Walls in discrete element method simulations of granular flows are sometimesmodeled as a closely packed monolayer of fixed particles, resulting in a roughwall rather than a geometrically smooth wall. An implicit assumption is thatthe resulting rough wall differs from a smooth wall only locally at theparticle scale. Here we test this assumption by considering the impact of thewall roughness at the periphery of the flowing layer on the flow ofmonodisperse particles in a rotating spherical tumbler. We find that varyingthe wall roughness significantly alters average particle trajectories even farfrom the wall. Rough walls induce greater poleward axial drift of particlesnear the flowing layer surface, but decrease the curvature of the trajectories.Increasing the volume fill level in the tumbler has little effect on the axialdrift for rough walls, but increases the drift while reducing curvature of theparticle trajectories for smooth walls. The mechanism for these effects isrelated to the degree of local slip at the bounding wall, which alters theflowing layer thickness near the walls, affecting the particle trajectorieseven far from the walls near the equator of the tumbler. Thus, the properchoice of wall conditions is important in the accurate simulation of granularflows, even far from the bounding wall.
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