Abstract
The dynamic behaviors of an oscillating bubble (e.g., underwater explosion bubble, cavitation bubble and air-gun bubble) are well known to be strongly dependent on the nature of boundary conditions. Many experiments demonstrated that a high-speed liquid jet is formed away from a free surface or towards a nearby rigid wall. The violent jet impact is believed to be one of the most important mechanisms of cavitation erosion and damages by an underwater explosion. In the previously published literature, the Kelvin impulse based on spherical bubble theory is adopted to determine the gross migration and jet direction of bubbles. However, the underlying mechanisms of jet inception and development are not fully understood and the characteristics of the jet impact still lack exploration. In the present work, both experimental and numerical methods are adopted to do some fundamental studies on bubble dynamics beneath a free surface and near a rigid wall. The electric discharge method is used to generate a bubble and the bubble motion is captured by a high-speed camera. Meanwhile, the boundary integral method is adopted to conduct numerical simulation. The presence of a nearby boundary alters the pressure gradient surrounding the bubble, which has a significant influence on the jet inception. Additionally, a local high-pressure region is generated near the bubble bottom, and it results in a positive feedback mechanism that further accelerates the jet. This mechanism reveals the fact that the jet can speed up to a hundred meters per second within a relatively short time. A localized high-pressure region is caused by the jet impact around the jet tip and the maximum pressure decreases gradually as the rebound of the toroidal bubble. At last, the effect of the dimensionless standoff parameter (defined as $\gamma = d / R_{m}$, where $d$ is the distance between the initial bubble center and the rigid wall and $R_{m}$ is the maximum bubble radius) on the jet impact pressure is discussed.
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