Effect of Modulation on Energy Conversion from Uniform Flow into Vortex-Induced Vibration of a Circular Cylinder

Abstract

We conducted an experiment to investigate the vortex-induced vibration (VIV) of an elastically supported rigid cylinder in fluid flow in order to develop a renewable energy system that utilizes the VIV. We constructed an experimental setup involving rotational machinery that can operate even under weak flow conditions, such as those in the deep sea. The displacement of the vibration and the lift force acting on the cylinder were measured and processed using Fourier and Hilbert transforms. The measured VIV exhibited modulations of amplitude and frequency, and a few response branches. Relationships between those and the efficiency of energy conversion from fluid flow into the VIV were analyzed. The modulations were observed only in the initial and upper branches. Our analysis found that the role for exciting the VIV periodically exchanges between locked-in and unlocked-in components of the lift force, thus causing the modulations. The maximum efficiency, about 10% in the upper branch, is insufficient and remains an issue. In addition, the present study has confirmed the predominance of the nonlinear damping force acting on the VIV, particularly in the upper branch. These experimental results will contribute to the next design of the energy system.