Settling behaviour of thin curved particles in quiescent fluid and turbulence

TL;DR

This study experimentally investigates the settling behavior of thin curved particles in quiescent and turbulent fluids, revealing how turbulence affects their velocity, motion patterns, and dispersion, with a new pendulum model explaining velocity fluctuations.

Contribution

It introduces an experimental analysis of thin curved particles in turbulence and proposes a simple pendulum model to explain velocity fluctuations and settling dynamics.

Findings

Turbulence decreases mean descent velocity of particles.

Unique descent events like 'long gliding' and 'rapid rotation' are observed in turbulence.

Turbulence increases radial dispersion and affects particle motion patterns.

Abstract

The motion of thin curved falling particles is ubiquitous in both nature and industry but is not yet widely examined. Here, we describe an experimental study on the dynamics of thin cylindrical shells resembling broken bottle fragments settling through quiescent fluid and homogeneous anisotropic turbulence. The particles have Archimedes numbers based on the mean descent velocity $0.75 \times 10^4 \lesssim Ar \lesssim 2.75 \times 10^4$. Turbulence reaching a Reynolds number of $Re_\lambda \approx 100$ is generated in a water tank using random jet arrays mounted in a co-planar configuration. After the flow becomes statistically stationary, a particle is released and its three-dimensional motion is recorded using two orthogonally positioned high-speed cameras. We propose a simple pendulum model that accurately captures the velocity fluctuations of the particles in still fluid and find that differences in the falling style might be explained by a closer alignment between the particle's pitch angle and its velocity vector. By comparing the trajectories under background turbulence with the quiescent fluid cases, we measure a decrease in the mean descent velocity in turbulence for the conditions tested. We also study the secondary motion of the particles and identify descent events that are unique to turbulence such as 'long gliding' and 'rapid rotation' events. Lastly, we show an increase in the radial dispersion of the particles under background turbulence and correlate the timescale of descent events with the local settling velocity.