Control of Air Bubble Cluster by a Vortex Ring Launched into Still Water

Abstract

An experimental study searching for the possible generation and transport of a bubble cluster by a vortex ring in water is performed. A vortex ring is launched vertically upward into a water tank by discharging the water from a cylinder mounted at the bottom of the tank with a piston. Air bubbles are successively injected into the vortex ring from a needle attached to the cylinder outlet. The cylinder inner diameter D 0 and the piston stroke L 0 are 42.5 mm and 100 mm, respectively. The circulation of the vortex ring is less than 20000, and accordingly laminar vortex rings are launched. The mean diameter of the bubbles is 3.4 mm. The generation of bubble cluster and transport of the cluster by the vortex ring can be classified into four patterns according to the piston velocity (strength of the vortex ring) and the air volumetric flow rate. When the strength of the vortex ring is low, the bubbles are less affected by the vortex ring and instead rise with the buoyant force at a higher velocity than the vortex ring. With an increase in the strength of the vortex ring, the bubbles are entrained in the vortex core and form a cluster. The bubbles entrained in the vortex core circumferentially disperse around the vertical axis of the vortex ring, and they are successfully transported by the convection of the vortex ring. The convection velocity of the vortex ring is scarcely affected by the entrained bubbles, but the radius is enlarged slightly. The circulation of a vortex ring that entrains and transports air bubbles in this study is nearly accurately predicted by the formula of Milenkovic et al., which gives the circulation of a vortex ring entraining a single bubble in the vortex core. Index Terms-Bubble cluster, bubble entrainment, vortex ring, vorticity, visualization.