LARGE SCALE EXPERIMENTS ON FARMS OF HEAVING BUOYS TO INVESTIGATE WAKE DIMENSIONS, NEAR-FIELD AND FAR-FIELD EFFECTS

The shrinking reserves of fossil fuels in combination with the increasing energy demand have enhanced the interest in renewable energy sources, including wave energy. In order to extract a considerable amount of wave power, large numbers of Wave Energy Converters will have to be arranged in arrays or farms using a particular geometrical layout. The operational behaviour of a single device may have a positive or negative effect on the power absorption of the neighbouring WECs in the farm (near-field effects). Moreover, as a result of the interaction between the WECs within a farm, the overall power absorption and the wave climate in the lee of the WECs is modified, which may influence neighbouring farms, other users in the sea or even the coastline (far-field effects). Several numerical studies on large WEC arrays have already been performed, but large scale experimental studies on near-field and far-field wake effects of large WEC arrays are not available in literature. Within the HYDRALAB IV European programme, the research project WECwakes has been introduced to perform large scale experiments in the Shallow Water Wave Basin of DHI, in Denmark, on large arrays of point absorbers for different layout configurations and inter-WEC spacings. The aim is to validate and further develop the applied numerical methods, as well as to optimize the geometrical layout of WEC arrays for real applications.

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Coupling Methodology for Studying the Far Field Effects of Wave Energy Converter Arrays over a Varying Bathymetry

Energies ◽

10.3390/en11112899 ◽

2018 ◽

Vol 11 (11) ◽

pp. 2899 ◽

Cited By ~ 13

Author(s):

Gael Verao Fernandez ◽

Philip Balitsky ◽

Vasiliki Stratigaki ◽

Peter Troch

Keyword(s):

Numerical Simulations ◽

Wave Energy ◽

Near Field ◽

Coupled Model ◽

Propagation Model ◽

Irregular Waves ◽

Computational Time ◽

Far Field ◽

Field Effects ◽

Numerical Tool

For renewable wave energy to operate at grid scale, large arrays of Wave Energy Converters (WECs) need to be deployed in the ocean. Due to the hydrodynamic interactions between the individual WECs of an array, the overall power absorption and surrounding wave field will be affected, both close to the WECs (near field effects) and at large distances from their location (far field effects). Therefore, it is essential to model both the near field and far field effects of WEC arrays. It is difficult, however, to model both effects using a single numerical model that offers the desired accuracy at a reasonable computational time. The objective of this paper is to present a generic coupling methodology that will allow to model both effects accurately. The presented coupling methodology is exemplified using the mild slope wave propagation model MILDwave and the Boundary Elements Methods (BEM) solver NEMOH. NEMOH is used to model the near field effects while MILDwave is used to model the WEC array far field effects. The information between the two models is transferred using a one-way coupling. The results of the NEMOH-MILDwave coupled model are compared to the results from using only NEMOH for various test cases in uniform water depth. Additionally, the NEMOH-MILDwave coupled model is validated against available experimental wave data for a 9-WEC array. The coupling methodology proves to be a reliable numerical tool as the results demonstrate a difference between the numerical simulations results smaller than 5% and between the numerical simulations results and the experimental data ranging from 3% to 11%. The simulations are subsequently extended for a varying bathymetry, which will affect the far field effects. As a result, our coupled model proves to be a suitable numerical tool for simulating far field effects of WEC arrays for regular and irregular waves over a varying bathymetry.

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Irregular Wave Validation of a Coupling Methodology for Numerical Modelling of Near and Far Field Effects of Wave Energy Converter Arrays

Energies ◽

10.3390/en12030538 ◽

2019 ◽

Vol 12 (3) ◽

pp. 538 ◽

Cited By ~ 7

Author(s):

Gael Fernández ◽

Vasiliki Stratigaki ◽

Peter Troch

Keyword(s):

