Hi SD,nThanks for the response. I already tried the DEFINE_FILM_REGIME with FILM_USER_0 example in the Fluent UDF manual. nThe DEFINE_IMPINGEMENT UDF was used to call the FILM_USER_0 from the DEFINE_FILM_REGIME . The only film regime was FILM_USER_0, so that the user film criteria will be met always.nIn the DEFINE_FILM_REGIME with FILM_USER_0, the fluent function Reflect_Particle(tp,f_normal,ND_ND,f,t, NULL, NULL_FACE) resulted in Fluent MPI error. So I changed this function with a velocity reflect UDF to calculate the rebound/reflect droplet velocity without the deviation angle. I updated the droplet velocity at the current and previous time steps. However, the droplets did not rebound after impinging the wall and the trajectory stopped after hitting the wall. nPlease let me know if there is something I am missing.nThanks for your help,nJo