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incompressible k-w SST turbulent model

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Old   April 22, 2010, 11:29
Default incompressible k-w SST turbulent model
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Please, I have some questions about implementation of \kappa-\omega SST turbulent model and wall functions. I hope Mr Weller can answer to me.

1. In most papers or threads of this forum I read that, when a wall function is used, y+ must be greater than 30, if possible closed to 30, so wall-adjacent first cells centroid is located within the log-law layer. But someone, with \kappa-\omega SST, sets y+ above 11 (or 20). Is it correct? Why? I can't find theoretical support for that.

2. Must I set initial condition for nut, as in motorBike tutorial? Even if I do not, my case works and C_L or C_D values seem good. Anyway, after the code running I find a nut file in my time folders, so I think it's calculated.

3. I'm in trouble with inlet boundary conditions for \omega. In FLUENT manual and other papers I read
\omega=\frac{\kappa}{\nu_t}
but I find also
\omega=\frac{0.09\cdot\kappa}{\nu_t}
In other words, my question is:
\omega=\frac{\epsilon}{0.09\cdot\kappa}
or
\omega=\frac{\epsilon}{\kappa}?

4. I set

Code:
wall
{
     type     omegaWallFunction;
     value    uniform ***;
}
with *** as internalField value. Is it wrong? Is there a better way to evaluate initial wall value for \omega?

Thanks for your help.

Last edited by vaina74; April 26, 2010 at 06:06.
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Old   August 2, 2010, 08:45
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Hi,

I would like to use the k-w SST model for car aerodynamics. I am a little bit confused because I have found a lot of formulas.

k = (3/2)*(U*I)^2
omega = k^(1/2)/(Cmu^(1/4)*l)

where
U = freestream velocity (40 m/s)
I = turbulence intensity (5%)
Cmu = 0.09 (constant)
l = turbulence length scale (= 0.07*L)
L = car length (1.044 m for ahmed body)

Are you agree with these formulas and values of k and omega ?

k = 6
omega = 63.89

Regards,

Stephane.
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Old   August 2, 2010, 09:28
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My knowledge is based on these guidelines. I'm in doubt with your definition of turbulence length scale: I don't believe L should be the length car. Anyway, if I'm right, one of the advantages of \kappa-\omega SST turbulent model is the small influence of freestream values, becouse in that region the \kappa-\epsilon equations are applied. The solutions should be not so sensitive to the \omega inlet value. However, your values seem not so absurd.
I'm not an expert, you can go on to explore internet and cfd-online forum. Maybe in the meantime others will give you some hints (they all seem to be on holidays now).
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