History effects in the sedimentation of light aerosols in turbulence: the case of marine snow

TL;DR

This study investigates how the Basset history force influences the sedimentation behavior of marine snow particles in turbulence, revealing a slow convergence to a steady settling velocity and diffusive spatial spreading.

Contribution

It demonstrates the significant impact of the history force on particle trajectories and settling velocities, providing new insights into sedimentation dynamics in turbulent flows.

Findings

History force causes slow, power-law convergence to steady settling velocity.

Ensemble spatial spread grows diffusively after initial ballistic phase.

Settling velocity approximates still fluid conditions with turbulence-induced fluctuations.

Abstract

We analyze the effect of the Basset history force on the sedimentation of nearly neutrally buoyant particles, exemplified by marine snow, in a three-dimensional turbulent flow. Particles are characterized by Stokes numbers much smaller than unity, and still water settling velocities, measured in units of the Kolmogorov velocity, of order one. The presence of the history force in the Maxey-Riley equation leads to individual trajectories which differ strongly from the dynamics of both inertial particles without this force, and ideal settling tracers. When considering, however, a large ensemble of particles, the statistical properties of all three dynamics become more similar. The main effect of the history force is a rather slow, power-law type convergence to an asymptotic settling velocity of the center of mass, which is found numerically to be the settling velocity in still fluid. The spatial extension of the ensemble grows diffusively after an initial ballistic growth lasting up to ca. one large eddy turnover time. We demonstrate that the settling of the center of mass for such light aggregates is best approximated by the settling dynamics in still fluid found with the history force, on top of which fluctuations appear which follow very closely those of the turbulent velocity field.