As Computational Directions, select the directions for which you want to calculate the flow. To obtain the full 3x3 permeability matrix and a pressure drop value for each direction in the results file, select all three directions.

For every selected direction, a boundary value problem is solved in which the Domain Boundary Conditions in Longitudinal Direction and the corresponding Experiment Input/Output are applied in the computational direction, and the Domain Boundary Conditions in Transverse Directions are applied on the other two space directions.  

Any directions not selected will not be calculated and will be marked as 'unknown' in the permeability matrix in the Report tab of the result file.

For Darcy Flow, the Boundary Conditions in Longitudinal Direction can be Periodic or Symmetric (Dirichlet). As Darcy Flow is used for structures with unresolved pores and low permeabilities, no inflow or outflow region should be given. Use symmetric boundary conditions for non-periodic structures if the percentage of solid voxels is high at the inlet. Otherwise, use periodic boundary conditions, as the simulation runtime is much faster.

In Darcy Flow simulations, the average Pressure at the Inlet surface and the average Pressure at the Outlet surface can be defined, where the pressure at outlet must be smaller than the pressure at inlet, since the pressure decreases from inlet to outlet. Select the desired pressure unit from the pull-down menu.

The Domain Boundary Conditions in Transverse Directions can be Periodic, Symmetric, No-Slip, 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 No-Slip (or Encase) boundary conditions, the domain boundary is treated as closed by solid material. When No-Slip is used, the solver internally adds a one-voxel layer in the required direction and solves with periodic boundary conditions. This is effectively equivalent to solving the structure with casing at both sides in the transverse direction. So, the structure size in the corresponding transverse direction becomes enlarged by one voxel internally.

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

The boundary conditions in the two directions transverse to the flow can also be set to be different by checking Expert boundary conditions. For example, when the fluid is chosen to flow in the Z-direction, the boundary conditions could be chosen to be No-slip in X-direction and Symmetric in the Y-direction.

For the Darcy Flow command, additionally to Periodic, Symmetric and No-Slip also No-Slip (fixed domain size) is available.

When No-Slip 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 No-Slip (fixed domain size) can avoid that, then the first layer of the structure is replaced by an impermeable material.