Velocity fluctuations and population distribution in clusters of settling particles at low Reynolds number

TL;DR

This study investigates how non-Brownian spherical particles settle in a viscous fluid, revealing that clustering significantly influences velocity fluctuations and that these clusters follow an exponential size distribution.

Contribution

It demonstrates that particle clustering, rather than random distribution, is the main factor behind velocity fluctuations in settling particles at low Reynolds number.

Findings

Particles form transient clusters during settling.

Cluster size distribution follows an exponential law.

Velocity fluctuations match theoretical predictions for clusters.

Abstract

A study on the spatial organization and velocity fluctuations of non Brownian spherical particles settling at low Reynolds number in a vertical Hele-Shaw cell is reported. The particle volume fraction ranged from 0.005 to 0.05, while the distance between cell plates ranged from 5 to 15 times the particle radius. Particle tracking revealed that particles were not uniformly distributed in space but assembled in transient settling clusters. The population distribution of these clusters followed an exponential law. The measured velocity fluctuations are in agreement with that predicted theoretically for spherical clusters, from the balance between the apparent weight and the drag force. This result suggests that particle clustering, more than a spatial distribution of particles derived from random and independent events, is at the origin of the velocity fluctuations.