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The stratified Boycott effect


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, $h_n\,{=}\,c_0(\rho_p-\rho_f)/|{\rm d}\rho/{\rm d}z|$, where $c_0$ is the particle concentration, $\rho_p$ the particle density, $\rho_f$ the mean fluid density and ${\rm d}\rho/{\rm d}z\,{<}\,0$ 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.

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Journal of Fluid Mechanics
  • ISSN: 0022-1120
  • EISSN: 1469-7645
  • URL: /core/journals/journal-of-fluid-mechanics
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