Use of Distance-Weighted Diffusivity Option in UDS Transport

What does the distance-weighted diffusivity option in user-defined scalar (UDS) transport do?

The distance-weighted diffusivity can be activated via a text user interface (TUI) command:
/define/user-defined> user-defined-scalars and yes to Use distance-weighted diffusivity?

This distance-weighted diffusivity option is relevant for models with non-orthogonal meshes (orthogonal meshes - hexa meshes with 90 degree and/or tetra meshes with 60 angles between the faces). In the case of a non-orthogonal mesh, the diffusive flux at the cell face center, which is derived from the gradient of the user defined scalar (UDS) at the face center, consists of two components: one in the normal direction (implicit term) and one in the tangential direction (explicit term). The gradient for the implicit term is computed through a finite differences approximation. The gradient for the explicit term is computed by default just as an arithmetic average of the values in at cell centers of the two neighboring cells, whereas the activation of the "distance-weighted diffusivity" option takes the distance of the cell centers to the face center as a weighting factor into account. The option is just another approximation scheme for the calculation of the explicit diffusive flux at cell faces, and in case of orthogonal meshes it is not relevant since the explicit term disappears.