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We propose an experimental study on the gravitational settling velocity ofdense, sub-Kolmogorov inertial particles under different background turbulentflows. We report Phase Doppler Particle Analyzer measurements in a low-speedwind tunnel uniformly seeded with micrometer scale water droplets. Turbulenceis generated with three different grids (two consisting on differentactive-grid protocols while the third is a regular static grid), allowing us tocover a very wide range of turbulence conditions in terms of Taylor-scale basedReynolds numbers ($Re_\lambda \in [30-520]$), Rouse numbers ($Ro \in [0-5]$)and volume fractions ($\phi_v \in[0.5\times10^{-5} - 2.0\times10^{-5}]$). Wefind, in agreement with previous works, that enhancement of the settlingvelocity occurs at low Rouse number, while hindering of the settling occurs athigher Rouse number for decreasing turbulence energy levels. The wide range offlow parameters explored allowed us to observe that enhancement decreasessignificantly with the Taylor Reynolds number and is significantly affected bythe volume fraction $\phi_v$. We also studied the effect of large-scale forcingon settling velocity modification. The possibility of changing the inflowconditions 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 andtheir role on particle settling. This is achieved by characterising thesettling velocity at two different positions, the centreline and close to thewall, with the same streamwise coordinate.

An experimental study on the settling velocity of inertial particles in different homogeneous isotropic turbulent flows