简介:Hydrodynamicinstabilityandthemechanismsforturbulenceonsetandself-sustainingaretworesearchquestionsthathavepuzzledthescientistsforoveracentury,yetrenewedinterestandsignificantprogressoccurredinthelastdecadeorso.WeknowonlyfewexactanalyticalsolutionsoftheNavier-Stokesequations,theequationsgoverningtheflowofsimplefluidslikewaterandair,andmostoftheseareneverobservedinapplicationsorlaboratory
简介:Experimentalandnumericalstudieshaveshownsimilaritiesbetweenlocalizedturbulenceinchannelandpipeflows.Byscalinganalysisofadisturbed-flowmodel,thispaperproposesalocalReynoldsnumberReMtocharacterizethethresholdoftransitiontriggeredbyfinite-amplitudedisturbances.TheReMrepresentsthemaximumcontributionofthebasicflowtothemomentumratiobetweenthenonlinearconvectionandtheviscousdiffusion.ThelowercriticalReMobservedinexperimentsofplanePoiseuilleflow...
简介:TheevolutionofG?rtlervorticesanditsinteractionwithotherinstabilitiesareinvestigatedinthispaper.BoththeMackmodeandtheG?rtlermodeexistinhypersonicboundary-layerflowsoverconcavesurfaces,andtheirinteractionsarecruciallyimportantinboundarylayertransition.WecarryoutadirectnumericalsimulationtoexploretheinteractionbetweentheG?rtlerandtheobliqueMackmode.TheresultsindicatethattheinteractionbetweentheforcedG?rtlermodeandtheobliqueMackmodepromotestheonsetofthetransition.TheforcedobliqueMackmodeissusceptibletononlinearinteraction.BecauseofthedevelopmentoftheG?rtlermode,theforcedMackmodeandotherharmonicmodesareexcited.
简介:Inapreviouspaper,amethodhasbeendevelopedtostudythestabilitycharacteristicsoflaminarboundarylayersovercompliantwalls.Inthispaper,theeffectofdoublelayeredcompliantwallandKramertypecompliantwallondelayingthetransitionisinvestigated,anditisshownthattheredoesexistthepossibilitytode-laythetransitionbyapplyingsuchcompliantwalls.
简介:AnartificialdisturbanceisintroducedintotheboundarylayeroveraflatplatetoinvestigatetheeffectonthetransitionprocessintheMach6.5windtunnelatPekingUniversity.Alinearstabilitytheory(LST)isutilizedtopredicttheevolutionoftheeigenmodes,andthefrequencyoftheartificialdisturbanceischosenaccordingtotheLSTresults.Theartificialdisturbanceisgeneratedbyglowingdischargeonthesurfaceoftheplateclosetotheleadingedge.TheRayleigh-scatteringvisualizationandparticleimagevelocimetry(PIV)measurementsareperformed.Bycomparingtheexperimentalresultswithartificialdisturbanceswiththoseunderthenaturalcondition(withoutartificialdisturbances),thepresentpapershowsthatthesecond-modeinstabilitywavesaresignificantlystimulatedbytheartificialdisturbances,andtheboundarylayertransitioniseffectivelytriggered.
简介:Thestress-inducedmagneticdomainswitchinginFeGathinfilmsisstudiedusingphase-fieldmethod.Inparticular,themagneticfieldisappliedalongthe[110]directionandbiaxialstressesareappliedalong[100]and[010].Acompressivepre-stresscorrespondstoasmallercoercivemagneticfieldwhileatensilepre-stresscorrespondedtoalargercoercivefield.Atthesametime,itisalsofoundthatthetransitionbetweenbutterflyandsquare-likemagnetostrictionloopsoccursatthecriticaloppositebiaxialstressstate.Thetwodifferentevolutionscorrespondtotwodifferentmechanisms:oneisthatthesingledomainswingsacrossafanareabackandforth;theotheristhatthesingledomainturnsaclockwisecircle.Theresultscanbeexplainedbythestresstunedanisotropyenergywell.
