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nutWallFunction questions

I'm investigating the wall function approaches used in OpenFOAM. Epsilon and omega wall functions were fairly straight-forward in the end. However, I am looking at nutWallFunction and I'm at a loss as to why nut at the wall is defined as so:

nutw[faceI] = nuw[faceI]*(yPlus*kappa_/log(E_*yPlus) - 1.0); //for y+>11

Why not use the values the computed values of k, epsilon, or omega?

 olesen November 29, 2009 04:51

Quote:
 Originally Posted by madad2005 (Post 237936) nutw[faceI] = nuw[faceI]*(yPlus*kappa_/log(E_*yPlus) - 1.0); //for y+>11 Why not use the values the computed values of k, epsilon, or omega?
I'm confused - do you mean not using any wall functions at all?

Sorry to have confused you; I'll elaborate more. In the k-epsilon and k-omega models, the very last thing they do is compute nut:

//from RealizableKE
// Re-calculate viscosity
nut_ = rCmu(gradU, S2, magS)*sqr(k_)/epsilon_; // (1) nut = Cmu*k/omega
nut_.correctBoundaryConditions();

nut is computed as I'd expected here. Why not use the same equation in nutWallFunction to compute the value at the boundary, as you have k, omega, and epsilon already computed at the boundary face? Is it because you require a general expression for application to all turbulence models?

The reason I've asked is because I'm not particularly conversant in wall functions and I wasn't expecting a different equation for nutWallFunction.

 chb November 30, 2009 05:09

• At the walls
nutw[faceI] = nuw[faceI]*(yPlus*kappa_/log(E_*yPlus) - 1.0);
is used to apply the wall shear stress
tau_wall = rho * (nu + nut) * du/dy
according to the log law.
• For the rest of the flow field
nut_ = Cmu*sqr(k_)/epsilon_;
is used to model turbulence effects.

Thanks, Christof.

"At the walls
nutw[faceI] = nuw[faceI]*(yPlus*kappa_/log(E_*yPlus) - 1.0);
is used to apply the wall shear stress
tau_wall = rho * (nu + nut) * du/dy
according to the log law. "

Ok, if that's the case, then this is a different form from what I've read other people doing in the past. Thank you, anyway, for helping clear that up.

 83_Ale_83 July 28, 2010 10:11

Hi to everybody,
i was looking for the "origin" of this nut equation, It's from:

nut=nu*[(Y+/U+)-1] and U+=1/k*ln(E*Y+) ???

I don't know the origin of the first equation, I have read Pope-Turbulent Flows but in that book nut=k*y*u_t

Alessandro

 83_Ale_83 August 11, 2010 04:49

Quote:
 Originally Posted by 83_Ale_83 (Post 269162) Hi to everybody, i was looking for the "origin" of this nut equation, It's from: nut=nu*[(Y+/U+)-1] and U+=1/k*ln(E*Y+) ??? I don't know the origin of the first equation, I have read Pope-Turbulent Flows but in that book nut=k*y*u_t Thank in advance Alessandro
Anyone ?
thanks

 83_Ale_83 October 13, 2010 06:08

Quote:
 Originally Posted by 83_Ale_83 (Post 271092) Anyone ? thanks

 Anne Lincke October 28, 2011 06:30

Hey Alessandro,

in this thread you might find help:

http://www.cfd-online.com/Forums/ope...-function.html

 Anne Lincke October 28, 2011 07:36

Quote:
 Originally Posted by madad2005 (Post 238157) Thanks, Christof. "At the walls nutw[faceI] = nuw[faceI]*(yPlus*kappa_/log(E_*yPlus) - 1.0); is used to apply the wall shear stress tau_wall = rho * (nu + nut) * du/dy according to the log law. " Ok, if that's the case, then this is a different form from what I've read other people doing in the past. Thank you, anyway, for helping clear that up.