Nonlinear behaviour of interacting oscillating water columns

A preliminary study is made of the dynamics of Oscillating Water Columns (OWCs), in which the amplitude of internal fluid displacement is nonlinear. The OWC is a well-studied class of ocean wave-energy converters. When the machine's natural frequency is appropriately tuned, it will resonate at the prevailing ocean-swell frequency. Large amplitudes at resonance are desirable since they maximise the power available for electricity generation; however, large amplitudes also imply nonlinear behaviour. Most analyses of wave-energy converters of any class assume linear dynamics. Furthermore, multiple machines are now constructed in close proximity to maximise economic returns, and hence they interact. The OWC is reduced to a single degree-of-freedom nonlinear oscillator based on appropriate simplifications of the Navier-Stokes equations. Nonlinear machines were then coupled together by inter-machine wave scattering. It is found that provided wave amplitudes are realistic for Southern Ocean conditions, linear theory remained reasonable for both single and coupled machines. However, nonlinearities could significantly reduce resonant power output.