The numerical error in fluid mixing simulations using CFD is due to the discretization of the problem, round-off in floating point operations and the use of time integration. As a matter of fact, it is known that the combination of these errors may lead to spurious mixing patterns even if the problem is a priori well stated. On the other hand, it appears that there is a current lack of a similar analysis in the field of granular mixing simulation, in particular with the discrete element method (DEM).

The objective of this work is to illustrate by means of two case studies the impact of the time integration error on the macroscopic flow patterns and mixing properties predicted with DEM, and gain insight into the importance of selecting an appropriate time step when simulating granular flow with this method. First, the effect of the time step on the accuracy of the flow of spherical particles through an orifice will be investigated. Next, the effect of the time step will be studied in the case of the mixing of bidisperse spherical particles in a cylindrical tumbler.