Numerical Study Of Flow On Asymmetric Confluence Zone

Due to the backwater effect, mixing of the mainstream and tributary in the confluence zone,the hydraulic characteristics such as flow field, water depth and sediment transportation are changed, thus, many unique and complicated hydraulic phenomena were performed, for example spiral flow, secondary flow, local flow stagnation, separation and uplifting of the water surface. These leads to bank erosion, sediment deposition and it also affect the diffusion and transportation of pollutants, which limits the development of shipping industry. Therefore, it is of great significance to research the flow on confluence zone.In this paper, the mainstream and tributary characteristics of the confluence zone were theoretically analyzed, and a physical water channel model with fixed bed was established where the intersection angle is 45°. In this condition, eight schemes set by different terrain conditions and confluence ratio were tested. Equally, a three dimensional channel model was established by the application of ANSYS ICEM CFD and simulated by the application of FLUENT. In the numerical simulation, QUICK format and SIMPLEC algorithm were applied coupling velocity field and pressure field, VOF method was used to capture the free water surface.The RNG k-εmodel was used to simulate the flow characteristics of the confluence zone at the intersection angle of 45 degrees under two terrains when the confluence ratio are 1:1, 1:2,1:3, 1:4, and three schemes were numerically simulated where the confluence angle are 30°、60°、90°, respectively. The characteristics of water flow at the intersection of asymmetric tributaries are analyzed under different terrain, confluence ratios and inlet angles.The main conclusions are as follows:(1) The variation of the piezometric water head along the course of the main stream is relatively slow, but the inlet of the branch will reduce the piezometric water head and increase the velocity head in the confluence zone.(2) The turbulent flow under the complex topography is strong, which makes the water surface line fluctuate more greatly than that without topography. Due to the constraint of paved terrain, the flow dynamic axis under the complex topography is more inclined to the central line of the sink than the non topographic condition. The separation zone has narrowed under the complex topography.(3) The flow dynamic axis is close to the sink center line when the confluence ratio is 1:3,and the erosion of both the left and right bank are minimized. Velocity gradient is small when the intersection angle is 45 degrees, and the flow dynamic axis is close to the sink center line which leads to the minimum erosion of left and right bank.(4) When the intersection angle is 90 degrees, the upstream backwater effect of the confluience zone is serious, the downstream flow velocity varies much bigger, the separation area becomes larger, and the energy loss reached to its maximum value.