Testing the integrity of POROUS media and por jump

Azman

I am testing the integrity of the porous jump and porous media model in Fluent, by simulating a flow of fluid through a channel with porous walls.

Here is how I did this test.

First of all, I did a simulation of flow through a normal long channel WITHOUT porous walls. My boundary conditions are inlet velocity 0.25 for the inlet on the left, and OUTFLOW for the outflow on the right. After the calculation has converged, I plotted a graph of static pressure vs position x, whereby x is in the direction of flow. ---- It gives me a straight line diagonally going down, and this agrees very well to my Navier Stokes equation.

Then, I did a simulation of flow with porous walls, using POROUS MEDIA model. My boundary conditions are the same for the inlet and outlet, but now I have a porous wall with a specified viscous resistance and thickness. After the calculation has converged, I plotted a graph of static pressure vs position x, whereby x is in the direction of flow. ------ It gives a curve that is going down, which is what I expect. (Refer to Journal of Membrane science, 191 (2001), 237-241, for a numerical research done by Sandeep K. Karode)

Lastly, I did a simulation of flow with porous walls, using the POROUS JUMP model. My boundary conditions are all the same as the second case, the only difference is that now my walls are now having Porous jump BC.

--- I get a curve of pressure vs position that is GOING UP!!!! And I really dont understand why it is going up.

Can anyone explain this strange result to me?