Settling velocity of weakly inertial particles in vertical flow

We investigate the settling velocity change of weakly inertial particles, whose density ratio to fluid ranges from 1.35 to 1.38, in vertical water flow. To assess the effect of turbulence, we experimentally examine the dependence of modifications of velocity on physical scales, including time, velocity, and length, between particles and turbulence. It is observed that the settling velocity is either enhanced or hindered by the turbulence compared to stagnant conditions. The change in settling velocity is observed to be responsive to both the inertia of particles and the turbulence intensity. In cases of weak turbulence or with larger particles, the settling velocity exhibits small changes and even decreases. Conversely, the change in settling velocity is more pronounced for smaller particles and in more intense turbulence, reaching a maximum increase at $St\approx O\left(10^{-3}\right)$. We compare our experimental results with existing studies conducted in solid–liquid two-phase flow, finding a consistent tendency. In both prior research and the present study, the length scale parameter, $StSv$, has consistently been important in discerning inertial conditions that determine the change in settling velocity under turbulent conditions.