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].

/pdf/energy-maximizing-control-of-wave-energy-converters-the-56go405abj.pdf

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

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.

/pdf/a-synthesis-of-numerical-methods-for-modeling-wave-energy-2fstcj9wlt.pdf

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

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

/pdf/mathematical-modelling-of-wave-energy-converters-a-review-of-26x5med31l.pdf

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

Numerical methods in laminar and turbulent flow

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.

/pdf/high-fidelity-numerical-modelling-of-ocean-wave-energy-40v9cfko54.pdf

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

RANSE Analysis of 2D flow about a submerged body using explicit incident wave models

Bilge keels are commonly used appendages that are effective in reducing roll motion. The complex fluid flow around such sharp-fin objects in the presence of hull geometry and a free surface has hardly been addressed. Two recently developed Navier-Stokes Solvers are utilized to examine the forced-motion hydrodynamic properties of rectangular cylinders fitted with bilge keels. One solver is the Free-Surface Random Vortex Method (FSRVM, Yeung & Vaidhyanathan, 1994) developed at the University of California at Berkeley, while the other is a Boundary-Fitted Finite- Difference Method (BFFDM, Alessandrini & Delhommeau, 1995) developed at the Ecole Centrale de Nantes. Cross-continents collaborations provided the opportunity for the authors to evaluate the consistency of these two different but equally successful methods. Where available, these theoretical calculations are validated against laboratory measurements taken from forced-oscillation experiments. The capabilities of such first-principle approaches to predict the added roll inertia and the roll damping are shown to be excellent. The shortcomings of traditional estimates of roll hydrodynamic coefficients based on potential-flow calculations are illustrated. The dependence of these coefficients on the parametric space of frequency, bilge-keel depths, and amplitude of roll is examined and discussed.

On roll hydrodynamics of cylinders fitted with bilge keels

In this study, the added resistance and seakeeping performance of the KVLCC2 ship is investigated by using the spectral wave explicit Navier-Stokes equations (SWENSE) method. The SWENSE method is based on the decomposition of the total field into an incident part explicitly obtained by the wave potential flow theory and a complementary part solved with a modified Reynolds-averaged Navier-Stokes solver. Therefore, the computational efficiency can be achieved by using a relatively coarse mesh in the far field, retaining the accuracy of the incident waves. A parametric study is performed under regular wave conditions with 3Degree of Freedom (DOF) motions of the hull. The results are compared with the large literature available. An additional case is simulated to demonstrate the capability of the present method in simulating seakeeping problems in irregular sea states. Good agreement between the computed results with the reference data can be observed for the hull model, which indicates that the added resistance and seakeeping performances can be well predicted by the present method.

Numerical Investigation on the Added Resistance and Seakeeping Performance of KVLCC2 with the SWENSE Method

This paper documents the SWENSE (Spectral Wave Explicit Navier-Stokes Equations) approach, an original method for studying the simulation of 6 DOF ships in nonlinear waves under viscous flow theory. This work has been motivated by the accuracy and efficiency requirements for simulating hulls maneuvering in waves. The validation of the method for a diffraction case is presented and followed by a 2 DOF simulation of a Wigley hull in head regular waves. Results show an overall good agreement with experiments. The accuracy and effectiveness confirm the viability of the method.

Lionel Gentaz

Papers

RANSE Analysis of 2D flow about a submerged body using explicit incident wave models

On roll hydrodynamics of cylinders fitted with bilge keels

Numerical Investigation on the Added Resistance and Seakeeping Performance of KVLCC2 with the SWENSE Method

Numerical simulation of ship seakeeping by the SWENSE approach

Experimental and Numerical Study of a Half Submerged Pile Supported Breakwater