PHYSICAL AND MATHEMATICAL MODELING OF A POINT ABSORBER WAVE POWER DEVICE WITH NONLINEAR DAMPING (ENERGY, OSCILLATOR)

A point absorber type, ocean wave power system has been under development at the University of Delaware since 1976. The system, referred to as Delbuoy, comprises a semi-submerged vertical cylinder driving a flexibly tethered, submerged, single acting positive displacement seawater pump. The pump and associated valving used in the Delbuoy to convert the wave induced buoy motions into high pressure seawater produce several nonlinearities in the system's response characteristics. This paper describes results from a series of physical and mathematical modeling experiments carried-out to investigate the effects of such nonlinearities on the heave response and efficiency of point absorber systems. Complete 1/10 scale models were tested in a wavetank under regular and random wave conditions. The single degree-of-freedom system was damped using miniature fluidics with the damping forces and the buoy's displacement monitored electronically. Data on the wave force amplitude, F(,o), system efficiency, and the frequency response function relating the incident wave and the buoy's heave response were obtained for several levels of damping.; In parallel, a mathematical model was developed which treated the Delbuoy system as a damped, single degree-of-freedom mechanical oscillator with either sinusoidal or random forcing. A fourth-order Runge-Kutta scheme was used to numerically integrate the nonlinear equation of motion. The model's input coefficients were determined from auxiliary experiments and existing data from the literature. Once validated by hindcasting the physical model tests, the mathematical simulation was used to predict the efficiency of a full-scale system operating in either ocean swells or random seas.; Data are presented comparing the mathematical and physical models along with projections for full-scale operation. The results indicate that nonlinear damping terms significantly reduce the system's maximum level of efficiency relative to linearly damped devices, but tend to produce an increase in the overall breadth of the response curves.