UDF for source term for secondary phase ONLY?! plz help
 

hi everyone!
I am using the VOF model to simulate a free surface flow of a non-Newtonian fluid (air is the first phase and the fluid is the secondary phase).
On one part of the domain I have to add a momentum source. I wrote an UDF for the momentum source and it works fine, but I don't know how to hook this UDF only to the secondary phase (to the fluid and not to the air!).
anyone knows how to solve that? thanks a lot!