Effect of Two-Way Coupling on Clustering and Settling of Heavy Particles in Homogeneous Turbulence

When inertial particles are dispersed in a turbulent flow at sufficiently high concentrations, the continuous and dispersed phases are two-way coupled. Here, we show via laboratory measurements how, as the suspended particles modify the turbulence, their behaviour is also profoundly changed. In particular, we investigate the spatial distribution and motion of sub-Kolmogorov particles falling in homogeneous air turbulence. We focus on the regime considered in Hassaini & Coletti (J. Fluid Mech., vol. 949, 2022, A30), where the turbulent kinetic energy and dissipation rate were found to increase as the particle volume fraction increases from 10(-6) to 5 x 10(-5). This leads to strong intensification of the clustering, encompassing a larger fraction of the particles and over a wider range of scales. The settling rate is approximately doubled over the considered range of concentrations, with particles in large clusters falling even faster. The settling enhancement is due in comparable measure to the predominantly downward fluid velocity at the particle location (attributed to the collective drag effect) and to the larger slip velocity between the particles and the fluid. With increasing loading, the particles become less able to respond to the fluid fluctuations, and the random uncorrelated component of their motion grows. Taken together, the results indicate that the concentrated particles possess an effectively higher Stokes number, which is a consequence of the amplified dissipation induced by two-way coupling. The larger relative velocities and accelerations due to the increased fall speed may have far-reaching consequences for the inter-particle collision probability.