Settling dynamics of non-Brownian suspension of spherical and cubic particles in Stokes flow

TL;DR

This study uses numerical simulations to analyze how spherical and cubic particles settle in a viscous fluid, revealing differences in microstructure, velocity fluctuations, and settling speeds influenced by particle shape and concentration.

Contribution

It provides new insights into the settling behavior of cubic particles compared to spheres, highlighting shape-dependent microstructure and velocity fluctuation effects in non-Brownian suspensions.

Findings

Spherical particles settle faster than cubes at low to moderate volume fractions.

Cubic particles exhibit higher velocity fluctuations and rebounding after collisions.

Microstructure anisotropy decreases with increasing volume fraction, affecting transport properties.

Abstract

The present study investigates the gravity-driven settling dynamics of non-Brownian suspensions of spherical and cubic particles within a triply periodic domain. The effect of solid volume fraction on the evolving microstructure of a suspension is numerically examined by employing the Rigid MultiBlob method under Stokes flow conditions. Our simulations accurately reproduce macroscopic trends observed in experiments and show strong agreement with established semi-empirical correlations over a wide range of volume fractions. It is observed that the hydrodynamic interactions between particles and the surrounding fluid dominate the settling mechanism at low to moderate solid volume fractions, whereas, frequent collisions between particles in a highly packed space tend to suppress velocity fluctuations at denser regime. The transport properties for dilute suspensions are found to be primarily shaped by an anisotropic microstructure, though this anisotropy decreases as many-body interactions significantly grow at higher volume fractions. Notably, cubic particles exhibit lower anisotropy in velocity fluctuations compared to spheres, due to more effective momentum and energy transfer from the gravity-driven direction to transverse directions. Moreover, cubic particles also display higher velocity fluctuations and particle rebounding after collisions, which may create a transient tendency for cubes to move against gravity more readily than spheres. The distinctive behaviors of cubic and spherical particles can notably influence the efficiency of size segregation in bidisperse mixtures. Interestingly, local velocity fluctuation intensity increases for both particle types in mixed suspensions, affecting the local sedimentation velocity field. Overall, spherical particles are found to settle faster than cubes at low to moderate volume fractions.