【2017年整理】LARGE EDDY SIMULATION.ppt

LARGE EDDY SIMULATION Chin-Hoh Moeng NCAR OUTLINE WHAT IS LES? APPLICATIONS TO PBL FUTURE DIRECTION WHAT IS LES? A NUMERICAL TOOL FOR TURBULENT FLOWS Turbulent Flows governing equations, known nonlinear term dissipation term no analytical solution highly diffusive smallest eddies ~ mm largest eddies --- depend 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 approach LES 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 functions LES EQUATIONS Different 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; largest eddies km; RAN or LES Geophysical turbulence PBL (pollution layer) boundary layer in the ocean turbulence inside forest deep convection convection in the Sun ….. Major difference between engineer and geophysical flows: near the wall Engineering flow: viscous layer Geophysical flow: inertial-subrange layer; need to use surface-layer theory The premise of LES Large eddies, most energy and fluxes, explicitly calculated Small eddies, little energy and fluxes, parameterized, SFS model The premise of LES Large eddies, most energy and fluxes, explicitly calculated Small eddies, little energy and fluxes, parameterized, SFS model LES solution is supposed to be insensitive to SFS model Caution near walls, eddies small, unresolved very stable region, eddies intermittent cloud physics, chemical reaction… more uncertainties A typical setup of PBL-LES 100 x 100 x 100 points grid sizes tens of meters tim