Sedimentation from particle clouds

Schematic illustrations of particle clouds settling in homogeneous (left) and stratified (middle, right) ambients.

We examine the settling of monodisperse heavy particles released into a fluid when the resulting motion is sufficiently vigorous that the particle cloud initially assumes the form of a turbulent thermal. A laboratory study is complemented by numerical simulations of particle cloud dynamics in both homogeneous and stratified ambients. In the homogeneous ambient, the cloud evolves in a manner consistent with a classical fluid thermal. The cloud grows through turbulent entrainment and decelerates until its speed is exceeded by that of the individual particles, at which point the particles rain out as individuals. Following fallout, the particles sink at their individual settling speeds in the form of a bowl-shaped swarm. In a stratified environment, the mode of fallout depends explicitly on the degree of stratification. nevertheless, following particle fallout, the fluid entrained by the thermal ascends and intrudes at a rebound height given to leading order by 3/4 times the fallout height. Criteria for three distinct modes of particle deposition in a stratified ambient (left) are developed.

See paper:  Bush, Blanchette & Thurber, JFM (2003).