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Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters

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A special issue of Water (ISSN 2073-4441). This special issue belongs to the section "Hydraulics and Hydrodynamics".

Deadline for manuscript submissions: closed (31 October 2020) | Viewed by 43525

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Special Issue Editors

Prof. Dr. Peter Troch

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Guest Editor

Dept. of Civil Engineering, Ghent University, Zwijnaarde (Gent), Belgium
Interests: coastal wave hydrodynamics; wave–structure interaction; coastal structures; wave energy; wave tank modeling; time domain modeling

Dr. Vicky Stratigaki

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Guest Editor

Department of Civil Engineering, Ghent University, Technologiepark 60, 9052 Zwijnaarder (Ghent), Belgium
Interests: marine renewable energy; wave energy converter arrays; experimental modelling; numerical coupling methodologies; WEC–WEC interactions; wave-structure interaction
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Dr. Matt FOLLEY

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Guest Editor

Marine Renewables Research Group, Queen's University Belfast, Belfast, United Kingdom
Interests: wave energy; spectral-domain modelling; wave energy resource; wave energy arrays; wave-tank modelling

Dr. Eva Loukogeorgaki

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Guest Editor

Department of Civil Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
Interests: ocean engineering; marine structures for renewable energy exploitation (offshore wind energy, offshore wind turbines, wave energy, wave energy converters); spatial planning of offshore renewable energy systems; risk assessment of marine structures; artificial neural networks
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Special Issue Information

Dear Colleagues,

Wave energy has a huge potential to contribute to the required renewable energy supply worldwide. Wave energy converters (WECs) will need to be installed in arrays (or farms) to capture a sufficient amount of energy from the incident waves. Conventionally, the power output, the power take-off system, and (conversion) efficiency of single WECs are investigated in detail during the design phase, aiming at WEC device optimization.

However, the individual WECs in an array configuration interact with each other, which affects the overall power output of the WEC array (“near field” effects). Moreover, large WEC arrays, called WEC farms, have a significant effect on the surrounding wave field. These are “far field” effects or “wake” effects for the WEC farms, featuring reduced wave heights in the shadow zone behind the farm due to the wave absorption in the farm. As a result, an accurate assessment of the environmental impact of WEC arrays and farms due to the wave field variations is also required during the design phase.

Papers are invited which present experimental or numerical methodologies for modelling both near field effects and far field wake effects of WEC farms on the surrounding wave field, on the coastal morphology, or on other users of the sea, as the relevant tools for the environmental impact assessment.

Apart from WECs, other floating devices placed in an array configuration are also targeted, including recent trends such as co-located wave and wind energy farms, offshore floating platforms, arrays of combined energy devices, etc. These topics are directly related to the new “WECANet COST Action CA17105 - A pan-European network for Marine Renewable Energy”.

Prof. Dr. Peter TROCH
Dr. Vicky STRATIGAKI
Dr. Matt FOLLEY
Assist. Prof. Dr. Eva LOUKOGEORGAKI
Guest Editors

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Keywords

marine renewable energy
wave energy converter arrays
array wave tank experiments
numerical modelling of WEC farm wake effects
arrays of moored floating structures, numerical coupling methodologies
co-located wave and wind energy farms
WEC arrays combined with other marine facilities (e.g., breakwaters, offshore platforms, offshore wind turbines)
WEC farm near and far field effects
WEC array environmental impact assessment
WEC array interactions

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Published Papers (11 papers)

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Research

20 pages, 6639 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Influence of Power Take-Off Modelling on the Far-Field Effects of Wave Energy Converter Farms

by Gael Verao Fernandez, Vasiliki Stratigaki, Nicolas Quartier and Peter Troch

Water 2021, 13(4), 429; https://doi.org/10.3390/w13040429 - 6 Feb 2021

Cited by 6 | Viewed by 2798

Abstract

The study of the potential impact of wave energy converter (WEC) farms on the surrounding wave field at long distances from the WEC farm location (also know as “far field” effects) has been a topic of great interest in the past decade. Typically, “far-field” effects have been studied using phase average or phase resolving numerical models using a parametrization of the WEC power absorption using wave transmission coefficients. Most recent studies have focused on using coupled models between a wave-structure interaction solver and a wave-propagation model, which offer a more complex and accurate representation of the WEC hydrodynamics and PTO behaviour. The difference in the results between the two aforementioned approaches has not been studied yet, nor how different ways of modelling the PTO system can affect wave propagation in the lee of the WEC farm. The Coastal Engineering Research Group of Ghent University has developed both a parameterized model using the sponge layer technique in the mild slope wave propagation model MILDwave and a coupled model MILDwave-NEMOH (NEMOH is a boundary element method-based wave-structure interaction solver), for studying the “far-field” effects of WEC farms. The objective of the present study is to perform a comparison between both numerical approaches in terms of performance for obtaining the “far-field” effects of two WEC farms. Results are given for a series of regular wave conditions, demonstrating a better accuracy of the MILDwave-NEMOH coupled model in obtaining the wave disturbance coefficient (

