## 3 Sectional Model Wind Tunnel Test
### Vortex-Induced Vibrations
\begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**States**} & \multirow{2}{*}{**Wind Attack Angle**} & \multirow{2}{*}{**Flow Field**} & \multirow{2}{*}{**Wind Speed Ratio**} & \multicolumn{2}{c}{**Vortex Vibration Limit**} \\ \cline{5-6}  & & & & & **Vertical (m)** & **Torsional (Deg)** \\ \hline construction & \(-5^{\circ}\), \(-3^{\circ}\), \(0^{\circ}\),+3\({}^{\circ}\), +5\({}^{\circ}\) & 41.39 & 1:2.2 & 0.1488 & 0.1684 \\ service & \(-5^{\circ}\), \(-3^{\circ}\), \(0^{\circ}\),+3\({}^{\circ}\), +5\({}^{\circ}\) & 58.24 & 1:2.5 & 0.1424 & 0.1620 \\ \hline \end{tabular}
\end{table} 
Table 3: Test conditions and limit values. As is shown in Figure 9a,b, there is no VIV phenomenon during the construction state. While in service state, the vertical and torsional amplitudes caused by VIV exceed the limit value defined by the Chinese Specification. Furthermore, both have two lock-in ranges, indicating that the bridge may appear hazardous vibrations at both low and high wind speeds. All these should be mitigated properly. To mitigate VIV, there are three basic measures: aerodynamic countermeasure, mechanical countermeasure, and structural countermeasure [32]. It is widely used since the aerodynamic countermeasure such as changing the angle of wind fairings is a method that the added cost of installation and maintenance could be low. Specific aerodynamic measures can be determined by a mixture of computational fluid dynamics (CFD) and wind tunnel tests. It has been proved that appropriate reduction of the wind fairings angle can mitigate the VIV amplitude with no adverse effect on flutter stability [25]. The application of aerodynamic countermeasure, as depicted in Figure 10.  
Figure 9: Vertical and torsional response. (**a**) Vertical amplitude of construction; (**b**) torsional amplitude of construction; (**c**) vertical amplitude of service; (**d**) torsional amplitude of service. Through the CFD results, not only the aerodynamic forces of the air flow acting on the deck can be simulated, but also the distribution of the air flow outside the deck can be obtained, which provides intuitive information for qualitatively explaining the impact of the change of wind fairings angle on the aerodynamic characteristics of the deck. Due to the influence of auxiliary structure, the triangular unstructured mesh is adopted to facilitate mesh division. The mesh size near the deck gradually increases, the growth rate is not greater than 1.15, and the mesh quality is controlled above 0.46. After mesh independence verification, the number of meshes in the final calculation scheme is about 100,000. Fluent is used for simulation and the commonly used SST \(k\)-\(\omega\) turbulence model is adopted. Two larger-scale vortices are created near the deck, including a big one on the deck and one on the downwind knuckle lines with blunt wind fairings while there is just one vortex on the downwind knuckle lines with sharp wind fairings, as shown in Figure 11. Only by changing the wind fairing angle does the wind flow around the deck change significantly. The formed vortices with blunt wind fairings are not only low in frequency and high in energy but they are also easy to detach from deck and form von Karman vortex street. Only a smaller vortex forms with sharp wind fairings, which will modify the vortices' action position and the aerodynamic forces on the deck. As a result, it can be concluded that sharping the wind fairings angle is useful in mitigating the VIV phenomenon by CFD. In the sectional model wind tunnel test, the VIV was investigated with 40\({}^{\circ}\) wind fairings. The VIV suppression effect is compared in Figure 12 when the wind attack angles are 0\({}^{\circ}\), +3\({}^{\circ}\), and +5\({}^{\circ}\) in the service state. Figure 11: Winding flow pattern in the area near the deck with different wind fairing angles. **(a)** 62\({}^{\circ}\) wind fairing; **(b)** 40\({}^{\circ}\) wind fairing. Figure 10: Aerodynamic measures.