简介:Inordertopredictthemechanicalperformanceofthepolyvinylchloride(PVC)atahighoperatingtemperature,aseriesofshort-termtensilecreeptests(onetenthofthephysicalagingtime)ofthePVCarecarriedoutat63Cwithasmallconstantstressbyadynamicmechanicalanalyzer(DMA).TheStruik-Kohlrausch(SK)formulaandStruikshiftingmethodsareusedtodescribethesecreepdataforvariousphysicalagingtime.AnewphenomenologicalmodelbasedonthemultiplerelaxationmechanismsofanamorphouspolymerisdevelopedtoquantitativelycharacterizetheSKparameters(theinitialcreepcompliance,thecharacteristicretardationtime,andtheshapefactor)determinedbytheagingtime.ItisshownthatthemomentarycreepcompliancecurveofthePVCat63CcanbeverywellfittedbytheSKformulaforeachagingtime.However,theSKparametersforthecreepcurvesarenotconstantduringtheagingprocessattheelevatedtemperatures,andtheevolutionoftheseparametersandthecreeprateversusagingtimecurvesatthedoublelogarithmiccoordinateshaveshownanonlinearphenomenon.Moreover,thecreepmastercurvesobtainedbythesuperpositionwiththeStruikshiftingmethodsareunsatisfactoryinsuchacase.Finally,thepredictedresultscalculatedfromthepresentmodelincorporatingwiththeSKformulaareinexcellentagreementwiththecreepexperimentaldataforthePVCisothermallyagedatthetemperaturerelativelyclosetotheglasstransitiontemperature.
简介:Anumericalsimulationisperformedtofindoutakeyvorticalstructureinthelaminar-turbulenttransition.Alow-speedstreakisgeneratedinsidealaminarboundarylayerusinganisolatedcuboidroughness,aimedatprovidinganenvironmentunstabletoouterdisturbances.Then,ashortdurationjetisissuedintotheboundarylayer.Whenthejetvelocityislow,somevorticesappearintheboundarylayer,butthetransitionoftheboundarylayerdoesnottakeplace.However,whenthejetvelocityexceedsacertainthreshold,twovorticesnewlyappearabovetheelongatedlegsofaV-shapedvortexandonlyoneofthemisstretchedandsurvives.Afterthat,vorticesaregeneratedoneafteranotheraroundthesurvivedone.Bycomparingthedecayedandthesurvivedvortices,itisfoundthatthedifferenceintheirheightsisthekeycharacteristicwhichleadstothetransition.
简介:Theelectromechanicalphasetransitionforadielectricelastomer(DE)tubehasbeendemonstratedinrecentexperiments,whereitisfoundthattheunbulgedphasegraduallychangedintobulgedphase.Previoustheoreticalworksonlystudiedthetransitionprocessunderpressurecontrolcondition,whichisnotconsistentwiththerealexperimentalcondition.Thispaperfocusesonmorecomplexfeaturesoftheelectromechanicalphasetransitionunderinternalpressureofconstantmass.WederivetheequilibriumequationsandtheconditionforcoexistentstatesforaDEtubeunderaninternalpressure,avoltagethroughthethicknessandanaxialforce.Wefindthatundermasscontrolconditionthevoltageneededtomaintainthephasetransitionincreasesastheprocessproceeds.Weanalyzetheentireprocessofelectromechanicalphasetransitionandfindthattheevolutionofconfigurationsisalsodifferentfromthatforpressurecontrolcondition.
