Lionel Gentaz

Bio: Lionel Gentaz is an academic researcher from École centrale de Nantes. The author has contributed to research in topics: Free surface & Potential flow. The author has an hindex of 11, co-authored 46 publications receiving 432 citations. Previous affiliations of Lionel Gentaz include École Centrale Paris.

Effect of Viscous Forces on the Performance of a Surging Wave Energy Converter

Abstract: A generic Oscillating Surge Wave Energy Converter (OSWC) has been tested numerically against the impact of the viscous forces. The study makes use of both the linear potential theory as well as the computational fluid dynamics (CFD). A state-of-the-art time domain wave-to-wire numerical model of the wave energy converter (WEC) is developed. Viscous damping is then included using an additional velocity squared term from the Morison equation. A range of possible values for the drag coefficient (following various literary resources) were tested so that to establish the scale of the viscous impact regarding the annual power production (APP) of the WEC. Wave resource considered in these numerical tests cover regular and irregular incident waves. Analysis of the APP demonstrates the importance/sensitivity of having an accurate prediction of the drag coefficient. Moreover CFD has been shown to be a valid tool for evaluation of the unknown drag coefficient. For this the CFD model has been validated by comparing its findings with the previously published experimental (and also numerical) results of a 3D square cylinder. This CFD model is then employed to 3D cases of the surging device in order to refine the estimates of the viscous drag coefficient.

A Potential/RANSE Approach for Regular Water Wave Diffraction about 2-d Structures

Abstract: A new formulation is proposed for the simulation of viscous flows around structures in waves. It consists in modifying the Reynolds-averaged Navier-Stokes equations: velocity, pressure or free-surface elevation fields are split into incident and diffracted fields to compute the diffracted flow only. The incident flow may be explicitly given by a stream function theory for non-linear regular waves, or by a spectral method for irregular waves. This method avoids classical problems (large CPU time, poor quality of generated wave) of numerical generation of waves in a viscous flow solver. The 2D flow around an immersed square in regular waves demonstrates the effectiveness of the method.

RANS Simulations of a Calm Buoy in Regular and Irregular Seas using the SWENSE Method

Abstract: This article recalls the recent developments of the SWENSE (Spectral Wave Explicit Navier-Stokes Equations) method and presents a first validation case for a multidirectional irregular wave field. The SWENSE approach is aimed at simulating fully nonlinear wave-body interactions including viscous effects. It combines the benefits of a potential flow theory used to compute the incident waves and of a Reynolds Averaged Navier-Stokes Equations (RANSE) solver used to obtain the diffracted field in the full domain. As an illustrative application, this article focuses on the case of a captive CALM (Catenary Anchor Leg Mooring) buoy. A full set of wave fields can be generated and the study deals successively with regular waves, head irregular waves and multidirectional head waves. The sharp geometry of the model makes this test case difficult for traditional potential codes and comparatively interesting for the SWENSE method.

Energy-Maximizing Control of Wave-Energy Converters: The Development of Control System Technology to Optimize Their Operation

Abstract: With the recent sharp increases in the price of oil, issues of security of supply, and pressure to honor greenhouse gas emission limits (e.g., the Kyoto protocol), much attention has turned to renewable energy sources to fulfill future increasing energy needs. Wind energy, now a mature technology, has had considerable proliferation, with other sources, such as biomass, solar, and tidal, enjoying somewhat less deployment. Waves provide previously untapped energy potential, and wave energy has been shown to have some favorable variability properties (a perennial issue with many renewables, especially wind), especially when combined with wind energy [1].

A synthesis of numerical methods for modeling wave energy converter-point absorbers

Abstract: During the past few decades, wave energy has received significant attention for harnessing ocean energy. Industry has proposed many technologies and, based on their working principle, these technologies generally can be categorized into oscillating water columns, point absorbers, overtopping systems, and bottom-hinged systems. In particular, many researchers have focused on modeling the point absorber, which is thought to be the most cost-efficient technology to extract wave energy. To model such devices, several modeling methods have been used such as analytical methods, boundary integral equation methods and Navier–Stokes equation methods. The first two are generally combined with the use of empirical solution to represent the viscous damping effect, while the last one is directly included in the solution. To assist the development of wave energy conversion (WEC) technologies, this paper extensively reviews the methods for modeling point absorbers.

Mathematical modelling of wave energy converters: A review of nonlinear approaches

Abstract: The wave energy sector has made and is still doing a great effort in order to open up a niche in the energy market, working on several and diverse concepts and making advances in all aspects towards more efficient technologies However, economic viability has not been achieved yet, for which maximisation of power production over the full range of sea conditions is crucial Precise mathematical models are essential to accurately reproduce the behaviour, including nonlinear dynamics, and understand the performance of wave energy converters Therefore, nonlinear models must be considered, which are required for power absorption assessment, simulation of devices motion and model-based control systems Main sources of nonlinear dynamics within the entire chain of a wave energy converter - incoming wave trains, wave-structure interaction, power take-off systems or mooring lines- are identified, with especial attention to the wave-device hydrodynamic interaction, and their influence is studied in the present paper for different types of converters In addition, different approaches to model nonlinear wave-device interaction are presented, highlighting their advantages and drawbacks Besides the traditional Navier-Stokes equations or potential flow methods, ‘new’ methods such as system-identification models, smoothed particle hydrodynamics or nonlinear potential flow methods are analysed

High-fidelity numerical modelling of ocean wave energy systems: A review of computational fluid dynamics-based numerical wave tanks

Abstract: For the research and development (R&D) of wave energy converters (WECs), numerical wave tanks (NWTs) provide an excellent numerical tool, enabling a cost-effective testbed for WEC experimentation, analysis and optimisation. Different methods for simulating the fluid dynamics and fluid structure interaction (FSI) within the NWT have been developed over the years, with increasing levels of fidelity, and associated computational expense. In the past, the high computational requirements largely precluded Computational Fluid Dynamics (CFD) from being applied to WEC analysis. However, the continual improvement and availability of high performance computing has led to the steady increase of CFD-based NWTs (CNWT) for WEC experiments. No attempt has yet been undertaken to comprehensively review CNWT approaches for WECs. This paper fills this gap and presents a thorough review of high-fidelity numerical modelling of WECs using CNWTs. In addition to collating the published literature, this review tries to make a step towards a best practice guideline for the applications of CFD in the field of wave energy.