scholarly journals Validation of CFD for the Determination of Damping Coefficients for the Use of Wave Energy Converters Modelling

2013 ◽  
Author(s):  
Olivia Thilleul ◽  
Aurélien Babarit ◽  
Aurélien Drouet ◽  
Sébastien Le Floch
Keyword(s):  
Wave Energy ◽  
Turbulence Models ◽  
Computational Time ◽  
Diffraction Radiation ◽  
Viscous Effects ◽  
Drag Coefficients ◽  
Damping Coefficients ◽  
Wave Energy Converters ◽  
Convergence Study ◽  
Energy Converters

Diffraction-radiation codes enable to model the behaviour of Wave Energy Converters (WEC) and seakeeping of ships on many sea-states with very little computational time. However, the viscous effects are neglected and therefore the simulations lead to relatively inaccurate values. The inaccuracy mainly occurs at the resonance frequency, especially in roll motions for which viscous effects are of major importance. Classically, the viscous effects are represented by adding viscous damping coefficients obtained either from experimental data or analytical approaches based on numerous approximations. In order to improve the accuracy of the diffraction-radiation solvers, the damping coefficients can also be calculated from Computational Fluid Dynamics (CFD) simulations. The first part of this paper presents the three CFD solvers and turbulence models used in this validation study: ICARE and ISIS-CFD are developed by Ecole Centrale de Nantes and Star-CCM+ is a general purpose solver developed by CD-adapco. For each case, a preferred solver is chosen and a second solver is used for verification in most cases. The second part briefly presents the theory that obtains drag coefficients in oscillatory flows, which are closely related to damping coefficients in waves. Each of the three following parts introduces the experimental test cases to which numerical results are compared to. The numerical parameter convergence study leads to a choice of around 200 timesteps per period with an adapted mesh enabling to obtain drag coefficients with errors lower than 5%. A mesh convergence study in the wake area leads to a mesh refinement of around 2 to 2.5 % of the body characteristic length. In order to reduce the computational time, the total number of cells can be decreased by mainly refining locations where specific flow detachment occurs, such as body corners or sharp edges. Turbulence models are also varied. Validation results are finally presented in terms of single or coupled damping coefficients and added mass coefficients. They are presented for various non-dimensional numbers such as Keulegan-Carpenters and Reynolds number.

scholarly journals Revisiting Theoretical Limits for One-Degree-of-Freedom Wave Energy Converters

2020 ◽  
pp. 1-11
Author(s):  
Nathan Tom
Keyword(s):  
Power Generation ◽  
Wave Energy ◽  
Degree Of Freedom ◽  
Upper And Lower Bounds ◽  
Complex Conjugate ◽  
Hydrodynamic Coefficients ◽  
Damping Coefficients ◽  
Wave Energy Converters ◽  
Heave Motion ◽  
Energy Converters

Abstract This work revisits the theoretical limits of one-degree-of-freedom wave energy converters (WECs). This paper considers the floating sphere used in the OES Task 10 WEC modeling and verification effort for analysis. Analytical equations are derived to determine bounds on displacement amplitude, time-averaged power (TAP), and power-take-off (PTO) force. A unique result found shows that the TAP absorbed by a WEC can be defined solely by the inertial properties and radiation hydrodynamic coefficients. In addition, a unique expression for the PTO force was derived that provides upper and lower bounds when resistive control is used to maximize power generation. For complex conjugate control, this same expression only provides a lower bound, as there is theoretically no upper bound. These bounds assist in comparing the performance of the floating sphere if it were to extract energy using surge or heave motion. The analysis shows because of differences in hydrodynamic coefficients for each oscillating mode, there are different frequency ranges that provide better power capture efficiency. The influence of a motion constraint on TAP while utilizing a nonideal power take-off is examined and found to reduce the losses associated with bidirectional energy flow. The expression to calculate TAP with a nonideal PTO is modified by the mechanical-to-electrical efficiency and the ratio of the PTO spring and damping coefficients. The PTO spring and damping coefficients were separated in the expression, allowing for limits to be set on the PTO coefficients to ensure net power generation.

Effect of Floaters’ Geometry on the Performance Characteristics of Tightly Moored Wave Energy Converters

2009 ◽  
Author(s):  
Spyros A. Mavrakos ◽  
Georgios M. Katsaounis ◽  
Michalis S. Apostolidis
Keyword(s):  
Wave Energy ◽  
Cylindrical Body ◽  
Performance Characteristics ◽  
Hydrodynamic Characteristics ◽  
Wave Forces ◽  
Diffraction Radiation ◽  
Wave Energy Converters ◽  
Internal Cone ◽  
Constant Displacement ◽  
Energy Converters

The paper deals with the investigation of the effect that floaters’ hydrodynamics has on the performance characteristics of tightly moored vertical axisymmetric wave energy converters. Several geometries of WEC’s floaters have been examined by assuming that they have constant displacement. Specifically, a cylindrical body with and without vertical and horizontal skirts at its bottom, a cone and a two–body, piston–like arrangement, which consists of an internal cone and an exterior torus, have been investigated and comparatively assessed. The WEC’s first-order hydrodynamic characteristics, i.e. their exciting wave forces and the correspondent hydrodynamic parameters, are evaluated using a linearized diffraction–radiation semi-analytical method. A dynamical model for evaluating of the floaters’ performance in time domain is developed that properly accounts for the floaters hydrodynamic behavior, the modeling of the hydraulic system and of the power take–off mechanism. The effect of the floaters geometry on the efficiency of the converter is analyzed through the results for the power absorption, under the excitation of several sea states.

