Linear and nonlinear hydrodynamic models for dynamics of a submerged point absorber wave energy converter

Abstract

This study compares the response of a submerged CETO-shaped point absorber wave energy converter using linear, partially-nonlinear, pseudo-nonlinear, and fully-nonlinear methods to model hydrodynamic effects. Linear potential flow models calculate hydrodynamic parameters to represent the fluid-structure interaction; typical dynamic models apply these parameters without pose-dependence. The partially-nonlinear method evaluates excitation forces at different poses to introduce a pose-dependent excitation force; in addition to the excitation force, the pseudo-nonlinear method calculates hydrodynamic coefficients using linear potential flow methods and includes pose-dependence through interpolating pre-calculated parameters to represent the radiation force. The fully-nonlinear CFD model is a numerical wave tank validated against published data. The applicability of linear-based methods has been explored by comparing the motion, force, and power of the system under various operating conditions against the fully-nonlinear results. It was expected that for low amplitude waves results tend towards the linear results; however, for both low amplitude waves and increased submergence depth, linear methods provided poor representations of the nonlinear CFD results. Geometric nonlinearities were insufficient to capture all the nonlinear behaviour. A frequency-dependent nonlinearity was identified in the water above the buoy resonating. For such submerged point absorbers, linear methods do not adequately represent the influential nonlinear effects.