Numerical Simulation of flow around the squareback of Ahmed body by PowerFlow文档格式.docx
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Numerical Simulation of flow around the squareback of Ahmed body by PowerFlow文档格式.docx
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Hiermitversichereich,XinleiMa,dassichdievorliegendeArbeitbzw.diedarinmitmeinemNamengekennzeichnetenAnteileselbstä
ndigverfasstundnurdieangegebenenQuellenundHilfsmittelbenutzthabe.Dabeihabeichallewö
rtlichodersinngemä
ß
ausanderenWerkenü
bernommenenAussagenalssolchegekenn-zeichnet.
DieArbeitistwedervollstä
ndignochinwesentlichenTeilenGegenstandeinesanderenPrü
fungsverfahrensgewesen.Ferneristsiewedervollstä
ndignochinTeilenbereitsverö
ffentlichtworden.DaselektronischeExemplarstimmtmitdenanderenExemplarenü
berein.
Stuttgart,20.08.2017
XinleiMa
Declaration
HerebyIdeclare,XinleiMa,whichthepresentworkandthesharesmarkedwithmynamewerewrittenindependentlyandonlytheindicatedsourcesandtoolswereused.Thequotationsandreferenceshavebeendulyacknowledgedintheconcernedplaces.
Theworkhasnotbeenthesubjectofanyotherexaminationprocedure,eitherwhollyorinparts.Furthermore,ithasnotbeenpublishedeithercompletelyorinpart.Theelectroniccopyagreeswiththeothercopies.
Content
AbstractIII
AbbreviationsIV
SymbolsV
ListofFiguresVII
ListofTablesIX
1Introduction1
1.1GeneralResearchBackgroundandImplication1
1.2ResearchMethod–ComputationalFluidDynamicsNumericalSimulation(CFD)2
1.3CurrentSituationofCFDAutomotiveResearch3
1.4TheResearchContent4
2CFDFundamentalTheory5
2.1CFDFundamentalTheory5
2.1.1FluidDynamicGoverningEquations5
2.1.2ABriefIntroductiontotheGridinCFD6
2.1.3NumericalSolutioninCFD7
2.1.4ABriefIntroductiontoTurbulence7
2.2TheProcessandCharacteristicsoftheOutflowFieldSimulation8
2.2.1TheProcessoftheOutflowFieldSimulation8
2.2.2TheCharacteristicsoftheOutflowFieldSimulation9
3TheSimulationontheAhmedBody10
3.1NumericalSimulationwithPowerFlow10
3.2AhmedBodyModel12
3.3SimulationEnviroment15
3.4GridGenerationandVRlevels16
4ResultandDiscussion18
4.1Streamlinesoftime-averagedvelocityintheWakeFlow18
4.2VelocityProfiles23
4.3TheInfluenceofResolutiononSimulationResults27
5Conclusions30
6Literature31
Appendix33
A.1ExamplesummaryinPowerFlow33
A.2CalculationcontentsinPowerFlow40
Abstract
Theautomobileindustryisdevelopingrapidly,thecarownershipisrisingsofast.Thegreatquantityofthevehiclesnotonlyconsumesthelargeamountofthepetroleumreserve,butalsotheenvironmentalpollutionproblemsaremuchmoreserious.Underthebackgroundoftheenergyconservationandtheenvironmentalprotection,thedemandofautomobileenergysavingandconsumptionisurgent.Theenergyconsumptionfromthevehicleislargelytoovercometheairresistanceduringthedriving,whichmakestheautomobileaerodynamicresearchbecomeoneofthehotspots.
Theapplicationofnumericalsimulationbasedoncomputationalfluiddynamics(CFD)isbecomingmoreandmorepopularinthestudyofautomobileflowfield.Comparingwiththewindtunneltest,thenumeriacalsimulationmethodhastheadvantagesoflowcostandshortcycle.
Theairresistancefromthevehicleislargelyduetothevorticesgeneratedbythetail.Therefore,thetailairflowundertheeffectivecontrollingbecomesthekeytothedragreduction.
InthisthesisthecommercialcomputationalfluiddynamicssoftwarePowerFlowisused,therewedotheresearchaboutthenumericalsimulationoftheflowfieldaroundtheAhmedbodyunderthedifferentaccuracyofthesimulationonthetailoftheAhmedbody,wecollectandcomparethedatatoexplorethepriciplesoftheairresistanceofthesquarebackonAhmedbody.Mesurethesteamlineofthetime-averagedvelocityandvelocityprofileatdifferentwakelocationwithdifferentresolutioncases.Findouttheinfluenceoftheresolutiononthesimulationwiththecomparisonwiththeexperimentdata.Summarizetheeffectofsimulationresultsunderdifferentaccuracy.
