Background: In particle-laden turbulent flows, the dispersed and continuous phases can have a significant effect on each other’s dynamics. This situation, usually termed two-way
coupling, is common and has been the focus of much of the recent research in the field (Poelma & Ooms 2006; Balachandar & Eaton 2010; Kuerten 2016).

Objectives: 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 behavior is also profoundly changed.

Methods: 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 1e−6 to 5e−5. The experiments are carried out in a zero-mean-flow facility which consists of a 5 cubic meter chamber where randomly actuated jets generate a region of homogeneous air turbulence, whose intensity is adjusted by varying the firing time of the jets. Here, glass micro-spheres are released at controlled rates via an adjustable hourglass, and enter the chamber after falling through a 3 m chute connected to its ceiling. A vertical plane at the centre of the chamber is illuminated by an Nd:YLF laser pulsed at 4 kHz and synchronized with two CMOS cameras. These cover a larger field of view (FOV) that captures the integral scales and the large-scale organization of the particles, and a smaller FOV nested in the larger one, to resolve the Kolmogorov scales and distinguish the closely clustered particles

Results:  In this regime, there is a 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.

Conclusions:  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.

 © The Author(s), 2023. Published by Cambridge University Press.  

 This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.