Unsteady flows generated when a body approaches or departs from a granular bed arise in swimming, burrowing, and manoeuvring devices. Yet, the threshold for grain motion in such transients remains poorly modelled due to the complexity of the flow. Building on recent characterisations of the squeeze/suction flow and vortex-ring dynamics generated by a translating disk near a rigid wall, we test how these flow scalings translate into erosion thresholds when the rigid wall is replaced by a granular bed. We report laboratory measurements of the onset of erosion when the rigid circular disk is subjected to a single vertical stroke through quiescent water above the granular bed. The stroke length and travel time were varied independently to determine the critical velocity at which the granular bed is eroded for different minimum distances from the bed. Two erosion mechanisms are observed for disk motion towards the bed: during the stroke, the outward squeezing flow erodes grains near the edge, while after stoppage, the starting vortex or associated secondary vortices impinge on the surface. For motion away from the bed, only the early interaction between the inward suction flow and the nascent vortex entrains grains. The resulting dimensionless thresholds clarify the respective roles of radial flows and vortices in transient, impulsively driven erosion.
Steiner et al. (Tue,) studied this question.