## 3 Aeroelasticity and Aerodynamics of Cable-Supported Bridges
### Vortex-Induced Vibration  
3.2.1. Mechanism of Vortex-Induced Vibration  
Vortex-induced vibrations (VIVs) are motions induced on a structure which is interacting with an external flow and are produced by vortex shedding of the flow [71]. When the wind velocity is beyond a critical value and within a critical range, the interaction of flow and bridge deck will produce a \"lock-in\" effect that synchronises the vortex shedding frequency with the frequency of the structure [51]. [PERSON] [9] summaries the four conditions under which VIVs can occur:  
1. The wind has a normal direction to the longitudinal axis of the bridge deck. 2. The turbulence intensity is normally less than 5%. 3. The critical wind velocity is within the range of 5 to 12 m/s. 4. The damping ratio of the system is less than 1%. It should be noted that VIVs only occur at a critical range of wind velocity, which indicates the vibration will not develop divergently, and so has not yet destroyed any bridges. However, it is commonly accepted that VIVs tend to accelerate fatigue damage and failure under serviceability criteria. It is reported that vibrations of large amplitudes are a common occurrence. For instance, a VIV was observed on the section model of the Stonecutters Bridge during the design stage [15]. The Great Belt Bridge encountered a series of VIVs during the construction of the girder, of which the maximum amplitude reached 0.32 m at the main span that is 1624 m long [72]. Similarly, VIVs with amplitudes of over 0.5 m occurred on 240 m spans of the Trans-Tokyo Bay Bridge after the girder was fully constructed [73]. 3.2.2. Wind Tunnel Studies of Vortex-Induced Vibrations  
There has been a significant amount of research into the use of wind tunnel tests for investigating VIVs. Experimental studies have been categorised into two groups depending on their focus: investigations on how to faithfully reproduce VIVs in wind tunnels and studies to find effective suppression strategies of VIVs. 1. Reproducing VIV in wind tunnel tests  
[PERSON] [74] classified vortex shedding effects of bridges into three mechanisms: one-shear-layer vortices, which are normally seen on the bridge deck and girder, two-shear-layer vortices, and 3D vortices, which are commonly observed in cable vibrations. They emphasise the vital role of the boundary layer in the formation of VIVs. It is suggested that the mechanism of creation or suppression of VIVs can be explained by the \"double vortex mode\" [75]. The \"double vortex mode\" considers effects of two types of vortices, those formed by model motions and those formed by secondary motions. The separation of these vortices generates periodic aerodynamic forces on upper and lower surfaces of the bridge girder which will constantly move the girder in the vertical direction. [PERSON] et al. [76] suggested that the study of VIVs and flutter instability are interdependent. For instance, twin-box girders are a new type of bridge girder designed to suppress flutter by increasing the critical flutter speed. However, [PERSON] et al. [15] proved that twin-box girders are more susceptible to VIVs than conventional trapezoidal girders or truss girders. The open gap between two box girders stimulates the formation of von Karman vortices which lead to VIVs. Since the triggering mechanism of VIVs is strictly related to vortex shedding, the Reynolds number effects on the boundary layer become significant. [PERSON] et al. [15] compared results of wind tunnel tests of a 1:80 model and 1:20 model of the Stonecutters bridge and found that the commonly used 1:80 scaled model cannot provide faithful results for the study of VIVs. In one specific case, with guide vanes installed on both models, the 1:80 model showed that the guide vanes stimulate extra vibrations while the larger model demonstrates that the guide vanes can effectively eliminate VIVs on the girder, which further explains why the low Reynolds number of the smaller model leads to an overestimation of the boundary layer thickness [15].