Parametric Evaluation of the Performance Characteristics of Tight Moored Wave Energy Converters

2005 ◽  
Author(s):  
Spyridon A. Mavrakos ◽  
Georgios M. Katsaounis
Keyword(s):  
Wave Energy ◽  
Wave Climate ◽  
The Body ◽  
Performance Characteristics ◽  
Hydrodynamic Characteristics ◽  
Wave Forces ◽  
Diffraction Radiation ◽  
Climate Conditions ◽  
Wave Energy Converters ◽  
Energy Converters

The paper aims at presenting a numerical model to predict performance characteristics of tight moored vertical axisymmetric wave energy converters that are allowed to move in heave, pitch and sway modes of motion. The hydrodynamic characteristics (exciting wave forces, hydrodynamic parameters) of the floats are evaluated using a linearized diffraction–radiation method of analysis that is suited for the type of bodies under consideration. According to this method matched axisymmetric eigenfunction expansions of the velocity potentials in properly defined fluid regions around the body are introduced to solve the respective diffraction and radiation problems and to calculate the floats’ hydrodynamic characteristics in the frequency domain. Based on these characteristics, the retardation forcing terms are calculated, which account for the memory effects of the motion. In this procedure, the coupling terms between the different modes of motion are properly formulated and taken into account. The floating WEC is connected to an underwater piston that feeds a hydraulic system with pressurized fluid. Numerical results showing parametrically the performance characteristics in terms of the expected power production for several types of floats that are exposed to the wave climate conditions commonly encountered in the Mediterranean area are presented and discussed.

A Machine Learning Approach to the Analysis of Wave Energy Converters

2015 ◽  
Author(s):  
Dripta Sarkar ◽  
Emile Contal ◽  
Nicolas Vayatis ◽  
Frederic Dias
Keyword(s):  
Wave Energy ◽  
Numerical Models ◽  
Computational Time ◽  
Wave Energy Converters ◽  
Machine Learning Approach ◽  
Input Variables ◽  
Training Examples ◽  
The Individual ◽  
Energy Conversion Devices ◽  
Energy Converters

The hydrodynamic analysis and estimation of the performance of wave energy converters (WECs) is generally performed using semi-analytical/numerical models. Commercial boundary element codes are widely used in analyzing the interactions in arrays comprising of wave energy conversion devices. However, the analysis of an array of such converters becomes computationally expensive, and the computational time increases as the number of devices in the system is increased. As such determination of optimal layouts of WECs in arrays becomes extremely difficult. In this study, an innovative active experimental approach is presented to predict the behaviour of theWECs in arrays. The input variables are the coordinates of the center of the wave energy converters. Simulations for training examples and validation are performed for an array of OscillatingWave Surge Converters, using the mathematical model of Sarkar et. al. (Proc. R. Soc. A, 2014). As a part of the initial findings, results will be presented on the performance of wave energy converters located well inside an array. The broader scope/aim of this research would be to predict the behaviour of the individual devices and overall performance of the array for arbitrary layouts of the system and then identify optimal layouts subject to various constraints.

scholarly journals Frequency-Based Performance Analysis of an Array of Wave Energy Converters around a Hybrid Wind–Wave Monopile Support Structure

2020 ◽  
Vol 9 (1) ◽  
pp. 2
Author(s):  
Sofia Gkaraklova ◽  
Pavlos Chotzoglou ◽  
Eva Loukogeorgaki
Keyword(s):  
Wave Energy ◽  
Wind Wave ◽  
Radial Distance ◽  
Boundary Integral ◽  
Irregular Waves ◽  
Diffraction Radiation ◽  
Power Absorption ◽  
Wave Energy Converters ◽  
Oblate Spheroidal ◽  
Energy Converters

In this paper, we investigate, in the frequency domain, the performance (hydrodynamic behavior and power absorption) of a circular array of four semi-immersed heaving Wave Energy Converters (WECs) around a hybrid wind–wave monopile (circular cylinder). The diffraction/radiation problem is solved by deploying the conventional boundary integral equation method. Oblate-spheroidal and hemispherical-shaped WECs are considered. For each geometry, we assess the effect of the array’s net radial distance from the monopile and of the incident wave direction on the array’s performance under regular waves. The results illustrate that by placing the oblate spheroidal WECs close to the monopile, the array’s power absorption ability is enhanced in the low frequency range, while the opposite occurs for higher wave frequencies. For hemispherical-shaped WECs, the array’s power absorption ability is improved when the devices are situated close to the monopile. The action of oblique waves, with respect to the WECs’ arrangement, increases the absorbed power in the case of oblate spheroidal WECs, while these WECs show the best power absorption ability among the two examined geometries. Finally, for the most efficient array configuration, consisting of oblate spheroidal WECs situated close to the monopile, we utilize an “active” Power Take-Off (PTO) mechanism, facilitating the consideration of a variable with frequency PTO damping coefficient. By deploying this mechanism, the power absorption ability of the array is significantly enhanced under both regular and irregular waves.

