y+ Dilemma

Mike

Hello everybody

In my experience a not too uncommon problem when using wall functions is that a cell with y+ ~30 is too large to resolve the flow domain geometry and you cannot afford to resolve the boundary layer (y+ ~1 in the first cell and a not too rapid cell growth in the wall normal direction).

I'm currently simulating a domain with a lot of jets with 2 mm diameter. I have just made a mesh with just below 10^6 cells. y+ in the wall cells in the region of interest is between 3 and 10. If i resolve this in a way that is suited for the two-layer model i will end up with something like 5*10^6 cells.

I know that Fluent modifies the wall functions if the first cell is located in the viscous sublayer. But does this really work or is it just a quick fix?

So dear CFD-community, what is your experience? Should I use too low y+ together with wall functions or should I use my badly resolved boundary layer together with the two-layer model? What will, generally of course, give the worst result?

Regards

Mike