Pulsating, buoyant bubbles close to a rigid boundary and near the null final Kelvin impulse state

This theoretical and computational study provides insight into the behaviour of large bubbles generated by underwater explosions near the seabed and airguns close to supporting structures when at least two opposing forces influence bubble behaviour. A null final Kelvin impulse occurs when these forces are in balance over a pulsation. Likewise for smaller bubbles such as occur in levitation phenomena for bubbles in a sound field, an 'equilibrium' bubble position is achieved. In both cases, energy dissipation mechanisms near minimum volume are important in determining subsequent bubble behaviour. Two cases typify the jetting behaviour near the null final Kelvin impulse state: (i) formation of an inward-flowing circular radial jet leading to bubble splitting, and (ii) formation of two opposite high-speed axial jets directed towards the bubble centre. The complex behaviour is attributed to a slight difference between the strength of the opposing forces acting on the bubble during growth and collapse. The present results indicate that the jetting behaviour in the neighbourhood of the neutral bubble collapse can be adequately described by the Kelvin impulse itself, but evaluated during the collapse phase of the bubble. Its direction determines the position of the radial jet in the initial phase of the collapse while its magnitude indicates the degree of asymmetry of the bubble-split and the intensity of the radial jet. Both factors are essential in estimating the final fate of the bubble at the neutral collapse state. Away from this null-state, the final Kelvin impulse is a valuable tool in predicting the migratory characteristics of the bubble and the direction of the axial jet developed during bubble collapse. (c) 2004 Elsevier Ltd. All rights reserved.