CONSIDERATIONS ABOUT THE ROLE OF ATMOSPHERIC STABILITY IN CFD MODELS
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Transcript of CONSIDERATIONS ABOUT THE ROLE OF ATMOSPHERIC STABILITY IN CFD MODELS
© 2010 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary© 2010 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary
CONSIDERATIONS ABOUT THE ROLE OF ATMOSPHERIC STABILITY IN CFD MODELS
CONSIDERATIONS ABOUT THE ROLE OF ATMOSPHERIC STABILITY IN CFD MODELS
C. Montavon, ANSYS UKC. Montavon, ANSYS UK
© 2010 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary
Contents
Atmospheric stability
Why
How
When
Effects
• Stable standard atmosphere in free stream
• Surface heat fluxes
• Buoyancy terms in momentum and turbulence equations
• Mountain waves• Possible changes in flow
topology• Affects
• Mixing• Boundary layer height• Boundary layer
regeneration
• Low Froude number
• Always,• More or less so depending
on surface stability
© 2010 ANSYS, Inc. All rights reserved. 3 ANSYS, Inc. Proprietary
How: Stability implementation
• Continuity • Momentum
• Turbulence quantities (e.g. k-e)
• Energy (potential temperature)
i
j
j
ieff
jiik
ki x
U
x
U
xp
xUU
xU
t
Px
k
xkU
xk
t jk
T
jj
j
j
T
pjj
j xCxU
xt
k
CPk
Cxx
Uxt j
T
jj
j
2
21
3ihh
hg
G
0,max1 Gk
C
z
gG
hH
eff
gravity waves
mixing, boundary layer height
© 2010 ANSYS, Inc. All rights reserved. 4 ANSYS, Inc. Proprietary
Effects: Mountain waves
Potential temperaturePotential temperature
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Effect: change in flow topology
NeutralNeutral StableStable
Low Froude number• High terrain elevation• Strong stability• Low wind speed
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Effect: reduced boundary layer height, reduced mixing
• Equilibrium profiles on flat terrain, roughness z0, Coriolis parameter f
Lfu /*
- u'w'/u* 2
0
0.1
0.2
0.3
0 0.5 1 1.5
zf/u
*
Nicholls, 1985
dT/dz = -9.7 K/km
dT/dz = -9 K/km
dT/dz = -8 K/km
dT/dz = -6.5 K/km
v'w'/u* 2
0
0.1
0.2
0.3
0 0.2 0.4
zf/u
*
Nicholls, 1985
dT/dz = -9.7 K/km
dT/dz = -9 K/km
dT/dz = -8 K/km
dT/dz = -6.5 K/km
0.0
0.5
1.0
0 0.05 0.1Tf/u*
2
zf/u
*
model, -9.7 K /km
model, -9.8 K/km
Hinze
Duynkerke
0
0.5
1
0 5 10k/u*
2
zf/u
*
model, -9.7 K/km
model, -9.8 K/kmHinze
Duynkerke
Turbulence kinetic energyTurbulence kinetic energy Eddy diffusivityEddy diffusivity
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Effect: reduced boundary layer height and momentum fluxes
• Equilibrium profiles on flat terrain, roughness z0, Coriolis parameter f
• Reduction of fluxes expected slower boundary layer regeneration downstream of forests or large arrays
Lfu /*
- u'w'/u* 2
0
0.1
0.2
0.3
0 0.5 1 1.5
zf/u
*
Nicholls, 1985
dT/dz = -9.7 K/km
dT/dz = -9 K/km
dT/dz = -8 K/km
dT/dz = -6.5 K/km
v'w'/u* 2
0
0.1
0.2
0.3
0 0.2 0.4
zf/u
*
Nicholls, 1985
dT/dz = -9.7 K/km
dT/dz = -9 K/km
dT/dz = -8 K/km
dT/dz = -6.5 K/km
Vertical momentum fluxesVertical momentum fluxes
Increasing free stream stability
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1S10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
h/(u*/f)
ln
0.7-0.8
0.6-0.7
0.5-0.6
0.4-0.5
0.3-0.4
0.2-0.3
0.1-0.2
0-0.1
Effect: Boundary layer height
Increasing free stream stability Increasing surface stability
Lfu /*fN /
Source: C. Montavon, 1998, Simulation of atmospheric flows over complex terrain for wind power potential assessment, Ph D thesis, EPF Lausanne, http://library.epfl.ch/en/theses/?nr=1855
25
75
100
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Example of An Suidhe
Normalised wind speedNormalised wind speed Normalised TINormalised TI
50m50m
30m30m
10m10m
RMS error [%]Wind speed
ratio TI ratioaverage over anemometers 8.9 12.8average highest meas. Heights 6.6 7.6average lowest meas. Heights 12.0 18.4
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Stability in ANSYS CFD
• Free stream stability tested as part of WindModeller developments
• Preliminary results– Validation successful of mountain waves from 2D theoretical test
case (witch of Agnesi mountain profile)– Application on real terrain (An Suidhe, Scotland), see poster
session for details inclusion of stability helped improve modelling accuracy,
particularly on the turbulence intensity predictions also observed strong sensitivity of results to domain vertical
extent when including stability. More work required to find best configuration for top boundary condition.