Wormhole formation in fluid-driven granular flow

Fluid-driven flow of granular material leads to complex behaviour and emergent instabilities in many natural and industrial settings. However, the effect of using fluid flow to vertically drive a dense bed of sedimenting grains is not well documented. Here we find contrasting behaviours in a submerged fluid-driven silo, including fingering patterns, porous flow, classical silo flow, and the formation of straight, semi-dilute wormhole-like channels. Once formed, these channels rapidly propagate towards the outlet and act as a bypass of the wider packing. The onset of this instability occurs when the gravity-driven grain flow at the free surface is insufficient to supply the fluid-assisted central region below the interface. Balancing empirical models of these flows predicts the height at which channels emerge as a function of grain size and flow rate. These findings provide a framework for predicting and controlling fluid-grain interactions in natural hazards, industrial processing, and geophysical flows.