Experimental Data ◽

Wave Field ◽

Wave Energy ◽

Near Field ◽

Coupled Model ◽

Propagation Model ◽

Irregular Waves ◽

Far Field ◽

Engineering Research ◽

Field Effects

Between the Wave Energy Converters (WECs) of a farm, hydrodynamic interactions occur and have an impact on the surrounding wave field, both close to the WECs (“near field” effects) and at large distances from their location (“far field” effects). To simulate this “far field” impact in a fast and accurate way, a generic coupling methodology between hydrodynamic models has been developed by the Coastal Engineering Research Group of Ghent University in Belgium. This coupling methodology has been widely used for regular waves. However, it has not been developed yet for realistic irregular sea states. The objective of this paper is to present a validation of the novel coupling methodology for the test case of irregular waves, which is demonstrated here for coupling between the mild slope wave propagation model, MILDwave, and the ‘Boundary Element Method’-based wave–structure interaction solver, NEMOH. MILDwave is used to model WEC farm “far field” effects, while NEMOH is used to model “near field” effects. The results of the MILDwave-NEMOH coupled model are validated against numerical results from NEMOH, and against the WECwakes experimental data for a single WEC, and for WEC arrays of five and nine WECs. Root Mean Square Error (RMSE) between disturbance coefficient (Kd) values in the entire numerical domain ( R M S E K d , D ) are used for evaluating the performed validation. The R M S E K d , D between results from the MILDwave-NEMOH coupled model and NEMOH is lower than 2.0% for the performed test cases, and between the MILDwave-NEMOH coupled model and the WECwakes experimental data R M S E K d , D remains below 10%. Consequently, the efficiency is demonstrated of the coupling methodology validated here which is used to simulate WEC farm impact on the wave field under the action of irregular waves.

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A COUPLING METHODOLOGY TO MODEL NEAR AND FAR FIELD EFFECTS OF STRUCTURES AND ENERGY DEVICES DUE TO WAVE INTERACTION

Coastal Engineering Proceedings ◽

10.9753/icce.v36.waves.47 ◽

2018 ◽

pp. 47

Author(s):

Vasiliki Stratigaki ◽

Peter Troch ◽

David Forehand

Keyword(s):

Wave Energy ◽

Hydrodynamic Interaction ◽

Near Field ◽

Wave Interaction ◽

Far Field ◽

Wave Fields ◽

Energy Devices ◽

Field Effects ◽

Wave Energy Converters ◽

Coastal Defense

This study focuses on the numerical modeling of wave fields around structures due to their interaction with waves, with the intention to simulate both the resulting near and far field effects. Examples from the wave energy world are employed such as Wave Energy Converters (WECs), fixed or oscillating devices usually arranged in farms, that interact with the incoming waves and extract wave energy from them. As a result of the hydrodynamic interaction between the devices within a farm (so-called near-field effects), the power absorption of the farm is affected. Moreover, wave dissipation has been observed numerically (e.g. Troch et al., 2010) and in scale tests (e.g. Stratigaki et al., 2014; 2015) between the WEC farm location and e.g. the shoreline (so called far-field effects). These wave field changes can affect neighboring sea activities, coastal eco-systems, the coastline and even coastal defense conditions/parameters.

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Coupling Methodology for Modelling the Near-Field and Far-Field Effects of a Wave Energy Converter

Volume 10: Ocean Renewable Energy ◽

10.1115/omae2017-61892 ◽

2017 ◽

Author(s):

Philip Balitsky ◽

Gael Verao Fernandez ◽

Vasiliki Stratigaki ◽

Peter Troch

Keyword(s):

Wave Energy ◽

Near Field ◽

Computational Cost ◽

Propagation Model ◽

Far Field ◽

Field Effects ◽

Wave Energy Converters ◽

The One ◽

Geometric Layout ◽

Low Computational Cost

In order to produce a large amount of electricity at a competitive cost, farms of Wave Energy Converters (WECs) will need to be deployed in the ocean. Due to hydrodynamic interaction between the devices, the geometric layout of the farm will influence the power production and affect the surrounding area around the WECs. Therefore it is essential to model both the near field effects and far field effects of the WEC farm. It is difficult, however, to model both, employing a single numerical model that offers the desired precision at a reasonable computational cost. The objective of this paper is to present a coupling methodology that will allow for the accurate modelling of both phenomena at a reasonably low computational cost. The one-way coupling proposed is between the Boundary Element Method (BEM) solver NEMOH, and the depth-averaged mild-slope wave propagation model, MILDwave. In the presented cases, NEMOH is used to resolve the near field effects whilst MILDwave is used to determine the far field effects.

