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Wind Velocity Profile in ANSYS CFX

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Old   December 4, 2009, 01:18
Default Wind Velocity Profile in ANSYS CFX
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Hi,

I'm trying to use CFX to model wind behaviors around tall buildings and have a few problems.

1. How can I insert the equation to the inlet to create a velocity profile?

2. Do I have to modify the inlet surface mesh to get accurate ABL or the automatic mesh with the prismatic elements near the ground and the equation for the velocity profile is enough?

Thank You Very Much in advance!
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Old   December 4, 2009, 03:36
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1-I assume you are using logaritmic law to define your ABL which is
U(z) = (U*/k) ln(z/z0)

U* (Ufrict) and Z0 should be determined by a curve fit to wind data recorded at the mast located around the boundary.

you also need to define k and epsilon values in your turbolence model, use the following expressions to do so:

Cmu = 0.09
ufrict = xxx [m s^-1]
hz = z-SurfaceElevation
kappa = 0.41
z0 = xxx [m]
Uz = (ufrict/kappa)*loge(max(1[m],hz)/z0)

tke = Cmu^(-0.5)*ufrict^2
eps = ufrict^3/(kappa*max(0.00001[m],hz))


Uz is the velocity variable you need to put into your inlet boundary

2- the inlet surface elevation is important in Uz expression, hence you either need to make the boundary flat (some people prefer this method by extending the booundary out) our create a line profile and define it as a function to return the surfcae elevation at each given xyz.
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Old   December 4, 2009, 06:31
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Thanks a lot AliTr

However I'm trying to use another function

u(z) = (Uref*ln(z/z0))/(ln(zref/z0))

my problem with this is that once the z=0, the expression becomes meaningless. How can I overcome this problem?
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Old   December 5, 2009, 05:56
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Your function is not meaningful for z=0 as it returns negative infinity. My guess is who ever developed this relation only meant it to be applied to some minimum value of z.
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Old   December 6, 2009, 18:47
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use max(1[m],z) instead of z
This will keep z>0 for all values.
The assumptions here are:
z0<1 and your mesh does not have elements thinner than 1m close to ground.

Last edited by AliTr; December 7, 2009 at 18:59.
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Old   December 7, 2009, 05:30
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Are you sure about this Alitr? This means the velocity at 1m the velocity will be a large negative number - this does not sound right to me! That is why I suggest that this equation has a much larger minimum value than 1m.
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Old   December 7, 2009, 18:58
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Quote:
Originally Posted by ghorrocks View Post
Are you sure about this Alitr? This means the velocity at 1m the velocity will be a large negative number - this does not sound right to me! That is why I suggest that this equation has a much larger minimum value than 1m.
Glen, Thank you for pointing to that. However, z0 is also described as the surface roughness and it is rarely more than 1, then I assumed that treatment should work as far as the first mesh elements are thicker than z0.
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Old   December 10, 2009, 23:50
Default U* vs Uref and Zref
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Thanks Everyone!
However I have another 2 problems.

1. Do we need to analyse this problem as steady state or transient? Since we are taking the turbulence into consideration, is it correct to assume that a steady state analysis will provide the dynamic response for the pressure on the building.

2. We have no data to calculate the U* value. Therefore I have omitted it and have rearranged the equation to use a Uref and Zref. Is it correct to assume a Uref and Zref using the Australian Wind code to get the wind profile?

Thanks in advance.
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