Vortical structures swept by a bubble swarm in turbulent boundary layers

Bubbles injected into a wall turbulent boundary layer can modify heat, mass, and momentum transfers of the wall. One factor responsible for the phenomenon is alternation of the vortical flow structures near the wall. To demonstrate this graphically, we directly visualized the interaction between the vortical flow structures and a bubble swarm composed of bubbles with various sizes, by means of two-color laser-sheet illumination of the wall turbulence with a dilute suspension of flakes. Image processing realized quantitative characterization of interaction events in the buffer and logarithmic layers, referring to multiple measured quantities such as the relative advection velocity between bubbles and turbulence, spacing of vortices both in streamwise and spanwise directions, persisting lengths of vortices in both directions, and statistical inclination angles of the streamwise vortices. A particular finding from the present visualization is that the streamwise vortices in the buffer layer were swept by the bubble swarm but brought to the backside of the swarm to survive until the swarm passed by.