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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

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large eddy simulation

LARGE EDDY SIMULATION

Chin-Hoh Moeng

NCAR

outline
OUTLINE
  • WHAT IS LES?
  • APPLICATIONS TO PBL
  • FUTURE DIRECTION
what is les
WHAT IS LES?

A NUMERICAL TOOL

FOR

TURBULENT FLOWS

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
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

slide6
LES

Resolved large eddies

turbulent flow

(important eddies)

Subfilter scale, small

(not so important)

first need to separate the flow field
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):
slide9

Example: An 1-D flow field

f

Apply filter 

large eddies

slide10

Reynolds averaged model (RAN)

f

Apply ensemble avg 

non-turbulent

les equations
LESEQUATIONS

Apply filter G

SFS

different reynolds number turbulent flows
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

geophysical turbulence
Geophysical turbulence
  • PBL (pollution layer)
  • boundary layer in the ocean
  • turbulence inside forest
  • deep convection
  • convection in the Sun
  • …..
slide14

LES of PBL

km

m

mm

resolved eddies

SFS eddies

L

inertial range,

energy input

dissipation

major difference between engineer and geophysical flows near the wall
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
The premise of LES
  • Large eddies, most energy and fluxes, explicitly calculated
  • Small eddies, little energy and fluxes, parameterized, SFS model
the premise of les17
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
Caution
  • near walls, eddies small, unresolved
  • very stable region, eddies intermittent
  • cloud physics, chemical reaction…

more uncertainties

a typical setup of pbl les
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
different pbl flow regimes
Different PBL Flow Regimes
  • numerical setup
  • large-scale forcing
  • flow characteristics
clear air convective pbl
Clear-air convective PBL

Convective updrafts

~ 2 km

oceanic boundary layer
Oceanic boundary layer

Add vortex force for Langmuir flows

McWilliam et al 1997

oceanic boundary layer25
Oceanic boundary layer

Add vortex force for Langmuir flows

McWilliams et al 1997

canopy turbulence
Canopy turbulence

< 100 m

Add drag force---leaf area index

Patton et al 1997

shallow cumulus clouds
Shallow cumulus clouds

~ 12 hr

~3 km

~ 6 km

Add phase change---condensation/evaporation

coupled with surface
COUPLED with SURFACE
  • turbulence heterogeneous land
  • turbulence ocean surface wave
coupled with heterogeneous soil

Surface model

Coupled with heterogeneous soil

Wet soil

LES model

Dry soil

the ground

Land model

coupled with heterogeneous soil31
Coupled with heterogeneous soil

wet soil

dry soil

(Patton et al 2003)

slide32

Coupled with wavy surface

stably stratified

slide33

U-field

flat surface

stationary wave

moving wave

so far idealized pbls
So far, idealized PBLs
  • Flat surface
  • Periodic in x & y
  • Shallow clouds
future direction of les for pbl research
Future Direction of LESfor PBL Research
  • Realistic surface
    • complex terrain, land use, waves
  • PBL under severe weather
slide37

mesoscale model domain

500 km

50 km

LES domain

computational challenge
Computational challenge

Resolve turbulent motion in Taipei basin

~ 1000 x 1000 x 100 grid points

Massive parallel machines

technical issues
Technical issues
  • Inflow boundary condition
  • SFS effect near irregular surfaces
  • Proper scaling; representations of ensemble mean
slide40

?

How to describe a turbulent inflow?

what do we do with les solutions
What do we do with LES solutions?

Understand turbulence behavior & diffusion property

Develop/calibrate PBL models

i.e. Reynolds average models

classic examples
CLASSIC EXAMPLES
  • Deardorff (1972; JAS)

- mixed layer scaling

  • Lamb (1978; atmos env)

- plume dispersion

future goal
FUTURE GOAL

Understand PBL in complex environment

and improve its parameterization

for regional and climate models

  • turbulent fluxes
  • air quality
  • cloud
  • chemical transport/reaction