# Aerodynamic Characteristics of Coupled Twin Circular Bridge Hangers with Near Wake Interference
###### Abstract
The VIV of two rigidly coupled circular cylinders of different diameters at low Reynolds numbers has also been investigated by researchers [20]. Setting a small gap between the two cylinders is an effective measure to mitigate the vibration by reducing the vibration amplitude and narrowing the lock-in regime. [PERSON] and [PERSON] [21] investigated the characteristics of wake galloping for two parallel/unparallel circular cylinders via wind tunnel tests at a Reynolds number of 2.0 \(\times\) 10\({}^{5}\). The unparallel disposition of two cylinders was effective to reduce wake galloping phenomena caused by unsynchronized motion along the cylinder with varying gap spacing. That study proposed a new method of vibration control for adjacent cylinders. Flow around twin cylinders arranged in tandem is extremely complex due to the aerodynamic or hydrodynamic interaction, and the flow characteristics are of great practical importance for determining the flow-induced behaviors of cylinders [29; 30]. Flow characteristics are mainly dependent on the characteristics (turbulence and incident angle) of incoming flow and the arrangement of cylinders. Researchers [9; 31; 32] classified the relative position of the downstream cylinder as three typical regions: proximity interference region, wake interference region, and region of no-interference. Meanwhile, flow regimes of the three patterns have been comprehensively studied via numerical simulations and experiments in the last several decades [10; 14]. Previous research indicated that critical spacing values for the three typical regions are mainly dependent on the turbulence and incidence angle of flow [14]. When the Reynolds number is in the subcritical range, wake interference takes place in tandem and slightly staggered cylinders whose spacing exceeds a critical value of 3.5D-4.0D [33]. In this case, the downstream cylinder wake is greatly affected by the upstream cylinder but not vice versa, thus the upstream cylinder is assumed to behave like a single isolated cylinder. The upstream cylinder flow is influenced slightly through a feedback mechanism by the downstream cylinder with the spacing at least up to 8D (with a reduced St and increased aerodynamic coefficients compared to those of an isolated cylinder) [34; 35]. In addition, researchers [36; 37] have discovered two completely distinct flow characteristics in laminar and turbulent regimes. [PERSON] and [PERSON] [38] studied the flow around twin tandem cylinders by both direct numerical calculation and numerical simulation. The results showed there was a critical Reynolds number for the transition from steady symmetric flow to an oscillatory flow, and also a certain range of the gap spacing where physical quantities (such as aerodynamic coefficients and Strowhal number) show an abrupt change. Moreover, the transition of flow regime is also connected with the flow incidence angle in the high subcritical range of the Reynolds number, and the critical incidence angle of transition is dependent on the Reynolds number and spacing of the cylinders [37]. Pressure measurement is a familiar efficient method for the study of structural wind engineering and has been widely used in previous investigations on the aerodynamic characteristics of twin cylinders [39; 40; 41]. As an essential aerodynamic parameter, the distribution and pulsation of pressure are sensitive to the surface roughness [42], Reynolds number, flow incidence angle, diameter ratio [43], and arrangement of the cylinders [14; 44]. [PERSON] [45] tested the pressure on two tandem identical cylinders at a Reynolds number range of subcritical values. Vortex shedding was detected from the upstream cylinder in a certain spacing near 3.5D, with a sharp peak of pressure pulsation emerging at \(\theta=40^{\circ}\) (where \(\theta\) is the angle on the circumference taken from the front stagnation point on a cylinder) of the downstream cylinder.