简介:Thenatureofthethree-dimensionaltransitionarisingintheflowpastacylinderisinvestigatedbyapplyingtheLifschitz-HameiritheoryalongspecialLagrangiantrajectoriesexistinginitswake.ResultsshowthatthevonKarmanstreetisunstablewithregardtoshort-wavelengthperturbations.Theasymptoticanalysispredictsthepossibleexistenceofbothsynchronous(asmodesAandB)andasynchronous(asmodeC)instabilities,eachassociatedtospecificLagrangianorbits.Theproposedstudyprovidesusefulqualitativeinformationontheoriginofthedifferentmodesbutnoquantitativeagreementbetweenthegrowthratespredictedbytheasymptoticanalysisandbyaglobalstabilityanalysisisobserved.Thereasonsforsuchmismatcharebrieflydiscussedandpossibleimprovementstothepresentanalysisaresuggested.
简介:Thephysicalmechanismforgenerationofstreamwisevortices(orribvortices)inthecylinderwakeisnumericallyinvestigatedwithafinite-differencescheme.Rayleigh’stheoryofcentrifugalinstabilityforinviscidaxisymmetricflowisextendedtoanalyzethe2-Dprimaryflows.Accordingly,ananalyticaldimensionlessgroupRay=-(r/vθ)■vθ/■r-1isderived,wherevθrepresentsthevelocityofafluidelementrelativetotheoncomingflow,risthelocalcurvatureradiusoftheelementpathline.CentrifugalinstabilityoccurswhenRay>0.StabilityanalysesarecarriedoutwiththisdiscriminantforprimaryflowsatdifferenttimelevelsinahalfsheddingperiodofthevonKarman(orvK)vortices.UnstableareasareidentifiedandthelocationsofribvorticesarecoincidentwellwiththeunstableareaswithinthefirstwavelengthofvKvorticesbehindthecylinder.Thenumericalresultsalsoshowthatribvorticesexperienceamplificationinthisregion.Itisapparentthatcentrifugalinstabilityplaysanimportantroleinthegenerationofribvorticesinthecylinderwake.
简介:Thepresentworkusesdynamicmodedecomposition(DMD)toanalyzewakeflowofNACA0015airfoilwithGurneyflap.ThephysicsofDMDisfirstintroduced.ThenthePIV-measuredwakeflowvelocityfieldisdecomposedintodynamicalmodes.Thevortexsheddingpatternbehindthetrailingedgeanditshigh-orderharmonicshavebeencapturedwithabundantinformationsuchasfrequency,wavelengthandconvectionspeed.Itisobservedthathigh-orderdynamicmodesconvectfasterthanlow-ordermodes;moreoverthewavel...
简介:Thepresentpaperderivedtemperaturedistributionsinpowerseriesinasteadylaminarwakebehindaslenderstreamilinedheatedbodyofrevolution.Thetemperaturedistributionofthewakebehindthebodyfromx/d=2to70werefullyexploredusingarecirculatingwaterchannel.ThethermalwakepatternsbehingthebodywerevisualizedwiththeaidofinfraredpicturesandH2bubbles.
简介:Intensiveturbulenceexistsinthewakesofhighspeedtrains,andtheaerodynamicperformanceofthetrailingcarcoulddeterioraterapidlyduetocomplicatedfeaturesofthevorticesinthewakezone.Asaresult,thesafetyandamenityofhighspeedtrainswouldfaceagreatchallenge.Thispaperconsidersmainlythemechanismofvortexformationandevolutioninthetrainflowfield.ArealCRH2modelisstudied,withaleadingcar,amiddlecarandatrailingcarincluded.Di?erentrunningspeedsandcrosswindconditionsareconsidered,andtheapproachesofunsteadyReynold-averagedNavier–Stokes(URANS)anddetachededdysimulation(DES)areutilized,respectively.ResultsrevealthatDEShasbettercapabilityofcapturingsmalleddiescomparedtoURANS.However,forlargeeddies,thee?ectsoftwoapproachesarealmostthesame.Inconditionswithoutcrosswinds,twolargevortexstreetsstretchfromthetrainnoseandinteractstronglywitheachotherinthewakezone.Withthereinforcementoftheground,acomplicatedwakevortexsystemgeneratesandbecomesstrengthenedastherunningspeedincreases.However,thelocationsofflowseparationsonthetrainsurfaceandtheseparationmechanismkeepunchanged.Inconditionswithcrosswinds,threelargevorticesdevelopalongtheleewardsideofthetrain,amongwhichtheweakestonehasnoobviousinfluenceonthewakeflowwhiletheothertwostretchtothetailofthetrainandcombinewiththehelicalvorticesinthetrainwake.Thus,optimizationoftheaerodynamicperformanceofthetrailingcarshouldbeaimingatreducingtheintensityofthewakevortexsystem.
