The main program sets up solution domain for 2-D advective diffusion for the same example shown in transient 2-D advective diffusion. However, here phi is set to a fixed value of 50 on a 3x3 block of volumes in the center of the domain.

• Source term in these volumes is set to:

• Then, phi = phi_int_BC is the solution for this volume.
• choose "big_num" large enough that other terms in equation for this volume are insignificant by comparison, but not so big that the matrix is rendered ill-conditioned.