Bubble Formation from an Air Jet Injected into a Turbulent Boundary Layer

Abstract

A theoretical and experimental study is conducted to investigate the bubble separation from an air jet injected into a liquid turbulent boundary layer. The following three patterns of bubble separation are dependent on the jet velocity: (a) a single bubble, (b) a coalescent bubble, and (c) a continuous jet. The critical jet velocity from (a) to (b) is theoretically estimated from the condition that the jet from the nozzle overtakes the rear end of the separated bubble, which shrinks due to surface tension. The calculated results generally reproduce the experimental results observed by a high-speed video camera. In addition, the process of bubble separation from the continuous jet (c) is classified into two patterns: (i) the bubble separates from the swell at the front end of the jet, or (ii) the jet breaks due to the instability at the liquid-gas interface. The separated bubble diameter of pattern (i) is theoretically determined by considering the force balance at the front end of the jet between the surface tension, the drag from the free stream, and the virtual mass force. The bubble diameter of pattern (ii) is calculated from the most unstable wavelength of Rayleigh instability. Both of these theoretical results also agree well with those obtained experimentally.