LARGE-EDDY-SIMULATION

LARGEEDDYSIMULATIONChin-HohMoengNCAROUTLINE•WHATISLES?•APPLICATIONSTOPBL•FUTUREDIRECTIONWHATISLES?ANUMERICALTOOLFORTURBULENTFLOWSTurbulentFlows•governingequations,known•nonlineartermdissipationterm•noanalyticalsolution•highlydiffusive•smallesteddies~mm•largesteddies---dependonRe-number(U;L;)Numericalmethodsofstudyingturbulence•Reynolds-averagedmodeling(RAN)modeljustensemblestatistics•Directnumericalsimulation(DNS)resolveforalleddies•Largeeddysimulation(LES)intermediateapproachLESturbulentflowResolvedlargeeddiesSubfilterscale,small(notsoimportant)(importanteddies)FIRSTNEEDTOSEPARATETHEFLOWFIELD•SelectafilterfunctionG•Definetheresolved-scale(large-eddy):•Findtheunresolved-scale(SGSorSFS):xdxxGxfxf),()()(~)(~)()(xfxfxfExamplesoffilterfunctionsTop-hatGaussianExample:An1-Dflowfield)()(~)(xfxfxfflargeeddiesReynoldsaveragedmodel(RAN))(')()(xfxfxffnon-turbulentLESEQUATIONS2201jiiijijixuxpTgxuutudxdydzGuuii~220~)~~(~1~~~~jijjijiiijijixuxuuuuxpTgxuutu~SFSDifferentReynoldsnumberturbulentflows•SmallReflows:laboratory(teacup)turbulence;largesteddies~O(m);RANorDNS•MediumReflows:engineeringflows;largesteddies~O(10m);RANorDNSorLES•LargeReflows:geophysicalturbulence;largesteddieskm;RANorLESGeophysicalturbulence•PBL(pollutionlayer)•boundarylayerintheocean•turbulenceinsideforest•deepconvection•convectionintheSun•…..LESofPBLkmmmmresolvededdiesSFSeddiesdissipationenergyinputfLinertialrange,3/5Majordifferencebetweenengineerandgeophysicalflows:nearthewall•Engineeringflow:viscouslayer•Geophysicalflow:inertial-subrangelayer;needtousesurface-layertheoryThepremiseofLES•Largeeddies,mostenergyandfluxes,explicitlycalculated•Smalleddies,littleenergyandfluxes,parameterized,SFSmodelThepremiseofLES•Largeeddies,mostenergyandfluxes,explicitlycalculated•Smalleddies,littleenergyandfluxes,parameterized,SFSmodelLESsolutionissupposedtobeinsensitivetoSFSmodelCaution•nearwalls,eddiessmall,unresolved•verystableregion,eddiesintermittent•cloudphysics,chemicalreaction…moreuncertaintiesAtypicalsetupofPBL-LES•100x100x100points•gridsizestensofmeters•timestepseconds•higher-orderschemes,nottoodiffusive•spin-uptime~30min,nouse•simulationtime~hours•massiveparallelcomputersDifferentPBLFlowRegimes•numericalsetup•large-scaleforcing•flowcharacteristicsClear-airconvectivePBLgUzkm5~QConvectiveupdraftsHorizontalhomogeneousCBLLocalTimeLIDARObservationOceanicboundarylayerzm300~AddvortexforceforLangmuirflowsMcWilliametal1997Oceanicboundarylayerzm300~AddvortexforceforLangmuirflowsMcWilliamsetal1997Canopyturbulence0Um200~zAdddragforce---leafareaindexPattonetal1997100mobservationLESComparisonwithobservationShallowcumuluscloudsgUzQlayercloudAddphasechange---condensation/evaporation~6km~12hrCOUPLEDwithSURFACE•turbulenceheterogeneousland•turbulenceoceansurfacewaveCoupledwithheterogeneoussoilSurfacemodelzWetsoilDrysoilkm30thegroundLESmodelLandmodelCoupledwithheterogeneoussoilwetsoildrysoil(Pattonetal2003)CoupledwithwavysurfacestablystratifiedU-fieldflatsurfacestationarywavemovingwaveSofar,idealizedPBLs•Flatsurface•Periodicinx&y•ShallowcloudsFutureDirectionofLESforPBLResearch•Realisticsurface–complexterrain,landuse,waves•PBLundersevereweather500km50kmLESdomainmesoscalemodeldomainComputationalchallengeMassiveparallelmachinesResolveturbulentmotioninTaipeibasin~1000x1000x100gridpointsTechnicalissues•Inflowboundarycondition•SFSeffectnearirregularsurfaces•Properscaling;representationsofensemblemean???Howtodescribeaturbulentinflow?WhatdowedowithLESsolutions?Understandturbulencebehavior&diffusionpropertyDevelop/calibratePBLmodelsi.e.ReynoldsaveragemodelsCLASSICEXAMPLES•Deardorff(1972;JAS)-mixedlayerscaling•Lamb(1978;atmosenv)-plumedispersionFUTUREGOALUnderstandPBLincomplexenvironmentandimproveitsparameterizationforregionalandclimatemodels–turbulentfluxes–airquality–cloud–chemicaltransport/reaction