Three-dimensional flow characteristics and structures in confluences based on large eddy simulation

Abstract

The flow structure at river confluences is complex, and is closely linked to sediment transport and fish migration. This study employed large eddy simulation to explore the three-dimensional flow field at an equal-width confluence under varying confluence angles and flow ratios. Based on flow characteristics, the confluence area was divided into six zones: stagnation zone, flow deflection zone, separation zone, shear layer, maximum velocity zone, and flow recovery zone. The results showed that the turbulent kinetic energy and Reynolds shear stress in the channel downstream of the confluence initially increased and then decreased along the channel. The width of the vortex in the shear layer showed various vertical distributions, reaching a maximum at the mid-water depth. The secondary flow intensity reached a maximum near the downstream confluence corner and then gradually decreased along the flow direction. Quadrant analysis revealed that sediment deposition predominantly occurred in the separation zone, driven by dominant inward interaction events. Riverbed erosion occurred in the shear layer and maximum velocity zone with strong sweep events. Furthermore, the width-to-length ratio in the separation zone was positively correlated with the confluence angle and negatively correlated with the flow ratio. The shape of the separation zone varied in the vertical direction, with the width-to-length ratio being higher at the mid-water depth than in other layers. There existed a positive linear relationship between the size of the separation zone and the confluence angle, while there was a negative linear relationship between the size of the separation zone and the flow ratio. Finally, the study discusses the relationship between fish migration and flow structure, suggesting that fish tend to prefer areas with vortex structures.