  Spatial clustering and intermittency in the relative velocity of heavy
particles of the same size settling in turbulent flows can be strongly affected
by gravity. We present a model for the timescale of the fluid velocity gradient
seen by particle pairs and propose an effective Kubo number based on this
timescale to explain the mechanism of gravity-enhanced clustering. We explore
the mechanisms of the gravity-induced reduction or enhancement of the
intermittency in the particle radial relative velocity (RRV) at different
Stokes numbers based on backward-in-time relative dispersion and preferential
sampling of the fluid field. These effects of gravity on clustering and the RRV
must be parameterized in the geometric collision kernel.
