Bubble deformation and migration under vortex breakdown in swirling pipe flows

In this study, we experimentally investigate the dynamics of a single bubble moving in a swirling pipe flow. By varying the inlet velocity (Reynolds number) and the blade design (to generate swirl), bubble trajectories and shapes were measured using high-speed shadowgraphy, and three-dimensional velocity fields were captured with stereo particle image velocimetry. While interacting with a swirling flow that is decaying along the streamwise (vertical) direction, the bubble tends to gradually migrate toward the center of the pipe, which was analyzed through a force model considering the contributions by pressure gradient, added mass, and lift forces. Interestingly, the bubbles located at the pipe center were observed to be decelerated substantially and simultaneously deformed (elongated) along the axial direction, reaching the aspect ratio up to 3.0. This deformation was hypothesized to be associated with vortex breakdown phenomena, which is caused by radial pressure gradients within the swirling flow. To explain this mechanism, we analyzed the axial velocity field in the frequency domain and identified both bubble- and conical-type vortex breakdowns, and we derived mathematical criteria for the onset of flow recirculation associated with vortex breakdown, which showed a reasonable agreement with experimental data. Finally, we will discuss the existence of an equilibrium aspect ratio of bubbles under external forcing by the swirling flow.