## 2 Background  
### Wind Tunnel Tests  
Measuring the wind effects on a structure is difficult because this process is highly sophisticated due to the random and spatiotemporally variable nature of the wind [1]. The traditional method of estimating wind effects on bridges is to conduct a wind tunnel test on a scaled model of the bridge. The wind tunnel test was originally designed for aeronautical use and was developed during war times in the twentieth century [9]. In terms of layout, there are two types of wind tunnel: the open-circuit type and the closed-circuit type. In the open-circuit wind tunnel, air is drawn into the wind tunnel from the external environment at the inlet and discharged at the outlet, as shown in Figure 1a,b. The model to be tested can be either placed upstream of the fan (Figure 1a) or downstream of the fan (Figure 1b). The former configuration is more convenient to control the flow within the tunnel, whereas the latter is more suitable for studies where atmospheric pressure is considered important [10]. Furthermore, the latter configuration facilitates large-scale testing as the model is placed outside the tunnel. Therefore, it is often used in the study of rain-wind-induced vibrations where cables are large. Figure 2 shows the layout of a closed-circuit wind tunnel, in which the air is circulated and can be continuously accelerated within the wind tunnel. This layout is more efficient in controlling the wind velocity and requires less floor space. Hence, it is favoured in the study of bridge aerodynamics and has been normally used to derive parameters such as aerodynamic forces and flutter derivatives. Figure 1: (**a**) Layout of the open-circuit wind tunnel. (**b**) Layout of the open-circuit wind tunnel. For wind tunnel tests conducted for bridges, the bridge girder, pylon or the full bridge may be validated and optimised under a given wind condition. For instance, during the design of the Stonecutters Bridge, wind tunnel tests were used to assess the impact of the complex weather conditions in Hong Kong and to subsequently refine the shape of the bridge deck [11]. Additionally, wind tunnel tests were applied to evaluate the wind effects on the bridge during its construction stage. In the construction of the Great Belt East Bridge, results of wind tunnel tests played important roles in the final decision of the erection scheme of the bridge girder [12]. During the construction of the Izmit Bay Bridge, a series of wind tunnel tests were used to assess the aerodynamic effects and confirm aeroelastic stability of the bridge [13]. More recently, before the construction of the Sutong Bridge, a series of wind tunnel tests were conducted to test the safety of the free-standing pylon and the incomplete bridge girder at the construction stage [14]. Although wind tunnel tests have been applied in the industry for many years, there are still several shortcomings and limitations of using this method to study bridge aerodynamics. Firstly, bridges are large structures. To test them in a wind tunnel, small-scale models are needed to be able to fit in a wind tunnel facility. The common range of scales used in wind tunnel tests for bridge models is 1/50 to 1/200 [9]. However, [PERSON] et al. [15] indicated that such small scales cannot provide reliable results for the study to mitigate VIV. Resulting from the small scale, the fluid boundary layer at structures such as guide vanes and deflectors on the bridge model will become too thick relative to the size of these structures, which will significantly influence the flow rate near these structures. For cases that require high resolutions of boundary layers, large-scaled sectional models should be used. However, the dimensions of the testing model in a wind tunnel are limited by the size of the testing facility and a factor termed the \"blockage ratio\" (the ratio of the cross-sectional area of the test model to the cross-sectional area of the wind tunnel).