Wall function in adverse pressure gradients

stephane baralon

I run a compressor computation with wall functions ( K-Epsilon ) to model the wall boundary layers. The pressure gradient is significantly adverse and my computation predict separation. My feeling is that wall functions should actually delay the advent of separation when compared to a low-Reynolds formulation of the k-e turbulence model with highly resolved B.L grid. Meaning that a more adverse gradient is necessary to separate a "wall function B.L.". Am I right? I base my reasoning on the fact that the wall friction velocity computed with wall functions would have a tendency to stay strictly positive whilst the separation point is characterized by a wall friction equal to zero. Is That correct ?

I am also wondering about the grid dependency of such computations.I know that for using wall func. , the cell center closest to the wall should be in the log-layer(y+ > 30). Unfortunately, for a same absolute distance, the y+ values of the cell center closest to the walls vary very much around the blade, whether you look at the leading edge , the first half of the chord on the suction side,etc. My question is: if the y+ values locally appears to be around 10, this means that i'm very close to the inner layer. What will be the effect of using a wall function based wall friction velocity for such a y+ value: too large or too low a value? Basically, am I "frictioning" too much ?

Thank you in advance.