Hi everyone, I am modelling a closed cylinder with piston from side and porous medium from the other side.

my issue is that when I set the porosity of porous zone = .5 and run the simulation for 360 degree (one revolution) .. the mass inside the system is increased during the compression stroke and decrease during the expansion until it reaches to initial state the increase is about 12% I am wondering why the mass value increases. It should be constant. By the way, if I deactivate the porous zone and run the simulation the mass still constant during the cycle. Please need your help.. thank you