Author(s): Yunqiang Zhu; Saiyu Yuan; Lei Xu; Guanghui Yan; Yuchen Zheng

Linked Author(s): Saiyu Yuan, Yuchen Zheng

Keywords: Confluence; Shear layer; Vortex

Abstract: Confluences are critical nodes in fluvial systems where complex vortex structures affect the mixing of the flow, sediment, and water quality. The large-scale vortex structure largely responsible for this mixing has been proposed to fall into three main classes: vertically orientated vortices (Kelvin-Helmholtz vortex or wake), secondary cells, and streamwise orientated vortices. Past studies of vertically orientated vortices have mainly focused on their development downstream. Little is known concerning the characteristics of the vertically orientated vortices and their interaction with other factors. A flume experiment was conducted at a 60-degree open channel confluence to investigate the K-H vortex variation over time in the shear layer along the water depth (H) using particle image velocimetry (PIV). The discharge of the mainstream and tributary were 12 L/s and 6 L/s, respectively, and the water depth at the post-confluence channel was 20cm. Preliminary results found that the number of clockwise vortices and the average radius are basically symmetrical about 0.5H, by identifying the K-H vortex in the shear at the different depths and counting the number and the average radius over time. The number of clockwise vortices generated by the shear decreases significantly away from the bed and the water surface as it gradually approaches the center of the depth, while the corresponding average radius increases significantly. The number and radius variation trend turns at 0.3H and 0.8H. The reason for the change in the trend of the former is due to the influence of the friction of the riverbed, while the latter may be due to the presence of the secondary cells and the change in the distribution of the velocity along the depth which caused by the inflow of the tributary. This study leads to an advancement in our current knowledge about the shear layer at the confluences.

DOI: https://doi.org/10.3850/978-90-833476-1-5_iahr40wc-p1386-cd

Year: 2023