LARGE EDDY SIMULATION

# LARGE EDDY SIMULATION

## LARGE EDDY SIMULATION

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1. LARGE EDDY SIMULATION Chin-Hoh Moeng NCAR

2. OUTLINE • WHAT IS LES? • APPLICATIONS TO PBL • FUTURE DIRECTION

3. WHAT IS LES? A NUMERICAL TOOL FOR TURBULENT FLOWS

4. 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; )

5. 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

6. LES Resolved large eddies turbulent flow (important eddies) Subfilter scale, small (not so important)

7. 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):

8. Examples of filter functions Top-hat Gaussian

9. Example: An 1-D flow field f Apply filter  large eddies

10. Reynolds averaged model (RAN) f Apply ensemble avg  non-turbulent

11. LESEQUATIONS Apply filter G SFS

12. Different Reynolds number turbulent flows • Small Re flows: laboratory (tea cup) turbulence; largest eddies ~ O(m); RAN orDNS • Medium Re flows: engineering flows; largest eddies ~ O(10 m); RAN orDNS or LES • Large Re flows: geophysical turbulence; largest eddies > km; RAN orLES

13. Geophysical turbulence • PBL (pollution layer) • boundary layer in the ocean • turbulence inside forest • deep convection • convection in the Sun • …..

14. LES of PBL km m mm resolved eddies SFS eddies L inertial range, energy input dissipation

15. 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

16. The premise of LES • Large eddies, most energy and fluxes, explicitly calculated • Small eddies, little energy and fluxes, parameterized, SFS model

17. 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

18. Caution • near walls, eddies small, unresolved • very stable region, eddies intermittent • cloud physics, chemical reaction… more uncertainties

19. A 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 computers

20. Different PBL Flow Regimes • numerical setup • large-scale forcing • flow characteristics

21. Clear-air convective PBL Convective updrafts ~ 2 km

22. Horizontal homogeneous CBL

23. LIDAR Observation Local Time

24. Oceanic boundary layer Add vortex force for Langmuir flows McWilliam et al 1997

25. Oceanic boundary layer Add vortex force for Langmuir flows McWilliams et al 1997

26. Canopy turbulence < 100 m Add drag force---leaf area index Patton et al 1997

27. Comparison with observation observation LES

28. Shallow cumulus clouds ~ 12 hr ~3 km ~ 6 km Add phase change---condensation/evaporation

29. COUPLED with SURFACE • turbulence heterogeneous land • turbulence ocean surface wave

30. Surface model Coupled with heterogeneous soil Wet soil LES model Dry soil the ground Land model

31. Coupled with heterogeneous soil wet soil dry soil (Patton et al 2003)

32. Coupled with wavy surface stably stratified

33. U-field flat surface stationary wave moving wave

34. So far, idealized PBLs • Flat surface • Periodic in x & y • Shallow clouds

35. Future Direction of LESfor PBL Research • Realistic surface • complex terrain, land use, waves • PBL under severe weather

36. mesoscale model domain 500 km 50 km LES domain

37. Computational challenge Resolve turbulent motion in Taipei basin ~ 1000 x 1000 x 100 grid points Massive parallel machines

38. Technical issues • Inflow boundary condition • SFS effect near irregular surfaces • Proper scaling; representations of ensemble mean

39. ? How to describe a turbulent inflow?

40. What do we do with LES solutions? Understand turbulence behavior & diffusion property Develop/calibrate PBL models i.e. Reynolds average models

41. CLASSIC EXAMPLES • Deardorff (1972; JAS) - mixed layer scaling • Lamb (1978; atmos env) - plume dispersion

42. FUTURE GOAL Understand PBL in complex environment and improve its parameterization for regional and climate models • turbulent fluxes • air quality • cloud • chemical transport/reaction