It has been shown that the extra condition needed to fix the stream function on an interior boundary in an incompressible liquid flow is that the integral of the pressure gradient along any path enclosing the boundary should vanish. More recently this constraint has been used to solve for a few special cases of compressible centrifuge flows. One way in which this constraint can be easily incorporated in a numerical scheme for solving the Onsager equation for the gas flow in a centrifuge is described.