The bubble dynamics behaviors and the pressure in the wall center are investigated through experimental method and numerical study. In the experiment, the dynamics of an underwater explosion (UNDEX) bubble beneath a rigid wall are captured by high-speed camera and the wall pressure in the wall center is measured by pressure transducer. To reveal the process and mechanism of the pressure on a rigid wall during the first bubble collapse, numerical studies based on boundary element method (BIM) are applied. Numerical results with two different stand-off parameters (γ=0.38 and γ=0.90) show excellent agreement with experiment measurements and observations. According to the experimental and the numerical results, we can conclude that the first peak is caused by the reentrant jet impact and the following splashing effect enlarged the duration of the first jet impact. When γ=0.38, the splashing jet has a strong impact on the minimum volume bubble, a number of tiny bubbles, formed like bubble ring, are created and collapse more rapidly owing to the surrounding high pressure and emit multi shock waves. When γ=0.90, the pressure field around the bubble is low enough only a weak rebounding bubble peak occurs.
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