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.
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