English: Supplementary Movie 1 Animation of particle motion in the r-z plane in a DEM simulation of cylindrical Couette flow. The thick black boundaries on the left, right and bottom are the inner cylinder, outer cylinder, and the stationary base, respectively; the sloping boundary at the top is the free surface. The particles in the entire Couette cell are projected onto the plane θ=0, with their r and z coordinates preserved. For clarity of viewing, the particles are rendered at 1/5 of their actual size, and only a small fraction of particles are made visible, which are initially in two layers: one of thickness 3dp at the free surface, and the other of thickness 2dp mid-way between the free surface and the base. The particles are colored according to their initial positions, to highlight visualization of the secondary flow—the coloring has no significance otherwise. The time span of the movie corresponds to 3 revolutions of the inner cylinder. The simulation is for fill height H =30dp, Savage number Sa =2×10-6, and rough walls.