nThanks. Is this the document you're referring to?:, the base must be defined as a stationary wall, and the face of the ball must be defined as a moving wall? However, I can't define its velocity or rotational speed as the purpose of the simulation is to determine these variables.nI defined the ball face as moving, relative to adjacent cell zone, translational, with speed 0 in direction [1,1,1], but the negative cell volume error occurred.n