K_{d}

) values around the considered WEC farms. Subsequently, the analysis is extended to study the influence of the PTO system modelling technique on the “far-field” effects by considering: (i) a linear optimal, (ii) a linear sub-optimal and (iii) a non-linear hydraulic PTO system. It is shown that modelling a linear optimal PTO system can lead to an unrealistic overestimation of the WEC motions than can heavily affect the wave height at a large distance in the lee of the WEC farm. On the contrary, modelling of a sub-optimal PTO system and of a hydraulic PTO system leads to a similar, yet reduced impact on the “far-field” effects on wave height. The comparison of the PTO systems’ modelling technique shows that when using coupled models, it is necessary to carefully model the WEC hydrodynamics and PTO behaviour as they can introduce substantial inaccuracies into the WECs’ motions and the WEC farm “far-field” effects. Full article

(This article belongs to the Special Issue Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters)

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23 pages, 1947 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Influence of the Drag Force on the Average Absorbed Power of Heaving Wave Energy Converters Using Smoothed Particle Hydrodynamics

by Nicolas Quartier, Pablo Ropero-Giralda, José M. Domínguez, Vasiliki Stratigaki and Peter Troch

Water 2021, 13(3), 384; https://doi.org/10.3390/w13030384 - 2 Feb 2021

Cited by 23 | Viewed by 4708

Abstract

In this paper, we investigated how the added mass, the hydrodynamic damping and the drag coefficient of a Wave Energy Converter (WEC) can be calculated using DualSPHysics. DualSPHysics is a software application that applies the Smoothed Particle Hydrodynamics (SPH) method, a Lagrangian meshless method used in a growing range of applications within the field of Computational Fluid Dynamics (CFD). Furthermore, the effect of the drag force on the WEC’s motion and average absorbed power is analyzed. Particularly under controlled conditions and in the resonance region, the drag force becomes significant and can greatly reduce the average absorbed power of a heaving point absorber. Once the drag coefficient has been determined, it is used in a modified equation of motion in the frequency domain, taking into account the effect of the drag force. Three different methods were compared for the calculation of the average absorbed power: linear potential flow theory, linear potential flow theory modified to take the drag force into account and DualSPHysics. This comparison showed the considerable effect of the drag force in the resonance region. Calculations of the drag coefficient were carried out for three point absorber WECs: one spherical WEC and two cylindrical WECs. Simulations in regular waves were performed for one cylindrical WEC with two different power take-off (PTO) systems: a linear damping and a Coulomb damping PTO system. The Coulomb damping PTO system was added in the numerical coupling between DualSPHysics and Project Chrono. Furthermore, we considered the optimal PTO system damping coefficient taking the effect of the drag force into account. Full article

(This article belongs to the Special Issue Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters)

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26 pages, 72125 KiB

Open AccessArticle

Integration of Wave Power Farms into Power Systems of the Adriatic Islands: Technical Possibilities and Cross-Cutting Aspects

by Damir Šljivac, Irina Temiz, Branka Nakomčić-Smaragdakis and Matej Žnidarec

Water 2021, 13(1), 13; https://doi.org/10.3390/w13010013 - 24 Dec 2020

Cited by 4 | Viewed by 2942

Abstract

Wave energy is of interest for regions with high wave power potential, as well as for regions with modest wave power potential such as the Adriatic/Mediterranean coastlines and islands. In the present paper, the possibility of integrating a wave power farm with the power system of an island in the Adriatic Sea, combining the wave power with a battery energy storage system (BESS) and solar photovoltaics (PVs) is explored and its impact on the local weak low voltage grid is investigated. The load profile is typical of the demand (consumption) of an Adriatic island, in which the demand substantially increases during summer (the tourist season). The wave power technology is a point-absorbing wave energy converter (WEC) with a direct drive linear permanent-magnet synchronous generator power take-off device. Wave power farms (WPFs) consist of two to ten WECs. In this study, we show that the integration of a WPF consisting of two WECs into the grid is optimal and helps to reach zero grid exchange, and a BESS reduces the intermittency of the power flow into the grid. Since a potential wave power farm is to be installed in a populated recreational area, the technical study is complemented by discussion on cross-cutting aspects such as its environmental and social impact. Full article

(This article belongs to the Special Issue Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters)

