Sorting of particles of different sizes strongly affects the dynamics of river beds. As a model for the sorting of fine particles on the lee face of an alluvial “dune”, we study the orbits of small spherical tracers in monodisperse particle beds in quasi-two dimensional rotating tumblers, both liquid- and air-filled. In addition to searching for clues on lee-face sorting, another purpose is to provide benchmark data for small-scale “qDNS” of bedload sediment transport. While sorting of fines in a rotating tumbler, and other canonical grain flows, has been studied in air, there is a dearth of corresponding studies in liquid. Compared to the “dry” case, our data show that immersion in liquid yields significant differences in bead trajectories at matched Froude number, in the intermittent avalanching regime at low rotation rate.

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