adding souce term
 

Dear all I have a question about adding source term. If I want to add a source term a(U*Nu*(a(Vol)/a(x)))/a(x) in u momentum equation, how can I do? Nu,Vol is turbulent viscosity and Volume fraction. The term a(Vol)/a(x)maybe calculated with utility routine ¡®GRADS¡¯, if F= U*Nu*(a(Vol)/a(x)), then the discretisation term is (Fi-F(I-1))/(Xi-X(i-1)), my difficulty is how to finish it with USER FORTION? I have been confused for a long time, please help me.