Porous media actual volume occupation
 

Hello

I'm trying to make a simple model: unsteady filling of tank with compressible gas

• geometry: tank with pipe attached to it;

• the only non-wall boundary condition is stagnation inlet at pipe end;

• initial pressure is lower than pressure at stagnation inlet;

• tank contains porous media (granules) resulting in inertial and viscous resistances.

The question is how to implement the fact that porous media actually occupies volume (0.6 of it) and thus - at the end of computation mass of the gas inside the tank is actually 0.4·ρ·V but not ρ·V?

I tried to define equation of state for gas:
This works well if all regions are filled with granules, but does not work (ε is a discontinuous function, this leads to divergence) in case when regions free of granules (like pipes) exist.