  Heavy particles suspended in a turbulent flow settle faster than in a still
fluid. This effect stems from a preferential sampling of the regions where the
fluid flows downward and is quantified here as a function of the level of
turbulence, of particle inertia, and of the ratio between gravity and turbulent
accelerations. By using analytical methods and detailed, state-of-the-art
numerical simulations, settling is shown to induce an effective horizontal
two-dimensional dynamics that increases clustering and reduce relative
velocities between particles. These two competing effects can either increase
or decrease the geometrical collision rates between same-size particles and are
crucial for realistic modeling of coalescing particles.
