An experimental investigation into WEC operation in realistic sea states using PIV

The focus of this study is on two key aspects of model scale experimentation when investigating the operation of ocean wave energy converters: the type of wave in which the device is exposed to; and the presence of three dimensional effects within and around the device. Scale model experiments were performed on an oscillating water column (OWC) in different conditions utilising particle image velocimetry (PIV). PIV is an advanced experimental technique which captures full velocity fields without interference with the flow and can be used to provide both qualitative and quantitative flow visualisation. To investigate the impact of wave type on the operation of an OWC, experiments were performed in three wave types: regular, polychromatic and irregular waves. While regular and irregular waves are often used, polychromatic waves offer an intermediate option which has properties of both. When investigating polychromatic waves phase averaging was shown to be a useful tool to generate averaged results for both PIV velocity fields and other more conventional data sources with a reduction in uncertainty. It was seen that testing in regular waves results in unrealistic harmonic effects which impact on device performance, in particular the formation and size of vortices. For experiments in irregular waves, a linear relationship was identified between the energy within vortices and the total energy within the velocity field. Identifying the presence of three dimensional effects was achieved by capturing two dimensional (2D) flow velocities at four transverse planes. This showed the transition between inflow and outflow conditions occurs at different times across the device. Velocity field divergence was calculated and large vortices were identified at the inside lip of the sidewall during inflow. It also indicated the device utilises the volume outside of its sidewalls during outflow, allowing an effective width greater than the extents of its sidewalls. This results in the potential for more power to be generated during outflow than inflow. This study has revealed the importance of performing experiments in realistic sea states and has highlighted the value of experiments in polychromatic and/or irregular waves early in the design process. The use of PIV provided a vast amount of information on the operation and performance of wave energy converters and should be strongly considered when performing quantitative flow visualisation, comparative studies and validation of numerical models.