Effects of particle size and background rotation on the settling of particle clouds

TL;DR

This study experimentally examines how particle size and background rotation influence the behavior and entrainment of turbulent particle clouds settling in water, revealing effects of inertia and Coriolis force on cloud dynamics.

Contribution

It provides systematic experimental data on particle cloud behavior under various rotation rates and particle sizes, highlighting the influence of inertia and rotation on entrainment and cloud evolution.

Findings

Particle clouds behave like salty thermals without rotation, with optimal entrainment at a specific inertia.

Rotation inhibits particulate enhancement of entrainment and transforms clouds into vortical flows.

Coriolis force dominates cloud dynamics at high rotation, increasing particle residence time.

Abstract

We experimentally investigate the behaviour of instantaneous localised releases of heavy particles falling as turbulent clouds in quiescent water, both with and without background rotation. We present the results of 514 systematic experiments for no rotation and for three rotation rates $\Omega = 5,10,20$rpm, and for the size of particles in the range $5\mu m$ to $1mm$, exploring four decades of the Rouse number $\mathcal{R} \in [6\times 10^{-4},4]$ which quantifies the inertia of particles. In the canonical framework of turbulent thermals described by Morton \textit{et al.}, [Proc. R. Soc. A: Math. Phys. Sci. \textbf{234}, 1 (1956)], we compare particle clouds with salt-water thermals to highlight specificities due to the particulate nature of the turbulence forcing. In the absence of rotation, particle clouds initially behave as salty thermals with a modulation of their entrainment capacity, which is optimally enhanced for a finite inertia $\mathcal{R}\simeq 0.3$ due to particulate effects. However this regime of turbulence is limited in time due to the inertial decoupling between turbulent eddies and particles. For the three values of $\Omega$ explored here, the particulate enhancement of entrainment is inhibited. Moreover the cloud's expansion is interrupted when the Coriolis force overcomes its inertia, forcing the cloud to transform into vortical columnar flows which considerably increase the residence time of particles.