简介:Westudythetransitiontoturbulenceofchannelflowoffinite-sizeparticlesuspensionsatlowvolumefraction,i.e.,Φ≈0.001.ThecriticalReynoldsnumberabovewhichturbulenceissustainedreducestoRe≈1675,inthepresenceoffewparticles,independentlyoftheinitialcondition,avaluelowerthanthatofthecorrespondingsingle-phaseflow,i.e.,Re≈1775.Inthedilutesuspension,theinitialarrangementoftheparticlesisimportanttotriggerthetransitionatafixedReynoldsnumberandparticlevolumefraction.Asinsinglephaseflows,streamwiseelongateddisturbancesareinitiallyinducedintheflow.Ifparticlescaninduceobliquedisturbanceswithhighenoughenergywithinacertaintime,thestreaksbreakdown,flowexperiencesthetransitiontoturbulenceandtheparticletrajectoriesbecomechaotic.Otherwise,thestreaksdecayintimeandtheparticlesimmigratetowardsthechannelcoreinalaminarflow.
简介:Thetransitionbetweenregularreflection(RR)andMachreflection(MR)oftypeVshock–shockinteractiononadouble-wedgegeometrywithhightemperaturenon-equilibriumeffectsisinvestigatedbyextendedshock-polarmethodandnumericalsimulation.First,thecriticalanglesoftransitionfromdetachmentcriterionandvonNeumanncriterionaredeterminedbytheextendedshock-polarmethodconsideringthenon-equilibriumeffects.Thenwavepatternsandthetransitionprocessarenumericallyobtained.Resultsofthecriticaltransitionanglesfromshock-polarcalculationandnumericalsimulationshowevidentdisagreement,indicatingtransitionmechanismbetweenRRandMRoftypeVinteractionischanged.Bycomparingwiththefrozencounterpart,itisalsofoundthatnon-equilibriumeffectsleadtoalargercriticalwedgeangleandalargerhysteresisinterval.
简介:Atheoreticalmodelthattakesintoaccountthefree-volumeaidedcooperativeshearingofsheartransformationzones(STZs)isdevelopedtoquantitativelyunderstandtheductile-to-brittletransition(DBT)ofmetallicglasses.TheSTZdilatationalstrainisdefinedastheratioofSTZ-activatedfreevolumetoSTZvolumeitself.ThemodeldemonstratesthattheSTZdilatationalstrainwillincreasedrasticallyandexceedthecharacteristicshearstrainofSTZastemperaturedecreasesbelowacriticalvalue.ThiscriticaltemperatureisingoodagreementwiththeexperimentallymeasuredDBTtemperature.OurresultssuggestthattheDBTofmetallicglassesisunderpinnedbythetransitionofatomic-clustermotionsfromSTZ-typerearrangementstodilatationalprocesses(termedtensiontransformationzones(TTZs)).
简介:Bacterialflagellarfilamentcanundergoastress-inducedpolymorphicphasetransitioninbothvitroandvivoenvironments.Thefilamenthas12differenthelicalforms(phases)characterizedbydifferentpitchlengthsandhelixradii.Whensubjectedtothefrictionalforceofflowingfluid,thefilamentchangesbetweenaleft-handednormalphaseandaright-handedsemi-coiledphaseviaphasenucleationandgrowth.Thispaperdevelopsnon-localfiniteelementmethod(FEM)tosimulatethephasetransitionunderad...
简介:Shape-inducedphasetransitionofvortexdomainstructures(VDSs)inBaTiO_3(BT)nanodotsunderopencircuitboundaryconditionhavebeeninvestigatedusinganeffectiveHamiltonianmethod.OurcalculationindicatesthetetragonalVDSmissingincubicBTnanodotscanbeinducedbyvaryingtheshapeofananodotfromcubetoplatelet.Interestingly,anovelVDSisfoundinBTnanoplateletsinoursimulations.Furtherinvestigationshowsthatitisaresultofcompromisebetweenthegroundstateandthesymmetryoftheshapeofthenanodot.Furthermore,basedonthenovelVDS,routesofcontrollingVDSsgovernedbyhomogeneouselectricfieldanduniformstressarediscussed.Inparticular,ourresultsshowthepossibilityofdesigningmulti-statesdevicesbasedonasingleVDS.