Thank you.nI found the DPM best practice presentation, and it was really helpfull. Reading some other posts helped me to understand how I can use the parcel size to increase the collision time. Unfortunately, I could not get the Hertzian model to work properly. nI think the problem is with my configuration. The relative velocity between particles is very low, but the relative velocity between particles and the wall is high, so I probably need different spring constants for the wall-particle pair and for the particle-particle pair. Using the equation for the spring constant I obtained 2000 N/m for the particle-wall case and k~1 N/m for particle-particle collisions, if I consider that the velocity difference is slow. nUnfortunately, sometimes I still have particles flying with the highest velocity allowed after colliding with the wall.nIf I use a small particle time step(1e-7), but a large number of particle iterations (1e4) between fluid iterations, will the collision be solved differently than when I have a small particle time step with a single particle iteration per fluid iteration?nIs there a way, with or without UDF, to:nIdentify the exact moment when a collision occured?nDebug the collision process (during particle iterations)?nDefine if a collision happened with the wall or with another particle?nWhat causes the particles to accelerate to max. velocity after collision?n