zero flux bc's on FDM 9-noded patch

E Siebrits

Given equation:

d/dx(X1*du/dx + X2*du/dy) + d/dy(Y2*du/dx + Y1*du/dy) = ...

where X1, Y1, etc are constants.

2nd-order FDM discretization of this equation system on a regular mesh (with rectangular elements) yields a 9-noded molecule in variable 'u' with the usual W, E, S, N, and C (center) coeffs, but also NW, NE, SW, and SE.

My question is how to correctly apply a zero flux bc, especially when acute or obtuse corners are present on the mesh boundaries of an irregular-shaped 2D domain?

Any pointers will be welcome!

Thanks