Experimental investigation of immersed granular collapse in viscous and inertial regimes

This paper presents an experimental investigation of immersed granular collapse with an initially dense packing, mainly focusing on the collapse characteristics of different flow regimes and the influence of the initial aspect ratio. A novel experimental setup and imaging method are introduced to simultaneously observe the motion of the particles and the fluid. The collapse dynamics, including the collapse acceleration, steady propagation velocity, and collapse duration, are analyzed based on the front propagation. It is found that the collapse procedures in the inertial and viscous regimes differ significantly, with the transitional regime possessing some unique characteristics of both. The inertial regime exhibits a faster collapse process, sharper final deposition, and a depression near the right wall in the case of high columns. The viscous regime collapses from the upper-left corner, from where particles drop to the bottom and form the flow front in advance of the particles initially at the bottom, and exhibits a triangular final deposition. The inertial regime exhibits swirling fluid motion, which helps the granular transport, whereas the fluid flow in the viscous regime mainly follows the granular flow. The collapse regime characteristics are more pronounced in higher columns.