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

A particle-laden homogeneous isotropic turbulent flow is studied experimentally to understand the role of collective effects (e.g. particle-particle aerodynamic interactions caused by local particle accumulation) on the settling velocity of inertial particles (Stokes number: 0.3 < St < 0.6). Conditional averaging of the particle vertical velocity on the local concentration identifies three settling regimes: modest enhancement by single particle-turbulence interactions in low concentration regions, rapid settling velocity increases in clusters at intermediate concentrations and saturation of the settling enhancement 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 Voronoi analysis. A model is developed following a classic volume-averaged multiphase flow methodology to provide an interpretation of the three settling regimes and quantitative predictions consistent with the measurements.