Effect of preferential concentration on the settling velocity of heavy particles in homogeneous isotropic turbulence

The behaviour of heavy particles in isotropic, homogeneous, decaying turbulence has been experimentally studied. The settling velocity of the particles has been found to be much larger than in a quiescent fluid. It has been determined that the enhancement of the settling velocity depends on the particle loading, increasing as the volume fraction of particles in the flow increases. The spatial and temporal distribution of the particle concentration field is shown to exhibit large inhomogeneities. As the particles interact with the underlying turbulence they concentrate preferentially in certain regions of the flow. A characteristic dimension of these particle clusters is found to be related to the viscous scales of the flow. Measurements of the settling velocity conditioned on the local concentration of particles in the flow have shown that there is a monotonic increase in the settling velocity with the local concentration (the relation being quasi-linear). A simple phenomenological model is proposed to explain this behaviour.