UDF for 3D Source

Mel

How do I add a 3D source to the momentum equations? Each source component is a function of the three velocity component, Sx(u,v,z), Sy(u,v,w), and Sz(u,v,w). How do I specify the dependent relationship between the sources (Sx, Sy, and Sz) and solution variables (u, v, and w) in my UDF? Do I take the partial derivative with respect to u, v, and w for the Sx component and so on?