Nonlinear Froude-Krylov and viscous drag representations for wave energy converters in the computation/fidelity continuum

Abstract

Designing, optimizing and controlling a wave energy converter requires the construction of a mathematical model in order to simulate the behaviour of the device. Given the nonlinear nature of fluid-structure interactions, the definition of the model is not straightforward and should take into account the specific application it is intended for. Two of the most important characteristics of a model are the computational time and the expected accuracy, which usually are mutually conflicting. The inclusion of nonlinearities potentially increases the model accuracy, but at a higher computational price. Considering a heaving wave energy converter with and without the application of latching control, this paper studies and compares nine different modelling options, eight of which are based on potential theory and consider nonlinear Froude-Krylov and viscous drag forces, while one is based on fully-nonlinear computational fluid dynamics. The value of including nonlinearities in the hydrodynamic model is discussed in relation to the computational cost of the eventual accuracy benefits, under a range of scenarios.

Ocean Energy Systems Wave Energy Modelling Task: Modelling, Verification and Validation of Wave Energy Converters

“…In recent years, there has been an increased interest in nonlinear hydrodynamic modelling of wave energy converters, and there are several approaches. The weakly nonlinear Froude-Krylov (FK) approach [8] is the most used and has been implemented in several of the existing linear codes, extending them to weakly nonlinear tools. The use of fully nonlinear potential flow (FNPF) is still rather scarce [9], but computational fluid dynamics (CFD) tools are frequently used [10].…”

Assessment of Scale Effects, Viscous Forces and Induced Drag on a Point-Absorbing Wave Energy Converter by CFD Simulations

“…They also highlight the difference in flow pattern surrounding the WEC for the different body shapes and put forward viscous correction factors, which are deduced from the deviation of the WEC decay motion from potential theory. In a recent series of papers Giorgi and co-authors [14,21,22] discuss a case study of a spherical floater in heave. They show that weakly non-linear potential flow simulations with calibrated parametrised drag force give results much closer to CFD prediction than simulations using only nonlinear FK corrections.…”

A Survey of WEC Reliability, Survival and Design Practices

“…This may include nonlinear hydrodynamics (see, e.g., [73][74][75][76][77]), nonlinear mechanical dynamics, such as due to a mooring system (see, e.g., [78]) or some arbitrary control input (see, e.g., [14,16]). Giorgi and Ringwood [79] performed an assessment of the relative importance of various nonlinear perturbations within a Cummins-style time-domain model and compared computational expenses. In this study, Giorgi and Ringwood found that viscous drag nonlinearities can be significant, even for the simple spherical WEC considered in their study, when motions are large.…”