Quasisteady theory for the hydrodynamic forces on a circular cylinder undergoing vortex-induced vibration

Vortex-induced vibrations of a rigid circular cylinder were studied by constructing a theory based on a wake oscillator model under quasisteady approximations, thereby evaluating vortex-induced hydrodynamic forces acting on the cylinder. A lock-in limit line representing the boundary for the occurrence of frequency lock-in was also theoretically derived. Hydrodynamic forces in forced oscillation problems estimated by the theory were compared with measured ones. Although some discrepancies were found, particularly in cases with high-frequency oscillations, good agreement was achieved in most cases. Accordingly, we conclude that the present theory captures well real phenomena in the wake downstream of a cylinder subjected to a flow.     

Development of a renewable energy system utilizing vortex induced vibration of a cylinder and principle of leverage aiming for application in deep sea

This study carried out an experiment to develop a renewable energy system utilizing vortex induced vibration (VIV) that can drive generators even in weak flow conditions. In our experimental setup, the translation motions of a cylinder undergoing VIV and a linear-type generator (coil and magnet) are connected through a rotational machinery, which enables us to apply the principle of leverage, leading to power generation even by small excitation force acting on the cylinder. The measurement shows that the present system performs optimally under the lock-in and upper branch conditions, and that there exists an optimum length of the lever of electromagnetic moment on a center of the rotation. Efficiencies of primary (from flows to the VIV) and secondary (from the VIV to electricity) energy conversions are estimated to show that the primary one is not enough high. We thus propose a few directions to improve it.