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On the scaling of large-scale structures in smooth-bed turbulent open-channel flows
Journal of Fluid Mechanics ( IF 3.7 ) Pub Date : 2020-02-18 , DOI: 10.1017/jfm.2020.73
C. Peruzzi , D. Poggi , L. Ridolfi , C. Manes

This paper investigates the existence and scaling of the so-called large-scale and very-large-scale motions (LSMs and VLSMs) in non-uniform turbulent open-channel flows developing over a smooth bed in a laboratory flume. A laser Doppler anemometry system was employed to measure vertical profiles of longitudinal and bed-normal velocity statistics over a wide range of hydraulic conditions. Pre-multiplied spectra of the longitudinal velocity fluctuations revealed the existence of two peaks occurring at wavelengths consistent with those associated with LSMs and VLSMs as detected in the past literature pertaining to wall turbulence. However, contrary to so-called canonical wall flows (i.e. flat-plate boundary layers, pipe and closed-channel flows), the LSM and VLSM peaks observed in the open-channel flows investigated herein are detectable over a much larger extent of the wall-normal coordinate. Furthermore, the VLSM peak appears at von Karman numbers $Re_{\unicode[STIX]{x1D70F}}$ as low as 725, whereas in other wall flows much higher values are normally required. Finally, as conjectured by a recent study on uniform rough-bed open-channel flows, the present paper confirms that LSM wavelengths scale nicely with the flow depth, whereas the channel aspect ratio (i.e. the ratio between channel width and flow depth) is the non-dimensional parameter controlling the scaling of VLSM wavelengths. The intensity and wavelengths of the VLSM peaks were also observed to depend on the spanwise coordinate. This result suggests that VLSMs might be dynamically linked to secondary currents, as these are also known to vary in strength and size across the channel width.

中文翻译:

光滑床湍流明渠流中大型结构的标度

本文研究了在实验室水槽的光滑床上发展的非均匀湍流明渠流中所谓的大规模和超大规模运动(LSM 和 VLSM)的存在和缩放。激光多普勒风速测量系统用于测量各种水力条件下纵向和床法向速度统计的垂直剖面。纵向速度波动的预乘光谱显示存在两个峰值,其波长与过去有关壁湍流的文献中检测到的与 LSM 和 VLSM 相关的波长一致。然而,与所谓的典型壁流(即平板边界层、管道和封闭通道流)相反,在本文研究的明渠流中观察到的 LSM 和 VLSM 峰在壁面法线坐标的更大范围内是可检测到的。此外,VLSM 峰值出现在 von Karman 数 $Re_{\unicode[STIX]{x1D70F}}$ 低至 725,而在其他壁流中通常需要更高的值。最后,正如最近对均匀粗糙床明渠流动的研究所推测的,本文证实 LSM 波长与流动深度很好地成比例,而通道纵横比(即通道宽度和流动深度之间的比率)是控制 VLSM 波长缩放的无量纲参数。还观察到 VLSM 峰的强度和波长取决于展向坐标。该结果表明 VLSM 可能与次级电流动态相关,
更新日期:2020-02-18
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