Sedimentation of Oblate Ellipsoids at low and Moderate Reynolds numbers

TL;DR

This study investigates how oblate ellipsoids settle and orient under gravity at low to moderate Reynolds numbers, revealing unique velocity and orientation behaviors compared to spherical particles, with implications for biophysics and environmental engineering.

Contribution

The paper provides new insights into the sedimentation dynamics of oblate ellipsoids, including velocity maxima and orientation effects, which differ from spherical particles and depend on Reynolds number.

Findings

Settling velocity exhibits a local maximum at intermediate densities.

Average orientation shows a similar local maximum that disappears at higher Reynolds numbers.

Oblate ellipsoids display orientational clustering and smaller density fluctuations compared to spheres.

Abstract

In many applications to biophysics and environmental engineering, sedimentation of non-spherical particles for example: ellipsoids, is an important problem. In our work, we simulate the dynamics of oblate ellipsoids under gravity. We study the settling velocity and the average orientation of the ellipsoids as a function of volume fraction. We see that the settling velocity shows a local maximum at the intermmediate densities unlike the spheres. The average orientation of the ellipsoids also shows a similar local maximum and we observe that this local maximum disappears as the Reynolds number is increased. Also, at small volume fractions, we observe that the oblate ellipsoids exhibit an orientational clustering effect in alignment with gravity accompanied by strong density fluctuations. The vertical and horizontal fluctuations of the oblate ellipsoids are small compared to that of the spheres.