This study experimentally examined the turbulent characteristics of decelerating open-channel flow based on particle imaging velocimetry measurement. The decelerating flow shows a similar velocity profile to that in uniform flow, but it exhibits greater turbulence intensity and Reynolds stress. Statistical evidence of the presence of very-large-scale motions (VLSMs) in decelerating open-channel flows was presented for the first time. The results indicate that VLSMs in decelerating flows can survive further away from the wall when compared to other wall-turbulence flows. The contribution rate of the VLSMs to the turbulent kinetic energy and the Reynolds stress in the decelerating open-channel flow is slightly lower than that in channels, boundary layers, and pipe flows.

The acoustic method, which enables continuous monitoring with great temporal resolution, is an alternative technique for detecting bedload movement. In order to record the sound signals produced by the impacts between gravel particles and detect the bedload motion, in this study, a hydrophone is placed close to the riverbed at the upper Yangtze River. Three categories of raw audio signals—moving gravel particles, ship engines, and flow turbulence—are collected and investigated. Signal preprocessing is performed using spectral subtraction to reduce the noise of the background sound, and the sound signal characteristic parameters are then calculated. In this paper, we propose a novel method for detecting and extracting bedload motion parameters, including peak frequency, pitch frequency, and energy eigenvector. When a segment of a speech signal meets the indicators for all three feature parameters simultaneously, the segment signal is classified as a bedload motion sound signal. Further work will be conducted to investigate bedload transport using the extracted audio signal.

Large-scale coherent structures (LSCSs) in rough-bed open-channel flow (OCF) are significant in turbulence research. A recent breakthrough is the bimodal feature of LSCSs on regular rough-bed OCF (i.e., LSCSs exhibit two typical motions: large-scale motions (LSMs) and very-large-scale motions (VLSMs)). However, the presence and characteristics of LSMs and VLSMs in irregularly arranged rough-bed OCF remain unclear. Thus, in this study, high-precision indoor flume experiments were performed under typical irregularly arranged rough-bed conditions, and time-resolved particle image velocimetry was used for velocity measurements. Statistical quantities of velocity fluctuations revealed that the friction Reynolds number and roughness exerted a certain modulation on the velocity fluctuating properties. The spectra of velocity fluctuations provided direct and statistical evidence for the presence of LSMs and VLSMs in irregularly arranged rough-bed OCF. VLSMs contributed more than 60% of the streamwise turbulent kinetic energy and 40% of the Reynolds shear stress in the outer region of the irregularly arranged rough-bed OCF, which was slightly higher than that in the smooth-bed or regular rough-bed OCF scenarios. No apparent dependence of the wavelength of VLSMs on the flow submergence (H/d50) was observed in the present irregularly arranged rough-bed OCF, which is in contrast to that reported for regular rough-bed OCF. Furthermore, the relationship between the peak wavelength of VLSMs and the aspect ratio did not strictly follow a linear increase, in contrast to that documented in the literature.