Can anyone explain the difference between normal diffusion and cross diffusion?

These terms occur in the STAR-CD methodology manual (chapter 4 discretisation practices) where it discusses how the diffusive fluxes are discretised. A two part expression is given, the first part represents normal diffusion between P and neighbouring cell-centred node N and the second part represents cross-diffusion.

My aim is to gain an understanding of how the diffusive fluxes are discretised in STAR.

Cheers

Andy.

They break the diffusion term into a part that is treated implicitly (normal diffusion) and a part that is treated explicitly (cross diffusion). If the line connecting the centroids of adjacent cells is perpendicular to the face, then cross diffusion will be zero; otherwise not. See the book by Ferziger and Peric for more details.