The stratified Boycott effect

Abstract

We present the results of an experimental investigation of the flows generated by monodisperse particles settling at low Reynolds number in a stably stratified ambient with an inclined sidewall. In this configuration, upwelling beneath the inclined wall associated with the Boycott effect is opposed by the ambient density stratification. The evolution of the system is determined by the relative magnitudes of the container depth, h, and the neutral buoyancy height, h n = c 0 (ρ p − ρ f )/|dρ/dz|, where c 0 is the particle concentration, ρ p the particle density, ρ f the mean fluid density and dρ/dz < 0 the stable ambient stratification. For sufficiently weak stratification, h < h n , the Boycott layer transports dense fluid from the bottom to the top of the system; subsequently, the upper clear layer of dense saline fluid is mixed by convection. For sufficiently strong stratification, h > h n , layering occurs. The lowermost layer is created by clear fluid transported from the base to its neutral buoyancy height, and has a vertical extent h n ; subsequently, smaller overlying layers develop. Within each layer, convection erodes the initially linear density gradient, generating a step-like density profile throughout the system that persists after all the particles have settled. Particles are transported across the discrete density jumps between layers by plumes of particle-laden fluid.

On basic equations and kinematic-wave theory of separation processes in suspensions with gravity, centrifugal and Coriolis forces

“…At low Reynolds numbers, instabilities that lead to fluctuations have been observed [91,92], in particular in sedimentation of polydisperse suspensions [93] and in a stratified ambient [94]. An analysis of those phenomena in the framework of kinematic-wave theory seems to be lacking, but is beyond the scope of the present paper.…”

Tracking velocity of multiple bubbles in a swarm

“…This could induce convection currents (Boycott effect, e.g. Peacock et al, 2005) but this should not alter the observed frother-type effect. Recent work holding bubbles stationary in down-flowing water continues to support this frother-type effect on fine bubbles (Navarra et al, 2009).…”

Optimization of Circular Plate Separators with Cross Flow for Removal of Oil Droplets and Solid Particles

“…Generally, the mechanisms of oil-water-solid gravity separation include the principles of Stokes' Law (Perry and Green, 1997) and the boycott effect (Peacock et al, 2005). Oil and solids in water are characterized by a spectrum of droplet sizes.…”