  We propose an experimental study on the gravitational settling velocity of
dense, sub-Kolmogorov inertial particles under different background turbulent
flows. We report Phase Doppler Particle Analyzer measurements in a low-speed
wind tunnel uniformly seeded with micrometer scale water droplets. Turbulence
is generated with three different grids (two consisting on different
active-grid protocols while the third is a regular static grid), allowing us to
cover a very wide range of turbulence conditions in terms of Taylor-scale based
Reynolds numbers ($Re_\lambda \in [30-520]$), Rouse numbers ($Ro \in [0-5]$)
and volume fractions ($\phi_v \in[0.5\times10^{-5} - 2.0\times10^{-5}]$). We
find, in agreement with previous works, that enhancement of the settling
velocity occurs at low Rouse number, while hindering of the settling occurs at
higher Rouse number for decreasing turbulence energy levels. The wide range of
flow parameters explored allowed us to observe that enhancement decreases
significantly with the Taylor Reynolds number and is significantly affected by
the volume fraction $\phi_v$. We also studied the effect of large-scale forcing
on settling velocity modification. The possibility of changing the inflow
conditions by using different grids allowed us to test cases with fixed
$Re_\lambda$ and turbulent intensity but different integral length scale.
Finally, we assess the existence of secondary flows in the wind tunnel and
their role on particle settling. This is achieved by characterising the
settling velocity at two different positions, the centreline and close to the
wall, with the same streamwise coordinate.
