The gradually varied open-channel flows (OCF) upstream of a run-of-river (RoR) dam are ubiquitous in natural rivers. In this flow type, the velocity profile shows some similarities to that in uniform open-channel flows, but the turbulence intensity and Reynolds shear stress are slightly greater. However, the presence and properties of very-large-scale motions (VLSMs) in such flows are still unclear. To fill this research gap, time-resolved particle image velocimetry measurements were performed upstream of a modeled RoR dam in an open-channel flume. Based on pre-multiplied spectra analysis, statistical evidence of the presence of VLSMs in the flow type is reported for the first time. The results reveal that although the typical streamwise wavelength of VLSMs in such gradually varied OCF is similar to that in other flows, such as turbulent boundary layers, closed-channel flows, pipe flows, and uniform OCF, the VLSMs in the present gradually varied OCF are stronger and contribute more streamwise turbulent kinetic energy as well as Reynolds shear stress than other flows.
The flows upstream of a run-of-river dam, commonly utilized as an overflow structure on rivers, are complex due to heterogeneities in both streamwise and spanwise directions. In particular, very-large-scale motions (VLSMs) are greatly influenced by the overflow structure, yet relevant understandings remain limited. Reported as novel coherent structures in turbulent flows, VLSMs are recognized with the scale up to several and tens of the outer-scaled unit, and they contribute significantly to turbulent transport and mixing. To fill the gap, experiments with particle image velocimetry were conducted to investigate the vitality of VLSMs upstream of a model dam. Measurements were designed to cover broad hydraulic scope with flow heterogeneities. The results reveal that VLSMs in the present flow scenario show noticeable characteristics in both streamwise and spanwise directions. Compared to those in uniform flows, the VLSMs in present flows are found to be more energetic and stress-active.
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