Rayleigh–Taylor-like effects in the settling of finite-size particle fronts

We study the settling of a heavy particle-laden phase layered above an unladen fluid. When the suspension has finite dilution, the dynamic of this set-up has remained largely unexplored. This system, which is fundamentally analogous to that found when studying the Rayleigh–Taylor instability, generates a mixing front whose width grows at an anomalous rate. In the present work, we explore the fluid and particle dynamics underpinning the mixing layer expansion. We perform particle-resolved direct numerical simulations to investigate the set-up: we use particle-to-fluid density ratios gamma equals StartSet 2 comma 4 comma 6 comma 8 EndSet γ = { 2 , 4 , 6 , 8 } γ =\2,4,6,8\ and particle-laden phase volume fractions phi Subscript b Baseline almost equals 0.1 ϕ b ≈ 0.1 φ b≈ 0.1 . The results show the emergence of flow structures analogous to Rayleigh–Taylor plumes. Downward-directed structures are preferentially sampled by the particles, and within their cores, particles are able to accelerate past their terminal velocity in still fluid, resulting in strongly enhanced settling rates. We quantify the intensity of turbulence in the mixing layer, showing possible evidence of an incipient energy cascade.