# Prediction of Dynamic Responses of Flow-Induced Vibration Using Deep Learning
## 2 Power Generation Technology of VIVACE Converter Using FIVs  
Contrary to previous efforts to suppress FIVs, which can destroy structures subject to fluid flows, the VIVACE converter utilizes and even enhances FIVs to harness the power from rivers and currents. The first prototype of the converter was developed at the end of 2003 and tested at MRELab in 2004. The left side of Figure 2 shows the schematic of the simplest unit of the VIVACE converter, comprising a single circular cylinder suspended by springs and a power take-off system. On the right side of Figure 2, four of the units installed on the LTFSW channel at the MRELab are shown [10]. The experiments were conducted in the flow range of 30,000 < Re < 120,000 for the Reynolds number (Re) to harness the optimal power envelopes at various flow velocities [12]. The Reynolds number is defined by:  
\[Re=\frac{U\,D}{\
u}\]  
where \(U,D,\) and \(\
u\) are the free stream velocity, the diameter of a circular cylinder, and the kinematic viscosity of fresh water, respectively. All tests with the single cylinder with passive turbulence control (PTC) presented herein were performed in the LTFSW channel of the MRELab. The measured turbulence intensity of the test section normalized by the free stream velocity was reported lower than 0.1% [1, 4, 6, 10] and it is suitable for the experiments [13]. A virtual spring-damping (Vck) system, a servo-motor controller that replaces physical damping and springs were designed and built to investigate the effects of damping and spring stiffness on power generation [11]. A schematic of the design model is shown in Figure 3.  
Figure 2: (Left) Simple schematic of the VIVACE module with coordinate system; (Right) four new VIVACE converters mounted on the LTFSW channel [10,11]. The first VIVACE converter was intended to utilize only VIV, as implied by the name. Because of the self-limiting characteristics of the VIV phenomenon, the harness power of the single-cylinder VIVACE converter is limited by its lock-in range and the self-limiting VIV characteristics. However, previous works on FIVs in circular cylinders have shown that fluid dynamic changes caused by attachments to the cylinders can cause galloping in circular cylinders [14]. In contrast to VIV, galloping is a vibration type with a short period compared to its high amplitude in a single degree of freedom and generally occurs in noncircular cross sections. Galloping is caused by motion-aiding forces, resulting in very high amplitude motions with the total system damping falling below zero and induction of negative aero/hydrodynamic damping. To overcome the shortcomings of VIV and initiate galloping, the MRELab team extensively investigated the effects of passive turbulence control (PTC), which is applied in the form of roughness strips attached to the cylinders [10; 12; 15]. This study shows that the selectively applied roughness on the cylinder surface can cause significant changes in the properties of the boundary or shear layers; accordingly, the cylinder responses may change. For some PTC positions, as shown in Figure 4, the PTC extends the upper VIV branch point to the reduced velocity \(U^{\star}\approx 11\) and increases fully from \(U^{\star}\approx 12\). The reduced velocity is defined by:  
\[U^{\star}=\frac{U}{f_{\text{nup}}D}\]  
Figure 3: Symmetric \(V_{\text{ck}}\) model with notations [11]. The natural frequency of the system including added mass is defined by:  
\[f_{nw}=\sqrt{\frac{K}{m_{osc}+m_{add}}}\]  
where \(K\), \(m_{osc}\), and \(m_{add}\) are the spring stiffness, oscillating mass, and added mass, respectively. The transition range in which both VIV and galloping mechanisms coexist is \(11<U^{*}<12\). Therefore, the synchronization region of FIVs, which covers both the alternately occurring VIV and galloping, increases rapidly. Thus, the VIVACE converter was able to harness higher quantities of power at over a wide range of speeds from 0.38 m/s to 1.45 m/s and maximum speed of the LTFSW channel.