  We investigate the motion of heavy particles with a diameter of several
multiples of the Kolmogorov length scale in the presence of forced turbulence
and gravity, resorting to interface-resolved DNS based on an IBM. The values of
the particles' relative density (1.5) and of the Galileo number (180) are such
that strong wake-induced particle clustering would occur in the absence of
turbulence. The forced turbulence in the two present cases (with Taylor-scale
Reynolds number 95 and 140) would lead to mild levels of clustering in the
absence of gravity. Here we detect a tendency to cluster with an intensity
which is intermediate between these two limiting cases, meaning that forced
background turbulence decreases the level of clustering otherwise observed
under ambient settling. However, the clustering strength does not monotonously
decay with the relative turbulence intensity. Various mechanisms by which
coherent structures can affect particle motion are discussed. It is argued that
the reduced interaction time due to particle settling through the surrounding
eddy (crossing trajectories) has the effect of shifting upwards the range of
eddies with a time scale matching the characteristic time scale of the
particle. In the present cases this shift might bring the particles into
resonance with the energetic eddies of the turbulent spectrum. Concerning the
average particle settling velocity we find very small deviations from the value
obtained for an isolated particle in ambient fluid when defining the relative
velocity as an apparent slip velocity (i.e. as the difference between the
averages computed separately for the velocities of each phase). This is
consistent with simple estimates of the non-linear drag effect. However, the
relative velocity based upon the fluid velocity seen by each particle has on
average a smaller magnitude (by 5-7%) than the ambient single-particle value.
