Vinay Neil Panda

Sorry for the delayed response.
Yes, the solids are modelled as the porous media, while the air should ideally be with a porosity of 1. The UDF is written such that mass is subtracted from the solids (phase 1) and added to the air surrounding air (phase 2). But the system demands the porosity of the air should be exactly equal to the porosity of the solids.

I am not able to reason why this is happening, theoretically. I would be really grateful for your help.