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18 pages, 5392 KiB

Open AccessArticle

Study of Snap Loads for Idealized Mooring Configurations with a Buoy, Inextensible and Elastic Cable Combinations for the Multi-Float M4 Wave Energy Converter

by Peter Stansby and Efrain Carpintero Moreno

Water 2020, 12(10), 2818; https://doi.org/10.3390/w12102818 - 11 Oct 2020

Cited by 9 | Viewed by 2489

Abstract

There has been considerable modelling and wave basin validation of the multi-mode, multi-float, moored wave energy converter M4. The 6 float (2 power take off) (PTO) configuration is considered here with mooring from a buoy with light inextensible cables. Large mean mooring forces and very large peak or snap forces were measured in large waves while the rotational response about the hinges (for power take off in operational conditions) was predominantly linear. Modelling has been extended with elastic mooring cables connected directly to the base of the bow float and to the buoy. The experimental mean force is input to the linear diffraction/radiation model. The device response is effectively unchanged. The peak mooring force and tensions remain large with direct connection to the base of the bow float but are only slightly greater than the mean forces with elastic cables to the buoy, and an elastic hawser provides a further slight reduction. For the largest waves measured, the force was about 10% of the dry weight of the platform. The idealized efficient modelling may inform more detailed design while efficient methods for determining highly nonlinear mean forces remain to be established. Full article

(This article belongs to the Special Issue Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters)

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21 pages, 7404 KiB

Open AccessFeature PaperArticle

Efficiency and Survivability of a Floating Oscillating Water Column Wave Energy Converter Moored to the Seabed: An Overview of the EsflOWC MaRINET2 Database

by Dogan Kisacik, Vasiliki Stratigaki, Minghao Wu, Lorenzo Cappietti, Irene Simonetti, Peter Troch, Alejandro Crespo, Corrado Altomare, José Domínguez, Matthew Hall, Moncho Gómez-Gesteira, Ricardo Birjukovs Canelas and Peter Stansby

Water 2020, 12(4), 992; https://doi.org/10.3390/w12040992 - 1 Apr 2020

Cited by 13 | Viewed by 6560

Abstract

Floating oscillating water column (OWC) type wave energy converters (WECs), compared to fixed OWC WECs that are installed near the coastline, can be more effective as they are subject to offshore waves before the occurrence of wave dissipation at a nearshore location. The performance of floating OWC WECs has been widely studied using both numerical and experimental methods. However, due to the complexity of fluid–structure interaction of floating OWC WECs, most of the available studies focus on 2D problems with WEC models of limited degrees-of-freedom (DOF) of motion, while 3D mooring effects and multiple-DOF OWC WECs have not been extensively investigated yet under 2D and 3D wave conditions. Therefore, in order to gain a deeper insight into these problems, the present study focuses on wave flume experiments to investigate the motion and mooring performance of a scaled floating OWC WEC model under 2D wave conditions. As a preparatory phase for the present MaRINET2 EsflOWC (efficiency and survivability of floating OWC) project completed at the end of 2017, experiments were also carried out in advance in the large wave flume of Ghent University. The following data were obtained during these experimental campaigns: multiple-DOF OWC WEC motions, mooring line tensions, free surface elevations throughout the wave flume, close to and inside the OWC WEC, change in the air pressure inside the OWC WEC chamber and velocity of the airflow through the vent on top of the model. The tested wave conditions mostly include nonlinear intermediate regular waves. The data obtained at the wave flume of Ghent University, together with the data from the EsflOWC tests at the wave flume of LABIMA, University of Florence, provide a database for numerical validation of research on floating OWC WECs and floating OWC WEC farms or arrays used by researchers worldwide. Full article

(This article belongs to the Special Issue Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters)

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21 pages, 3803 KiB

Open AccessArticle

Performance of an Array of Oblate Spheroidal Heaving Wave Energy Converters in Front of a Wall

by Eva Loukogeorgaki, Ifigeneia Boufidi and Ioannis K. Chatjigeorgiou

Water 2020, 12(1), 188; https://doi.org/10.3390/w12010188 - 9 Jan 2020

Cited by 19 | Viewed by 3323

Abstract

In this paper, we investigate the performance of a linear array of five semi-immersed, oblate spheroidal heaving Wave Energy Converters (WECs) in front of a bottom-mounted, finite-length, vertical wall under perpendicular to the wall regular waves. The diffraction and radiation problems are solved in the frequency domain by utilizing the conventional boundary integral equation method. Initially, to demonstrate the enhanced absorption ability of this array, we compare results with the ones corresponding to arrays of cylindrical and hemisphere-shaped WECs. Next, we investigate the effect of the array’s distance from the wall and of the length of the wall on the physical quantities describing the array’s performance. The results illustrate that the array’s placement at successively larger distances from the wall, up to three times the WECs’ radius, induces hydrodynamic interactions that improve the array’s hydrodynamic behavior, and thus its power absorption ability. An increase in the length of the wall does not lead to any significant power absorption improvement. Compared to the isolated array, the presence of the wall affects positively the array’s power absorption ability at specific frequency ranges, depending mainly on the array’s distance from the wall. Finally, characteristic diffracted wave field patterns are presented to interpret physically the occurrence of the local minima of the heave exciting forces. Full article

