Effects of the wake flow on bubble patterns downstream of a cylindrical nozzle

The aim of the presented study is to reveal patterns of the bubbles traveling in liquid flow. Air was injected into flowing water via a cylindrical nozzle that protrudes in the flow. Bubble images were recorded using high-speed photography. Water flows under no-bubble condition were measured using particle image velocimetry (PIV). Effects of the mean water velocity and the air flow rate were considered. Three bubble patterns with distinct features were unveiled. They were purely rising bubbles, peeling-off bubbles from large air pocket, wake-dominated bubbles. As the mean water velocity increases, the probability density function of bubble size varies from unimodal to multi-mode distribution and eventually recovers to unimodal distribution. During this process, overall bubble size decreases continuously. The carrier flow imposes a significant effect on bubble patterns. For low mean water velocities, the driving force for bubble motion in streamwise direction is weak and bubbles rise in a well-organized manner. At the nozzle outlet, the sharp turning of streamlines results in the formation of large air pocket. Vorticity elements downstream of the nozzle are related to bubble patterns. As the mean water velocity increases, collapse of large bubbles is intensified and resultant small bubbles manifest wake flow features.