I looked at the code of the diffusive operator, which is great (second order accuracy) for structured/orthogonal grids.

On non orthogonal meshes this approach gives some problem, however, it seems that many commercial codes use it to estimate diffusive terms too.

There are other approaches for unstructured meshes, but they present some difficulty in implementation:

1) Add a term to keep into account of the cross diffusion term, which becomes relevant only when the centres distance direction is far from being orthogonal to the surface normal vector. This tecnique has been proposed by Chow et al. It's problem is the definition of the cross-diffusion term. If I'm not wrong, the same authors did some work on that (I'm looking for it).

2) Use an higher order approximation scheme, which leads to deep changes in the whole discretization scheme.

I'd prefer to find something easier and more efficient, if possible.

I'm looking for more information on this subject talking to some other people too, and I'll make you know what I find as soon as possible.

P.S. I like the way this code is growing. You're doing a great job :-)

Hi :-)

ap