Unsteady Flow in Diffusion Section at the Outlet of Large Water Conveyance Tunnel

Abstract

【Objective】 The inverted siphon tunnel in big long-distance open channel spanning across a river is a critical hydraulic infrastructure. Investigating the mechanisms and strategies for addressing the unsteady flow during transition from the pressurized tunnel to the non-pressurized diffusion section of the open channel is essential to safeguarding water delivery and optimizing flow capacity. 【Method】 Using the principles of flow similarity and boundary layer control theory, this study focuses on the transition section at a large water transfer siphon outlet. Through hydraulic model testing, it examines the boundary layer separation, derived backflow interference, mainstream instability, and flow fluctuations in the diffusion section. We also investigate three additional turbulent effects resulting from unfavorable flow patterns. 【Result】 ① The boundary layer separation in the diffusion section leads to the entrainment of a significant number of vortices by the main flow, resulting in an increased energy consumption and the superposition of derived waves. These phenomena disrupt the smooth flow in the diffusion section. ②Introducing diversion piers in the diffusion section effectively suppresses boundary layer separation, eliminates vortex shedding, and mitigates side backflow in the confluence area. This leads to a substantial improvement in wave dissipation, reducing wave intensity at the end of the diffusion section from 1.11 to 0.2 m. Furthermore, it straightens the flow path within the diffusion section. 【Conclusion】 Installing diversion piers in the diffusion section is highly effective in eliminating vorticity and reducing wave interference. Specifically, B-type diversion piers prove to be superior in optimizing flow patterns and reducing energy consumption while ensuring a well-balanced flow velocity distribution. These research findings hold significant practical value for design and renovation of large-scale water transfer projects.

Keywords

• diffusion region
• boundary layer separation
• shedding vortex
• derived wave
• streamliner
• diversion pier