Experimental study of forces on freely moving spherical particles during resuspension into turbulent flow

Abstract

Turbulent resuspension, a process of lifting solid particles from the bottom by turbulent flow, is ubiquitous in environmental and industrial applications. The process is a sequence of events that start with an incipient motion of the particle being dislodged from its place, continue as sliding or rolling on the surface, ending with the particle being detached from the surface and lifted up into the flow. In this study the focus is on the resuspension of solid spherical particles with the density comparable to that of the fluid and the diameter comparable with the Kolmogorov length scale. We track their motion during the lift-off events in an oscillating grid turbulent flow. We measure simultaneously the Lagrangian trajectories of both the particles freely moving along the bottom smooth wall and the surrounding flow tracers. Different force terms acting on particles were estimated based on particle motion and local flow parameters. The results show that: \emph{i}) the lift force is dominant; \emph{ii}) drag force on freely moving particles is less relevant in this type of resuspension; \emph{iii}) the Basset (history or viscous-unsteady) force is a non-negligible component and plays an important role before the lift-off event. Although we cannot estimate very accurately the magnitude of the force terms, we find that during the resuspension they are within the range of $2\div10$ times the buoyancy force magnitude. The findings cannot be extrapolated to particles, which are much smaller than the Kolmogorov length scale, or much denser than the fluid. Nevertheless, the present findings can assist in modeling of the sediment transport, particle filtration, pneumatic conveying and mixing in bio-reactors.