乱流基础工学2

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1、乱流基礎工学"求一卜Areviewofparticle-turbulenceinteractionandcloudsinengineering2.Parameterspertinenttoparticle-turbulenceinteractionTheparticle-flowinteractionisatwo-waycoupling;thatis,theturbulentflowmayinfluencethespatialdistributionoftheparticleswhiletheparticlesmayalsomodifythepropertiesof

2、theflow.Inthisreview,wewillmostlystudythefirstissueandbrieflydiscusswhythesecondissueislesspertinenttocloudphysics.Particlesinteractwiththeflowthroughthedragforce(mechanicalcoupling)andthroughthermodynamiceflects.Thedragforceisnonzerowhenevertherelativevelocitybetweentheparticleandthef

3、lowisnonzero.Thisarisesbecauseparticlesfallundertheinfluenceofgravityandbecausetheypossessinertiasothattheydonotrespondinstantaneouslytoaccelerationsoftheflow.Thermodynamiceffectsmaybeimportantwhenphasechangesoccur(e.g・，growingclouddroplets)orforchemicallyreactingflows(e.g.,combustion)

4、.Wewillnotdiscusschemicallyreactingflowshereandjustbrieflycommentonthepossibilitythatparticleslikeclouddropletsmayinfluencetheturbulentflowthroughlatentheatexchanges.Thefbllowingfourdimensionlessparametersareusefultopredictthenatureordegreeoftheparticle-flowmechanicalcouplingofindividu

5、alparticlesortheoveralleflectofagroupofparticlesontheflow:A.theratiod/1,wheredistheparticlediameterandlisacharacteristiclengthscaleoftheflow[eithertheKolmogorovlengthscale(q)ortheintegralscale(L)];B・theparticle'sReynoldsnumber,ReP=UR/v,whereUistherelativevelocitybetweentheflowandthepar

6、ticle,Ristheradiusoftheparticle,andvisthekinematicviscosityofthefluid;C.theStokesnumber,whichistheratiobetweentheparticle^sresponsetime(tP)andacharacteristictimescaleoftheflow(iF),St=tP/tF;andD.themassloading(Mp/Mf),thatis,theratioofthetotalmassofparticles(Mp)andthemassofthecarrierflui

7、d(Mf).Particles,assumedspherical,areessentiallydescribedbytheirmass(mp)andradius(R).Withthesetwoparameters,andthedynamicviscosityofthecarrierfluid,p,wecancalculatetheparticle'sresponsetimetP(alsocalledtherelaxationorStokestimescale).ThetimescaletPisthecharacteristictimetheparticletak