Effects of various backward-facing steps on flow characteristics and sediment behaviors

Abstract

The study of flow over various steps behind the hydraulic structures, such as weirs, dams, spillways, barrages, and flood gates, is still of particular interest to researchers due to the complexity of flow characteristics and sediment behaviors at near-bed region. This study investigates the effect of various inclined backward-facing step (BFS) angles on near-bed turbulent flow structures and bedload transport rate in surface jet flow regime. A combined numerical technique of large eddy simulation and discrete element method, based on the open sources package OpenFOAM, is employed. The validation of the numerical model shows a good agreement between the simulation results and observed data obtained from different experiments. The simulation results reveal that the flow does not form a separation zone when the BFS angle is less than 20°, wherein the near-bed turbulence intensity is insufficient to induce substantial sediment movements. As the step angle is increased to a certain value (20°), the separation flow is formed. Consequently, the near-bed turbulence intensity is significantly increased due to the splat effect, and the sediment flux is also drastically increased. As the BFS angle further increases to 30° and 90°, the reattachment length is extended without notable changes in maximum turbulence intensity near the bed. The peaks of the mean bedload transport rate are located further downstream along the extended reattachment length. The quadrant analysis for bedload transport is performed to quantify the interactions between the flow and sediment movement. The results demonstrate that the sweep event plays significant role in moving most sediment to downstream of the reattachment point. On the other hand, right upstream of the reattachment point, the burst becomes the dominant turbulence event to move the majority of the sediment backward in the upstream direction.