Enhancement and reduction of settling velocity of heavy particles in homogeneous turbulence

Several mechanisms have been identified by which turbulence may either enhance or reduce the settling velocity of heavy particles. However, the conditions under which some of them may dominate over others are not well known, thwarting our ability to predict the net outcome. We investigate this issue experimentally, focusing on spherical particles of size comparable to the Kolmogorov scale in homogeneous turbulence. We combine results obtained in two zero-mean-flow chambers working with water and air and various particle densities, leveraging high-resolution imaging of both phases and spanning a substantially wider range of parameters compared with previous studies. The change in settling velocity is found to be dictated by a single parameter: the ratio between the still-fluid fall speed and the characteristic velocity of the turbulent fluctuations, with settling enhancement and reduction occurring when this is smaller and larger than unity, respectively. This marks the transition from particles being preferentially swept by downward fluctuations to them loitering in upward ones. Counterintuitively, nonlinear drag effects are most significant in the regime of settling enhancement, while their contribution to settling reduction is modest.