Wake structures and mixing of falling spheres and rising bubble in density-stratified fluids

Density stratification is common in nature. Oceans, lakes, and the atmosphere have density-stratified environments. Vertical movement of particles, droplets, and bubbles can induce mixing and subsequently enhance the transport of heat, carbon dioxide, and nutrients from the upper layer to the lower layers of the ocean. Studying the wake structure of moving bodies in the stratified fluid would help us to understand the underlying process that enhance mixing. Previous studies confirmed the presence of a reverse jet in the wake of descending spheres in a stratified fluid. As the sphere moves, it drags down the lighter fluid to deeper layers with higher density. Due to buoyancy forces, the lighter fluid moves back to upper layers and forms the inverse jet, making the mixing process less effective. The same phenomenon has been observed for bubbles rising in the stratified fluids. Even though the wake structure of different bodies has been studied extensively, there hasn’t been a study comparing the wake structure between falling spheres or rising bubbles in a stratified fluid. In this study, we investigate the wake structure of a falling sphere and a rising bubble with similar non-dimensional numbers (Re and Fr). The flow field and the reverse jet is quantified using 3-D Particle Tracking Velocimetry (PTV) method. The comparison between the two cases of rising bubbles and falling spheres will help us identify the more efficient way of induced mixing by moving bodies in stratified fluids.