Dynamics of a Single Bubble Rising in Confined Stratified Flow*

In oceans, density varies with depth due to varying salinity, which captures myriads of pollutants such as plastics, rubber, etc. that drastically affect marine wildlife. The rising motion of a single or cluster of bubbles creates a vertical upflow that can transport the buoyant sediments to the surface for efficient waste removal. To begin realizing this complex multi-phase flow system, we start with a simplified problem of a single millimeter-sized air bubble rising in a 2.5mm confined channel. We performed a time-resolved stereoscopic 2D3C Particle Image Velocimetry (PIV) measurement to characterize the bubble wake. Pure water and varying salt concentration were used to achieve a linear density stratification corresponding to Froude numbers (Fr) ranging from 20-40. Due to the large dynamic velocity range (60) for PIV, we enhance our cross-correlation algorithm with pyramid correlations. The rising bubble generates vortices that shed downstream and decay with varying timescale for different Fr. The wake of the bubble carries the higher density fluid to the top, which later releases from the wake to form the reverse jet. This process enhances the mixing causing destratification. The coherent structures and the jet are characterized as a function of the Froude number.