CFD Online Discussion Forums - Wall shear stress definition

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Wall shear stress definition

Hello.

I am using SST - Automatical wall function.

I plot wall shear stress x CFX variable. Then I make an expression based on the definition of the shear stress i.e. I make the following expression

Mixture.Eddy Viscosity*Mixture.Velocity u.Gradient Y.

When I compare the results are quite different.

1. I search in CFX theory guide and I cannot find the way that the CFX variables are calculated. Does anybody know if there is somewhere the way and the expressions tha CFX calculates the variables?

2. I search in CFD online forum and I see many posts regarding my problem i.e. other users are wondering about how shear stress is calculated but I have not found any "clear" solution/reply. Does anybody have found out why these two plots give quite different values?

Thanx a lot in advance

Anthi

It is well described in the documentation

Quote:

CFX-Solver Theory Guide
Chapter 2: Turbulence and Wall Function Theory | 2.8. Modeling Flow Near the Wall |

2.8. Modeling Flow Near the Wall

....

2.8.1.3. Automatic Near-Wall Treatment for Omega-Based Models

While the wall-functions presented above enable a consistent mesh refinement, they are based on physical assumptions that are problematic, especially in flows at lower Reynolds numbers (Re<105), as the sublayer portion of the boundary layer is neglected in the mass and momentum balance. For flows at low Reynolds ......

If you know the relationship between wall shear and momentum flux, everything you need should be there.

Thank you Opaque.

I would appreciate any further information / hint / suggestion of another paper regarding shear stresses since I am a little confused.