Time persistency of floating particle clusters in free-surface turbulence

TL;DR

This study investigates how floating particles form long-lasting clusters on turbulent water surfaces, revealing that buoyancy causes persistent fractal distributions that outlive the turbulent structures that create them.

Contribution

It provides new insights into the long-term persistence and fractal nature of particle clusters in free-surface turbulence through detailed numerical simulations.

Findings

Particle buoyancy leads to long-lived fractal clusters.

Clusters persist longer than surface turbulent structures.

Cluster dynamics are quantified via correlation dimension evolution.

Abstract

We study the dispersion of light particles floating on a flat shear-free surface of an open channel in which the flow is turbulent. This configuration mimics the motion of buoyant matter (e.g. phytoplankton, pollutants or nutrients) in water bodies when surface waves and ripples are smooth or absent. We perform direct numerical simulation of turbulence coupled with Lagrangian particle tracking, considering different values of the shear Reynolds number (Re{\tau} = 171 and 509) and of the Stokes number (0.06 < St < 1 in viscous units). Results show that particle buoyancy induces clusters that evolve towards a long-term fractal distribution in a time much longer than the Lagrangian integral fluid time scale, indicating that such clusters over-live the surface turbulent structures which produced them. We quantify cluster dynamics, crucial when modeling dispersion in free-surface flow turbulence, via the time evolution of the cluster correlation dimension.