Formation mechanism of water jets induced by the interaction between bubble and free surface

Abstract

Explosion in shallow water or small depth water will generate upward water jet, mainly because bubbles generated by explosion will interact with the surface of water. Different underwater depths can result in upward water jets with different kinds of shapes, such as water column, water plume, jet, spall dome, splash, spike, etc. To reveal the formation mechanisms of different types of water jets, a spark bubble experiment platform is set up, and the motions of bubble and free surface are studied experimentally by high-speed photography. The dynamic images for the formation process of the water jets under different initial depths of bubble are obtained. Through theoretical analysis and direct observation of the experimental data, the interaction process between the oscillating bubble and free surface are clarified, and the evolution rule of water jets is obtained. It is found that the key factor affecting the formation of different shapes of the water jets is the superposition of the disturbance of the second bubble pulse and the simple-shape jet induced by the first bubble pulse. Five types of the superpositions are summarized:1) All-fit type, with a large depth of initial bubble, the first and the second bubble impulse fit well to form a smooth and slightly arched water dome; 2) partial-fit type, with a less large depth of initial bubble, higher arched water dome is formed due to the raising effects of second bubble pulse partially fit the initial water dome shape; 3) catch-up type, with a mediate depth of initial bubble, the free-surface jet caused by first bubble pulse will be caught up from the bottom by the second pulse, and form a thin and high velocity jet; 4) run-after type, with a smaller depth of initial bubble, the free-surface jet caused by first bubble pulse will be raised from the bottom by the second pulse, and form a jet with thin head and thick pedestal, sometimes form a crown-type splash; 5) non-superposition type, the depth of initial bubble is so small that the bubble will break up, and no superposition will happen. In summary, the ratio of the initial depth to the maximum radius of bubble is found to be a decisive factor of the superposition type. The initial bubble is described by a dimensionless distance. These conclusions well explain the phenomena observed in experiment, and can provide a new vision and reference to the understanding of the formation mechanism of water jets induced by the interaction between bubble and free surface.