## 2 Experiment Setup and Model Configuration  
### Model Configuration  
The experiment was conducted in a small-scale closed-circuit wind tunnel, i.e., SMC-WT1 (Harbin Institute of Technology, Harbin, China) at the Joint Laboratory of Wind Tunnel and Wave Flume, Harbin Institute of Technology. The test section was made of transparent plexiglass with a cross-section of 505 mm \(\times\) 505 mm and length of 1050 mm, as shown in Figure 1. The wind speed can be continuously adjusted from 0 to 25 m/s with minimum steps of 0.05 m/s, and the turbulence intensity of the incoming airflow is less than 0.4%. The Great Belt East Bridge in Denmark is the third longest suspension bridge in the world with a main span of 1624 m. The cross-section of the main girder is a single box with a width and height of 31.0 m and 4.34 m, respectively. With a relatively small aspect ratio (width/height), VIVs excited by the vortex shedding excitation at a low wind speed range of 4-12 m/s was observed in this bridge [5]. Therefore, the main girder of the Great Belt East Bridge was chosen for the research objective in the present experimental study. A high-precision 3D printing technology was employed to manufacture a segment model of the main girder by using a PLA (polylactic acid) plastic material. A geometric scale ratio of 1:125 was determined, although it was relatively smaller because of the limitation of the test section size. Scaling the model by a factor of 125 has an impact on the Reynolds number of the flow. As reviewed by [PERSON] and [PERSON] [19], the Reynolds number sensitivity of the aerodynamics of bluff bodies with sharp edges and concluded that ignoring the Reynolds number effect of bluff bodies such as box-like bridge decks can lead to systematic errors. [PERSON] et al. [20] conducted an investigation on the vortex characteristics of a twin box bridge section at different Reynolds numbers. In our group, the Reynolds number effects of twin-box girders have always been a hot topic. A recent report can be found in [PERSON] et al. [21].It should be mentioned that we could never reach the real Reynolds number level in the atmospheric boundary layer wind tunnel, due to the limitations of test section and the maximum wind speed. We also noticed that the Reynolds number was much smaller in comparison with real bridges. However, the present study focuses on the fundamental side, and we aim to reveal the flow characteristics around the scaled models with and without control. For the segment model, the accessory members of the single box girder bridge were neglected and the slope of the upper surface was considered as shown in Figure 2.  
Figure 2a shows the geometric information for the cross-section of the segment model, where the width and height are 248.0 mm and 34.7 mm, respectively, according to the geometric scale ratio. The dimension of the segment model in the span-wise direction is 480.0 mm, due to the size limitation of the test section. With this relatively small aspect ratio, the vortex shedding of the test segment model will be easily observed, and the potential vortex-induced vibration can be controlled when the alternating vortex shedding is suppressed. In the present study, a bypass passive jet flow control method was proposed to manipulate the vortex shedding in the test model. Figure 1: A sketch view of the test section. Five test models with bypass passive jet flow control are shown in Figure 2b-f, respectively. The test models for the five control cases have the same external dimensions as Case 0 (Figure 2a). To transport the airflow from the positive side to the negative pressure region and ensure that the upper surface of the girder is not affected, suction/jet holes were only manufactured on the leading/trailing edges of the test models, respectively, which were connected by the hollow channels. As shown in Figure 2e, the height of the connected channel is 2.5 mm, the thickness of the wall is 1.5 mm, and the height of the suction/jet hole is one tenth of the cross-sectional height of the test model (i.e., 3.47 mm).