Author(s): Miao Wei; Chen Huai; Chen Qigang; Li Danxun
Linked Author(s):
Keywords: Bed-load transport; Closed channel; Image subtraction; Cross-correlation; Sampling parameters
Abstract: We introduce a novel instrument for measuring bed-load transport in closed channel flows. The instrument is based on modern innovations in high-frequency acquisition and processing of digital images, and its novelty allows for quantifying instantaneous sediment transport rate in an efficient and non-intrusive way. With this instrument, experiments were conducted in a closed rectangular channel under various water pressures. For each experiment, uniform sediment particles were paved on the flat bed, and their motions were recorded with a high-speed camera. Velocities of active particles (particle entrained to motion) were calculated by identifying and tracking particles in motion. The effects of various parameters were analyzed, including sampling frequency, sampling area, flow intensity, and water pressure. Preliminary analysis of the experimental results yields interesting new findings. Bed load transport displays a streak-like pattern on the bed, i. e, sediment particles tend to be entrained into motion in patches rather than individually. This nonuniformity indicates that large-scale coherent structures near channel bed may play an important role in“picking up”sediment particles. The sampling frequency affects the percentage of particles in motion as well as their velocities; the sediment transport rate, however, seems to be irrelevant to sampling frequency. The streamwise and spanwise particle velocities exhibit Gaussian and exponential probability density functions, respectively. The absolute pressure exerts negligible influence on the entrainment of non-cohesive particle, and the threshold of Shields number for bed load initiation is about 0. 03. The experiments show that variation in instrumental parameters may substantially alter the final results.
Year: 2015