# Aerodynamic Characteristics When Trains Pass Each Other in High-Speed Railway Shield Tunnel
## 1 Introduction
[9] analyzed the formation mechanism, amplitude distribution, and attenuation of the pressure wave through a series of on-site measurements. Compared with field tests, model test applications have the advantages of low costs and simple operations. However, dynamic model tests began relatively late [10], and their advantage is that they can simulate the relative motion between the train and the ground. For example, the dynamic model test bench of the Institute of Mechanics of the Chinese Academy of Sciences can simulate the relative movement of a train and the ground and verify the reliability of numerical simulations [11; 12]. [PERSON] used dynamic model tests to reveal that the peak winds of trains under all closed conditions were greater than those of open-air conditions. In a tunnel, the \"piston effect\" seems to be the main reason for the increase in the peak intensity of the train wind [13]. [PERSON] et al. [14] used model experiments to study the pressure wave generated by a train passing through a branch and the pulse wave that radiated from the main tunnel through the exit of the branch. At the same time, simple acoustic theory was used to clarify the correlation between the pressure wave in the tunnel and the pulse wave radiating from the exit. At present, with the rapid development of computer technology, numerical simulations are not only convenient to perform but also yield accurate results. Thus, numerical simulations have been widely used [15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. They can also greatly reduce the costs of actual vehicle tests and dynamic model tests. Some scholars have used numerical simulations to study the impact of aerodynamic effects when two trains meet, mainly focusing on the flow field distribution [15], traffic safety issues [16], changes in the pressure wave amplitude on the surface of the train [17; 18; 19], and the influence of different train head lengths on the average slipstream velocity and tunnel wall pressure [20; 21]. [PERSON] et al. [22] also studied the influence of four streamlined structures on aerodynamic performance and flow mechanism based on numerical simulations. Other scholars [23; 24; 25; 26; 27; 28] studied the propagation of aerodynamic loads and pressure waves when trains intersected in a tunnel through numerical simulations and compared the aerodynamic performances of open lines and tunnels. For the intersection of high-speed trains in a tunnel, different tunnel lining structures may cause different aerodynamic effects when the trains meet. At the same time, the intersection position of the train in the tunnel may not be at the midpoint of the tunnel, which could affect the pressure wave. When two high-speed trains travel at different speeds, the aerodynamic load acting on the surface of the train may also change. Although many scholars have studied the aerodynamic loads on the surfaces of trains and the wall pressures of tunnels, there have been few studies on the wall pressure and the aerodynamic load on the surface of a train based on the shield tunnel lining structure. Therefore, it is necessary to establish a simulation model with tunnel-train intersection coupling based on the three-dimensional, compressible, and unsteady Reynolds-averaged Navier-Stokes equations to compare and study the aerodynamic characteristics of the train surface and different tunnel lining structures when two trains pass each other in a tunnel.