Revisiting Theoretical Limits for One-Degree-of-Freedom Wave Energy Converters

2020 ◽  
Author(s):  
Nathan M. Tom
Keyword(s):  
Wave Energy ◽  
Wave Energy Converter ◽  
Degree Of Freedom ◽  
Upper And Lower Bounds ◽  
Complex Conjugate ◽  
Hydrodynamic Coefficients ◽  
Energy Converter ◽  
Damping Coefficients ◽  
Wave Energy Converters ◽  
Energy Converters

Abstract This work revisits the theoretical limits of one-degree-of-freedom wave energy converters. This paper considers the floating sphere used in the Ocean Energy Systems Task 10 Wave Energy Converter modeling and verification effort for analysis. Analytical equations are derived to determine bounds on the motion amplitude, time-averaged power, and power-take-off (PTO) force. A unique result was found that shows the time-averaged power absorbed by a wave energy converter can be defined solely by the inertial properties and radiation hydrodynamic coefficients. In addition, a unique expression for the PTO force amplitude was derived that has provided upper and lower bounds when resistive control is used to maximize power generation. For complex conjugate control, this same expression can only provide a lower bound, as there is theoretically no upper bound. These bounds are used to compare the performance of a floating sphere if it were to extract energy using surge or heave motion. The analysis shows that because of the differences in hydrodynamic coefficients of each oscillating mode, there will be different frequency ranges that provide better power capture efficiency. The influence of a motion constraint on power absorption while also utilizing a nonideal power take-off is examined and found to reduce the losses associated with bidirectional energy flow. The expression to calculate the time-averaged power with a nonideal PTO is modified by the mechanical-to-electrical efficiency and the ratio of the PTO spring and damping coefficients. The PTO spring and damping coefficients were separated in the expression, which allows for limits to be set on the possible values of PTO coefficients to ensure a net flow of power to the grid.

Assessment of damping coefficients of power take-off systems of wave energy converters: A hybrid approach

Energy ◽  
2019 ◽  
Vol 169 ◽  
pp. 1022-1038 ◽  
Author(s):  
Claudio A. Rodríguez ◽  
Paulo Rosa-Santos ◽  
Francisco Taveira-Pinto
Keyword(s):  
Wave Energy ◽  
Hybrid Approach ◽  
Damping Coefficients ◽  
Wave Energy Converters ◽  
Energy Converters

scholarly journals InWave: A New Flexible Design Tool Dedicated to Wave Energy Converters

2014 ◽  
Author(s):  
Adrien Combourieu ◽  
Maxime Philippe ◽  
François Rongère ◽  
Aurélien Babarit
Keyword(s):  
Wave Energy ◽  
Time Domain ◽  
Potential Flow ◽  
Computational Time ◽  
Test Case ◽  
Linear Potential ◽  
Flow Solver ◽  
Wave Energy Converters ◽  
Multibody Dynamic ◽  
Energy Converters

This article presents the novel methodology used in the software InWave to address the problem of wave energy converters (WEC) modelling. The originality compared to other recently developed tools lies in a fast semi-recursive multibody dynamic solver which integrates a flexible hydrodynamic solver. The multibody solver works in time domain and is fully nonlinear. It solves the dynamic of systems formed of a fixed or free base articulated with any number of bodies that can be floating or not, with branchy structure ([1]). The integrated hydrodynamic solver is a linear potential flow solver based on boundary elements method. It uses the generalized degrees of freedom approach ([11]). Combined with a relative coordinate parameterization, it allows for a minimization of the number of hydrodynamic boundary value problems that have to be solved, thus allowing a reduction of computational time both for BEM computations and time domain simulations. Time domain reconstruction is performed to link hydrodynamic loads with the multibody dynamic solver. Interaction between bodies through radiation is thus taken into account. InWave is a complete WEC modelling tool including incident wave generation, multibody dynamic solver, hydrodynamic solver, power take-off and mooring models, post-processing and visualization. A successful comparison with the linear potential flow solver Aquaplus ([5]) on a basic cylinder test case is carried out. Finally, a complex test case on a Langlee-like device is presented, comparing InWave results with those from the NumWec project ([2]). A good agreement between both models is found, which increases the confidence in InWave algorithms and implementation.

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