The main program sets up geometry of solution domain for 2-D advective diffusion with uniform density rho. Conductivity Gamma is now a function of phi, and the solution is achieved iteratively.

• The boundaries of this solution domain are on finite-volume faces, but additional nodes are added on the boundaries (Patankar's Practice B, p.69).

• Boundary conditions can be either fixed phi or fixed phi gradient at each face on the boundary. For example, in bc_e, first column is numerical value of BC, second column specifies BC type - 1 for value, 2 for gradient. A value of -2 indicates a specified diffusive flux,rather than a phi gradient (that's because Gamma is changing.)
• Gamma_phi m-file uses 1/phi dependence
• Gamma_phi_bad m-file uses linear dependence on phi (can get negative Gamma)
• u.m and w.m find velocities on appropriate volume boundaries
• A.m and F_upwind.m implement power-law advection scheme

solve.m solves the resulting system of linear equations.