Currently, research on wind fields of U-shaped valleys is rarely reported, and anemometers or wind observation towers are usually used for field measurement, but the measured position is limited and the cost is high. In order to study the wind characteristics in a mountainous U-shaped valley, a long-range, all-weather, high-precision Wind3D 6000 lidar was placed at a bridge site located in a U-shaped valley. Then, according to the data effective ratio and wind speed, nearly 6 months of original data ranging from 0 m to 810 m were analyzed statistically. It was found that the spatio-temporal distribution of wind speed and direction is obviously not uniform, and the wind parameters are correlated among different virtual wind towers (VWTs). By classification, the effective data of midspan position is taken as the research object, and the wind speed profile is divided into three categories. Type-1 shows disorderly characteristics; Type-2 shows a linear relationship; and Type-3 shows a nonlinear relationship. The wind direction is consistent with the main wind direction at the bridge site and the average wind direction of different VWTs has a high consistency. The concept of wind-direction deflection rate is put forward to describe the variation of wind direction with height. These measured wind parameters could be used as a reference for bridge wind-resistant design.

In order to explore the inducing mechanism of negative damping of bending–torsional coupling flutter, an ideal plate with a width of 0.45 m was taken as the research object. The changes in frequency, critical wind speed, aerodynamic stiffness, and aerodynamic damping were systematically analyzed by using the “incentive-feedback” mechanism theory. The source of modal damping and the inducing mechanism of bending–torsional coupling flutter were identified. The research results show that the torsional modal damping of the ideal plate mainly comes from the aerodynamic positive damping of the torsional velocity self-excitation ($A_2^{*}$) and the aerodynamic negative damping of the torsional displacement incentive feedback ($A_1^{*}{H}_3^{*}$). Among them, the aerodynamic negative damping of the item ($A_1^{*}{H}_3^{*}$) causes the torsional mode damping to be negative, and the ideal plate undergoes bending–torsion-coupled flutter under the drive of the torsional mode aerodynamic negative damping. The reason why the aerodynamic damping of the item ($A_1^{*}{H}_3^{*}$) is negative depends on two aspects: one is that the flutter derivatives $A_1^{*}$ and $H_3^{*}$ have opposite signs; the second is that the torsional displacement self-excited lift excites the vertical vibration to produce negative stiffness $-m_h{\omega }_{s\alpha }^2$. This results in the phase difference between the torsional displacement self-excited lift and the vertical displacement response in the range of (90–180°).