Numerical modelling of a box-type bottom-detached oscillating water column wave energy conversion device

Abstract

Utilization of Boundary Element Method (BEM) based on linear potential flow for modelling Oscillating Water Column (OWC) devices has gained popularity in the last two decades. The commercial BEM solver WAMIT has been used widely for modelling OWCs and validated using experimental modelling. Literature has shown that when using the thin disk approach for modelling the imaginary piston in OWCs in BEM solver Nemoh, the results have been poor since the solver fails to provide good results when the source and field points are very close. In this research, the two-body interaction problem has been adopted in modelling a box-type and bottom-detached OWC device in Nemoh, where the first modelled body is the OWC hull (fixed) and the second body is an imaginary piston modelled to the same length as the internal water column. An average linear damping coefficient relating pressure and discharge is used within a frequency domain model and the obtained response is compared with experimental data. A direct comparison is also conducted with the numerical method of Computational Fluid Dynamics (CFD), which has shown to be accurate for modelling OWC devices, however it does require significantly higher computational resources. A two-dimensional CFD numerical wave tank which allows for wave generation and absorption has been implemented within the open-source package OpenFOAM® and is used for comparative purposes. Results show that while CFD matches well with the experimental results, the BEM model under-estimates the response. To further improve this, an instantaneous frequency dependent damping coefficient relating pressure and discharge is derived, which then provides very close results to both the experiments and CFD model.