incompressible k-w SST turbulent model

vaina74 · April 22, 2010, 11:29

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

or

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.

openfoam_user · August 2, 2010, 08:45

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.

vaina74 · August 2, 2010, 09:28

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).

April 22, 2010, 11:29	incompressible k-w SST turbulent model	#1
vaina74 Senior Member Join Date: Feb 2010 Posts: 213 Rep Power: 17	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 or 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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August 2, 2010, 08:45		#2
openfoam_user Senior Member stephane sanchi Join Date: Mar 2009 Posts: 314 Rep Power: 18	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)(UI)^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 (= 0.07L) 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.

August 2, 2010, 09:28		#3
vaina74 Senior Member Join Date: Feb 2010 Posts: 213 Rep Power: 17	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).