Sedimentation of a dilute suspension of rigid spheres at intermediate Galileo numbers: the effect of clustering upon the particle motion

TL;DR

This study uses direct numerical simulations to investigate how particle clustering affects settling velocities in dilute suspensions at intermediate Galileo numbers, revealing that clustering enhances settling speed and velocity fluctuations.

Contribution

It demonstrates the link between clustering and increased settling velocity, and analyzes the fluid-particle interactions responsible for this effect at specific Galileo numbers.

Findings

Clustering occurs only at higher Galileo number (Ga=178).

Particle clustering increases average settling velocity by 12%.

Velocity fluctuations are significantly enhanced during clustering.

Abstract

Direct numerical simulation of the gravity-induced settling of finite-size particles in triply-periodic domains has been performed under dilute conditions. For a single solid-to-fluid density ratio of 1.5 we have considered two values of the Galileo number corresponding to steady vertical motion (Ga=121) and to steady oblique motion (Ga=178) in the case of one isolated sphere. For the multi-particle system we observe strong particle clustering only in the latter case. The geometry and time scales related to clustering are determined from Voronoi tesselation and particle-conditioned averaging. As a consequence of clustering, the average particle settling velocity is increased by 12% as compared to the value of an isolated sphere; such a collective effect is not observed in the non-clustering case. By defining a local (instantaneous) fluid velocity average in the vicinity of the finite-size particles it is shown that the observed enhancement of the settling velocity is due to the fact that the downward fluid motion (with respect to the global average) which is induced in the cluster regions is preferentially sampled by the particles. It is further observed that the variance of the particle velocity is strongly enhanced in the clustering case. With the aid of a decomposition of the particle velocity it is shown that this increase is due to enhanced fluid velocity fluctuations (due to clustering) in the vicinity of the particles. Finally, we discuss a possible explanation for the observation of a critical Galileo number marking the onset of clustering under dilute conditions.