On the Numerical Modelling of the Non Linear Behaviour of a Wave Energy Converter

Abstract

Wave energy converters of the wave activated body class are designed to have large amplitudes of motion, even in moderate sea states, because their efficiency is directly related with the amplitude of their motion. Hence, classical seakeeping numerical tools based on linear potential theory, which are widely used in the design process of offshore structures, are not accurate enough in the case of wave energy conversion. So, large differences between numerical predictions and wave tank experiments are often observed. On the other hand, the use of CFD models theoretically able to provide more accurate results is still difficult for wave energy applications, mainly because this requires a huge computation time. Moreover, it is well known that viscous solver have difficulties in propagatating gravity waves accurately. In this paper, we assess the potential of two advanced hydro-dynamic numerical models in the numerical modelling of wave energy converters. These numerical models are expected to provide more accurate results than classical linear theory based numerical models and faster results than CFD models. Particularly, these tools are expected to be able to deal efficiently with large motions of wave energy converters. In the first one, the hydrostatic forces and the Froude-Krylov forces are computed on the exact wetted surface of the wave energy converter, whereas radiation and diffraction forces are computed using the standard linear potential theory. Using this model, it is shown that we were able to predict the parametric roll phenomenon in the case of the SEAREV wave energy converter. In the second one, a Navier Stokes solver, based on RANS equations, is used. Comparisons are made with experiments and it is showed that this tool is able to model quite accurately viscous effects such as slamming. However, computation time is found to be long with this last tool.

Identifying Models Using Recorded Data

“…The identification of linear hydrodynamic models is not restricted to data generated with BEM; indeed, it is possible also to employ data from physical wave tanks or from NWT (Davidson et al, 2015b;Armesto et al, 2014). The linear models can be enriched introducing nonlinear terms able to describe specific nonlinear physical effects, such as viscosity (Bhinder et al, 2012), nonlinear Froude-Krylov force (Babarit and Laporte-Weywada, 2009;Guérinel et al, 2011;Lawson et al, 2014) or nonlinear restoring force (Zurkinden et al, 2014).…”

Large Scale Experiments on Farms of Heaving Buoys to Investigate Wake Dimensions, Near-Field and Far-Field Effects

“…At the Ecole Centrale de Nantes the SEAREV device has been extensively studied both numerically and experimentally. Validations of (non-)linear models are presented in Durand et al (2007), Gilloteaux et al (2007) and Babarit et al (2009) …”

Numerical and Experimental Modelling of a Wave Energy Converter Pitching in Close Proximity to a Fixed Structure

“…Quasi-linear codes (or weakly non-linear codes) are based on linear theory but include some non-linear effects. These non-linear effects commonly include some or all of instantaneous hydrodynamic forces, quadratic drag loads or the non-linear power-take-of forces [11][12][13]. Finally, codes that account for strong non-linearities include both fully non-linear time domain boundary element models and computational fluid dynamics models (CFD).…”