# Experimental Investigation on Flow-Induced Rotation of Two Mechanically Tandem-Coupled Cylinders
## 1 Introduction  
Flow-induced motion (FIM) is a complicated non-linear vibration phenomenon caused by the interaction between moving fluid and elastic bodies [1], which is widely used in practical engineering, such as marine risers [2; 3; 4; 5], bridge latches [6; 7] and high-rise buildings [8]. The FIM phenomenon could cause great damage to engineering. Therefore, much research [9; 10] has focused on how to predict and suppress FIM to ensure the safety of engineering structures in the early stages. However, with the development and utilization of ocean energy and in-depth research on FIM, more scholars are focusing on exploiting ocean energy by means of the FIM phenomenon. A series of new ocean current and tidal current power generation devices have been proposed [11; 12; 13]. Among them, a typical one is the Vortex Induced Vibration for Aquatic Clean Energy (VIVACE), a new low-speed current power generation device, proposed by [PERSON] from the University of Michigan [14; 15; 16; 17]. At the same time, scholars have also carried out a large number of studies on oscillation characteristics and energy conversion of FIM for multi-oscillators, most of which have concentrated on circular cylinders [18; 19; 20]. Previously, scholars mainly studied the effect of spacing on the wake and oscillation characteristics of the upstream and downstream cylinders through experimental methods. [PERSON] and [PERSON] [21] found that when two identical cylinders were partially, or completely, immersed in fluid, with a spacing ratio of \(0.25<L/D<6\), a complex interaction wouldoccur between the wake vortex and the deflection of the cylinders. [PERSON] [22; 23; 24] systematically studied the changes of wake vortex and flow force of tandem cylinders through the experimental method, and summarized the evolution process of the wake vortex of the upstream and downstream cylinders with the change of spacing. On this basis, [PERSON] [25] analyzed the effect of spacing ratio and Reynolds number on vibration and wake of upstream and downstream cylinders, through experiments for a Reynolds number of 8.7 \(\times\) 10\({}^{3}\)\(\leq\)\(Re\)\(\leq\) 5.2 \(\times\) 10\({}^{4}\) and the spacing ratio of 1.03 \(\leq\)\(L/D\)\(\leq\) 5.0, and defined the wake path with different spacing ratios. [PERSON] and [PERSON] [26; 27] studied the vibration characteristics of upstream and downstream cylinders with different sizes through a series of physical experiments. It was shown that the oscillation wake of the upstream cylinder would be affected by the amplitude variation of the downstream cylinder. [PERSON] et al. [28] studied the force of the upstream and downstream cylinders through the experimental method, under the condition that the Reynolds number range was 4.5 \(\times\) 10\({}^{4}\) < \(Re\) < 5.8 \(\times\) 10\({}^{5}\) and the spacing ratio was 2.5 \(\leq\)\(L/D\)\(\leq\) 5.02, and proved that the mutual disturbance between the two cylinders could disturb the lift and drag coefficients of the system. [PERSON] and [PERSON] [29; 30] conducted an experimental analysis on the wake of tandem double cylinders and their research showed that the wake of the double cylinder system was more complex and varied than that of a single cylinder under different phase conditions. [PERSON] and [PERSON] [31] conducted an experimental study on the vibration of tandem cylinders, which showed that the resonance region of the tandem cylinders was larger than that of an isolated cylinder. Additionally, the resonance region decreased with increase of \(L/D\). Subsequently, with the continuous progress of CFD technology, and the diversification of test equipment, scholars have further analyzed the flow field and vibration characteristics of tandem double cylinders. [PERSON] et al. [32] observed the flow field characteristics of two cylinders in series and calculated the vorticity lines and the force on the cylinders. [PERSON] et al. [33] used particle image velocimetry (PIV) to measure the flow field of two tandem cylinders.