Challenges of entrapping microbubbles in near-wall streamwise vortices*

It is well-known that bubbles and light particles tend to cluster inside vortices (low-pressure, high-enstrophy regions), even in complicated flows such as homogeneous isotropic turbulence. However, this trend does not appear to hold as well for sparse microbubbles in the presence of quasi-streamwise vortices of wall-bounded turbulence. To understand the fundamental physics of this phenomenon, we use DNS to study very small bubbles in simplified flow cases, such as a single vortex pair, and extend this to a 3D turbulent channel flow with a moderate Reynolds number of . We also simulate other small particles with different densities to understand the role of density in particles' behavior near quasi-streamwise vortices in wall-bounded turbulence. All particles are on the order of the Kolmogorov length scale of the turbulent channel and their paths are integrated using modified Maxey-Riley equations, and then correlated to Eulerian vortex structures—identified using the swirl criterion—to determine what limits bubble clustering in these structures.