Thanks for replaying, I have run with DPM without including collision, and it was done very successfully. The problem appeared when I implement DEM model with 5*10^-4-time step. nThe particles were injected in low volume fraction 0.175, and I standard initialize using the default values. nI believe the problem in the coupling between particle and the fluid, in another word k-omega viscose model and the solution methods. I am mainly using second order solvers, also for the dissipation rate solver.nSince my model is simple particle flow, it is possible to solve the volume fraction equation only (neglecting fluid)?n nRegardsnn