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A common type of device for wave-energy extraction is an oscillating water column (OWC) with a compression chamber. Peak performance of most OWC systems occurs at resonance with the driving waves. At resonance, oscillations increase linearly in time until damping inhibits further growth. Parametric resonance is introduced as a means of exciting the oscillations of the water column. In parametric resonance, oscillations increase exponentially in time. The use of this kind of resonance may increase the performance of OWC systems. This type of resonance occurs when one of the parameters in an oscillator varies periodically. Asymptotic methods are used to study the nonlinear dynamics of an OWC with parametric resonance. These results are compared with those of a numerical model of a real experimental laboratory setup.

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Performance of wave-energy devices of the oscillating water column (OWC) type is greatly enhanced when a resonant condition with the forcing waves is maintained. The natural frequency of such systems can in general be tuned to resonate with a given wave forcing frequency. In this paper we address the tuning of an OWC seawater pump to polychromatic waves. We report results of wave tank experiments, which were conducted with a scale model of the pump. Also, a numerical solution for the pump equations, which were proven in previous work to successfully describe its behavior when driven by monochromatic waves, is tested with various polychromatic wave spectra. Results of the numerical model forced by the wave trains measured in the wave tank experiments are used to develop a tuning criterion for the sea-water pump.