In this paper, a circular cylinder attached by a pair of fin-shaped strips is proposed with intent to harvest hydrokinetic energy. It is found via numerical simulations of fluid-structure interaction that vortex-induced vibration and galloping occur back-to-back by placing the strips on the front surface. With the increasing of flow speed, the hydrodynamic instability is enhanced, especially the cases with strips at 20° and 45°. More than 60W/m power is harnessed at a typical ocean current speed (un=1.5m/s), and the conversion efficient keeps above 35% at the galloping region. However, when the strips move to 90°, the galloping is disappeared, and the vibration is suppressed as the fin-shaped strips further move to 120°.
Ocean currents and tidal-stream are an abundant and consistent source of renewable energy. Currently, the kinetic energy of currents is mostly converted into pollution-free electrical energy by means of horizontal- axis or vertical-axis turbines (Khan et al., 2009; Pacheco and Ferreira, 2016; Yuce and Muratoglu, 2015). Nevertheless, large-diameter open propellers pose a great threat on marine life and livelihoods linked to fisheries, and the installation and maintenance charges are relatively high. Moreover, turbines are financially viable in average flow speeds of 2.5-3.6 m/s or higher (Lee et al., 2005), the energy conversion efficiency drops significantly when the turbines run at currents below 2 m/s. It is a challenge to commercialize this energy harnessing technology in a large scale, as the majority of ocean currents are slower than 1.55 m/s (Bahaj and Myers, 2003).
Bernitsas and his co-workers (Bernitsas et al., 2008, 2009; Lee and Bernitsas, 2011; Sun et al., 2016) at the University of Michigan developed a novel hydrokinetic power generating device called Vortex- Induced Vibration Aquatic Clean Energy (VIVACE) converter, which can harvest the mechanical energy of structural vibration induced by a cross flow as slow as 0.4 m/s. Vortex-induced vibration (VIV), as a common fluid-structure interaction (FSI) phenomenon, is frequently observed in nature and most practical applications. For a circular cylinder, when the Reynolds number, Re (﹩﹩uinD/v, where uin is the free stream velocity, D is the diameter of the cylinder and υ is the fluid kinematic viscosity), is higher than 47 (Henderson, 1997), vortex is shed from the two lateral surfaces of the cylinder, exciting time-dependent hydrodynamic forces on the cylinder and hence the vibration. Lock-on phenomenon occurs when the vortex shedding frequency matches the natural frequency of the structure, resulting in larger amplitude response. More hydrokinetic energy is extracted and converted into the mechanical energy of the cylinder in this lock-on region of VIV, which is beneficial to further convert into electrical energy. In the experiments carried out by Lee et al. (2011), the maximum power of 15.85W was successfully generated by a single circular cylinder converter at a flow velocity of 1.11 m/s.