简介:TheintrinsicphysicalrelationshipofvorticitybetweenmodesAandBinthethree-dimensionalwaketransitionisinvestigated.Directnumericalsimulationsfortheflowpastasquare-sectioncylinderarecarriedoutatReynoldsnumbersof180and250,associatedwithmodesAandB,respectively.Basedontheanalysisofspacialdistributionsofvorticityinthenearwake,characteristicsoftheverticalvorticityinmodesAandBareidentified.Moreover,therelationshipofthreevorticitycomponentswithspecificsignsissummarizedintotwosignlaws,asintrinsicphysicalrelationshipsbetweentwoinstabilitymodes.Bythetheoryofvortex-inducedvortex,suchtwosignlawsconfirmthattherearetwoandonlytwokindsofvortex-sheddingpatternsinthenearwake,justcorrespondingtomodesAandB.Inbrief,alongthefreestreamdirection,modeAcanbedescribedbytheparallelsheddingverticalvorticeswiththesamesign,whilemodeBisdescribedbytheparallelsheddingstreamwisevorticeswiththesamesign.Finally,itisfoundoutthatthe-typevortexisabasickindofvortexstructureinbothmodesAandB.
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简介:TheresultsofplaneturbulentwakegivenbyChouPeiyuanareconsideredasthefirstorderap-proximationandputintotheequationsofturbulentfluctuation.Theequationsaresolvednumericallywithintherangeofmicro-scalebymeansofspectrummethod.Thedouble,tripleandquadruplefluctuatingvelocitycorrelationsareobtainedbycomputation.Theyareingoodagreementwithexperimentalresults.
简介:Thomsonscatteringimaging(TSI)isproposedandexperimentallydemonstratedtoobservethefinestructureofthelaserwakefield.ByThomsonscatteringaco-propagatinglaserpulse,weobtainc/earimagesindicatingthatthewakefieldislikeanacalephswimmingbehindthepumplaser.Thewavelengthofthewakefieldobservedatdifferentelectrondensitiesagreeswellwiththetheory.Sincenomathematicstransformationisinvolved,TSIcouldbepotentiallyusedasanonlinemonitorforfuture'tabletop'plasmaaccelerators.
简介:Thispaperinvestigatestheeffectofupstreamwakeonunsteadyseparatedflowfieldofdownstreamadjacentvanesbysolving2DunsteadyReynolds-averagedNavier-Stokesequationsdiscretizedbyahigh-orderscheme.Theresultsindicatedthatthemaximumrelativereductionoflosscoefficientwas27.2%whentherelativepassingfrequency(theratioofwakepassingfrequencytothecharacteristicfrequencyoftrailing-vortexsheddingofdownstreamadjacentrotor)islessthan2.5.Theamplitudeofwakedefectexistsathresholdvalueandtheaerodynamicperformanceisenhancedmonotonicallywiththeamplitudeofwakedefectbasically.Theeffectiverangeofincidencemustbegreaterthan8°,whichisnearthestallboundary.
简介:Cerenkovwakeamplificationcanbeusedasanacceleratingscheme,inwhichatriggerbunchofelectronspropagatinginsideacylindricalwaveguidefilledwithanactivemediumgeneratesaninitialwakefield.Duetothemultiplereflectionsinsidethewaveguide,thewakemaybeamplifiedsignificantlymorestronglythanwhenpropagatinginaboundlessmedium.Sufficientlyfarawayfromthetriggerbunchthewake,whichtravelswiththesamephasevelocityasthebunch,reachessaturationanditcanaccelerateasecondbunchofelectronstrailingbehind.ForaCO2gasmixtureournumericalandanalyticalcalculationsindicatethatashortsaturationlengthandahighgradientcanbeachievedwithalargewaveguideradiusfilledwithahighdensityofexcitedatomsandatriggerbunchthattravelsatavelocityslightlyabovetheCerenkovvelocity.Toobtainastablelevelofsaturatedwakethatwillbesuitableforparticleacceleration,itiscrucialtosatisfythesingle-moderesonancecondition,whichrequireshighaccuracyinthewaveguideradiusandtheratiobetweentheelectronphasevelocityandtheCerenkovvelocity.Forsingle-modepropagationourmodelindicatesthatitisfeasibletoobtaingradientsashighasGVm-1inawaveguidelengthofcm.
简介:3-DevolutionofKarmanvortexfilamentsandvortexfilamentsinbraidregionsintheturbulentwakeofa2-Dcirculaxcylinderisinvestigatednumeri-callybasedoninviscidvortexdynamicsbyanalyzingtheresponseoftheinitially2-Dspanwisevortexfilamentstoperiodicspanwisedisturbanceofvaryingmagnitude,wavelengthandinitialphaseangles.Ourresultsrevealakindof3-Dvortexsysteminthewakewhichconsistsoflargescalehorseshoe-shapedvorticesandsmallscaleλ-shapedvortexfilamentsaswellasvortexloops.Themechanismandthedynamicprocessaboutthegenerationofstreamwisevorticalstructureandthe3-Dcoherentstructurearereported.