Single bubble dynamics and entrainment-driven destratification mechanism in density-stratified aquatic environment

The bubble plume technique is a promising approach for enhancing water quality through destratification and oxygenation. Although the applications of bubble plume technology in density stratification mixing have been investigated, there is limited research on bubble dynamics and their mixing characteristics under these conditions. The present study investigates the dynamic characteristics of a single bubble rising and traversing the pycnocline in a density-stratified environment through visualization experiments. The findings reveal that the velocity and trajectory of a single bubble change as it approaches and passes through the pycnocline. The bubble exhibits acceleration phases in both the heavy and light layers, reaching terminal velocities in each. Crucially, during pycnocline transit, a secondary acceleration based on its terminal velocity in the heavy layer is observed, followed by deceleration to velocities below the terminal velocity. Following pycnocline penetration, alterations in amplitude and frequency of zigzag trajectory oscillations are observed, with the extent of variation exhibiting a direct correlation with bubble dimensions. A single bubble penetration through the pycnocline induces entrainment of heavy layer fluid, with observed fluid detachment and subsequent sedimentation phenomena elucidating the mechanism of bubble-induced destratification. For single bubbles, larger volumes correlate with enhanced entrainment capabilities. The critical condition that characterizes secondary acceleration in a density-stratified environment is modeled through dimensionless numbers. This approach enables the assessment of the secondary acceleration occurrence of a single bubble within the pycnocline under varying operating conditions.