Time-resolved particle image velocimetry measurements were performed in smooth-walled open channels to investigate the contributions of very large-scale motions (VLSMs) to the turbulence characteristics in open channel flows. The focal point is to clarify the free surface effects on the characteristics of VLSMs and the contributions of VLSMs to the unique statistical features in open channel flows (i.e., the turbulent kinetic energy (TKE) redistribution and smaller wake strength of the mean velocity profile). The resulting wavelength of VLSMs in present smooth-walled open channels is approximately $20h$ ( $h$ is water depth), which is comparable to that in pipe and closed channels while smaller than that in rough-walled open channels, and they are shown to make a great contribution to turbulence statistics with over 50 % of streamwise turbulence intensity, Reynolds shear stress and negative net force coming from VLSMs in the outer layer. Compared with other wall-bounded flows, VLSMs maintain higher strength in the outer layer of open channel flows with non-negligible strength even in the near surface region ( $y\sim >0.8h$ ), indicating that the free surface seems to sustain/promote VLSMs. This strength difference of VLSMs closely relates to the TKE redistribution and smaller wake strength of the mean velocity in the outer layer of open channel flows. The higher streamwise turbulence intensity is mainly contributed from the higher strength of VLSMs therein. The decelerating role of VLSMs combining with their higher strength is vital for shaping the mean velocity profile, which therefore is speculated to make a great contribution to the smaller wake strength phenomenon.

中文翻译：

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超大规模运动对明渠流湍流统计的贡献

在光滑壁明渠中进行时间分辨粒子图像测速测量，以研究超大规模运动 (VLSM) 对明渠流动湍流特性的贡献。重点是阐明自由表面对 VLSM 特性的影响以及 VLSM 对明渠流中独特统计特征的贡献（即湍动能 (TKE) 重新分布和平均速度剖面的较小尾流强度） . 目前光滑壁明渠中 VLSM 的最终波长约为 $20h$（$h$ 是水深），与管道和封闭通道中的波长相当，但小于粗糙壁明渠中的波长，并且它们被证明对湍流统计做出了巨大贡献，超过 50% 的流向湍流强度、雷诺剪切应力和来自外层 VLSM 的负净力。与其他壁面流动相比，VLSMs 在明渠流动的外层保持更高的强度，即使在近表面区域（$y\sim >0.8h$）也具有不可忽略的强度，表明自由表面似乎可以维持/ 促进 VLSM。VLSMs 的这种强度差异与明渠流外层平均速度的 TKE 再分布和较小的尾流强度密切相关。较高的流向湍流强度主要是由于其中 VLSM 的强度较高。VLSM 的减速作用与其更高的强度相结合，对于塑造平均速度剖面至关重要，