## 4 Wind Environment around the Bridge Pylon
### Wind Speed Distribution at the Bridge Pylon Zone
#### 4.2.2 Wind Speed Coefficient Result  
The experimental and numerical results of the wind speed coefficient at the upstream and downstream main lanes under C1 and C2 conditions are shown in Figures 13-16. In these figures, the x-axis represents the wind speed coefficient, and the y-axis represents the distance in the bridge's longitudinal direction from the central of two pylon columns. For comparison, the mean wind speed at  
Figure 11: Vortex zone (at 0.25\(H\) above the deck, C2 condition). Figure 12: Boundary layer flow profile to equivalent uniform profile. locations 5 m above the upstream and downstream main lane, and 100 m in from the center of the pylon columns, is given. It is 8.68 m/s upstream and 8.67 m/s downstream in the C1 condition, and 8.60 m/s for both upstream and downstream in the C2 condition. The study on wind speed reveals that its distribution has a certain pattern along the longitudinal direction; the common details are as follows:  
1. The wind speed coefficient increases at first, then decreases along the longitudinal direction, consistent with the analysis in the flow visualization section. 2. The wind speed coefficient is very small from the bridge pylon center to 0.5\(W\), and has a sudden increase from 0.5\(W\) to \(W\), which is unfavorable for bridge driving safety. \(W\) represents the width of the bridge pylon of 15 m.
3. \(\alpha\) tends to be static at a distance further than 3\(W\), and the largest \(\alpha_{max}\) appears near the distance of \(W\) distant from the pylon column's center, which indicates that the flow around the pylon is dominant in this range. 4. The magnitude of \(\alpha_{max}\) increases as the height above the bridge deck increases, which is caused by the boundary layer flow of the bridge deck. 5. \(\alpha\) is a unimodal curve at higher locations above the bridge deck, which means that the flow around the bridge pylon is dominant at higher locations. 6. In the range of 0.8 to 1.2\(H\) above the bridge deck, \(\alpha\) is basically coincident. In addition, the magnitudes of \(\alpha_{max}\) are very close in the C1 and C2 conditions. This indicates that the wind environment within 0.8\(H\) is controlled by the flow around the bridge girder. 7. The numerical simulation result has the same trend and same location of \(\alpha_{max}\), and corresponds with the experimental result, though its value is generally larger than in the experimental results. A possible reason for this is that the wind speed is considered as a 3D resultant velocity in the numerical simulation, but as 1D along the inflow direction in the experimental results. If the measuring points are in the vortex zone, the velocity cannot be accurately obtained from the wind tunnel experiment. Figure 13: Wind speed coefficient at upstream main lane in C1 condition. i. Upstream Main Lane  
For both the numerical and experimental results, the pattern of the wind speed coefficient of the upstream main lane in the C1 condition can be summarized as follows:  
1. \(\alpha_{max}\) is located 0.67\(W\) from the center of the pylons, and within 0.3\(H\) above the deck (\(H\) is the girder height 4 m and \(W\) is the bridge pylon width 15 m at bridge deck elevation). The magnitudes of \(\alpha\) within 0.3\(H\) are all smaller than the inflow velocity. 2. From 0.4 to 0.6\(H\) above the bridge deck, \(\alpha_{max}\) appears at 1.33\(W\) from the center of the pylon columns. Figure 14: Wind speed coefficient at upstream main lane in C2 condition. Figure 15: Wind speed coefficient at downstream main lane in C1 condition. For the upstream main lane in C2 of the numerical and experimental results, similar conclusions can be drawn. The differences are listed as follows:  
1. Under the C2 condition, \(\alpha_{max}\) for all heights above the bridge deck of the upstream main lane is located near the distance of \(W\) from the center of pylon columns. 2. The magnitude of the wind speed coefficient within \(0.3H\) above the deck is larger than that of the C1 condition, and the curve is more complicated in the C1 condition.