Chapter 4 - Spectral-Domain Models

Abstract

Spectral-domain models are a relatively efficient method of producing an estimate of the expected response and power capture for wave energy converters (WECs) that are subject to nonlinear forces such as Coulomb (constant) or viscous (quadratic) damping. They are generally faster than time-domain models and more accurate than frequency-domain models. However, these models can only be used for spectral excitation and are not appropriate for use with monochromatic waves. The estimates of the expected responses and power captures are made using the assumption that the individual frequency components in the wave spectra are uncorrelated. Because the results of a spectral-domain model are fundamentally statistical they are not able to provide details of extreme values. The only spectral-domain models that have so far been implemented effectively linearize the nonlinear forces and iterate the linearized equations of motion to determine the expected response. This technique has been validated using time-domain models and wave-tank experiments. The linearization of the WEC dynamics effectively assumes that the response is Gaussian; however, spectral-domain modelling techniques used in other fields suggest that it should be possible to model non-Gaussian responses, which is expected to increase the range of nonlinearities for which there are solutions in the spectral domain.