Colm J. Fitzgerald

Nonlinear hydrodynamic and real fluid effects on wave energy converters

Ocean wave energy has been of interest since at least the mid-1970s and great advances in the understanding of the fundamental principles of wave energy extraction and converter modelling have been made using linear hydrodynamic analysis. This paper reviews efforts that have been made to use nonlinear hydrodynamics to analyse wave energy converter behaviour and performance. Both ‘partially nonlinear’ and fully nonlinear potential flow methods, as well as computational fluid dynamics methods solving the Navier–Stokes equations and smoothed particle hydrodynamics have been used for this purpose and are reviewed here. These more complex methods have been applied primarily to single devices, so array and mooring line interactions are not considered. While a number of studies have been performed in these areas and results are encouraging, further advances are required to give accurate prediction or reproduction of experimental results.

Phase manipulation and the harmonic components of ringing forces on a surface-piercing column

A general phase-based harmonic separation method for the hydrodynamic loading on a fixed structure in water waves of moderate steepness is proposed. An existing method demonstrated in the experimental study described by Zang et al. (Zang et al. 2010 In Proc. Third Int. Conf. on Appl. of Phys. Modelling to Port and Coastal Protection. pp. 1–7.) achieves the separation of a total diffraction force into odd and even harmonics by controlling the phase of incident focused waves. Underlying this method is the assumption that the hydrodynamic force in focused waves possesses a Stokes-like structure. Under the same assumption, it is shown here how the harmonic separation method can be generalized, so that the first four sum harmonics can be separated by phase control and linear combinations of the resultant time-histories. The effectiveness of the method is demonstrated by comparisons of the Fourier transforms of the combined time-histories containing the harmonics of interest. The local wave elevations around the focus time are also visualized for the first three harmonics in order to reveal the local dynamics driving components within the wave force time-history.

Generalized eigenfunction method for floating bodies

We consider the time domain problem of a floating body in two dimensions, constrained to move in heave and pitch only, subject to the linear equations of water waves. We show that using the acceleration potential, we can write the equations of motion as an abstract wave equation. From this we derive a generalized eigenfunction solution in which the time domain problem is solved using the frequency-domain solutions. We present numerical results for two simple cases and compare our results with an alternative time domain method.