(This article belongs to the Special Issue Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters)

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19 pages, 8484 KiB

Open AccessArticle

The Expected Shoreline Effect of a Marine Energy Farm Operating Close to Sardinia Island

by Florin Onea and Eugen Rusu

Water 2019, 11(11), 2303; https://doi.org/10.3390/w11112303 - 3 Nov 2019

Cited by 11 | Viewed by 3410

Abstract

Coastal areas are defined by numerous opportunities and threats. Among them we can mention emerging renewable projects and on the other hand coastal erosion. In the present work, the impact of a generic wind–wave farm on the nearshore waves and currents in the vicinity of the Porto Ferro inlet (northwest Sardinia) was assessed. Using a reanalysis wave dataset that covers a 40-year interval (1979–2018), the most relevant wave characteristics in the target area were identified. These can reach during winter a maximum value of 7.35 m for the significant wave height. As a next step, considering a modeling system that combines a wave model (simulating waves nearshore (SWAN)) and a surf model, the coastal impact of some generic marine energy farms defined by a transmission coefficient of 25% was assessed. According to the results corresponding to the reference sites and lines defined close to the shore, it becomes obvious that there is a clear attenuation in terms of significant wave heights, and as regards current velocities, although the general tendency for them to decrease, there are, however, some situations when the values of the nearshore current velocities can also decrease. Finally, we can mention that the presence of a marine energy farm seems to be beneficial for the beach stability in this particular coastal environment, and in some cases the transformation of the breaking waves from plunging to spilling is noticed. Full article

(This article belongs to the Special Issue Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters)

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16 pages, 1221 KiB

Open AccessArticle

WECANet: The First Open Pan-European Network for Marine Renewable Energy with a Focus on Wave Energy-COST Action CA17105

by Vasiliki Stratigaki

Water 2019, 11(6), 1249; https://doi.org/10.3390/w11061249 - 14 Jun 2019

Cited by 13 | Viewed by 4219

Abstract

Growing energy demand has increased interest in marine renewable energy resources (i.e., wave energy, which is harvested through wave energy converter (WEC) arrays. However, the wave energy industry is currently at a significant juncture in its development, facing a number of challenges which require that research re-focuses on a holistic techno-economic perspective, where the economics considers the full life cycle costs of the technology. It also requires development of WECs suitable for niche markets, because in Europe there are inequalities regarding wave energy resources, wave energy companies, national programs and investments. As a result, in Europe there are leading and non-leading countries in wave energy technology. The sector also needs to increase confidence of potential investors by reducing (non-)technological risks. This can be achieved through an interdisciplinary approach by involving engineers, economists, environmental scientists, lawyers, regulators and policy experts. Consequently, the wave energy sector needs to receive the necessary attention compared to other more advanced and commercial offshore energy technologies (e.g., offshore wind). The formation of the first open pan-European network with an interdisciplinary approach will contribute to large-scale WEC array deployment by dealing with the current bottlenecks. The WECANet (Wave Energy Converter Array Network) European COST Action, introduced in September 2018 and presented in this paper, aims at a collaborative and inclusive approach, as it provides a strong networking and collaboration platform that also creates the space for dialogue between all stakeholders in wave energy. An important characteristic of the Action is that participation is open to all parties interested and active in the development of wave energy. Previous activities organised by WECANet core group members have resulted in a number of joint European projects and scientific publications. WECANet’s main target is the equal research, training, networking, collaboration and funding opportunities for all researchers and professionals, regardless of age, gender and country in order to obtain understanding of the main challenges governing the development of the wave energy sector. Full article

(This article belongs to the Special Issue Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters)

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30 pages, 12443 KiB

Open AccessArticle

Analysing the Near-Field Effects and the Power Production of Near-Shore WEC Array Using a New Wave-to-Wire Model

by Philip Balitsky, Nicolas Quartier, Vasiliki Stratigaki, Gael Verao Fernandez, Panagiotis Vasarmidis and Peter Troch

