For the Computation Directions, choose the directions of the calculated flux. To obtain the whole diffusivity matrix in the result file, it is necessary to choose all three directions.

When computing the relative gas diffusivity, Boundary Conditions must be entered for the computations of the diffusion solver.

If Periodic is selected, the solution is computed assuming that the structure is periodically repeated in the diffusion direction(s).

If Symmetric (Dirichlet) is selected, a constant concentration is ensured along both border planes in the diffusion direction.

For highly porous structures, the difference between these two settings is not significant and but it gets significant for low porous structures (e.g. digital rocks). In general, we suggest using symmetric boundary conditions.