## 5 Aerodynamic Loads and Safety Analysis  
### Flow Structure and Aerodynamic Loads  
Figure 14 shows the variations of the pressure and flow structure around the train when approximately half of the middle carriage reached the tunnel entrance for a train speed of 250 km/h and a crosswind speed of 15 m/s, and those at 25 m/s are shown in Figure 15.  
Figure 14 shows that in open air, the flow structure showed remarkable asymmetry where crosswind was applied to the train. A series of vortexes considerably originated from the tunnel entrance and stretched to the train tail along the leeward side. The vortexes originated from the tip of the head carriage and stretched far away behind the tip of the rear carriage when the train ran completely outside the tunnel. The vortexes gradually retreated from the entrance as the train went on. In open air, the pressure on the train showed the positive values at the windward side and negative values at the leeward side because of crosswind. Relatively, inside the tunnel, the vortex around the carriage rapidly disappeared, and pressure in both sides of the train showed a basic symmetry. Sudden variations along the longitudinal of flow structures around the train at the tunnel entrance were discovered. During these instances, such a difference of the flow fields between inside and outside the tunnel entrance can explain the sudden variation of aerodynamic forces and moments on the train. Figure 12: The numerical model corresponding to the field test. Figure 13: Comparison of the time history of pressure on the train surface between the field test and the present numerical simulation (train speed = 200 km/h; wind velocity = 0 m/s). According to Figure 15, the difference of flow fields between the leeward and windward sides of the train outside the tunnel became considerable as the wind speed increased. Figure 15b shows that the two larger vortexes in the leeward side and the pressure difference on both sides of the train increased outside the tunnel. Nevertheless, compared with Figure 14c, there was no substantial change in the flow structure and pressure around the train inside the tunnel (see Figure 15c). Thus, the sudden variation of the aerodynamic forces and moments under condition of higher crosswind velocity may be remarkable. Figure 14: Pressure and flow distribution for train speed = 250 km/h, crosswind velocity = 15 m/s, (**a**) z = 1.55 m from top of rail, (**b**) 10 m outside tunnel entrance, and (**c**) 10 m inside tunnel entrance. Figure 15: Pressure and flow distribution for train speed = 250 km/h, crosswind velocity = 25 m/s, (**a**) z = 1.55 m from top of rail, (**b**) 10 m outside tunnel entrance, and (**c**) 10 m inside tunnel entrance. Figure 16a-e show the time-histories of the five aerodynamic loads (i.e., side, lift forces, and rolling and pitching and yawing moments, respectively) during the train entering the tunnel under conditions of train speed = 250 km/h and crosswind velocity = 25 m/s. The red, green, and black lines represent the head, middle, and rear carriages, respectively. Figure 16 shows that the five aerodynamic loads of the three carriages all characterized a sudden variation when entering tunnel. For example, the side force, regardless of the head, middle, and rear carriages, showed a dramatic decrease (see Figure 16a). For the lift force, except for the head carriage, both the middle and rear carriages showed an upward shock (see Figure 16b). This phenomenon may be attributed to the jet flow, which was exhausted outside the entrance along the hoop space between the train body and the tunnel. The hoop space, which was larger in the train top side than the bottom, was asymmetrical in the vertical direction, thereby causing the aerodynamic pressure on the bottom of the train to be larger than that on the top. Such a phenomenon mainly occurred in open air approximately 20 m away from the tunnel entrance. Therefore, the impact caused by the jet flow did not affect the side and lift forces of the head carriage.