Compressible Flow Phenomena at Inception of Lateral Density Currents Fed by Collapsing Gas‐Particle Mixtures

Many geological flows are sourced by falling gas‐particle mixtures, such as during collapse of lava domes, and impulsive eruptive jets, and sustained columns, and rock falls. The transition from vertical to lateral flow is complex due to the range of coupling between particles of different sizes and densities and the carrier gas, and due to the potential for compressible flow phenomena. We use multiphase modeling to explore these dynamics. In mixtures with small particles, and with subsonic speeds, particles follow the gas such that outgoing lateral flows have similar particle concentration and speed as the vertical flows. Large particles concentrate immediately upon impact and move laterally away as granular flows overridden by a high‐speed jet of expelled gas. When a falling flow is supersonic, a bow shock develops above the impact zone, and this produces a zone of high pressure from which lateral flows emerge as overpressured wall jets. The jets form complex structures as the mixtures expand and accelerate and then recompress through a recompression zone that mimics a Mach disk shock in ideal gas jets. In mixtures with moderate to high ratios of fine to coarse particles, the latter tend to follow fine particles through the expansion‐recompression flow fields because of particle‐particle drag. Expansion within the flow fields can lead to locally reduced gas pressure that could enhance substrate erosion in natural flows. The recompression zones form at distances, and have peak pressures, that are roughly proportional to the Mach numbers of impacting flows. Key Points Dynamics of geological flows fed by collapsing mixtures strongly depends upon particle‐gas coupling Fine particles can reduce sound speed in the collapsing mixtures, resulting in additional compressible flow phenomena in proximal lateral flows Polydisperse cases exhibit hybrid behavior determined by both particle‐gas coupling and Mach number, depending upon relative proportions of particle sizes