Settling-driven Rayleigh–Taylor instability in a pre-existing turbulent field

Chih-Lun Liu , Adam Jiankang Yang

Vertical particle transport controls environmental processes ranging from sediment deposition and the export of organic material and microplastics to the fate of minerals released for carbon dioxide removal. When a particle-laden layer overlies denser water, particles destabilize the interface and descend collectively as convective plumes, far faster than individual settling. Yet this mechanism has commonly been studied in quiescent fluids, whereas the surface ocean is turbulent. We use direct numerical simulations to examine how pre-existing turbulence modifies each stage of settling-driven convection, varying the turbulence amplitude $\mathcal{A}$ and particle diameter $D$. Stronger pre-existing turbulence advances the onset of the Rayleigh--Taylor instability by increasing the effective seed amplitude. This earlier onset leaves less time for sediment to accumulate, resulting in a shallower nose and a weaker buoyancy gradient when growth begins. Consequently, the growth rate is reduced and the fastest-growing and marginal wavenumbers shift to lower values. Because the buoyancy gradient evolves almost identically at a given particle size, pre-existing turbulence controls when the instability begins rather than the interface it grows from. The disturbance also loses coherence, the vertical velocity field being rearranged into filaments within one e-folding at large $\mathcal{A}$. In the nonlinear phase, increasing $\mathcal{A}$ weakens both the sediment flux and the effective settling velocity, with stronger effects for smaller particles. For $D=10\ \mu$m, the settling velocity decreases by a factor of 2 from the weakest to the strongest $\mathcal{A}$, whereas for $D=30\ \mu$m, both bulk and frontal estimates are nearly $\mathcal{A}$-independent.