A permeability can be input using the Darcy equation. Unlike loss coefficients which have different resistance values in the three directions, a permeability provides a constant resistance in all directions. An example is a packed bed of stones.
The governing equation for pressure drop as a function of permeability is:
where C is the viscosity coefficient, 
is the viscosity (of the surrounding fluid) and
u
i
is the velocity in the global i coordinate direction.
To represent a porous media, select Permeability from the Variation pull-down menu, and enter the value of the permeability, 
, as shown in the following equation:
The units of permeability are length squared.
The reason is that the Length term in the above equation is determined from the meshed geometry. Unlike the loss coefficient (K) variation method, the length over which a permeability acts is not divided out of the equation.