Transport, evolution, and induced mixing of bubble swarms across pycnoclines

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The density of water in lakes and oceans is far from uniform; it varies over length scales from meters to a kilometer. This variation in density (or density stratification) can be due to two reasons: i) thermal, wherein the density varies due to variations in local temperature or, ii) saline, wherein the density varies due to variations in the salt concentration. The subsurface layers across which this change in density is observed are called pycnoclines. Researchers often neglect the effects of density stratification on the motion of rising bubbles and droplets and the induced mixing of the background fluid. However, the PIs showed that such density gradients can significantly affect the ascent of small drops by enhancing the drag experienced by them. Quantifying the impact of density stratification on bubbles and drops is an essential, yet under-explored area of fluid dynamics, and the need to better understand this area is prevalent in different multiphase phenomena pertaining to lakes and oceans, for example, destratification of water reservoirs by air-bubble plume systems, release of methane in marine sediments, dispersion of an oil plume during an oil spill, and the use of artificially generated bubble plumes to lower surface temperatures in order to arrest excessive evaporation from lakes. The new physical insights to be gained in this study, along with the numerical techniques that have been and will be developed, will be significantly beneficial for a broad range of other researchers who are devoted at the interface between fluid dynamics, hydrology, and oceanography. This research will use state-of-the-art, experimentally validated computational tools to quantify the effects of density stratification on (1) the mixing of the background fluid by laminar bubbly upflows in density stratification, (2) the migration velocity and microstructure formation of bubble/droplet swarms, and (3) the mixing and the motions of bubbles/drops in the presence of background turbulence. The research will pave the way to systematically investigate the motion of multiphase flows at pycnoclines and the resultant mixing. The extent of mixing will be quantified by computing the mixing efficiency (a measure of the rate of increase in background potential energy due to mixing against the rate of dissipation of kinetic energy in the process), the diapycnal eddy diffusivity (a measure of mixing due to vertical transport of the fluid) and the temperature micro-structure. The properties of the bubble/droplet swarms will be quantified by calculating the drift velocity and the fluctuation velocities of the dispersed and continuous phases, and pair probability distribution functions for the dispersed phase.