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-   -   Spalart-Allmaras boundary condition (https://www.cfd-online.com/Forums/openfoam-solving/74746-spalart-allmaras-boundary-condition.html)

 vaina74 April 7, 2010 17:04

Spalart-Allmaras boundary condition

I have a few questions about Spalart-Allmaras model, I applied it to an airfoil (blade section of a marine propeller).

1. I guess a turbulent viscosity ratio equal to 1, so nut = nu. I don't find a shared opinion about nuTilda: nuTilda = 3 * nu? nuTilda = 5 * nu? Maybe it's not so important.

2. Wich is the best range for y+?

3. My reference case is 2DAirfoil tutorial (incompressible/simpleFoam). I don't know the meaning of freestreamPressure or freestream BC types, I've used fieldValue or zeroGradient so far. Could you explain this to me? For istance, I used (k-w SST model):

U = uniform (7.30 0 0);
k = uniform 0.24;
omega = uniform 1.78

outlet: p = uniform 0;

U = uniform (0 0 0);
k = uniform 0.24; type: kqRWallFunction;
omega = uniform 1.78; type: kqRWallFunction;

side: p,U,k,omega = symmetryPlane;
frontAndBack: p,U,k,omega = empty;

Thanks for your help (and patience).

 vaina74 April 16, 2010 08:17

I really don't understand. For external flows, FLUENT suggests =1-10, ESI a range of 0.1-0.2, other sources about Spalart-Allamaras model suggest a 0.1 value... help me, please!

 malaboss April 3, 2013 11:53

Hi !
I really don't understand how to set the BC for the Spalart model either.
What I read in the litterature explains that it is suitable to choose something between 3*nu and 5*nu.

On the wiki, you can also find a formula which depends on the turbulent length scale and the turbulence intensity. When I apply it for a cylinder in a wind tunnel of turbulence intensity of 1%, a turbulence length scale of 0.11m (estimated with the dimensions of the wind tunnel used by Roschko), and for a velocity of 15 m/s, I have nutilda/nu >> 5

Conclusion 1: my flow, is not fully turbulent...
But lets make it more turbulent : we will have to increase velocity. But if we increase it, nutilda is going to increase, and never come closer to 5*nu

Conclusion 2 : there is something I don't understand on the way you choose the Spalart BC.

If anyone has an idea, I would be very delighted to hear it !