problem with using colocated code

Jack

When I am using the FVM with colocated grid (Rhie&Chow's momentum interpolation for decoupling problem of velocity and pressure), the code works well for the single phase flow. When I apply it to the bed with a porous media and non porous media, the code can't converge due to the large existed gradient of velocity and pressure at the interface of a porous media and non porous media.

As I have browsed the whole forum, I realized that this is well known problem for colocated grid. However, I can't get the suitable answer to solve this problem. I am a student in the chemical engineering field, not in the CFD field. I really need some helps from CFD staff to comment on this problem because my research project (focus on concentration distribution in the chemical reactor ) needs the codes for irregular geometry.

I really appreciate your comments, hints and papers for this problem.

Thanks for your attention. Merry X'mas and Happy New Year