Eulerian-Eulerian approach UDF
 

I am a new player for Fluent. I have a two phase problem air and sand, I use Eulerian-Eulerian approach and want to write a UDF modify the source term of momentum for secondary phase (sand). could someone give me an example how to write this UDF? for example, in the UDF, I want use the following term as the source term of momentum for secondary phase:

the air density*air velocity*sand velocity, something like this. (This is random example)

Could someone show me an example how to write this kind of UDF.

Thank you very much!