In FlowDict, the relationship between the predicted mean flow velocity (or pressure drop), the fluid viscosity and media thickness expressed in Darcy's law is used to compute and output the material permeability.

where is the fluid flow velocity, is the permeability, is the fluid viscosity, is the pressure, and is a force density.

Since the viscosity of the fluid is written separately in our formulation of Darcy’s law, permeability is a true material property. Instead of pulling the viscosity into the definition of permeability, the viscous resistivity is considered, i.e., the quotient of fluid viscosity and material permeability. Thus, if quantities such as water or air permeability are needed, consider the reciprocal of the viscous resistivity.

Defining permeability as a material property also means that the pressure drop cannot be so high as to result in deformation of the media.

where is total discharge (m3/s or l/s), is the intrinsic permeability of the medium (m2), is the cross-sectional area to flow (m2), is the total pressure drop (Pa), is the viscosity (Pa ∙ s), and is the length over which the pressure drop is taking place (m).