## 4 Results and Discussion  
### Influence of Initial Conditions on Wind Parameter  
#### 4.1.1 Surface Roughness  
Under the influence of surface roughness, the wind speed near the ground will be reduced to 0. The simulation of surface roughness was summarized by [PERSON] et al. [30], which was divided into two methods: wall function and adding original resistance term. The wind speed calculation results of the two methods were quite different at low altitude. [PERSON] et al. [30], [PERSON] et al. [31], and others used the wall function to simulate the roughness of complex terrain. Therefore, in this section, the surface roughness was simulated by setting the wall function. The setting of roughness value \(z_{0}\), referred to as [PERSON]'s recommendations [24; 32; 33], as shown in Equation (10):  
\[z_{0}=\frac{K_{S}C_{S}}{9.793} \tag{10}\]  
where \(z_{0}\) is the surface roughness (m); \(K_{S}\) is roughness height (m), which is related to the grid size and should not be less than two times the vertical distance \(h_{p}\) from the centroid of the first layer of grid to the bottom; and \(C_{S}\) is the roughness coefficient, which was 0.5 by default, indicating uniform roughness, and for uneven roughness, the value was 0.5-1.  
As for Equation (10), it is suitable for use for CFD based on ANSYS. As the \(C_{S}\) ranged from 0.5-1, the required cell height was limited to some extent by the maximum of \(C_{S}\), especially for the lowest boundary layer cell [32].  
Therefore, roughness \(z_{0}\) was successively set as 0, 0.03, 0.06, 0.09, and 0.12 m, meeting the requirements of Equation (10). The inlet wind speed was taken as 27.1 m/s, according to the 100-year return period wind speed [34] of Lijiang meteorological station, which is the nearest station to the bridge site, so the wind speed at gradient wind height was 27.1 m/s. Taking 102\({}^{\circ}\) oncoming flow as an example, the influence of surface roughness on wind parameters is shown in Figure 5. Then taking the condition with the surface roughness of 0 m as the reference condition, and according to Equations (7)-(9), the nondimensional factors are shown in Figure 5b,d,f.  
It can be seen from Figure 5 that, compared with the working condition without setting surface roughness, the influence of surface roughness on wind parameters cannot be ignored. The higher the surface roughness, the more obvious the influence on wind speed, wind attack angle, and wind azimuth angle. The surface roughness has a greater influence on the wind attack angle and wind azimuth angle than on wind speed profile. Specifically, the higher the roughness, the more obvious the influence on low-level wind speed below 1000 m. The maximum wind speed factor was about 1.05. The wind speed factor decreased gradually with increased height, and wind speed above 1000 m increased by about 1%. The influence of surface roughness on the wind attack angle varied with height and surface roughness. The higher the surface roughness, the higher the change in wind attack angle at a 5000 m altitude, with a maximum of about 2.5 times. The wind attack angle factor first decreased and then increased with height. Second, the higher the surface roughness, the larger the wind azimuth angle, and the influence on the wind azimuth angle at low altitude was more obvious. From the wind azimuth angle factor, the influence of roughness increased and then decreased, and the maximum amount was about 2.4 times. The wind azimuth angle factor first increased and then decreased with height.  
According to the survey of the landform of the bridge site, there are weeds on both sides of the river, and some low trees are scattered on the ridges on both sides. The roughness length in the study was set as 0.12 m.