Spalart Allmaras
Hey buddys,
got a question to turbulence modeling in general. There are the several ways of dealing with turbulence. Lets talk about RANS. The TModels try to predict the turbulent shear stress. My question is, do they try to predict the elements of the stress tensor over the whole boundary layer thickness or do they start to be valid after a special wall distance? And does this gap been closed by the logarithmic wall law in general? I'm specially interested in Spalart Allmaras, because the damping functions, specially for the production term of the viscosity, are nearly zero till y+<=4, which means that there is no production in viscosity or that the logarithmic wall law is valid there instead. I'm wee bit confused. Can somebody help me out my confusion, please? 
Re: Spalart Allmaras
...right now I proved myself wrong. The logarithmic wall law is valid for y+ > 30. But what about the gap between 5<y+<30? Is the SAModel valid till the wall?...

Re: Spalart Allmaras
SA model can be integrated up to the wall. You will need a grid of y+ = O(1).

Re: Spalart Allmaras
As stated by Praveen the SA model has LowRe capability and can be used down to the wall (if the grid is fine enough to let the model works properly)
However there is no attempt to predict the stress tensor elements (this is what happens in 2nd order models like "Reynoldsstress models") because this is a socalled "turbulent viscosity model" Also the Kepsilon model has a lowRe extension which works properly down to the wall (again with the right conditions) and maybe there are some few other models with this capability. But, actually, the biggest part of the models doesn't have this feature and there is no reason (if not totally wrong) to have any grid point inside the viscous sublayer. Usually the first grid point is put inside or at the end of the loglayer and a properly chosen boundary condition is selected (which uses the loglaw, so this approach is only valid where this law is valid) to let everything works fine enough (that is to have the right stress at the wall). Even with the LES approach, some very popular models have the same problems at the wall (e.g. smagorinsky) if not corrected I hope this is what you were looking for 
Re: Spalart Allmaras
Thanks Paolo for this comprehensive answer.
I understand the difference. 
All times are GMT 4. The time now is 03:14. 