Flow pattern and pressure loss characteristics in the curved section of road tunnels

For curved tunnels that have been widely adopted in recent years, a thorough understanding into the flow pattern and the associated pressure loss in the curved sections is crucial for the design and operation of their ventilation system. In this work, a computational fluid dynamics (CFD) model is constructed to capture the airflow structures. Results show that owing to the large curved length Lc and curvature ratio R/d, secondary flow (i.e., a pair of counter-rotating vortices) in curved tunnels can evolve sufficiently and then maintain a stable stage before its decay. This leads to a three-stage (i.e., developing-stable-decaying) pattern of pressure loss along the curved section, which is distinct from the two-stage (i.e., developing-decaying) pattern in conventional elbow bends with small Lc and R/d. Based on this mechanism, a criterion between the three-stage and two-stage pattern is proposed, and an improved model covering both patterns is proposed to predict the pressure loss in curved tunnels, the error range of which is within ±20% for most of the conditions and is far smaller than that of the existing models. Finally, a scaled experiment is conducted to further demonstrate the applicability of the improved model.