# Study on Mechanism of Series-Flow of Pollutants between Consecutive Tunnels by Numerical Simulation
## 1 Introduction  
With the booming of economy and increasing demand of transportation, more and more tunnels are built to relieve traffic pressure [1; 2; 3]. A tunnel is a special semi-closed section of road, often with long and narrow structure which makes it difficult to diffuse the exhaust gas (such as CO, NOx) and particles raised by moving cars, seriously degenerating the traffic environment and harming human health [4; 5; 6; 7]. In order to improve the air quality and the safety and comfort of passengers and workers inside the tunnel, longitudinal ventilation, which includes jet fans and vehicle piston wind, is often used, to exchange the air inside and outside the tunnel [8; 9] so as to dilute pollutants and discharge them from the tunnel exit. However, for two tunnels nearby, the exhaust gas discharged out from one tunnel can enter the other by circulating wind if the two tunnels are side by side, or by series-flow [10; 11] if they are consecutive. That will cause secondary pollution and increase the cost of tunnel ventilation.  
There have been many studies made for the recirculating air between laterally adjacent tunnels [12; 13; 14; 15], but still few for series-flow so far. [PERSON] et al. analyzed the series-flow characteristics of pollutants between two consecutive tunnels with a spacing of 100 m [16], by use of a 1/30 scale model. The results showed when only the upstream tunnel jet fan is started, the series-flow ratio of pollutants is irrelevant to the air velocity of the upstream tunnel exit jet and the concentration of pollutants. [PERSON] et al. [17] used computational fluid dynamics (CFD) technology to study the effects of the tunnel distance within 100 m and the air velocity on series-flow. They concluded that increasing the tunnel distance or the air velocity of upstream tunnel or lowering the air velocity of downstream tunnel, results in an alleviated series-flow. With a 1/30 scale model, [PERSON] et al. [18] studied the relationship between series-flow ratio of consecutive tunnels, where the model tunnel distance was set 3 m, 2.3 m, and 1.7 m. They found that the airflow velocity ratio of upstream and downstream is the key to affect series-flow, and while the change of traffic speed affects series-flow ratio in a limited way the traffic jam does significantly. It should be noted that, with two consecutive tunnels, it requires the tunnel distance in between not to be more than 250 m [19], and the series-flow is essentially the result of interaction between the three-dimensional wall jet of the upstream tunnel exit and the air flow sink of the downstream tunnel entrance. In existing studies, there is a lack of not only systematic analysis on the jet development characteristics of two consecutive tunnels, but also works for the series-flow between two consecutive tunnels with a distance greater than 100 m.  
In this paper, series-flow between consecutive tunnels with a tunnel distance ranging from 20 m to 250 m was simulated using CFD at different operation wind speeds. The characteristics of the flow field structure and the change rule of the series-flow ratio were analyzed. A series-flow model was built on basis of the three-dimensional wall jet theory to support environment-friendly ventilation design and economical operation of the consecutive tunnels.