平板湍流边界层瞬时摩擦阻力的光学测量和统计分析

采用近壁粒子图像测速（particle image velocimetry，PIV）技术实现了中低Reynolds数光滑平板湍流边界层瞬时摩擦阻力的测量.发展了单行互相关算法和迭代拟合技术估算瞬时摩擦阻力，其法向空间分辨率可达到2~4 pixel.作为对比，同时分析了相似Reynolds数范围的直接数值模拟数据.统计结果表明:摩擦阻力的脉动强度随Reynolds数升高呈现对数增长趋势，证实了外区大/超大尺度结构对壁面的影响.谱分析结果表明近壁区的低速条带结构主导着摩擦阻力的多尺度脉动特性，外区大/超大尺度结构对摩擦阻力的印迹作用较小，其主要通过调制作用实现对摩擦阻力的影响，这一结论可以从瞬时摩擦阻力的偏度以及概率密度函数曲线具有Reynolds数不变性得到进一步证实. 

Optical Measurement and Statistical Analysis of Instantaneous Wall-Shear Stress in a Turbulent Boundary Layer

The near-wall PIV technique was utilized to measure the instantaneous wall-shear stress (WSS) in a smooth-wall turbulent boundary layer at low-to-moderate Reynolds numbers. The algorithm of single-row cross correlation, together with a newly proposed iterative fitting method, was used to estimate the WSS. The spatial resolution of WSS was about 2~4 pixel in the wall-normal direction. Existing direct numerical simulation datasets in the same Re_τ range were also analyzed for direct comparison. The fluctuating intensity of WSS was found to follow the empirical log-law correlation with Re_τ, indicating the existence of the influence of outer-layer large-scale or very-large-scale motions (LSMs/VLSMs) on the wall. A further spectra analysis indicates that the near wall low-speed streaks dominate the dynamic behavior of the WSS. Combined with weak Re-dependency of both the skewness and probability density function of fluctuating WSS, it can be inferred that the LSMs/VLSMs leave a mild footprint on the wall. Instead, the amplitude modulation effect is inferred to play a more prominent role in affecting the statistics and multi-scale characteristics of WSS.