how to do BC of p_in depending on p_out and u_in
 

Hi

I am wondering how to set the boundary conditions for this situation:
I am simulating the velocity and pressure of water going thru a cylinder.
I give the pressure at the outlet and the velocity at the inlet. zeroGradient of velocity at the outlet and the pressure at the inlet depends on the pressure at the outlet and the velocity at the inlet. I know how to set BC for the previous three, but I have no clue how to set the fourth one.

I have searched thru the web and the manual, but I have not found a satisfying answer. I am wondering if anyone here can nod a direction for me.
Thanks!

Hsingtzu