We present an experimental study on the variation in wave impact location and present a mechanism for the development of free surface instabilities on the wave crest for repeatable plunging wave impacts on a vertical wall. The existence of free surface instabilities on an impacting wave is well known, but their characteristics and formation mechanism are relatively unknown. The development of the global wave shape is measured using a visualization camera, whereas the local wave shape is measured with an accurate stereo-planar laser-induced fluorescence technique. A repeatable wave is generated with negligible system variability. The global wave behavior resembles that of a plunging breaker, with a gas pocket cross-sectional area defined by an ellipse of constant aspect ratio. The variability of the local wave profile increases significantly as it approaches the wall. The impact location varies by ∼0.5% of the wave height or more than a typical pressure sensor diameter. Additionally, the wave tip accelerates to a velocity of 1.5gh0 compared to the global wave velocity of 1.2gh0. The difference in impact location and velocity can result in a pressure variation of ∼25%. A mechanism for instability development is observed as the wave tip becomes thinner and elongates when it approaches the wall. A flapping liquid sheet develops that accelerates the wave tip locally and this triggers a spanwise Rayleigh–Taylor instability.

中文翻译：

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冲击波波尖变化的实验研究

我们提出了一项关于波浪冲击位置变化的实验研究，并提出了一种在波峰上产生自由表面不稳定性的机制，用于可重复的俯冲波冲击垂直壁。撞击波存在自由表面不稳定性是众所周知的，但其特征和形成机制相对未知。使用可视化相机测量全局波形的发展，而使用精确的立体平面激光诱导荧光技术测量局部波形。产生可重复的波，系统可变性可忽略不计。整体波浪行为类似于下沉式破碎机的行为，具有由恒定纵横比椭圆定义的气袋横截面面积。当接近壁面时，局部波剖面的可变性显着增加。撞击位置的变化范围为波高的 0.5% 或超过典型的压力传感器直径。此外，与全局波速 1.2gh0 相比，波尖加速到 1.5gh0 的速度。撞击位置和速度的差异可导致约 25% 的压力变化。当波尖在接近壁时变得更薄和伸长时，观察到不稳定发展的机制。拍动的液体层会产生局部加速波尖，这会触发展向瑞利-泰勒不稳定性。5gh0 与 1.2gh0 的全球波速相比。撞击位置和速度的差异可导致约 25% 的压力变化。当波尖在接近壁时变得更薄和伸长时，观察到不稳定发展的机制。拍动的液体层会产生局部加速波尖，这会触发展向瑞利-泰勒不稳定性。5gh0 与 1.2gh0 的全球波速相比。撞击位置和速度的差异可导致约 25% 的压力变化。当波尖在接近壁时变得更薄和伸长时，观察到不稳定发展的机制。拍动的液体层会产生局部加速波尖，这会触发展向瑞利-泰勒不稳定性。