# minimum formulation in function calcUTau (S-A wall function)

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 May 8, 2010, 19:21 minimum formulation in function calcUTau (S-A wall function) #1 New Member   Join Date: Jan 2010 Posts: 12 Rep Power: 9 Sponsored Links Hi, I took a closer look at nutSpalartAllmarasWallFunction.C. In function calcUTau the calculation of kUu reads kUu = min(50, kappa*Up/ut) Why is this minimum formulation introduced here and why is it restricting to the particular value of 50? Maybe it should keep the argument of the taylor expansion in a converging range, but 50 seems much too large for this. Does anybody have an idea? Thanks in advance, Luise

May 11, 2010, 03:38
#2
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Nobody knows anything?

In my calculations (a simple airfoil at 5deg incidence) the velocity profiles (sampled on the suction side at x/chord={0.07, 0.17, 0.3}) in the boundary layer show a strange behaviour, which seems to be a rather systematic problem:

For the cell closest to the wall the yPlus/uPlus mark is glued to the viscous sublayer, i.e. uPlus=yPlus.

Now, the Newtons method for finding the zero of f(uTau) is a gradient based method, while the function f shows a hyperbolic behaviour. So it might be better to engage other methods, which do not use gradients.
Otherwise, if all this has its reasons in some other part of the wallfunction or whatever (maybe the restriction of kUu), working on this would be a waste of time.

Thats what I figured!
Maybe someone knows better? I'm looking forward for suggestions!

Regards,
Luise
Attached Images
 Gs.07.gif (4.9 KB, 9 views) Gs.17.gif (4.9 KB, 5 views) Gs.30.gif (4.9 KB, 6 views)

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