Effect of Stratification and Crossflow on the Dynamics of Bubble Plumes

Bubble plumes in stratified environments are very common in both environmental and industrial processes, with stratification influencing the mixing and material transport mechanisms. In this study, Eulerian–Lagrangian large-eddy simulations (LESs) of bubble plumes in density stratified environments are performed to investigate the peculiar features triggered by stratification and its influence on turbulent mixing and flow advection for different gas flow rates and crossflow velocities. The numerical solver is validated against experimental data of a bubble plume in a density stratified tank, where LES predictions show a remarkable agreement with the previously measured trap and peel heights. Spectral analysis at these heights coincides with the experimental energy peaks, although they are not as pronounced. For the first time we show that 3D Lagrangian particle tracking data of our model correctly captures the behavior of Type 1* plumes in agreement with experiments. With regard to stratified plumes in crossflow, transported material spreads laterally downstream of the plume in weaker crossflow, while stronger crossflow leads to vortex shedding. In both cases, initial stratification is largely broken in the top half of the tank, while the bottom half remains stratified. Both cases show a sharp increase in potential energy (PE) past the bubble plume release point; past the PE peak, weaker gas flow rate and crossflow cause a gradual decrease in PE, while stronger flow leads to a sharp decrease, followed by a small localized secondary peak corresponding to the location of the downward detrained fluid flow. Despite different conditions, similar mixing levels are achieved in the far downstream region. The analysis of the effect of crossflow on the plume width confirms that it deviates from the linear assumption below the separation height.