Numerical investigation of airflow patterns and pollutant dispersions induced by a fleet of vehicles inside road tunnels using dynamic mesh part Ⅰ: Traffic wind evaluation

Abstract

The transient piston effect induced by a fleet of vehicles that passes through a tunnel can dominate pollutant dispersion, human exposure, and tunnel ventilation. This study employed the realizable k–ε model together with the dynamic mesh technique to analyze the characteristics of the transient traffic piston effect inside a tunnel. A tunnel model with a length of 450 m and a 450-m extension from the outlet in the driveway direction was developed, which contained 36 gasoline-powered cars located in nine rows with four vehicles per row. The unsteady three-dimensional model was validated by comparing the simulation results with the field-measurement data of a real tunnel. The simulation results show that traffic wind was significant, and is sufficient for diluting the pollution inside the tunnel if the vehicle speed is over 20 km/h. The peak-to-valley ratio of traffic-wind velocity below 60 km/h in three specific planes (Z = 0.5 m, Z = 1.5 m, and the tunnel outlet) was in the range of 1.47–1.77, which was slower than the 40-km/h (2.00–2.36) and 20-km/h (2.45–3.09) cases. Ventilation fans should be added to ensure fresh air supply as well as human health and safety when the vehicle speed is not faster than 40 km/h. A drag coefficient of 0.1–0.3 should be used to increase the accuracy of the pressure analysis if a fleet of vehicles moves inside the tunnel at a speed of not less than 40 km/h.