No abstract

Oscillating Water Columns (OWCs) are some of the most-studied wave energy converters (WECs). Previous work showed that the geometric characteristics of the OWC can play a significant role in the efficiency of the device. In this study, we investigate the behaviour of different designs of OWC making geometric modifications to the classic design of OWC and the U-OWC, initially suggested by Boccotti [1]. The multi-chamber OWCs examined here are fixed on the seabed and have a slit opening at the seaward side. The physical modelling was undertaken in the COAST laboratory of the University of Plymouth. The devices were tested in regular and irregular wave conditions, with and without power take-off (PTO) mechanism, essentially also testing absorbing seawalls.The aim of the study is to present a preliminary comparison related to the geometry of OWCs under some typical wave conditions and suggest potential shape improvements towards an overall optimization of the devices that takes into account both the hydrodynamic efficiency of the OWC and other design aspects, such as the wave run-up. The present study also endeavours to highlight potential benefits from incorporating OWCs in coastal defence as absorbing seawalls.

Various studies investigated the behaviour and the performance of Oscillating Water Columns (OWCs) suggesting many alternative design concepts to improve the efficiency of the device. The OWCs examined here are fixed on the seabed and have a slit opening at the seaward side. The present study investigates the applicability of a multiphase Reynolds Averaged Navier-Stokes (RANS) numerical model for simulating the interaction between an OWC and regular and irregular waves. An initial validation of the open-source computational fluid dynamics (CFD) software package OpenFOAM with the wave generation and absorption toolbox waves2Foam is performed against experimental results obtained at the COAST laboratory of the University of Plymouth. The main aim of the study is to complement to the validation of RANS CFD models and later employ the broadly used numerical tool for further studies for better understanding the behaviour of the OWCs. A method based on mechanical damped oscillations for calculating the eigenfrequency of the device from a decay test is presented and compared with the performance curve. The strength of CFD modelling for obtaining better insight to the hydrodynamics of OWCs is also demonstrated.

Hydrodynamic performance of a fixed U-shaped Oscillating Water Column (U-OWC) Wave Energy Converter is numerically investigated. Based on the timedomain higher-order boundary element method (HOBEM), a two-dimensional fully nonlinear numerical model is implemented to simulate the nonlinear wave interaction with a U-OWC device. In the model, the inner-domain-source method is adopted to generate the incident waves and a linear pneumatic model is used to determine the air pressure which is imposed on the free surface inside the chamber. The numerical model is well validated against the published experimental data of the free surface elevation at the chamber center, air pressure inside the chamber and hydrodynamic efficiency.

NewWave-type focused wave groups are commonly used to simulate the design wave for a given sea state. These extreme wave events are challenging to reproduce numerically by the various Numerical Wave Tanks (NWTs), due to the high steepness of the wave group and the occurring wave-wave interactions. For such complex problems, the validation of NWTs against experimental results is vital for confirming the applicability of the models. Intercomparisons among different solvers are also important for selecting the most appropriate model in terms of balancing between accuracy and computational cost. The present study compares three open-source NWTs in OpenFOAM, SWASH and HOS-NWT, with experimental results for limiting breaking focused wave groups. The comparison is performed by analysing the propagation of steep wave groups and their extracted harmonics after employing an accurate focusing methodology. The scope is to investigate the capabilities of the solvers for simulating extreme NewWave-type groups, which can be used as the “design wave” for ocean and coastal engineering applications. The results demonstrate the very good performance of the numerical models and provide valuable insights to the design of the NWTs, while highlighting potential limitations in the reproduction of specific harmonics of the wave group.