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首页> 外文期刊>Water resources research >Statistical analysis of turbulent super-streamwise vortices based on observations of streaky structures near the free surface in the smooth open channel flow
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Statistical analysis of turbulent super-streamwise vortices based on observations of streaky structures near the free surface in the smooth open channel flow

机译：基于光滑明渠流动中自由表面附近的条纹结构观测值的湍流超涡流统计分析

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摘要

Long streamwise-elongated high- and low-speed streaks are repeatedly observed near the free surface in open channel flows in natural rivers and lab experiments. Super-streamwise vortex model has been proposed to explain this widespread phenomenon for quite some time. However, statistical evidence of the existence of the super-streamwise vortices as one type of coherent structures is still insufficient. Correlation and proper orthogonal decomposition (POD) analysis based on PIV experimental data in the streamwise-spanwise plane near the free surface in a smooth open channel flow are employed to investigate this topic. Correlation analysis revealed that the streaky structures appear frequently near the free surface and their occurrence probability at any spanwise position is equal. The spanwise velocity fluctuation usually flows from low-speed streaks toward high-speed streaks. The average spanwise width and spacing between neighboring low (or high) speed streaks are approximately h and 2h respectively. POD analysis reveals that there are streaks with different spanwise width in the instantaneous flow fields. Typical streamwise rotational movement can be sketched out directly based on the results from statistical analyses. Point-by-point analysis indicates that this pattern is consistent everywhere in the measurement window and is without any inhomogeneity in the spanwise direction, which reveals the essential difference between coherent structures and secondary flow cells. The pattern found by statistical analysis is consistent with the notion that the super-streamwise vortices exist universally as one type of coherent structure in open channel flows.
机译：在天然河流和实验室实验中，在明渠的自由表面附近反复观察到长的沿河道伸长的高速和低速条纹。提出了超流涡模型来解释这种普遍现象已有相当长的一段时间。然而，关于作为一种相干结构的超流涡旋存在的统计证据仍然不足。基于PIV实验数据的相关性和适当的正交分解（POD）分析，该数据在光滑明渠流中靠近自由表面的流向-展向平面中进行研究。相关分析表明，条纹结构频繁出现在自由表面附近，并且它们在任何翼展方向位置上的出现概率均相等。翼展方向速度波动通常从低速条纹流向高速条纹。相邻低速（或高速）条纹之间的平均翼展方向宽度和间距分别约为h和2h。 POD分析表明，在瞬时流场中存在跨度不同的条纹。可以基于统计分析的结果直接勾画出典型的沿流方向的旋转运动。点到点分析表明，该模式在测量窗口中的每个位置都是一致的，并且在翼展方向上没有任何不均匀性，这揭示了相干结构与二级流通池之间的本质差异。通过统计分析发现的模式与以下观点相一致：超流涡旋作为明渠流中的一种相干结构而普遍存在。

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