## 3 Train-Bridge System Aerodynamic Characteristics
### The HST Aerodynamic Characteristics
#### 3.1.2 Wind Pressure Distribution on HST  
Testing the surface pressure distribution of trains can help understand the reasons for changes in train aerodynamics. Figure 6 shows the distribution of mean wind pressure on HST upstream. When the train is on the upstream side of the track, Figure 6a shows that after installing the WGB, the leeward part of the train roof and bottom, as well as the mean wind pressure on the leeward side of the train, remain almost unchanged, but the mean wind pressure on the windward side of the train changes from positive to negative. Compared with vertical wind barriers, the installation of the guide vanes does not significantly change the overall wind pressure distribution of the train, which verifies the view that WGBs have the same train protection effect as the vertical wind barriers mentioned earlier. It is worth noting that when the WGBs are installed, there is an increase in suction force at the shoulder of the train, which may be due to the upward deflection effect of the secondary deflection angle on the airflow. When \(\theta_{1}=10^{\circ}\), as shown in Figure 6b, compared with \(\theta_{1}=20^{\circ}\), the barrier blocks more airflow, resulting in lower pressure in the low-pressure area behind the barrier, which enhances the overall negative pressure of the train. The shoulder part is more sensitive to the changes in the inflow as it is immersed in the separated airflow, so it shows more obvious differences, which can also be observed in cases when \(h_{1}\) is changed. Surprisingly, as shown in Figure 6c, changing \(B_{1}\) does not significantly affect the mean wind pressure on the bottom of the train. When \(B_{1}\) is increased from 0.25 m to 0.3 m, only the trend of the separation point moving up and the weakening of the suction force at the shoulder of the train can be observed, and further increasing \(B_{1}\) will not significantly change the mean wind pressure on the train. When HST is on the downstream side of the track, as shown in Figure 7, compared with on the upstream side, the positive pressure on the windward side of the train decreases, and the negative pressure at the shoulder of the train increases to \(-7.328\). After installing the WGB, the separation of airflow at the shoulder of the train essentially disappears, and the overall aerodynamic level of the train is reduced, with no significant pressure jumps at each measurement point, indicating that the various WGB used in the test can meet the windproofing needs of downstream trains. Overall, when HST is on the upstream side, it is the worst bridge-train system case and should be given priority consideration in design. Figure 6: Effect of WGB parameters on HST mean pressure (windward): (**a**) without a wind barrier; (**b**) angle of the main deflector plate \(\theta_{1}\); (**c**) width of the WGB root \(B_{1}\); (**d**) size of the upper deflector angle \(h_{1}\). To further analyze the causes of the extreme aerodynamic forces on the train, we plotted the fluctuating wind pressure coefficient at each measurement point (Figures 8 and 9). For ease of observation, the figures also show the differences in fluctuating wind pressure coefficients under different parameter configurations. When various types of WGB are installed, the max fluctuating wind pressure coefficient values are all found at measurement point 9. The max fluctuating wind pressure coefficient in all cases is less than 0.3 and only greater than 0.1 near the train shoulder. This indicates that the guide function of the WGB mainly affects the train shoulder and does not cause an increase in turbulence at the bottom of the train or on the leeward side. The variation in the fluctuating wind pressure coefficient at the train shoulder is the main cause of the extreme aerodynamic forces on the train. Overall, \(\theta_{1}\) and \(h_{1}\) are the most important factors affecting the fluctuating wind pressure coefficient.