Vortex-induced vibrations of a single-degree-of-freedom circular cylinder in the vicinity of the free surface

Hydrokinetic energy from vortex-induced vibrations (VIV) have become popular recently. The potential is higher in seas and oceans but the technology is also applicable in rivers. However, many rivers in the world have limited depths and the VIV response heavily depends on the boundaries. In this study, we focus on the free-surface effects on the VIV response of a smooth circular cylinder having one-degree-of-freedom. Experiments at six different depths for three different mass ratios are conducted at the Flow Induced Motions Laboratory of Istanbul Technical University (ITU FIMLab). Results show that the free surface alters the vortex shedding frequency and the flow approaches to a stationary cylinder case as the cylinder approaches the free-surface. The range of synchronization narrows down and the amplitudes reduce. As a result, harnessed power also decreases. We also observed a multi-frequency behavior close to the free-surface: a second dominant frequency exists in the vicinity of the free-surface and this dominant frequency is found to be the second harmonics of the oscillation frequency. Linear mathematical model utilizing experimental results shows that the existence of the free surface boundary alters the added mass, leading to differences in the VIV response of the cylinder.