeamountoftimerequiredtoperformsimulations.Thegridmustbegenerated,assembledandrecalculatedateachtimestepofthesimulation.Finegridsandsmalltimestepsareoftenrequiredforaccuracyandstability,whichincreasescomputationaltime.Themosttime-consumingaspectoftheFigure2:

Surfacemeshesforcoarse,nominal,andfinegridsCopyright2005FluentInc.JA212-Page3of4simulationisusuallythegridgenerationandassembly.Thisisespeciallytrueforcomplexstoregeometries,andinparticular,forthecaseofstoresreleasedfromweaponsbays.Thesebaysoftencontainintricategeometricfeaturesthataffecttheflowfield.UnstructureddynamicmeshingapproachToimproveonthesemethods,Sverdrupengineerssetouttodevelopanunstructureddynamicmeshapproachforstoreseparationsimulations.Gridgenerationtimewithunstructuredmeshesisreducedbecausetheusersinputislargelylimitedtothegenerationofasurfacemesh.Also,becausetherearenooverlappinggridregions,fewergridpointsarerequired.TheSverdrupcomputationalapproachconsistsofthreedistinctcomponents:

aflowsolver,asix-degree-of-freedom（6DOF）trajectorycalculator,andadynamicmeshalgorithm.TheFLUENTflowsolverfromFluentIncorpor,Lebanon,NewHampshire,isusedtosolvethegoverningfluiddynamicequationsateachtimestep.Fromthissolution,theaerodynamicforcesandmomentsactingonthestorearecomputedbyintegratingthepressureoverthesurface.Theforcesandmomentsareusedtocomputethemovementofthestorebythe6DOFtrajectorycode.ThiscodeintegratestheNewton-EulerequationsofmotionwithinFLUENTasauser-definedfunction（UDF）thatisdynamicallylinkedwiththeFLUENTsolveratruntime.Alocalremeshingalgorithmisusedtoaccommodatethemovingbodyinthediscretizedcomputationaldomain.Finally,theunstructuredmeshismodifiedtoaccountforstoremovementviathedynamicmeshalgorithm.Whenthemotionofthebodyissmall,alocalizedsmoothingmethodisused.Whenthemotionofthemovingbodyislarge,poorqualitycells,basedonvolumeorskewnesscriteria,areagglomeratedandlocallyremeshed.ValidatingthenewmethodThisapproachwasdemonstratedonagenericwing/pylon/storegeometryforwhichbenchmarkexperimentaldatawasavailable.Thewingisa45-degreeclippeddeltawitha25-footrootchordlength.Theogive-flatpylonextends2feetbelowthewingleadingedge.Thestoreconsistsofatangent-ogiveforebody,aclippedtangent-ogiveafterbody,andacylindricalcenterbody.Fluentpreprocessor,Gambit,wasusedtogeneratetrianglesurfacemeshesfromCADgeometryfiles.ThemesheswereimportedintoFluentsmeshingpreprocessor,TGrid,forgenerationofthetetrahedralvolumemesh.TheautomatedmeshingtoolsinGAMBITandTGridmadeitpossibletocreateallthreemeshesinjustafewhours.Nonpermeablewallboundaryconditionswereusedforthewing,pylon,andstoresurfaces.Apressurefarfieldconditionwasusedattheupstreamanddownstreamdomainextents,asymmetryplanewasusedatthewingroot,andthedownstreamboundarywasassignedapressureoutletcondition.TheinitialconditionusedfortheseparationanalysisFigure3:

Comparisonofnumerical（blue）andexperimental（shadow）separationeventsCopyright2005FluentInc.JA212-Page4of4wasafullyconvergedsteady-statesolution.Timestepsofthreesizes,0.01sec.,0.002secand0.0004sec.,wereevaluatedforeachofthreegridrefinementsused.TheresultsshowedthatCFDperformedverywellinmodelingaseparationeventatMach1.2atanaltitudeof38,000feet.ThecenterofgravitylocationmatchedverycloselywiththeexperimentaldataforallthreegridrefinementsThestoremovedrearwardandslightlyinboardasitfell.TheCFDresultsslightlyunderpredictedtherearwardacceleration.Thepitchandyawanglesofthestorecalculatedbythesimulationalsoagreedwellwiththeexperimentalresults.Therollangleisespeciallydifficulttomodelbecausethemomentofinertiaabouttherollaxisismuchsmallerthanthatofthepitchandyawaxes.Consequentlytherollisverysensitivetoerrorsinaerodynamicforceprediction.Inthiscase,thestoreinitiallypitchednose-upinresponsetothemomentproducedbytheejectors.Oncefreeoftheejectors,thenose-downaerodynamicpitchingmomentreversedthetrend.Thestoreyawedinitiallyoutboarduntilabout0.55seconds,afterwhichitbeganturninginboard.Thestorerolledcontinuouslyoutboardthroughoutthefirst0.8secondsoftheseparation.Thistrendwasunderpredictedbythesimulationandthecurvetendedtodivergefromtheexperimentsafterapproximately0.3seconds.Experimentalsurfacepressuredatawasalsoavailablefromthewindtunneltests.Thedatawascomparedwiththesimulationalongaxiallinesofthestorebodyatfourcircumferentiallocationsandthreeinstantsintime.Theagreementbetweenthesimulationandexperimentswasexceptional.Ofparticularinterestedwasthe5degreecircumferentiallocationlineatt=0.0becauseitislocatedinthesmallgapbetweenthepylonandstore.Thesimulationdidagoodjobofcapturingthedecelerationneartheleadingedgeofthepylon.Allinall,thisstudyshowedthatCFDwithunstructureddynamicmeshingcanbeapowerfultoolformodelingtransonicstoreseparation.Thesimulationcapturedtheexperimentallocation,velocity,orientationandangularratetrends.Surfacepressuredistributionswerealsoinexcellentagreementwiththeexperimentsatthethreetimesatwhichtheywerecomparedduringtheseparationevent.Thisnewapproachofferstheabilitytoobtainaccuratestoreseparationpredictionswithquickturn-aroundtimes.Gridgenerationcanbeaccomplishedinamatterofhours,andrunssuchasthenominalgridcaseexaminedinthisstudycanbecompletedovernightonadesktopworkstation.Theuseofunstructuredtetrahedralmeshesandafullyparallelized,accurateandstablesolverallowsforsmallgridsandrelativelylargetimestepswithoutanexcessivecomputationalburden.Figure4:

Rigidboundarylayermeshattachedtothestore