# Steady-Suction-Based Flow Control of Flutter of Long-Span Bridge
## 2 Experimental Set-Up
### Wind Tunnel Set-Up and Test Measurements  
The experiments were conducted in a closed-circuit wind tunnel located in the Department of Civil Engineering at the Harbin Institute of Technology. The test section of the wind tunnel was 3.0 m in height, 4.0 m in width, and 25 m in length; the available wind velocity range was 2-45 m/s. Moreover, the maximal free-stream turbulence intensity was 0.46%, and the maximal free-stream nonuniformity was 1%. In this study, the oncoming flow was uniform and smooth. A very fine scan of the wind speed range of interest was conducted for each test case. During the tests, the bridge model was mounted inside the wind tunnel with a suspension system consisting of eight coil springs and two supporting frames, which established the heave and pitch stiffness of the bridge section model. The natural vertical and torsional frequencies of the overall test model in still air were 2.586 and 6.8 Hz,  
Figure 1: Size of the section model: (a) cross section; (b) longitudinal section. Figure 2: Suction slot arrangement. respectively, with an acceptable deviation within 5% compared to the design values of 2.548 and 7.092 Hz, respectively. The vertical and torsional damping ratio of the overall test model were 0.44% and 0.74%, respectively. It should be mentioned that, whether or not suction control was applied, the bridge model was connected to the suction control device throughout the test process, including the measurement stage of frequencies and damping ratios. Two laser reflection sensors set in pairs were installed underneath one side of the test section and aimed at the reflected target set on the lever arm. They were used to measure the vertical and torsional responses of the model. The signals from the reflection gauges were sampled at 1000 Hz. Furthermore, steady suction was realized by installing a self-designed suction control system. Figure 3a depicts details of the control system. The suction pump pumped the air out of the vacuum tank to generate differential pressure. This provided suction power at the bottom of the bridge deck. The digital readout flowmeter used to monitor the flow rate and the pneumatic junction throttle employed to adjust the suction flow were installed in the control system to ensure a precise control device. Moreover, the suction control device was connected to the test model through air suction pipes. The overall model configuration of the wind tunnel is shown in Figure 3a,b. In this study, the effects of the test variables, including the suction slot position, suction interval, and suction rate, on the control were studied in detail. To distinguish between the positions of the suction slots, positions on the windward side were denoted by \(l\), positions in the middle were denoted by \(m\), and positions on the leeward side were denoted by \(b\). The suction interval was expressed in a nondimensional form as \(\lambda=d/H\), where \(\lambda\) is the relative suction interval, \(d\) is the distance between two adjacent slots, and \(H\) is the model section height. The suction rate was also described in a nondimensional form as \(C_{q}=Q/U_{\text{max}}LH\), where \(C_{q}\) is the relative single-slot suction flow rate, \(Q\) the single-slot suction flow rate, \(U_{\text{max}}\) the maximal upcoming wind velocity, and \(L\) the model length. Three test cases with three different locations of suction slot rows (i.e., on the windward side, in the middle, and on the leeward side) were investigated to determine the best suction position. In addition, to systematically investigate the influence of different suction intervals and suction flow rates on the flutter instability of the bridge, four representative relative suction intervals (\(\lambda\)) (i.e., \(\lambda=1\), 2, 3, 4) and three relative suction flow rates (i.e., \(C_{q}=0.00174\), \(0.0244\), and \(0.00348\) corresponding to \(Q=5\), 7, and 10 L/min, respectively) were selected for the specified suction positions. In total, nine test cases (Table 1) were adopted with a naming standard.