## 4 Vortex Shedding Characteristics by Numerical Simulations
### Influence Mechanism of the Railing Ventilation Rate  
Wind tunnel tests revealed that decreasing the pedestrian railing ventilation rate suppressed the torsional VIV response of the truss girder significantly. The wind speed time-history curve recorded by measurement point P22 indicates a frequency close to the first-order torsional fundamental frequency of the truss girder, suggesting that vortex shedding in the wake region of the lower deck may dominate the torsional VIV. Reducing the railing ventilation rate may disrupt this region's original pattern of vortex shedding. In order to investigate the mechanism behind this vibration suppression, numerical simulations were conducted on the girder cross-section using various pedestrian railing ventilation rates, and the resulting flow fields were analyzed. The simulations were conducted with a wind angle of attack of +5\({}^{\circ}\) and a wind speed of 16.67 m/s (equivalent to an actual wind speed of 16.67 m/s, consistent with the major torsional VIV peak response point wind speed in the wind tunnel tests). This working condition produced the most significant torsional VIV in the wind tunnel tests. Figure 20 displays the variation in vorticity contours in the wake region of the lower deck in a complete vortex shedding cycle. \begin{table}
\begin{tabular}{c c c} \hline
**Measurement Point** & **Frequency (Hz)** & \(\mathbf{S_{t}}\) \\ \hline P22 & 1.47, 3.08 & 0.31, 0.65 \\ P1–P6, P13 & 2.39 & 0.51 \\ P23–P29 & 4.94 & 1.04 \\ P23, P29 & 7.36 & 1.56 \\ P1–P12, P16–P21, P24–P29 & 8.56 & 1.81 \\ P14, P15 & 10.02 & 2.11 \\ P11, P12 & 11.64 & 2.47 \\ P1–P3, P7–P9, P14, P16–P19 & 15.02 & 3.18 \\ P1–P3, P23–P25 & 18.55 & 3.92 \\ \hline \end{tabular}
\end{table} 
Table 7: Characteristic frequency and Strouhal number of measurement points. Figure 19: Layout of measurement points. Reducing the pedestrian railing ventilation rate alters the vortex shedding mode in the wake region of the lower bridge deck significantly: the vortex intensity at the top of the railing decreases markedly; the blocking effect of the railing increases, and the vortices generated by the airflow passing through the gap of the railing were gradually decreased to zero; the vortex on the sidewalk surface also decreases significantly with the reduction in the gap of the railing until it vanishes when using an airtight railing; and the slowed airflow above the lower bridge deck reduces the pressure difference on the upper and lower sides of the bridge deck, leading to a marked reduction in the strength of the vortex behind the lower chord. Overall, the decrease in the railing ventilation rate reduces the vortex intensity and quantity in the wake region of the lower bridge deck, resulting in weak vortex shedding. Figure 21 displays the frequency spectrum analysis of wind speed time histories detected at the measurement point P22. The vortex shedding frequency at this point did not change significantly, consistent with wind tunnel tests, but the amplitude decreased considerably, indicating a marked reduction in vortex intensity. When the pedestrian railing ventilation rate was zero, the amplitude of the peak corresponding to the minor torsional VIV was almost zero, possibly due to the significant reduction in vortex because of the inability of flow to pass through the railing, resulting in the nearly complete disappearance of minor torsional VIV in wind tunnel tests. The torsional VIV of the truss girder at low wind speeds may be related to vortex shedding caused by flow passing through the railing gap and vortex shedding at the top of the railing. Furthermore, the torsional VIV at high wind speeds may be related to vortex shedding caused by flow being blocked by the lower chord. By reducing the railing ventilation rate, the vortex intensity and quantity in the wake region of the lower bridge deck decrease significantly, achieving the goal of suppressing VIV.