Membrane and Darcy's law
 

Hello,

I'm new to OpenFOAM, and as a starting project, I'm trying to simulate the crossflow over a permeable membrane. The setup is roughly sketched in this figure:

http://img291.imageshack.us/img291/1232/drawingg.png


For the flow through the membrane, I simply wish to put an "internal" boundary condition on the membrane, setting:

velocity_through_membrane=k*Delta_P_over_membrane

So, my question is; how do I implement such an internal boundary condition?

Also, what would be a proper boundary condition for the pressure at the membrane?