Originally Posted by sbaffini

Indeed, if y+ =4 is relative to the finest grid, it is confirmed to be a wall function problem. I can't double check now, but I'm pretty sure that the k-omega sst model in CFX uses an all y+ wall function, which means that a wall function is always active. While, in theory, such wall functions should be insensitive to the specific y+ value, they are not perfect and your case is very far from the typical wall function scenario (equilibrium boundary layer), so what you obtain is actually expected.

The only viable solution here, and I suggest you to investigate it also for your other models, is to redistribute cells in your grid to be always within y+ = 1-2, but no more. In any case, the important thing is that you can't have y+ changing between the grids when doing a grid refinement.

EDIT: I know, it sucks...