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):
inlet: p = zeroGradient;
U = uniform (7.30 0 0);
k = uniform 0.24;
omega = uniform 1.78
outlet: p = uniform 0;
U = zeroGradient;
k = zeroGradient;
omega = zeroGradient;
wall: p = zeroGradient;
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).
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 !
thanks in advance !
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