# Aerodynamic Characteristics of Coupled Twin Circular Bridge Hangers with Near Wake Interference
## 5 Conclusions  
The aerodynamic forcing characteristics of coupled twin circular cylinders were investigated experimentally to study the synchronous galloping. Pressure measurements were performed on a stationary section model with a series of cylinder spacings and wind incidence angles. Based on the pressure data, mean and fluctuating pressure distributions, reduced frequencies and total aerodynamic forces of the cylinders are discussed. Moreover, numerical simulations were performed for flow visualization. The conclusions are summarized as follows:  
Figure 12: Velocity contours for twin cylinders arranged in tandem: (**a**) W/D = 2.5; (**b**) W/D = 4.3.  
Figure 13: Velocity contours for twin cylinders arranged in staggered (\(\alpha=10^{\circ}\)): (**a**) W/D = 2.5; (**b**) W/D = 4.3.  
(1) The flow around twin cylinders shows two distinct patterns with various spacings, and the corresponding critical spacings at wind incidence angles of 0\({}^{\circ}\) and 9\({}^{\circ}\) are in the range of 3.8D-4.3D and 3.5D-3.8D, respectively.  
(2) When W/D is greater than the critical value, regular vortex shedding will be generated from the upstream cylinder and the vortexes impact upon the downstream cylinder. In consequence, large fluctuating pressure on the downstream cylinder is induced, while the fluctuation decreases with increasing W/D. When W/D is smaller than the critical value, the vortex shedding of the upstream cylinder is suppressed by the downstream cylinder: the symmetric region between the twin cylinders is almost static at a wind incidence angle of 0\({}^{\circ}\); a negative pressure region emerges on the top edge of the downstream cylinder and the pressure distribution is asymmetrical at a wind incidence angle of 9\({}^{\circ}\).  
(3) When \(2.5\leq\text{W/D}\leq 3.5\), there is a critical wind incidence angle near 9\({}^{\circ}\). In this case, the wake flow of the upstream cylinder flows rapidly near the top edge and impacts on the inlet edge of the downstream cylinder, which causes a negative and positive pressure region near the corresponding edge regions, respectively. Hence, the lift force of the downstream cylinder comes to a peak while the drag force jumps to a higher value. In consequence, the total lift force of the twin cylinders comes to a peak while the total drag force jumps to a higher value. Furthermore, there is a sharp drop in the total lift coefficient in the range of 9\({}^{\circ}<\alpha<12^{\circ}\), and this may lead to aeroelastic instability of the coupled twin cylinders when the flow velocity increases to a critical value. Therefore, these cases (i.e., 9\({}^{\circ}<\alpha<12^{\circ}\) while \(2.5\leq\text{W/D}\leq 3.5\)) should be carefully checked in engineering applications involving avoiding aeroelastic instability.  
Conceptualization, X.H.; methodology, C.W. and Q.W.; software, C.W.; validation, X.H. and Z.H.; investigation, C.W., Z.H. and Q.W.; writing--original draft preparation, C.W.; writing--review and editing, X.H. All authors have read and agreed to the published version of the manuscript.  
This study is sponsored by the National Science Foundation for Distinguished Young Scholars of China (No. 52025082), the National Science Foundation of China (No. 51422806), and the Hunan Provincial Innovation Foundation for Postgraduate, China (No. CX20190288), which are greatly acknowledged.  
The help from [PERSON], former graduate student, is also appreciated for conducting wind tunnel tests.  
The authors declare no conflict of interest.