# Experimental Investigation on Flow-Induced Rotation of Two Mechanically Tandem-Coupled Cylinders
## 1 Introduction
Based on previous research results, [PERSON] and [PERSON] [34] concluded that there were three wake interference regions: extended-body regime, reattachment regime and co-shedding regime. [PERSON] and [PERSON] [35; 36] simulated the two cylinders in tandem arrangement at low Reynolds number and large spacing ratio (\(L/D\) = 5.5). The numerical results showed that the locking interval of the downstream cylinder was significantly larger than that of an isolated cylinder, due to the influence of the upstream cylinder, while the downstream cylinder had almost no influence on the upstream cylinder. [PERSON] et al. [37] experimentally studied the vibration of tandem double cylinders with the upstream cylinder fixed, through a flume test. The results showed that wake galloping could be observed in the downstream cylinder, and the amplitude increased by nearly 50%, compared with that of a single cylinder, when the spacing ratio was 3.0 \(<\)\(L/D\) < 5.6. Moreover, [PERSON] et al. [38] proposed the concept of wake-induced vibration (WIV) in their further research. In order to develop a clearer understanding of the WIV excitation mechanism, the response of the downstream cylinder in a steady shear flow was compared with typical VIV responses of a single cylinder. The test results showed that the WIV phenomenon of the downstream cylinder was excited by the interaction between the unstable vortices from the wake of the upstream cylinder and the downstream cylinder. [PERSON] et al. [39] performed a numerical simulation on the VIV of the tandem double cylinders with two degrees of freedom. The results showed that the double resonance phenomenon occurred in all frequency ratios, and the vibration of the upstream cylinder was similar to that of an isolated cylinder under various working conditions, while the dynamic response of the downstream cylinder was more sensitive to the incoming direction than to the lateral direction, due to the change of the frequency ratio. In later papers, scholars paid more attention to analyzing the internal mechanism of vibration characteristics. [PERSON] et al. [40] conducted a comprehensive analysis on the mechanism of the large-amplitude response of the downstream cylinder in tandem double cylinders at \(Re\) = 100, and found that the shedding vortices of the upstream cylinder induced a low pressure area, which, in turn, stimulated the downstream cylinder to generate a large amplitude response. [PERSON] et al. [41] numerically investigated the three tandem cylinders in the range of \(L/D=1.2\) to \(5.0\), and analyzed the influence of three key factors on the galloping phenomenon, including balance position offset, vortex shedding and timing between cylinders. [PERSON] et al. [42] experimentally studied tandem double cylinders. It was found that there were different vibration mechanisms, such as locking, VIV and galloping, under various frequency ratio conditions. Furthermore, differing from previous research on the FIM of cylinders, [PERSON] and [PERSON] [43, 44] identified four mechanisms of vibration with two mechanically coupled circular cylinders that were free to rotate around a pivot, and they proposed the concept of gap switching-induced vibration (GSIV). [PERSON] et al. [45] numerically investigated the flow-induced rotation of a circular cylinder with a detached splitter plate at \(Re=100\), and found the occurrence of bifurcation at \(0\leq G/D\)\(\leq 0.5\), which disappeared at \(0.55\leq G/D\leq 2.0\). [PERSON] and [PERSON] [46] undertook extensive simulations to investigate the VIV of a piggyback circular cylinder system at \(Re=100\). It was found that the motion trajectories would take the form of a classical pattern of 8 when the system was in tandem arrangement. Furthermore, many scholars also conducted in-depth research [47, 48, 49] on the FIM power generation of tandem double cylinders.