Hydrodynamic Characteristics of Strong, Unsteady Open-Channel Flow

Keke Hu, Jian Hu, Tianwei Huang, Xiangwei Ye, Shu Jiang () and Ying-Tien Lin ()
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Keke Hu: Zhejiang Institute of Hydraulics & Estuary (Zhejiang Institute of Marine Planning and Design), Hangzhou 310020, China
Jian Hu: Wenzhou Communications Develop Group Co., Ltd., Wenzhou 325000, China
Tianwei Huang: Institute of Port, Coastal and Offshore Engineering, Ocean College, Zhejiang University, Zhoushan 316021, China
Xiangwei Ye: Qiantang River Basin Management Center of Zhejiang Province, Hangzhou 310000, China
Shu Jiang: School of Economics and Management, Zhejiang Ocean University, Zhoushan 316022, China
Ying-Tien Lin: Institute of Port, Coastal and Offshore Engineering, Ocean College, Zhejiang University, Zhoushan 316021, China

Sustainability, 2023, vol. 15, issue 17, 1-20

Abstract: Due to climate change, the intensity of extreme rainfall has been observed to increase with a shorter duration, causing flash floods (strong, unsteady flow) that lead to serious loss of life and economic damage all over the world. In this study, by repeating the same flume experiments twenty times over a bare bed or with a submerged vane installed, the hydrodynamic characteristics of a strong, unsteady open-channel flow were investigated. Acoustic Doppler velocimetry (ADV) was used to measure the instantaneous three-dimensional velocity, and the ensemble average method was then adopted to obtain the time-varying mean flow velocities. Reynolds decomposition was applied to disintegrate the instantaneous velocity to time-varying average velocity and fluctuating velocity. Turbulence characteristics such as turbulent intensity, turbulent bursting, and power spectral density (PSD) were analyzed against water depth variations. The results show that the loop pattern of the streamwise velocity against the water depth variations could significantly affect the turbulence characteristics of unsteady flow. Near the bed, the peaks of the turbulent intensity and the TKE lag behind the peak of the water depth. The PSD revealed that the turbulent energy increases at the rising and falling stages were due to the generation of small-scale turbulence vortices or eddies. As a submerged vane was present, the increase in the angle of attack caused the increase in the turbulent intensity and TKE, which means that the induced vortex became stronger and the wake region was larger. When the angle of attack was equal to 20 ° , the TKE abruptly enlarged in the falling stages, implying the breaking-up of the induced vortex. The PSD of the transverse fluctuation velocity showed multiple spikes at the high-frequency part, possibly denoting the shedding frequency from the induced vortex. Further downstream, behind the submerged vane, the peak frequencies of the PSD became imperceptible, likely because of the induced vortex decays or other factors such as the turbulence generated from the free surface or the channel bed mixing with the turbulence from the induced vortex.

Keywords: strong unsteady flow; ensemble average method; acoustic Doppler velocimetry; submerged vane; turbulence characteristics; power spectral density (search for similar items in EconPapers)
JEL-codes: O13 Q Q0 Q2 Q3 Q5 Q56 (search for similar items in EconPapers)
Date: 2023
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