# Aerodynamic Characteristics of Coupled Twin Circular Bridge Hangers with Near Wake Interference
###### Abstract  
Much work has been devoted to the investigation and understanding of the flow-induced vibrations of twin cylinders vibrating individually (e.g., vortex-induced vibration and wake-induced galloping), but little has been devoted to coupled twin cylinders with synchronous galloping. The primary objective of this work is to investigate the aerodynamic forcing characteristics of coupled twin cylinders in cross flow and explore their effects on synchronous galloping. Pressure measurements were performed on a stationary section model of twin cylinders with various cylinder center-to-center distances from 2.5 to 11 diameters. Pressure distributions, reduced frequencies and total aerodynamic forces of the cylinders are analyzed. The results show that the flow around twin cylinders shows two typical patterns with different spacing, and the critical spacing for the two patterns at wind incidence angles of 0\({}^{\circ}\) and 9\({}^{\circ}\) is in the range of 3.8D\(\sim\)4.3D and 3.5D\(\sim\)3.8D, respectively. For cylinder spacings below the critical value, vortex shedding of the upstream cylinder is suppressed by the downstream cylinder. In particular, at wind incidence angles of 9\({}^{\circ}\), 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, respectively. As a result, the total lift force of twin cylinders comes to a peak while the total drag force jumps to a higher value. Moreover, there is a sharp drop of total lift coefficient for \(\alpha=\) 9-12\({}^{\circ}\), indicating the potential galloping instability. Finally, numerical simulations were performed for the visualization of the two flow patterns. twin cylinders; interference effects; pressure measurement; wake-induced vibrations; synchronous galloping; wind tunnel tests +
Footnote †: journal:flow switching, vortex impingement, and quasi-periodic vortices, involving most of the generic flow features associated with various structures [14]. Flow around twin cylinders provides an excellent model for gaining insight into the underlying flow physics around multiple structures and has attracted wide attention [7; 15; 16; 17; 18; 19]. There are three arrangements for parallel twin cylinders: tandem arrangement, side-by-side arrangement and stagger arrangement. The effects of aerodynamic interactions on cylinders are significantly different for the three arrangements. For instance, the downstream cylinder is sucked by the upstream cylinder when twin cylinders are arranged in tandem closely, while it may suffer strong vibration when they are arranged in stagger with a certain relative position, which can lead to disastrous results on engineering structures. Previous studies have focused on multiple aerodynamic instabilities of the downstream cylinder such as vortex-induced vibration (VIV) [20], wake induced galloping [21; 22; 23; 24], and a combination of them [25]. Instability may occur on both cylinders in a tandem arrangement with a small spacing (W/D \(<\) 3.2) at a low Reynolds number [26]. Additionally, the vibration amplitude of one cylinder is sensitive to the spacing between the two cylinders and whether one of them is fixed or not. For the downstream cylinder, strong vibration results in particular cases for staggered arrangement [27]. It is worth mentioning that flow-induced vibration can also occur on two rigidly coupled identical circular cylinders in tandem, staggered and side-by-side arrangements [25; 28]. [PERSON] [25] studied the synchronous vibration of coupled twin cylinders in the cross-flow direction at a low Reynolds number, and found that the gap between the two cylinders has a significant effect on the response. For a tandem arrangement, the lock-in regime of the reduced velocity for VIV is very different from that of a single cylinder; for a side-by-side arrangement, there is a specific phenomenon that the combination of VIV and galloping emerges at a certain range of spacing and reduced velocity.