For the Computational Directions, choose the direction for which the diffusivity should be calculated. To obtain the whole 3x3 diffusivity matrix and a tortuosity value for each direction in the result file, select all three directions.

The directions that are not checked will not be calculated and will appear marked as unknown in the diffusivity matrix in the Result Viewer of the result file.

The choice of boundary conditions depends on the structure and the application.

If Periodic is checked, DiffuDict assumes that the structure is periodically repeated in the diffusion direction. If Symmetric (Dirichlet) is checked, a constant concentration is ensured along both border planes in the diffusion direction. For non-periodic structures, Symmetric (Dirichlet) boundary conditions give more accurate results than Periodic boundary conditions.

In previous versions of GeoDict, the computational memory requirements for Symmetric (Dirichlet) boundary conditions case were almost two times higher and therefore, the computational time was longer. This is no longer the case since GeoDict 2022.

For structures with large porosity (>80%), the difference between these two cases is insignificant and can be compared with a computational error.

In few situations, however, due to the structure’s geometry, using Dirichlet boundary conditions is unavoidable. An example, with pores artificially closed by using a periodic boundary condition, is shown below. When repeating the structure periodically it becomes clear, that periodic boundary conditions cannot be used in this case because the periodic repetition artificially closes the pores, reducing the true diffusivity. The use of Dirichlet boundary conditions eliminates this artifact.

In the Experiment Input panel, the difference in concentration of the diffusing fluid across the material (Concentration at Inlet, Concentration at Outlet) model for a specific experiment should be entered.

Besides the boundary conditions chosen for the main diffusion direction, the boundary conditions in the transverse (or tangential) directions can be set to be Periodic, Symmetric, Encase or Expert.

With Periodic boundary conditions, the structure is repeated in the transverse directions.

With Symmetric boundary conditions, the structure is mirrored at the domain boundary. The LIR and EJ solver solve the symmetric boundary condition by using Neumann (or zero flux) condition at the domain boundary.

With Encase, the domain boundary is treated as closed by solid material.

With Expert, it is possible to define different boundary conditions for each side in each computation:

When Encase is used, the solver internally adds a one-voxel layer in the required direction and solves with periodic boundary conditions. That effectively is equivalent to solve the structure with casing in two ends in the direction of interest. So, the size of computation in this direction becomes n+1. However, if changing the computational size is not preferable, using Encase (fixed domain size) can avoid that, then the first layer of the structure is replaced by a non-diffusing material.