Erosion induced by a disk translating toward or away from a granular bed

TL;DR

This study investigates the conditions under which a translating disk causes erosion of a granular bed, revealing how flow dynamics and vortices influence grain motion during approach and retreat.

Contribution

It provides experimental measurements and analysis of erosion thresholds for a disk moving vertically near a granular bed, highlighting different mechanisms during approach and retreat.

Findings

Erosion occurs via edge flow during approach and vortex impingement after stopping.

Thresholds depend on stroke length, travel time, and distance from the bed.

Radial flows and vortices play distinct roles in transient erosion.

Abstract

Unsteady flows generated when a body approaches or departs from a granular bed arise in swimming, burrowing, and maneuvering devices. Yet, the threshold for grain motion in such transients remains poorly modeled due to the complexity of the flow. In this study, we report laboratory measurements of the onset of erosion when a rigid circular disk is subjected to a single vertical stroke through quiescent water above a 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.