Abbreviations
CFD
ComputationalFluidDynamics
RSM
ReynoldsStressModel
DNS
DirectlyNumericalSimulationmethod
LBM
Lattice-Boltzmann-Method
NS-CFD
Navier-StokesComputationalFluidDynamicsmethods
UDDS
UrbanDynamometerDrivingSchedule
FDM
FiniteDifferenceMethod
FEM
FiniteElementMethod
FVM
FiniteVolumeMethod
CAD
ComputerAidedDesign
RANS
Reynolds-AveragedNavier-Stokessolver
VR
VariablesResolution
FeV
FinestregionofVoxels
FeS
FinestregionofSurfels
PIV
ParticleImageVelocimetry
LES
LargerEddySimulation
Symbols
Volumen
Density
Time
Thetotaldragcoefficientincludingfrictionalresistancecoefficientanddifferentialpressureresistancecoefficient
Thetotalfrictionalresistancecoefficient
Thefrontenddifferentialpressurecoefficient
Thedifferentialpressurecoefficientoftheverticalplaneofthetail
Thedifferentialpressurecoefficientofthetailslope
Free-streamvelocity
Taylormicroscale
Minimalcellsize
Reynoldsnumber
∆t
Onetimestepinseconds
Averaged-time
ElapsedtimethroughtheAhmedbodylength
y*
Thedistancefromthecentertothetoptrailingedge
ListofFigures
Figure1.1Theproportionofpneumaticresistancetototalresistance1
Figure2.1Thesketchmapsofthegrid7
Figure2.2Themeasuredvelocityatapointinaturbulentflow7
Figure2.3CFDBasicFlowDiagram8
Figure3.1Theelementsinthelattice10
Figure3.2TheoreticalApproachesofDIGITALPHYSICSandTraditionalCFD11
Figure3.3ThebasicdimensionsoftheAhmedmodel13
Figure3.4ThedragcoefficientsoftheAhmedbodyfrom0-40angle14
Figure3.5Theexperimentset-up15
Figure3.6ThemeshmodebyANSA16
Figure3.7DifferentVRlevelsaroundtheAhmedbodymodel17
Figure4.1ComparisonbetweenexperimentalPIVdataandthesimulationdatainthelongitudinalsymmetricalplane.(a)time-averagedPIV;
(b)time-averagedLES;
(c)time-averagedPowerFlow19-20
Figure4.2ThevorticeslocationsinPowerFlowsimulation21
Figure4.3Theboundarylayerdevelopedbetweenthevortices21
Figure4.4SurfacestreamlineshowingtheconvergentpointN22
Figure4.5Thevorticesweregeneratedaroundthesquareback22
Figure4.6Comparisonofthetime-averagedstreamwisevelocitycomponent,ufordifferentlocationsinwake.Shearlayerprofile(0.03H)downstreamofthetoptrailingedge.y∗=y+0.5H23
Figure4.7Comparisonprofileat(a)0.17Hdownstream(b)0.34Hdownstream(c)0.5Hdownstream(d)0.67Hdownstream(f)0.84Hdownstream24-26
Figure4.8Streamwisevelocitydistributionwirhfinestresolution27
Figure4.9(a)SteamwiseVelocityDistributionChangewiththedifferentgridsize(b)VelocityProfileDistributionChangewiththedifferentgridsize27-28
ListofTables
Table
3.1:
Numberoflatticesandcomputationaleffortassociatedwiththeresolution17
Table4.1:
Differentresolutionwithtotalsimulationtime28
1Introduction
1.1GeneralResearchBackgroundandImplication
Automotiveaerodynamicsisthesicencethatstudiestheinteractionbetweenthevehiclesandair.Asoneimportantperformanceofthevehicles,theaerodynamicscharacteristicsofthevehicleshasthegreatrelationwiththevehiclefueleconomy,controllingstability,safetyandcomfort.Especiallyundertheageoftheenergyconservationandtheenvironmentalprotection,thereisagreatvaluableapplicationinreducingfuelconsumptioninautomotiveaerodynamicsfield.
Thefigure1.1showstheproportionofpneumaticresistancecomparedtototalresistance.Atthevehiclespeed80km/h,thepneumaticresistanceisalmostequaltotherollingdynamicresistace;
atthevehiclespeed150km/h,thepneumaticresistanceisequivalenttothe2~3timesoftherollingdynamicresistance[1].
Figure1.1Theproportionofpneumaticresistancetototalresistance
Thus,thereductionoftheaerodynamicdragissignificantforreducingthetotalresistanceofthevehicle,whichcangreatlyreducefuelconsumption.
Thepneumaticresistanceofthevehicleismainlydividedintotwoparts;
pressureresistanceandfrictionresistance.TheGermanProfessorS.R.Ahmedresearchshowsthatforagroundvehicleshapebluntbodymodel-Ahmedbodymodel,thediffentialpressureresistanceisasmuchas85%ofthetotalresistance,therestisthefrictionresistance;
onlythe9%ofthediffentialpressureresistanceisproducedfromthefront,mostofthediffentialpressureresistancearegeneratedbythetail[2].
Insummary,wehavethereasontobelievethatthekeyofthereductionoftheenergy-efficientvehicleistoreducethevehicle'
saerodynamicdragandthecoreofthereductionofthepneumaticresistanceistosuppressthediffentialpressureresistancebytail.
1.2ResearchMethod–ComputationalFluidDynamicsNumericalSimulation(CFD)
Atpresent,ourresearchmethodsare:
theoreticalanalysis,roadtest,windtunneltest,computationalfluiddynamicsnumericalsimulation(CFD).
Foralongperiodofyears,thewindtunneltesthasbeenamajortoolforevaluatingthevehicleaerodynamicperformance.However,inrecentyears,thedevelopmentofcomputerhardwareandsoftwaremakespossibleforthehigh-qualityautomoti
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