Flow loss characteristics in parallel confluence sections of tunnels

The construction of bifurcated tunnels is essential to advancing urban infrastructure systems, as they conserve land, reduce carbon emissions, and optimize traffic. However, the bifurcation structure of the parallel confluence section of such tunnels poses significant challenges in the design and operation of the tunnel ventilation system, in terms of both the internal and external environment. In this work, the flow and loss characteristics of parallel confluence sections are studied with numerical simulations and model experiments. The influences of the confluence ratio q and the confluence angle θ on the flow characteristics and loss mechanisms of the parallel confluence section are revealed theoretically. The results indicate that when q is small, the high-velocity airflow from the mainline entrains the low-speed airflow from the ramp, leading to flow separation at the upper connection between the parallel section and the gradual transition section; when q is large, the high-velocity airflow from the ramp entrains the low-speed airflow from the mainline, resulting in flow separation on the side of the confluence section adjacent to the mainline. Additionally, the mismatch between the airflow ratio Q and cross-sectional area ratio φ of the mainline tunnel and the ramp prior to confluence enhances the jet entrainment effect, increases the curvature of the streamline, expands the range of the flow separation area, and generates higher confluence loss coefficients ∣K13∣ and ∣K23∣ of the mainline and the ramp. For small q, K13 and K23 remain relatively constant with respect to θ, whereas for large q, both ∣K13∣ and ∣K23∣ decrease as θ increases. Finally, a semi-empirical formula is proposed to predict the loss coefficients for parallel bifurcated tunnels with confluence angles ranging from 5° to 15°. This study provides insights into the aerodynamic behaviour and loss mechanisms in bifurcated tunnels, offering guidelines for enhancing the efficiency of tunnel ventilation systems in tunnel-like underground infrastructure.