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在光滑明渠紊动水流的研究中,黏性底层具有重要的作用,壁面剪切应力与涡量均源于此流区。由于充分发展明渠紊流的黏性底层的物理尺度在0.1 mm量级,常规方法难以实际测量该区内的速度分布。本文构建了测量黏性底层的高分辨率HR-PTV与测量全流区的普通PIV的耦合测量系统,对光滑明渠紊流黏性底层的流动特性进行了研究。HR-PTV系统采用阈值递增法、灰度截断、椭圆度与充实度指标等方法进行图像处理得到清晰的粒子图像;使用匹配几率法对粒子坐标进行匹配,计算粒子速度。得到了明渠紊流黏性底层(0.3 mm以内)的平均流速、紊动强度、偏态系数和峰度系数的垂线分布。从拟合的平均流速分布得出摩阻流速u*,结果与牛顿流体本构方程一致;黏性底层与全流区的时均流速、紊动强度和雷诺应力均能用拟合所得的u*合理无量纲化,并能光滑连接,表明由此得出的u*适用于全流区。将明渠紊流的各种分布与封闭槽道流DNS数据进行对比,验证了HR-PTV在黏性底层所得各阶速度矩的准确性。
In the study of turbulent water flow in smooth open channel, the viscous underlayer plays an important role, and both the wall shear stress and vorticity originate from this flow regime. Since the physical scale of the viscous underlayer that fully develops the open channel turbulence is on the order of 0.1 mm, it is difficult for the conventional method to actually measure the velocity distribution in this area. In this paper, a coupled measuring system of high-resolution HR-PTV for measuring viscous underlayer and ordinary PIV for measuring full-flow region was constructed, and the flow characteristics of the smooth bottom channel with turbulent flow were studied. The HR-PTV system uses the threshold increment method, the gray-scale truncation, the ovality degree and the degree of substantialness index to carry out the image processing to obtain the clear particle image. The matching probability method is used to match the particle coordinates to calculate the particle velocity. The vertical distribution of average velocity, turbulence intensity, skewness coefficient and kurtosis coefficient of the open channel turbulent viscosity bottom (within 0.3 mm) was obtained. The friction velocity u * is obtained from the fitted average velocity distribution, and the result is consistent with the Newtonian fluid constitutive equation. The average velocity, turbulence intensity and Reynolds stress in the bottom and the full flow zone can be calculated using the fitted u * Reasonable dimensionless, and smooth connection, indicating that the resulting u * for the whole area. The distribution of open channel turbulence was compared with the closed channel flow DNS data to verify the accuracy of HR-PTV at each step of the viscous bottom layer.