Abstract

This paper presents the numerical investigations of an oscillating flaptype wave surge converter device operating in extreme sea states. We use the open-source CFD library OpenFOAM to carry out the twodimensional numerical simulations. A preliminary study is done to verify the convergence of our results while scalability tests confirm the high-performance computing capabilities of OpenFOAM and the possibility to extend this study to large three-dimensional configurations. The Oyster flap-type device is simulated with both incompressible and compressible solvers and results are compared against previous numerical and experimental results. It is shown that an incompressible solver can capture well the dynamics and general behavior of the flap device. Nevertheless, the slamming events captured in the laboratory can only be reproduced with the aid of a compressible solver, which takes into account density changes in the air and water phases. The numerically captured slamming events only take place if there is a preliminary formation of trapped air pockets on the seaward side of the flap. The entrapped air is subject to further compression and expansion thus confirming the importance of compressibility and aeration effects in agreement with thoughts based on past experiments and numerical simulations.

Introduction

The use of renewable energies such as wind and solar have been experiencing a noticeable increase in recent years. However, other sources of renewable energies, such as the one produced by the dissipation of ocean waves in the near shore region, still remain largely underexploited at present. Therefore, more experiments and accurate numerical simulations need to be carried out in this area with special focus on structure survival as a consequence of harsh ocean conditions.

This paper focuses on the Oyster oscillating wave surge converter (Whittaker & Folley 2012), which consists of a flap device hinged at the seabed and driven back and forth by the action of waves. The energy taken from the waves' interaction is utilized to pump fresh water to a hydraulic plant inshore where it is finally converted into electricity. Oyster gets its maximum efficiency in nearshore locations of shallow water depths, where it acquires larger motions from the waves. One of the current challenges of Oyster is its survivability during extreme sea states in which large and infrequent extreme waves may compromise its structural integrity leading to an increase of its maintenance costs.

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