Fluid Motions Associated with Engineered Area Bubble Plumes

Current bubble oil booms (BOBs) create bubble curtains to protect fixed installations in calm waters such as harbors, but fail in more energetic environments. Field and lab studies reveal that area bubble plumes enable BOB applications in such environments. Area bubble plumes exhibit dynamics distinct from point- or line-source plumes, including thicker and more robust outwelling flows and relatively smaller turbulence scales. Area bubble plume studies broaden our understanding of natural systems, where currents and waves are ubiquitous, leading to a variety of marine applications. Laboratory studies in a large flume have investigated the fluid dynamics of shallow line and area bubble plumes. These studies focused on area-plume application to oil spill response, which, based on preliminary tests, featured improved outflows with a downstream barrier. A range of ambient currents, current–plume angles, and airflows were tested. Key to understanding current–plume interactions was the upstream rotor or near-surface recirculation vortex. The strongest outwelling was for a $0.25{\text{ms}}^{-1}$ current and 5-sparger plume (∼unity width-to-depth aspect ratio, $\epsilon$). Owing to acceleration by the rotor, however, the rotor collapsed into this plume at a current of $0.40{\text{ms}}^{-1}$, destroying plume coherency. The fastest surface outwelling was slightly outside the bubble plume edge, and horizontal profiles were well described by two piecewise exponentials. A 10-sparger plume with $\epsilon \sim 2$ was slower but more robust (thicker), allowing its rotor to survive in stronger currents. Vertical profiles of the outwelling flow were well described by an offset Gaussian (subsurface peak) with significantly lower velocity fluctuations than for line-source plumes. Streamline convergence from the rotor was important in reducing turbulence fluctuations. Oblique angle currents led to greater rotor acceleration with the surface outwelling flow largely perpendicular. However, velocities deeper than the main outwelling flow were deflected downstream, showing a highly nonlinear, helical flow in the rotor. These results highlight the importance of better understanding the complex flow fields of area bubble plumes in natural systems and marine engineering applications. Better characterization of the rotor is needed for a range of ambient conditions to enable BOB design optimization.