Use Bulk (Laplace) Diffusion to determine the tortuosity and the relative diffusivity tensor of the porous media. These quantities are properties of the porous media itself and as such, independent of the diffusing species and surrounding fluid.

This command solves the dimensionless Laplace equation in the pore space to compute a stationary diffusive flux and concentration distribution. Fick's first law is then utilized to derive the diffusivity of the media. To obtain the complete three-dimensional diffusivity tensor, the Laplace equation has to be solved three times using a concentration step in each space direction as boundary condition.