Enhancement and reduction of particle settling velocity in homogeneous turbulence*

The settling of inertial particles in turbulent flows, relevant for environmental and engineering settings alike, is complicated by the stochastic nature of the carrier-phase flow and dispersed-phase distribution. Turbulence can both enhance or reduce the mean settling velocity, and predicting the dominant mechanisms is an ongoing challenge.

Here we experimentally investigate the behavior of heavy particles settling in homogeneous turbulence, where the particle diameter is comparable to the Kolmogorov scale. We utilize a zero-mean-flow water chamber featuring randomly actuated jet arrays, in which spheres of different densities are released. The Taylor-scale Reynolds number falls within the range of 234-587, and the particle Stokes number ranges from O(0.1) and O(1). The motion of the particles and the turbulent flow field are simultaneously measured using particle tracking velocimetry and particle image velocimetry, respectively.

We confirm that the enhancement/reduction of the settling velocity depends primarily on the ratio between the quiescent-fluid settling velocity and the root-mean-square fluid velocity fluctuation, with a trend that reconciles previous experiments in both air and water. We discuss the respective roles of preferential sampling of upward/downward fluid fluctuations, as well as non-linear drag effects.