Numerical modelling and wave tank testing of a self-reacting two-body wave energy device

This paper summarises a series of studies in the early design phase of a wave energy device. The device consists of an outer body in the form of a buoy with a vertical tube, inside which an inner body can slide relative to the former. To understand the behaviour of the device, a linear mathematical model is first developed in the frequency domain. Parametric studies show that a device with an enlarged tube cross section at the bottom has a better power performance relative to its size compared to a device with a uniform tube. Measurements of the device displacements, absorbed power and mean surge force obtained through scaled model tests in a wave tank are generally in good agreement with predictions from the frequency-domain model, but some discrepancies are also observed. A notable discrepancy is the occurrence of a Mathieu-type instability, where amplified pitch/roll motions with a period twice the wave period are triggered by heave motions. Once this happens, the relative motion between the two bodies and hence the absorbed power are found to be greatly reduced. A weakly nonlinear time-domain model is then developed and is able to predict, to some extent, this behaviour. It is demonstrated that numerical and experimental analyses going hand in hand are indispensable in the early design phase of a wave energy device.