Hydrodynamic Modelling of Wave Energy Converter Arrays

Wave energy from wind-generated waves in the ocean or sea is absorbed by wave energy converters (WECs). In this research, floating point absorber (FPA) WECs are studied which are floating devices on the water surface. FPA WECs installed in the ocean or sea respond to the incoming waves and start moving in six degrees of freedom. The WECs extract energy from the waves by using a power take-off (PTO) system which converts the WEC's motion into electricity. In order to absorb a considerable amount of wave energy at a location in a cost-effective way, a number of WECs are arranged in an array layout using a particular geometrical configuration. If the individual WECs are installed close to each other, they will interact with each other, affecting the overall electricity production of the array (near-field effects). Firstly, the presence of a WEC unit disturbs the incoming wave field by both wave reflection and wave diffraction. Secondly, the WEC's motion leads to the generation of waves, called radiated waves. The wave field around a WEC is thus perturbed by a combination of incoming, reflected, diffracted and radiated waves. This results in zones with higher or lower wave heights compared to the incident wave field. The case where one WEC is positioned in the wake region of another WEC where lower wave heights are observed must be avoided. By positioning the individual WECs in the zones with higher wave heights, the total energy extraction of the WEC array is significantly improved, increasing the electricity production. In addition to these near-field effects, a WEC array also influences the wave climate further away (far-field effects). The wave height reduction behind an entire WEC array affects other users in the sea, the environment or even the coastline.

In this research, only the near-field effects are considered. The WECs are tested in a three-dimensional (3D) non-linear viscous numerical wave tank (NWT). The NWT is implemented in the computational fluid dynamics (CFD) toolbox OpenFOAM and consists of two fluid phases: water with air on top. The 3D incompressible Navier-Stokes equations, which represent the physics with a very high accuracy, are solved on a mesh in a computational domain. The interface between water and air is resolved by a conservation equation formulated by the volume of fluid (VoF) method. Compared to traditional linear potential flow solvers based on a boundary element method (BEM), CFD is necessary to resolve complex physical processes. Examples are survivability simulations of WECs subjected to breaking waves and WECs operating in resonance mode by applying control methods resulting in significant non-linear and viscous effects combined with large WEC motions. The present research focusses on filling two knowledge gaps for a NWT. The first one is related to enhanced turbulence modelling for NWTs using a two-phase fluid solver and therefore applicable for a wide range of coastal and offshore processes such as wave-structure interaction, wave-current interaction, wave breaking, sediment transport, etc. The second gap is related to fluid-structure interaction simulations of a floating body. Instabilities between the fluid solver and the motion solver might happen due to added mass effects. During this research, enhanced prediction tools for turbulence modelling and efficient fluid-structure interaction simulations in a NWT have been developed. All these developed methods are coupled and validated by using experimental data obtained in a physical wave flume or basin.