明渠瞬时床面切应力粒子图像测量技术

床面切应力瞬时值的实验测量极为重要但富有挑战。为了利用粒子图像技术准确测量瞬时床面切应力，采用模拟不同图像和流动参数的合成粒子图片对粒子图像测速技术（PIV）、粒子图像示踪技术（PTV）和直线相关PIV技术（LCPIV）的测量精度进行分析，发现各技术在床面附近的测量精度均因床面影响而变差，其中PIV技术受影响的范围距床面不小于判读窗口高度，PTV及LCPIV技术受影响的范围等于粒子图像半径；在床面影响范围外，PTV技术可实现无偏差测量，并在粒子图像密度较小且粒径适中时拥有高于PIV和LCPIV的精度。比较而言，PTV更适于测量瞬时床面切应力。将PTV技术用于雷诺数分别为8007和11462的明渠均匀紊流瞬时床面切应力实验测量，进一步验证了该技术无偏、高精度测量的能力，发现了明渠紊流瞬时床面切应力不符合高斯分布，离散系数约为0.4，偏度系数和峰度系数随雷诺数增大而逐渐变大。研究结论为明渠紊流瞬时床面切应力的实验测量提供理论依据和实践参考。

Particle Image Techniques for Measuring Instantaneous Bed Shear Stress in Open Channel Flows

Measurement of instantaneous bed shear stress in open channel flows is challenging but extremely important. In order to accurately measure the instantaneous bed shear stress with particle image techniques, synthesized particle images with different image and flow parameters were applied to evaluate the measurement precision of particle image velocimetry (PIV), particle tracking velocimetry (PTV), and line correlation PIV (LCPIV). The measurement precision of all techniques in the near bed region were found to be degraded by the bed. The wall-normal height of the bed influence equals the particle image radius for PTV and LCPIV, and is no shorter than the height of the interrogation window for PIV. Outside of the bed influenced region, the measurement of PTV is unbiased and is with the highest precision when both the particle image density is relatively small and the particle image diameter is medium. Therefore, PTV is more suitable for the measurement of instantaneous bed shear stress. The ability of PTV for unbiased and high precision measurement of instantaneous bed shear stress was further validated with experiments in two open channel flows with a Reynolds number of 8007 and 11462, respectively. The experiment results also indicated that the distribution of instantaneous bed shear stress in open channel turbulence, with a coefficient of variation of about 0.4, is non-Gaussian. Meanwhile, both the skewness and kurtosis increase with the Reynolds number of the flow. The results provide theoretical foundation and practical reference for measuring instantaneous bed shear stress in open channel flows.