# Investigation on the use of wall reflections to simulate wave energy converter array effects

Winship, BJ ORCID: 0000-0001-7400-9137 2019 , 'Investigation on the use of wall reflections to simulate wave energy converter array effects', PhD thesis, University of Tasmania.

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## Abstract

Wave Energy Converter (WEC) technologies are under development internationally. WECs are of value to society as alternatives to geological hydrocarbons, with depleting reserves and concerns of the implications of anthropogenic climate change. The development of WECs will at some point require devices to be placed in groups or arrays; such arrangements induce hydrodynamic effects which have the potential to materially affect energy capture. These hydrodynamic array effects need to be understood to maximise return on investment. However, simulation of large arrays, both experimentally and numerically, can be prohibitively expensive. This work explores the use of reflections to simulate WEC array effects as an approach to reducing the cost of array investigations; this is done numerically with linear potential ow and experimentally in the Australian Maritime College's towing tank. The experimental investigation includes the use of novel experimental techniques including stereo-videogrammetry and the implementation of a real-time control system for modelling a WEC Power Take Off (PTO).
The results show it is possible to explore WEC array interactions with the use of wall reflections. This approach can reduce the overall cost of experimental array investigations, thereby allowing further studies to be completed and ultimately reducing the cost of commercial WEC arrays. Through this exploration it has been shown that phase is just as important as amplitude for understanding WEC array interactions. As amplitude describes the potential interaction effect, but without an understanding of phase it is not possible to know if the interactions are constructive, destructive or neutral.
Adjunct to this, the stereo-videogrammetry approach employed is shown to be able to measure an area of approximately 12 m$$^2$$ with a 3 mm horizontal discretisation and sub-millimetre accuracy on the vertical measurement. This novel measurement approach has the potential to be useful to investigations ranging from validation of theoretical wave models through to assessment of wave-structure interactions in commercial development. While the approach, at present, is expensive to implement, it has capabilities which conventional wave probes cannot match. For example the large spatial area considered and velocity measurements.
In the numerical study, the linearized investigations of diffraction and heave radiation showed very good correlation with experimental results, with respect to both loads and free surface deformation. For the relaxed condition where a PTO system was employed it is shown that the linearized assumption allowed for accurate prediction of power capture, however did not allow for accurate prediction of the resulting wave climate. This suggests that for different array arrangements that more complicated modelling may be required.
Through the numerical modelling it was evident that wave shoaling must be taking place. This implicitly means that the results are incomplete, as shoaling requires that there must be additional harmonics for loads and free surface deformations. Such harmonic components are not able to be considered by linear potential flow.
There is further work which can be completed based on the material in this thesis. While the linear PTO system employed was functional, extending the study to more advanced control strategies would be beneficial to enable higher energy capture. In addition, comparing the experimental results presented here with more advanced numerical models, such as Computational Fluid Dynamics, that are able to better capture the nonlinear dynamics of the WEC should be considered. Lastly, the novel use of stereo-videogrammetry to look at the free surface effects could be applied to other WEC devices (and hydrodynamic problems) where complex fluid-structure interactions are important to understand their design including a range of analytical models.

Item Type: Thesis - PhD Winship, BJ WEC Array Effects, Stereo-Videogrammetry, Linear Potential Flow, Method of Images, CETO 5, Point Absorber 10.25959/100.00031864 Copyright 2019 the author Chapters 2 and 3 appear to be, in part, the equivalent of a pre-print version of an article published as: Winship, B., Fleming, A., Penesis, I., Hemer, M., Macfarlane, G., 2018. Preliminary investigation on the use of tank wall reflections to model WEC array effects, Ocean engineering, 164, 388-401 View statistics for this item