采用高速摄像和数字图像处理技术求解强紊动明渠水流自由面

Application of High Speed Photography and Digital Image Processing to the Surveying of the Free Surface of Strongly Turbulent Open Channel Flow

针对强紊动明渠水流流速大、自由面形状难以测定的情况,使用高速摄像机以5000 fps的速率采集了陡槽中强紊动水流的高清晰数字图像,根据数字图像处理的基本原理,采用最大类间差法确定了所采集图像的阈值,并使用该阈值对所采集的图像进行了分割,得到了紊动水流的二进制图像,并基于八连通准则对该二进制图像中水流的边界进行了搜索检测,求得了水流边缘像素的位置坐标,确定了水流自由面的形状。结果表明,当陡槽中水流的平均流速达到7m/s后其自由面极不规则,存在着明显的柱状、团状突起,甚至破碎;这些突起的运动变形,造成了自由面的形状在短时间内变化剧烈,有可能发生包裹、卷吸作用而促进水气交换。最后对误差可能的产生来源进行了分析,指出了进一步的研究目标。

For the high speed and strongly turbulent open channel flow, it is difficult to determine its free surface. The high speed video camera was used to take the definitional photographs with resolution of 5000 frames per second. The thresholds of those photographs were computed using the Otsu＇ s method. Those collected images were converted to the binary images based on the calculated thresholds. According to the principle of 8th communicating set, the edges of the flow in the images were examined and found, then the coordinates of the boundary pixels were ascertained and the shapes of the free surfaces were determined. It was found that the free surfaces deform remarkably and such deformations lead to surface roughness, breakup and disintegration as the velocity of the flows is about 7.1 m/s. The splashing, projections and bulks were generated on the surfaces and strongly distorted. The kinematics of these projections and cusps may be the reason of aeration.