## 3 Train-Bridge System Aerodynamic Characteristics  
### The HST Aerodynamic Characteristics  
#### 3.1.1 Aerodynamic Coefficients of HST  
The aerodynamic coefficients acting on a train can intuitively reflect the windproof performance of wind barriers. Figures 4 and 5 show the mean and extreme aerodynamic coefficients of the train when it is located on the windward and leeward sides of the bridge. The red and blue lines in Figures 4 and 5, respectively, represent the mean aerodynamic coefficient of the train without barriers and with a vertical wind barrier installed. For both train-bridge cases, installing a vertical wind barrier can effectively reduce the train's drag. Moreover, when a type 4 WGB is installed, the mean aerodynamic coefficient of the train is close to that when a vertical wind barrier is installed, indicating that WGB can provide a similar windproof performance for trains as vertical wind barriers.  
In order to facilitate the comparison of the effects of different wind barrier parameters on the aerodynamic coefficients of the train, the comparison cases of each parameter were separately plotted. As shown in Figure 4a, when \(\theta_{1}\) is slightly adjusted, the mean aerodynamic coefficient of the train remains almost unchanged, but the extreme aerodynamic coefficient of the train is greatly affected by the angle of the windward surface. When \(\theta_{1}=20^{\circ}\), the \(C_{H}\) extreme value increases from 0.24 to 0.45, and at the same time, the \(C_{V}\) extreme value increases from 0.52 to 0.71. As shown in Figure 4b, when \(B_{1}\) is increased from 0.25 m to 0.3 m, the mean \(C_{H}\) and mean \(C_{V}\) of the train will be reduced to varying degrees, but the extreme \(C_{H}\) of the train will increase. At the same time, it should be noted that when \(B_{1}\) is further increased, the mean aerodynamic coefficient of the train does not change significantly, so the optimal value of \(B_{1}\) should be between 0.25 m and 0.3 m. As shown in Figure 4c, when \(h_{1}\) is increased from 0.35 m to 0.5 m, the mean and extreme aerodynamic coefficients of the train will increase. The max CV of the train increases by 60%, which will increase the train's derailment coefficient. Overall, \(B_{1}\) and \(h_{1}\) are WGB parameters that have a relatively large impact on the aerodynamic coefficients of the train, while \(\theta_{1}\) has a relatively small impact. When optimizing the WGB parameters, the adjustment of the first two parameters should be emphasized. It should be noted that the extreme aerodynamic coefficient is sometimes several times larger than the mean aerodynamic coefficient, which means that the extreme aerodynamic coefficient may become a controlling factor in wind barrier design.  
When the train is on the leeward side of the track, due to the influence of the bridge deck on the wind field, different lane positions can also cause differences in the aerodynamic characteristics of trains [24]. As shown in Figure 5, the effects of various WGB parameters  
Figure 4: Effect of WGB parameters on HST aerodynamic coefficients (windward): (**a**) angle of main deflector plate \(\theta_{1}\); (**b**) width of WGB root \(B_{1}\); (**c**) size of upper deflector angle \(h_{1}\).  
Figure 5: Effect of WGB parameters on HST aerodynamic coefficient (leeward): (**a**) angle of main deflector plate \(\theta_{1}\); (**b**) width of WGB root \(B_{1}\); (**c**) size of upper deflector angle \(h_{1}\).  
on the mean aerodynamic coefficient of the train are similar. However, the magnitude of the mean aerodynamic coefficient and the difference in the mean aerodynamic coefficient caused by the parameter changes are greatly reduced compared to when the train is on the upstream side. This may be because the entire train is in the low-pressure wake of the barrier, away from the flow separation point.