  Sedimentation of a dispersed solid phase is widely encountered in
applications and environmental flows, yet little is known about the behavior of
finite-size particles in homogeneous isotropic turbulence. To fill this gap, we
perform Direct Numerical Simulations of sedimentation in quiescent and
turbulent environments using an Immersed Boundary Method to account for the
dispersed rigid spherical particles. The solid volume fractions considered are
0.5-1%, while the solid to fluid density ratio 1.02. The particle radius is
chosen to be approximately 6 Komlogorov lengthscales. The results show that the
mean settling velocity is lower in an already turbulent flow than in a
quiescent fluid. The reduction with respect to a single particle in quiescent
fluid is about 12\% and 14\% for the two volume fractions investigated. The
probability density function of the particle velocity is almost Gaussian in a
turbulent flow, whereas it displays large positive tails in quiescent fluid.
These tails are associated to the intermittent fast sedimentation of particle
pairs in drafting-kissing-tumbling motions. The particle lateral dispersion is
higher in a turbulent flow, whereas the vertical one is, surprisingly, of
comparable magnitude as a consequence of the highly intermittent behavior
observed in the quiescent fluid. Using the concept of mean relative velocity we
estimate the mean drag coefficient from empirical formulas and show that non
stationary effects, related to vortex shedding, explain the increased reduction
in mean settling velocity in a turbulent environment.
