On convection and mixing driven by sedimentation

Abstract

The sedimentation of small particles from a suspension and the concomitant release of light interstitial fluid may constitute a buoyancy source for the development of convective motions. When the dense suspension is emplaced beneath a stratified fluid an intermediate convecting layer between the sedimenting front and the density gradient above gradually grows in depth by erosion of the overlying stratified fluid. Novel laboratory experiments involving sedimentation below a two-layer stratified region show that turbulent mixing and entrainment across the top density interface is significant for a broad range of the Richardson number. A simple theoretical model predicting the rate of erosion of the stratification above the convecting layer agrees well with these experiments. The model is then extended to include the case of an overlying continuous density gradient and compared successfully with both new experimental data and the original data of Kerr (1991). Owing to the effects of dispersion of grain sizes, small particles in the convecting fluid may lower the efficiency of the interfacial mixing by the turbulent eddies. Our model calculations suggest that turbulent mixing and entrainment driven by sedimentation may be significant in the atmospheric and oceanic contexts, in both of which stratification is weak. Such mixing may also occur in molten magma chambers following the sedimentation of suspended crystals, and in this case it may suppress large-scale overturning events.