I'm currently working on simulating the flow through an exhaust system with filters and I need to find the pressure drop across the entire system at various given volumetric flows. I have the general flow set up I'm just struggling with the porous media part. I have 4 filters, 2 are HEPA filters and the other two are carbon glove box filters. I have the porous media set up as an axisymmetrical flow (because all of the filters are cylindrical). I have the calculation formula set to pressure drop, flowrate, and dimension. I have the manufacturing details for the filters so I have the flow rate vs pressure difference for the linear interpolation curve. I'm just struggling with n and r. I currently have n as the length and r as the radius (or layer thickness of the activated carbon). Is n the cross sectional area and r the surface area? And should the surface area be the entire surface area of the filter dimensions or should it be the surface are of the activated carbon?