Spalart-Allmaras boundary condition

vaina74 · April 7, 2010, 17:04

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).

vaina74 · April 16, 2010, 08:17

I really don't understand. For external flows, FLUENT suggests $\frac{\mu_t}{\mu}$ =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 !
thanks in advance !

April 7, 2010, 17:04	Spalart-Allmaras boundary condition	#1
vaina74 Senior Member Join Date: Feb 2010 Posts: 213 Rep Power: 17	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).

April 16, 2010, 08:17		#2
vaina74 Senior Member Join Date: Feb 2010 Posts: 213 Rep Power: 17	I really don't understand. For external flows, FLUENT suggests $\frac{\mu_t}{\mu}$ =1-10, ESI a range of 0.1-0.2, other sources about Spalart-Allamaras model suggest a 0.1 value... help me, please! Last edited by vaina74; April 16, 2010 at 15:52.

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April 3, 2013, 11:53		#3
malaboss Member Malik Join Date: Dec 2012 Location: Austin, USA Posts: 53 Rep Power: 13	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 3nu and 5nu. 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 !