The role of collective effects on the enhancement of the settling velocity of inertial particles in turbulence

A particle-laden homogeneous isotropic turbulence experiment is used to study the role that collective effects (e.g. particle-particle aerodynamic interactions ) have on the settling velocity of inertial particles (Stokes $0.1 < \mathrm{St} < 0.3$). Conditional averaging of the particle vertical velocity on the local concentration identifies three settling regimes: low concentration fast-tracking, rapid increase in settling velocity at intermediate concentrations, and saturation at large concentrations. The latter effect, associated with four-way coupling, displays qualitative agreement with simulations in the literature and is a new experimental observation. Fluctuations up to an order of magnitude larger than the background volume fraction are measured using Voronöi analysis which is used in a model developed in the spirit of volume-averaged multiphase flow methods, that gives a consistent interpretation, and quantitative predictive power, of the three settling regimes measured.