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Near-field and far-field effects in thermal radiation from metallic carbon nanotubes

10.1117/12.752152 ◽

2007 ◽

Cited By ~ 1

Author(s):

Andrei M. Nemilentsau ◽

Gregory Ya. Slepyan ◽

Sergey A. Maksimenko

Keyword(s):

Carbon Nanotubes ◽

Thermal Radiation ◽

Near Field ◽

Far Field ◽

Field Effects

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Benchmark Modeling of the Near-Field and Far-Field Wave Effects of Wave Energy Arrays

10.2172/1060889 ◽

2013 ◽

Author(s):

Kenneth E Rhinefrank ◽

Merrick C Haller ◽

H Tuba Ozkan-Haller

Keyword(s):

Wave Energy ◽

Near Field ◽

Far Field ◽

Wave Effects ◽

Field Wave

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WAVELET-BASED POSTPROCESSING OF JET LES DATA FOR ACOUSTIC FAR-FIELD EXTRAPOLATIONS

Journal of Computational Acoustics ◽

10.1142/s0218396x13500173 ◽

2013 ◽

Vol 21 (04) ◽

pp. 1350017

Author(s):

RAMIN KAVIANI ◽

VAHID ESFAHANIAN ◽

MOHAMMAD EBRAHIMI

Keyword(s):

Large Scale ◽

Stokes Equations ◽

Near Field ◽

Flow Simulation ◽

Computational Time ◽

Jet Flows ◽

Small Scale ◽

Frequency Noise ◽

Far Field ◽

Noise Sources

The affordable grid resolutions in conventional large-eddy simulations (LESs) of high Reynolds jet flows are unable to capture the sound generated by fluid motions near and beyond the grid cut-off scale. As a result, the frequency spectrum of the extrapolated sound field is artificially truncated at high frequencies. In this paper, a new method is proposed to account for the high frequency noise sources beyond the resolution of a compressible flow simulation. The large-scale turbulent structures as dominant radiators of sound are captured in LES, satisfying filtered Navier–Stokes equations, while for small-scale turbulence, a Kolmogorov's turbulence spectrum is imposed. The latter is performed via a wavelet-based extrapolation to add randomly generated small-scale noise sources to the LES near-field data. Further, the vorticity and instability waves are filtered out via a passive wavelet-based masking and the whole spectrum of filtered data are captured on a Ffowcs-Williams/Hawkings (FW-H) surface surrounding the near-field region and are projected to acoustic far-field. The algorithm can be implemented as a separate postprocessing stage and it is observed that the computational time is considerably reduced utilizing a hybrid of many-core and multi-core framework, i.e. MPI-CUDA programming. The comparison of the results obtained from this procedure and those from experiments for high subsonic and transonic jets, shows that the far-field noise spectrum agree well up to 2 times of the grid cut-off frequency.

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Theoretical and experimental studies of three-dimensional wavemaking in narrow tanks, including nonlinear phenomena near resonance

Journal of Fluid Mechanics ◽

10.1017/s0022112094002533 ◽

1994 ◽

Vol 276 ◽

pp. 211-232 ◽

Cited By ~ 4

Author(s):

Yitao Yao ◽

Marshall P. Tulin ◽

Ali R. Kolaini

Keyword(s):

Nonlinear Wave ◽

Near Field ◽

Experimental Studies ◽

Three Dimensional ◽

Analytical Techniques ◽

Nonlinear Phenomena ◽

Far Field ◽

Wave Groups ◽

Near Resonance ◽

Matching Techniques

In view of several practical ramifications of this problem, computational-analytical techniques for calculating waves induced by heaving arbitrary bodies in narrow tanks have been developed, including nonlinear wave groups produced near tank resonance. These feature computational near-field solutions matched with appropriate far-field solutions. In the linear case, the far field is provided by linear mode superposition. In the nonlinear case, the far field is described by a suitable nonlinear evolution equation of the cubic Schrödinger type. Matching techniques were developed. Calculations were successfully carried out and the results confirm the important effect of tank walls on added mass and damping.Results of computations have been compared with some data obtained with a conical wavemaker in a narrow tank. Pronounced nonlinear wave groups were obtained near resonance, and these are well reproduced in some detail by the nonlinear theory and computations, without considering any effects of dissipation.The related problem of resonant wave groups produced by a segmented paddle wavemaker has also been treated by analysis and subject to computation, with good general agreement with past experiments. The technique features matching near- and far-field computations using energy considerations.

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