Numerical analysis of arrays of wave energy converters

A wave energy converter (WEC) has the potential to become a viable technology for clean, renewable energy production. This technology may prove invaluable to meet the growing demands for electrical power and the apparent changing climate conditions. WEC designs and power output rely heavily on the sea conditions and bathymetry surrounding their deployment sites and require extensive testing prior to the deployment of a commercially operating device. With the challenges and high costs of prototype testing in wave tanks at model scale and in open ocean conditions, numerical studies have been employed extensively to study the performance and estimate potential power-take-off (PTO) capabilities of WECs. Much of the numerical work however, is based on linear wave theory which has the capability of validating the potential of a design and modelling it's performance in various sea states but lacks the capability to capture complex, nonlinear behaviour such as viscous, turbulent or wave breaking dynamics. With model and full scale WECs being tested, there is now a growing need to understand their dynamics beyond the capabilities of linear modelling. It is necessary to understand their combined interaction when positioned in close proximity to each other, as the diffracted waves from one WEC to another can influence the hydrodynamic forces for better or worse and influence the overall power generated. This problem has largely been addressed using linear modelling techniques providing optimised array configurations for various WECs and sea conditions. These studies are however limited by the linear wave assumptions. This work aims to fill this knowledge gap by applying fully nonlinearmodelling techniques to assess the validity of linear modelling methods. In this sense the notion of superposition used in constructing linear models of WECs is held by modelling the diffraction and radiation problems separately. This method ensures no coupling between the nonlinear effects generated in each problem allowing for the detailed study of the nonlinearities caused solely from either the WECs presence in a wave field or its contribution to the wave field. This represents a novel approach to the problem of modelling a wave energy converter and offers a unique perspective into the dynamics of a WEC. The method of applying computational fluid dynamics (CFD) to the diffraction and radiation problems of WEC modelling offers the benefit of computational speed to simulations which are a major limiting factor in it's applications. In combination with linear modelling techniques, device performance can be predicted quickly across a wide range of conditions allowing further focussed studies to be carried out with CFD. The effects of high order waves, viscosity, turbulence and free surface interaction on the hydrodynamic coefficients near the WECs resonance are quantified. The samemethods and principles are then applied to array configurations of WECs. The method of superposition of the hydrodynamic diffraction and radiation components developed and applied here allows for detailed predictions of device performance and optimisation of any WEC type, and a means of analysing complex WEC dynamics efficiently through the reduction of long simulation times. By using a generic WEC device to study these hydrodynamic behaviours we have highlighted important factors that effect the performance of a WEC which are not captured by linear methods.