# nutWallFunction questions

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 November 27, 2009, 08:15 nutWallFunction questions #1 Senior Member   Anonymous Join Date: Mar 2009 Posts: 110 Rep Power: 10 Sponsored Links 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?

November 29, 2009, 04:51
#2
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Mark Olesen
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Quote:
 Originally Posted by madad2005 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?

 November 29, 2009, 15:03 #3 Senior Member   Anonymous Join Date: Mar 2009 Posts: 110 Rep Power: 10 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.

 November 30, 2009, 05:09 #4 New Member   CHB Join Date: Oct 2009 Posts: 3 Rep Power: 9 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. fumiya likes this.

 November 30, 2009, 06:36 #5 Senior Member   Anonymous Join Date: Mar 2009 Posts: 110 Rep Power: 10 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.

 July 28, 2010, 09:11 #6 Member     Alessandro Join Date: Nov 2009 Posts: 67 Rep Power: 9 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 __________________

August 11, 2010, 03:49
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Alessandro
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Quote:
 Originally Posted by 83_Ale_83 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
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October 13, 2010, 05:08
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Alessandro
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Quote:
 Originally Posted by 83_Ale_83 Anyone ? thanks
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 October 28, 2011, 05:30 #9 Senior Member   Anne Gerdes Join Date: Aug 2010 Location: Hamburg Posts: 168 Rep Power: 9 Hey Alessandro, in this thread you might find help: http://www.cfd-online.com/Forums/ope...-function.html

October 28, 2011, 06:36
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Anne Gerdes
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Quote:
 Originally Posted by madad2005 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.

can you please explain how the nut computed by the nutWallFunction applies the wall hear stress according to the log law?
Am I right that there is no explicit near wall treatment for the velocity and the wall function is only enabled via omega, k and nut?

Thank you for an answer in advance.
Anne

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