Retardation of a sphere settling in a vertically oscillating fluid

Abstract

It has been shown that a particle settling through a vertically oscillating flow settles more slowly than through a quiescent fluid. The phenomenon is referred to as retardation and has found fruitful application in a multitude of industrial separation processes, such as in baffled tubes and jigs.While published experimental works partly document the influence of material properties (particle to fluid density ratio rho_s =rho_p/rho_f, fluid kinematic viscosity v) and flow characteristics (oscillation frequency f, ratio of amplitude to particle diameter A/D_p) on retardation, the mechanism behind slowed settling remains unclear. Furthermore, a universal law describing the retarded settling velocity remains unknown. To this end, a parametric study across the aforementioned material and flow properties is conducted by means of grain-resolved Direct Numerical Simulations (DNS).The fully resolved flow data around the oscillated particle obtained by this study allowed precise measurement and detailed description of the pressure and skin drag force. A strong link between retardation magnitude and the contribution of the pressure force Fdp to the net-lift was found.Differences in flow-structures during the particle’s upward and downward-movement are brought forward, we suggest they are linked to an asymmetry in the pressure force Fdp increasing along retardation. Additionally, the influence of a horizontally confined domain on particle retardation was found to be significant. A novel dimensionless parameter is proposed for which a wide range of prior experimental data and simulation data obtained from this study collapse. Finally, levitation(infinitely suspended particle) as an extreme case of retardation was observed. However, the observation of this behaviour might be an artefact of geometric limitations.