乱流基础工学1 5页

Areviewofparticle-turbulenceinteractionandcloudsinengineeringAbstractThisreviewsummarizesrecentprogressontheparticle-turbulenceinteractionanddiscussesthepossibleimplicationsforcloudsinengineering.Theparticle-turbulenceinteractionandcloudsinengineeringhavebeenstudied.Theresultsofthisanalysisindicatethattheeffectofpreferentialconcentrationduringdiffusionalgrowthcannotexplainadequatelytheobserveddropletspectralbroadeninginadiabaticcloudcores-ThereasonisattributabletotherelativelysmallStokesnumberoftheclouddropletsunderconsiderationandtherelativelyshortdurationassociatedwithstrongvariationsintheconcentrationofdroplets-Fluidturbulenceisubiquitousandsoisitsabilitytotransportparticulatemattersuchasdust,sootordroplets.Thedynamicsofparticlesinaturbulentflowisfundamentaltoeverydaylife・examplesofopenscientificandtechnologicalissuesincluderainformationinclouds,pollution\ndispersionintheatmosphere,andoptimizationandemissionreductionincombustion,planktonpopulationdynamics-andconstituteamajorscientificchallengewithimmediatepracticalimplicationsandapplications.Openscientificissuessuchasinertia,finiteparticlessizes,collisions,advectionincomplexflowgeometriesareexamplesoffundamentalkeyingredientswhichposechallengingtheoreticalproblemsandneedtobeunderstoodinordertohaveanimpactonapplications.Byjoiningforceswithintheexperimentalandnumericalcommunityofturbulencemajorbreakthroughscanbeachieved.However,whystudyClouds?Cloudsareextremelyeffectiveintheirinteractionwithvisibleandinfraredradiation.Cloudscanbecome"colloidallyunstable59andformraindrops.Cloudsplayacrucialroleintransportingwaterandenergyintheatmosphere.WhenviewedfromspaceEarthrevealsmuchmorecloudandOceanisthanland.Approximately70%oftheearthwascoveredbywater.Cloudsrepresentthehighestmassconcentrationofanyatmosphericaerosol.Cloudcharacteristicsareinherentlylinkedtotheformationandevolutionofclouddropletsizedistribution.Theparticle-turbulenceinteractionanddiscussesthepossibleimplicationsforcloudphysicshasbeenstudied.Interactionsbetweenturbulenceandcloudmicrophysicsoccuronverysmallscales.However,thespatialresolutionofmostaircraft-basedmeasurementstypicallyarelimitedtomscalesandabove.Cloudsareanintegralpartoftheclimatesystembecausetheycouplethedynamics,thetransferofradiation,andthedistributionofwaterintheatmosphere.CloudsareinherentlyturbulentandasaresultoftheirenormousReynoldsnumbersfluctuationsinenergydissipationandvelocityandscalargradientsarehighlyintermittent.Recentmeasurementsof\nenergydissipationrateatmeter-scales,forexample,arequalitativelyconsistentwithalognormal(K62)distribution(Siebertetal.2006b).Howturbulentfluctuationsinfluencetheevolutionofcloudparticlesizedistributionsishighlyrelevanttoourabilitytodescriberelevantcloudprocessesinnumericalandtheoreticalmodelsoftheatmosphere(bothregionalandglobal).Takingrainformationviadropletcoalescenceasanexample,turbulenceisthoughttoinfluencethedropletcollisionratebymodulatingrelativedropletvelocity,dropletcollisionandcoalescenceefficiencies,anddropletspatialdistributions.Aminimalistmodelofrainformationdemonstratesthatrainformationishighlysensitivetorarecollisionsearlyinthelifetimeofacloud,andthereforefluctuationsinthesequantitiesnecessarilywilldominatethetimetoinitiaterain(KostinskiandShaw2005).Theseturbulence-cloudinteractionshavebeenstudiedmostlywiththeoreticalandcomputationaltools,butsomerecentexperimentalresultsaresummarizedhere.Dropletspatialdistributionsarerelevanttocollisionrates,condensationgrowthrates,andtoradioactivetransfe匚Briefly,itisexpectedthatparticleswithStokesnumbers[St=(particleinertialtimescale)/(fluidtimescale)]oforder1willclusterinregionsoflowvorticesinaturbulentflow(Shaw2003).Thehypothesishasbeenvalidatedinnumericalmodelsandinarecentexperimentinahigh-Reynolds-numberwindtunnelflowseededwithparticles-Particleclusteringisobservedtoincreasewithdecreasingscaleinapower-lawform,asexpectedfromtheory,andatagivenscaletheclusteringstrengthincreaseswithdropletStokesnumbe匚Similarexperimentshavebeencarriedoutincloudsandinitialdataanalysisshowsanalogousclusteringsignaturesinregionsofintenseturbulence.RecentcloudmeasurementsincollaborationwiththeACTOSgroupfromtheInstituteforTroposphericResearch(Leipzig,Germany)showrichinteractionsbetweenthermodynamicandcloudmicrophysicalprocessesatfinescales(Siebertetal.2006a).Forexample,wehaveobservedsuddenappearanceof\nlargedropletsinmixedregionsofcloudswhentheconditionsformixingareinhomogeneous(e.g.，Andrejczuketat2004).Laboratoryandcloudmeasurementsrevealthecomplexityofcloud-turbulenceinteractionsandarepoisedtoconstrainandstimulatetheoreticalandcomputationalworkontheseproblems.Inrecentyears,newinstruments[FastForwardScatteringSpectrometerProbe(FFSSP)一Brenguieretal.(1998);holographicimagery(HODAR)—Kozikowskaetal.(1984)]andbetterstatisticalanalysismethods(Baker1992)incloudphysicshaveemerged.Thesedevelopmentshaveallowedthedirectobservationoranalysisofthespatialdistributionofclouddropletsonthecentimeterscaleorevenindividualclouddropletsinsmallcloudvolumes(HODAR).Inthisrespect,recentworksinmechanicalengineeringonthesubjectofparticlescarriedbyaturbulentflowareofinteresttocloudphysicists.ReferencesA・B.Kostinski,andR.A.Shaw(2005):Fluctuationsandluckindropletgrowthbycoalescence.Bull.Ame匚Meteo匸Soc.，86,235-244.A・K.Kozikowska,Haman,andJ・Supronowicz,(1984):Preliminaryresultsofaninvestigationofthespatialdistributionoffogdropletsbyaholographicmethod.Quart.J.Roy.Meteor.Soc.,110,65-73.B.A.Baker,(1992):Turbulententrainmentandmixinginclouds:Anewobservationalapproach.J.Atmos.Sci.,49,387-404.H.Siebert,H.Franke,K.Lehmann,R.Maser,E.W.Saw,R.A.Shaw,D.Schell,andM.Wendisch(2006a):Probingfine-scaledynamicsandmicrophysicsofcloudswithhelicopter-bornemeasurements.Bull.Ame匚Meteo匚Soc.,inreview.H.Siebert,K.Lehmann,andM・Wendisch(2006b):Observationsofsmall-scaleturbulenceandenergydissipationratesinthecloudyboundarylaye匚J.Atmos.Sci.,63,14511466.\nJ.L.Brenguier,andW.Cooper,(1998):Unknownsourceofreflectivityfromsmallcumulusclouds.Preprints,Conf.onCloudPhysics,Everett,WA,Ame匚Meteo匚Soc.,148-151.J.K-Eaton,andJ.R.Fessler,(1994):Preferentialconcentrationofparticlesbyturbulence.Int.J.MultiphaseFlow,20,169-209.M・Andrejczuk,W.W.Grabowski,S.P.Malinowski,andP.K.Smolarkiewicz(2004):Numericalsimulationofcloud-clearairinterfacialmixing.J.Atmos.Sci・,61,1726-1739.R.A.Shaw(2003):Particle-turbulenceinteractionsinatmosphericclouds.Ann.Rev.FluidMeeh.,35,183-227.Z.S.She,E.Jackson,andS.A.Orszag,(1990):Intermittentvortexstructuresinhomogeneousisotropicturbulence.Natu©344,226-228.Z.S.She,E.Jackson,andS.A.Orszag,(1991):Structureanddynamicsofhomogeneousturbulence:Modelsandsimulations.Proc.Roy.Soc.London,434A，101-124.