A fluid domain together with a model for momentum loss. The effects of porosity are accounted for only through this loss term; all other terms in the governing equations are not changed. As a result, this formulation can be called the "superficial velocity formulation". For information on the momentum loss models available with this formulation, see Momentum Sources.

A porous domain that involves one or more fluids and an optional solid. Porosity modifies all terms in the governing equations as well as the loss term. As a result, this formulation can be called the "true velocity formulation" or the "full porous model". This method supports solid models (for example, for modeling thermal conductivity of the solid), and models for the interaction between the fluid and solid parts of the domain.