porous media divergence

Phil

I have a problem with a rotationally periodic model I am running with porous media. The whole model apart from the porous media converges well with periodic boundaries, however when i change the porous media boundary from wall to periodic i get divergence. This may not be a problem as in this part of the model the flow through the media should be radial only and so i would expect no mass transfer across this periodic boundary. I would like to know why the model is diverging though as I can't have much confidence in the results. The far side of the porous media is set as a pressure outlet. Can anyone give any reasons for this happening?