Interaction of two approximately equal-size bubbles produced by sparks in a free field

Abstract

A series of experiments are performed to study the interaction of two bubbles of approximately equal size in free-field water for different parameter combinations (i.e., the phase difference $\theta$ and the initial distance $d^{*}$ ). First, a high-voltage discharge device that can generate multiple bubbles of different sizes in water and accurately control the time of bubble generation is introduced. For different discharge voltages, the maximum radius and the first oscillation period of a single bubble and the shock wave energy and bubble energy conversion rate are studied through high-speed photography, current monitoring, voltage probes, and an underwater free-field pressure sensor. Then the distance of the center of mass and average radius of two bubbles generated at different distances at the same time are compared with a theoretical model and good consistency is obtained when the initial distance is greater than the bubble diameter. Finally, the interaction phenomena of two bubbles generated with different initial distances and phase differences are divided into five categories, namely, coalescence, failed catapult, piercing jet, jet toward, and jet away, and the boundary conditions used to distinguish different phenomena are given. For the piercing jet, a high-speed fine jet will be generated in the bubble generated later. The jet will penetrate the wall of the bubble and propagate steadily in the water at high speed (∼79 m/s). The velocity of the jet in the bubble and in water generated with different parameter combinations is statistically analyzed, and the velocity distribution cloud diagrams are given.