Modal switching of bubbly Taylor–Couette flow investigated by particle tracking velocimetry

Injection of bubbles into Taylor–Couette flow creates new flow modes which have not been experienced in single-phase flow conditions. An array of spiral vortices array is one such mode departing from the original toroidal Taylor vortices. The aim of the present study is to explore the mode transition of the two-phase flow using particle tracking velocimetry. Vertical-axis concentric cylinder with a rotating inner cylinder was used to measure the motion of both phases in the range 600 < Re < 4500. From time–space mapping of the experimentally measured stream functions, we found that bubble injection intensified the axial displacement of the wavy Taylor vortices in the toroidal array, and there was a smooth switching to a spiral array when the azimuthal traveling velocities coincided between two phases. During this vortical reconnection, the two counter-rotating vortices were attenuated asymmetrically. After the two phases co-organized a spiral array, the spiral-maintaining mechanism was weakened to restore the toroidal vortices. These forward and backward mode transitions occurred periodically as bubble injection flow rates increased. We, therefore, concluded that modal switching was an evidence of two-way bubble–vortex interaction for a maximized state, causing the largest drag reduction reported in this regime.