Color-VTR image analysis on catastrophic deformations of water waves

发展了高速彩色录像机图像的空间相关分析技术，用于研究破碎、漫顶等灾害性波浪变形的流动机理。采用聚苯乙烯微珠作为示踪剂进行流动显示。为了确定图像分析中示踪剂混合、照明、视频记录和帧大小的最佳条件，反复试验和错误。像素数据的统计表明，需要进一步改进可视化技术，以获得具有广泛色调的图像。在本研究中，利用分级亮度数据的定量增加，即结合红、绿、蓝三个分量图像的三个相关系数，实现了高可靠性的流速检测。通过对流动连续性残差的评估，验证了所得到的速度矢量场的准确性和可靠性，并在实验水池中再现了固定坡度上的孤立波，提取了内部流速。通过对速度场的全面描述，可以推导出压力场和涡量场。因此，在波峰区发现的极温和的压力梯度可能在破裂现象中起到重要作用。为了研究漫顶孤立波中的流场，建立了垂直防波堤模型，并对其进行了数值模拟。在防波堤的上前角附近出现了一个以切变流为主的流动，在那里发现了强烈的涡旋。漫顶流速一般在波峰以下较高，最大值出现在水面以下。

Technology of spatial correlation analysis of high-speed color-VTR imageries was developed for investigating flow mechanisms of catastrophic wave deformations such as breaking and overtopping. Polystirene beads were used as tracers for flow visualization. Trials and errors were repeated to determine the optimum conditions of tracer mixing, lighting, video recording, and frame sizes in image analysis.Statistics of pixelwise data implies that further improvement in visualization techniques are required to obtain imageries with widely ranged colortones. In this study, flow velocity detection with high reliability was realized by taking advantage of quantitative increase in graded brightness data, that is, by incorporating three correlation coeffcients for red, green, and blue component imageries. Accuracy and reliability of velocity vector fields thus obtained were examined through evaluation of residual error in terms of flow continuity.Solitary wave overturning over a fixed slope was reproduced in an experimental basin, and internal flow velocities were extracted. Thorough description of velocity fields allowed the deduction of pressure and vorticity fields. Extremely mild pressure gradient thus found in the wave crest region may play an essential role in the breaking phenomenon. Notable vorticity was observed below the water surface on the rear side of the crest.A vertical breakwater model was installed for investgation of the flow field in an overtopping solitary wave. A predominant shear-flow appeared near the upper-front corner of the breakwater, where strong vorticities were detected. The overtopping flow velocity is high in general under the wave crest, and its maximum appeared slightly below the surface.