Propagation scaling and micromechanics of buoyant granular column collapses

Granular avalanches exhibit striking parallels to natural geophysical flows, including debris flows, landslides, and pyroclastic flows, whose instability, rheology, and deposition morphology are governed by particle-scale dynamics and bulk interactions within granular systems. To rigorously investigate fluid-particle coupling in such multiphase environments, this study employs coupled lattice Boltzmann-discrete element method (LB-DEM) simulations, which are validated by our experimental observations, to analyze the collapse of buoyant granular columns in subaqueous settings. The insights gained are extended to elucidate the behavior of large-scale geophysical flows, such as volcanic cloud propagation and submarine gravity currents. Notably, the dynamics of buoyant granular flows are shown to follow a scaling relationship analogous to that observed in the 2022 Hunga Tonga-Hunga Ha'apai volcanic plume. This scaling transition signifies a dual regime shift, occurring both within the granular phase and at the interstitial fluid-grain interface. By incorporating buoyancy effects where the particle density is lower than the ambient fluid, this work advances existing scaling laws for granular column collapses. Furthermore, the results provide critical insights into the multiphase physics that governs airborne geophysical flows, bridging microscale interactions to macroscale flow behaviors in natural and industrial contexts.