We experimentally investigate the behavior of instantaneous localized 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 $\mathrm{\Omega }=5,10,20\mathrm{rpm}$ and for the size of particles in the range $5\mu \text{m}$ to $1\text{mm}$, 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 et al. [Proc. R. Soc. A: Math. Phys. Sci. 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 $\mathrm{\Omega }$ 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.

• Received 19 May 2022
• Accepted 27 October 2022

DOI:https://doi.org/10.1103/PhysRevFluids.7.124302