DrAmine
Ansys Employee

Again I won't coomment on the UDF.

The UDF is required to adjust all associated injection properties as particles are injected to the domain. As mentioned several times: MASS and DIAM and theier INIT values should be referenced in the UDF (so check the UDF when the IF condition is not fulfilled). 

Do work first without additional collision model. I can think the latter wil require smaller particle integration time step sizes.

 

What is your application? Perhaps we can guide on a different /  built-in modeling approach.