Influence of auxiliary cylinders on the dynamics of overland flow upstream of the main cylinder based on particle image velocimetry

摘要

The position of auxiliary cylinders in flow experiments noticeably affects soil erosion and pollutant transportation processes by altering the dynamic flow field in front of the main cylinder. However, few studies have been conducted for overland flow due to the shallow flow depth and limited observation technology. In this study, we conducted fixed-bed flume experiments and developed a high-resolution and high-frequency particle image velocimetry system to investigate the two-dimensional dynamic field of overflow using a symmetrical surface (i.e., the vertical plane passing through the axis of the main cylinder in the flow direction) upstream of the main cylinder as the observation area. Two auxiliary cylinders in each position were placed upstream, parallel, and downstream (i.e., pre-column condition, parallel condition, and post-column condition, respectively). Results showed that: For a given flow discharge, the ranking of the cylinder configurations in terms of the stability of the hydrodynamic conditions (velocity, turbulent energy, shear force, and Reynolds stress) was pre-column condition parallel condition post-column condition. The number of horseshoe vortices and the rotation intensity of the main horseshoe vortex were similar for the three configurations. The position of the main horseshoe vortex was closest to the main cylinder in the pre-column condition, causing substantial soil erosion around the main cylinder. For different flow rates, the water depth and turbulent intensity showed monotonic increasing trends as the flow discharge increased, whereas the shear force and Reynolds stress exhibited a single peak. As the Reynolds number of the cylinder increased, the flow separation point moved upstream, and the rotation intensity of the main horseshoe vortex was negatively correlated with the water depth. Therefore, a shallower water depth corresponded to significantly greater erosion damage by the horseshoe vortex. Overall, the configuration of the auxiliary cylinders downstream of the main cylinder resulted in the optimum water flow control. The findings of this study provide theoretical support for designing vegetation management strategies that minimize soil erosion and protect water resources. (C) 2020 Elsevier Ltd. All rights reserved.