## 3 Experimental Setup  
All the tests were performed in a closed-circuit wind tunnel at Hunan University, China. One of the test sections in which the 3D prototype prism tests were performed had a width of 5.5 m, a height of 4.5 m, and a maximum wind velocity up to 16 m/s. The other test section had smaller cross-sections with a width of 3.0 m, a height of 2.5 m, and a higher wind velocity up to 58 m/s. The 2D section model tests were conducted in the smaller test section. All the tests were designed to perform smooth flow conditions.  
The 3D prototype rectangular prism was made of a rectangular steel tube with the dimensions of \(50\times 70\times 4500\) mm. As shown in Figure 1, the prism was well fixed between the top and bottom parts of the wind tunnel. The flow condition distortion caused by the boundary condition was alleviated considerably with the use of a plane end-plate. Four accelerometers were employed to record the along-wind and cross-wind responses at the mid-span and quarter-span of the prototype prism, as illustrated in Figure 2, and the sampling frequencies were 200 Hz.  
The experimental setup of the 1:1 2D section model, which was made of organic glass (a type of polymethylmethacrylate) material, is shown in Figure 3. The 2D rigid section model adapts eight springs and two circular endplates (diameter = 210 mm) at both ends to guarantee the 2D flow conditions around the model. The section model system has six possible degrees of freedom (DOFs), as shown in Figure 4. There are three translational DOFs and three rotational DOFs, along with X-X, Y-Y, and Z-Z axes, respectively. The translational DOFs in the directions of the X-X and Z-Z axes were restricted in the section modal system, as was the rotational DOF in the direction of the Y-Y  
Figure 1: Wind tunnel test setup of the prototype prism.  
Figure 2: Accelerometers installed on the prototype prism at mid-span.  
axis. The other three available DOFs of the section model system were the translational DOFs in the directions of the Y-Y axis (heaving mode) and Z-Z axis (rolling mode), respectively, as well as the rotational DOF in the direction of the X-X axis (pitching mode).  
Both the 3D prototype prism and the 2D section model were perpendicular to each other and faced the oncoming wind flow with the long edge of the cross-section (70 mm side) during wind tunnel tests. Two model tests were performed in separate wind tunnel sections. It is possible to vary the flow conditions even in the controlled smooth flow conditions. The onset wind velocity of the vortex-induced vibration was \(>\)8 m/s. This was measured such that the wind profile was very close to the turbulence intensities.  
The comparison of the main parameters of the 3D prototype prism with the 1:1 2D section model is shown in Table 1. The surface roughness of the prototype prism and section model was different. The steel prototype prism was painted as smoothly as possible to make it similar to the organic glass material section model. Both the 3D prototype prism and the 2D section model had the same cross-sectional dimensions and almost the same mass, damping, and natural frequencies. The Reynolds number corresponded to the peak of the VIV lock-in.  
Figure 4: Possible degrees of freedom (DOFs) of the section model.  
Figure 3: Section model setup.