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1、LARGE EDDY SIMULATIONChin-Hoh Moeng NCAROUTLINE WHAT IS LES? APPLICATIONS TO PBL FUTURE DIRECTIONWHAT IS LES?A NUMERICAL TOOL FORTURBULENT FLOWSTurbulent Flows governing equations, known nonlinear term dissipation term no analytical solution highly diffusive smallest eddies mm largest eddies - depen
2、d on Re- number (U; L; )Numerical methods of studying turbulence Reynolds-averaged modeling (RAN)model just ensemble statistics Direct numerical simulation (DNS)resolve for all eddies Large eddy simulation (LES)intermediate approachLESturbulent flowResolved large eddiesSubfilter scale, small(not so
3、important)(important eddies)FIRST NEED TO SEPARATE THE FLOW FIELD Select a filter function G Define the resolved-scale (large-eddy): Find the unresolved-scale (SGS or SFS):Examples of filter functionsTop-hatGaussianExample: An 1-D flow fieldfApply filter large eddiesReynolds averaged model (RAN)fApp
4、ly ensemble avg non-turbulentLES EQUATIONSSFSApply filter GDifferent Reynolds number turbulent flows Small Re flows: laboratory (tea cup) turbulence; largest eddies O(m); RAN or DNS Medium Re flows: engineering flows; largest eddies O(10 m); RAN or DNS or LES Large Re flows: geophysical turbulence;
5、largest eddies km; RAN or LESGeophysical turbulence PBL (pollution layer) boundary layer in the ocean turbulence inside forest deep convection convection in the Sun LES of PBLkmmmmresolved eddiesSFS eddiesdissipationenergy inputLinertial range, Major difference between engineer and geophysical flows
6、: near the wall Engineering flow: viscous layer Geophysical flow: inertial-subrange layer; need to use surface-layer theoryThe premise of LES Large eddies, most energy and fluxes, explicitly calculated Small eddies, little energy and fluxes, parameterized, SFS modelThe premise of LES Large eddies, m
7、ost energy and fluxes, explicitly calculated Small eddies, little energy and fluxes, parameterized, SFS modelLES solution is supposed to be insensitive to SFS modelCaution near walls, eddies small, unresolved very stable region, eddies intermittent cloud physics, chemical reactionmore uncertaintiesA
8、 typical setup of PBL-LES 100 x 100 x 100 points grid sizes tens of meters time step seconds higher-order schemes, not too diffusive spin-up time 30 min, no use simulation time hours massive parallel computersDifferent PBL Flow Regimes numerical setup large-scale forcing flow characteristicsClear-ai
9、r convective PBLConvective updrafts 2 kmHorizontal homogeneous CBLLocal TimeLIDAR ObservationOceanic boundary layerAdd vortex force for Langmuir flowsMcWilliam et al 1997Oceanic boundary layerAdd vortex force for Langmuir flowsMcWilliams et al 1997Canopy turbulenceAdd drag force-leaf area indexPatto
10、n et al 1997 100 mobservationLESComparison with observationShallow cumulus cloudsAdd phase change-condensation/evaporation 6 km3 km 12 hr COUPLED with SURFACE turbulence heterogeneous land turbulence ocean surface waveCoupled with heterogeneous soilSurface modelWet soilDry soilthe groundLES modelLan
11、d modelCoupled with heterogeneous soilwet soildry soil(Patton et al 2003)Coupled with wavy surfacestably stratifiedU-fieldflat surfacestationary wavemoving waveSo far, idealized PBLs Flat surface Periodic in x representations of ensemble mean ?How to describe a turbulent inflow?What do we do with LES solutions?Understand turbulence behavior JAS)- mixed layer scaling Lamb (1978; atmos env)- plume dispersion FUTURE GOALUnderstand PBL in complex environment and improve its parameterization for regional and climate models turbulent fluxes air quality cloud chemical transport/reaction
