Breaking Mechanism of Water Surface and Detailed Structure of Wave Breaking-Caused Air-Entrained Turbulence

本研究旨在研制一种新型超高速摄像机，尽可能准确地测量气-水界面破波的开始和气泡的形成和破裂，并通过室内实验和数值计算来研究破波引起的湍流。取得的主要成果如下：(1)新研制了一种带线电荷耦合器件（CCD）的1,000,000 fps的原位存储图像传感器（ISIS），并在国际上首次成功研制出1,000,000 fps的高灵敏度超高速摄像机。(2)卡尔曼滤波和卡方法（KC法）在跟踪波浪破碎后的水粒子运动方面得到了很大改进。利用新型超高速摄像机（1,000,000 fps）和改进的KC方法，在世界上首次以高精度拍摄了气泡的形成和崩塌过程。研究了粘度和表面张力对气泡形成和破裂的影响。(3)表面张力波在波的前表面形成并长大。在临界表面张力波生长的空气包裹作用下，前缘水面被打破，并在水体中引入气泡。(4) FFT方法和离散小波分析表明，气泡形成后，水压高频分量迅速增加，声波是由气泡在水中运动产生的。(5)声波压衰减与气泡形成点的距离成反比，用球面波传播理论可以很好地估计其大小。(6) DOLFIN是一种新的三维数值计算方法，通过扩展CIP和SMAC方法来评价固-水-气组合流动。

This research aims to develop a new ultra-high speed video-camera to measure the wave breaking inception and formation and collapsing of air bubbles at the air-water interface as accurately as possible and to investigate the breaking wave-caused turbulence by means of laboratory experiments and numerical calculation.Main results obtained are as follows :(1) An In-situ Storage Image Sensor (ISIS) of 1,000,000 fps with linear Charge Coupled Device (CCD) has been newly developed and an ultra-high speed video-camera of 1,000,000 fps with high sensitivity has been successfully manufactured for the fist time in the world.(2) Kalman filtering and Chi-square Method (KC Method) has been much improved to trace the water particle motion after wave breaking. Using the new ultra-high speed video-camera (l,000,000 fps) and the improved KC Method, air bubble formation and collapsing process are filmed with very high accuracy for the first time in the world. The effect of viscosity and surface tension to the air bubble formation and collapsing are investigated.(3) Surface tension waves are formed on the front surface of wave and are grown up. The front water surface is broken by the air-wrapping effect of the grown critical surface tension wave and air bubbles are introduced in the water body.(4) FFT method and discrete wavelet analyses have revealed that high frequency components of water pressure rapidly increase after formation of air bubbles and that the sound wave is generated by the motion of air bubbles in water.(5) The sound wave pressure decays inverse proportional to the distance from air bubble formation point and its magnitude is well estimated theoretically with the theory of spherical wave propagation.(6) 3-dimensional numerical computation method DOLFIN has been newly developed, by extending CIP and SMAC methods, to evaluate the solid-water-air combined flow.