Wave Energy Absorption Spectrum and Energy Budget of an Oscillating Horizontal Cylinder Near Free Surface

In this talk we present control volume analysis from a series of experiments with a wave energy converter (WEC) consisting of a neutrally buoyant horizontal slender circular cylinder (diameter D=0.27m) fully submerged below the free water surface with the cylinder axis aligned in the direction perpendicular to the incident wave propagation. Results are presented for two different experimental configurations. The first experiment was performed in a small (120ft) USNA towing tank where the long axis of the cylinder extended across the full width of the tank and cylinder oscillated in a single degree of freedom (heave). In the second experiment, the same oscillating cylinder was tested in large (380ft) tow tank that has sufficient width to account for three-dimensional effects in both single and two degree of freedom (heave and surge) motions. Discussion will concentrate on the cylinder response and its wave energy absorption spectrum in a range of wave steepness 0.02 < kA < 0.2 where k is the wavenumber and A is incident wave amplitude and wave periods from 0.8 to 2.2 seconds. Energy budget results suggest that loss of wave energy flux through the control volume equals in magnitude to measured absorbed power by the cylinder plus viscous losses on the tank walls. However, at resonance conditions, this approach fails due to non-linear regime characterized by localized increased wave steepness and wave breaking occurring on the downstream side of the control volume.