## 4 Results and Discussion
### Mean Wind Characteristics
#### 4.1.3 Wind Attack Angle  
The wind attack angle \(\beta\) is a critical parameter to describe the state of structural elements such as the main beam and towers, which is used to assess the influence of the wind on bridges. A significant change in the velocity and pressure fields results in different attack angles. The attack angle is of great significance for studying the wind-induced vibration performance of components. The definition of wind attack angle is given by Equation (4).  
The distribution of wind attack angles on the southwest bridge tower is shown in Figures 18 and 19. It can be observed that the wind attack angle is the largest at the \(180^{\circ}\) wind direction, and this indicates that significant upward airflow exists at the southwest bridge tower. The wind attack angle is significantly influenced by the oncoming wind speed, with the angle increasing as the wind speed increases, whether the oncoming flow  
Figure 16: Wind yaw angles of the main beam under uniform inflow with different wind speeds and directions.  
Figure 17: Wind yaw angles of the main beam under ABL inflow with different wind speeds and directions.  
is uniform or the ABL inflow. Overall, when the oncoming wind is ABL inflow, the value of the wind attack angle is lower than uniform inflow. This is due to the obstruction of the wedges and rough elements in the wind tunnel, which reduces the wind speed as it reaches the bridge, resulting in a lower overall value than that of the uniform inflow. This is consistent with the conclusion that the higher the wind speed, the larger the attack angle.  
The distribution of wind attack angles on the northeast bridge tower is shown in Figures 20 and 21. When the oncoming wind is uniform, the wind attack angles are relatively small at the northeast tower under the \(0^{\circ}\) and \(30^{\circ}\) wind directions. The reason for this phenomenon is that the measuring points are higher and the obstruction effect of the mountains is smaller. Additionally, the higher the oncoming wind speed, the greater the wind attack angle at the same measuring point. For the \(180^{\circ}\) wind direction, the bridge site is located on the leeward side, and the distribution of wind attack angles is more complex, initially decreasing and then increasing with height. The wind attack angle changes very little with the oncoming wind speed.  
The distribution of wind attack angles along the main beam is shown in Figures 22 and 23. For both uniform and ABL inflow, the variation in wind attack angle with locations is essentially the same, and the values for the ABL are slightly lower. When the oncoming wind is uniform inflow, the wind attack angle of the main beam is relatively small, influenced by the oncoming wind speed. When the oncoming wind is ABL inflow, the wind attack angle shows an increasing trend with the increase in oncoming wind speed. The wind attack angles at the bridge site are all positive, indicating the presence of significant upward airflow throughout the bridge site, which typically increases the lift component in  
Figure 19: Wind attack angles of the southwest tower under ABL inflow with different wind speeds and directions.  
Figure 18: Wind attack angles of the southwest tower under uniform inflow with different wind speeds and directions.