Turbulent structure at a 30° confluence of pipes in a sewer system

Pipe confluence frequently occur within urban drainage network systems, where domestic and industrial wastewater discharge introduces sediments and pollutants. This deposition within the confluence area significantly impacts the operational efficiency of the drainage network. This study analyzes the mean and turbulent structures within the shear layer, focusing on parameters such as the mean flow field, turbulent kinetic energy, Reynolds shear stress (τuv, τuw), and quadrant analysis. The findings indicate that shear layer position is best identified by τuv and τuw values. In addition, the spiral flow occurs only at larger main and tributary inlet flows or when the discharge ratio is relatively large and disappears gradually as it progresses downstream. Meanwhile, the shear layer exhibits the highest proportion of ejection and sweep in the vicinity of the bed surface, with ejection dominating, which positively affects sediment initiation. As a result, sediment and pollutants are less likely to accumulate in this area. With the increase in flow or discharge ratio, the parameters such as velocity, turbulent kinetic energy, Reynolds shear stress, and Reynolds stress contribution value increase. Consequently, this study provides a deeper understanding of the flow structure and turbulence in the confluence zone, offering a reference for sediment reduction in confluence pipes.