Water 2019, 11(6), 1137; https://doi.org/10.3390/w11061137 - 30 May 2019

Cited by 15 | Viewed by 3863

Abstract

In this study, a series of modules is integrated into a wave-to-wire (W2W) model that links a Boundary Element Method (BEM) solver to a Wave Energy Converter (WEC) motion solver which are in turn coupled to a wave propagation model. The hydrodynamics of the WECs are resolved in the wave structure interaction solver NEMOH, the Power Take-off (PTO) is simulated in the WEC simulation tool WEC-Sim, and the resulting perturbed wave field is coupled to the mild-slope propagation model MILDwave. The W2W model is run for verified for a realistic wave energy project consisting of a WEC farm composed of 10 5-WEC arrays of Oscillating Surging Wave Energy Converters (OSWECs). The investigated WEC farm is modelled for a real wave climate and a sloping bathymetry based on a proposed OSWEC array project off the coast of Bretagne, France. Each WEC array is arranged in a power-maximizing 2-row configuration that also minimizes the inter-array separation distance

d_{x}

and

d_{y}

and the arrays are located in a staggered energy maximizing configuration that also decreases the along-shore WEC farm extent. The WEC farm power output and the near and far-field effects are simulated for irregular waves with various significant wave heights wave peak periods and mean wave incidence directions

β

based on the modelled site wave climatology. The PTO system of each WEC in each farm is modelled as a closed-circuit hydraulic PTO system optimized for each set of incident wave conditions, mimicking the proposed site technology, namely the WaveRoller^® OSWEC developed by AW Energy Ltd. The investigation in this study provides a proof of concept of the proposed W2W model in investigating potential commercial WEC projects. Full article

(This article belongs to the Special Issue Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters)

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25 pages, 8329 KiB

Open AccessArticle

Wake Effect Assessment in Long- and Short-Crested Seas of Heaving-Point Absorber and Oscillating Wave Surge WEC Arrays

by Gael Verao Fernandez, Vasiliki Stratigaki, Panagiotis Vasarmidis, Philip Balitsky and Peter Troch

Water 2019, 11(6), 1126; https://doi.org/10.3390/w11061126 - 29 May 2019

Cited by 6 | Viewed by 3221

Abstract

In the recent years, the potential impact of wave energy converter (WEC) arrays on the surrounding wave field has been studied using both phase-averaging and phase-resolving wave propagation models. Obtaining understanding of this impact is important because it may affect other users in the sea or on the coastline. However, in these models a parametrization of the WEC power absorption is often adopted. This may lead to an overestimation or underestimation of the overall WEC array power absorption, and thus to an unrealistic estimation of the potential WEC array impact. WEC array power absorption is a result of energy extraction from the incoming waves, and thus wave height decrease is generally observed downwave at large distances (the so-called “wake” or “far-field” effects). Moreover, the power absorption depends on the mutual interactions between the WECs of an array (the so-called “near field” effects). To deal with the limitations posed by wave propagation models, coupled models of recent years, which are nesting wave-structure interaction solvers into wave propagation models, have been used. Wave-structure interaction solvers can generally provide detailed hydrodynamic information around the WECs and a more realistic representation of wave power absorption. Coupled models have shown a lower WEC array impact in terms of wake effects compared to wave propagation models. However, all studies to date in which coupled models are employed have been performed using idealized long-crested waves. Ocean waves propagate with a certain directional spreading that affects the redistribution of wave energy in the lee of WEC arrays, and thus gaining insight wake effect for irregular short-crested sea states is crucial. In our research, a new methodology is introduced for the assessment of WEC array wake effects for realistic sea states. A coupled model is developed between the wave-structure interaction solver NEMOH and the wave propagation model MILDwave. A parametric study is performed showing a comparison between WEC array wake effects for regular, long-crested irregular, and short-crested irregular waves. For this investigation, a nine heaving-point absorber array is used for which the wave height reduction is found to be up to 8% lower at 1.0 km downwave the WEC array when changing from long-crested to short-crested irregular waves. Also, an oscillating wave surge WEC array is simulated and the overestimation of the wake effects in this case is up to 5%. These differences in wake effects between different wave types indicates the need to consider short-crested irregular waves to avoid overestimating the WEC array potential impacts. The MILDwave-NEMOH coupled model has proven to be a reliable numerical tool, with an efficient computational effort for simulating the wake effects of two different WEC arrays under the action of a range of different sea states. Full article

(This article belongs to the Special Issue Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters)

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18 pages, 4984 KiB

Open AccessArticle

Internal Wave Generation in a Non-Hydrostatic Wave Model

by Panagiotis Vasarmidis, Vasiliki Stratigaki, Tomohiro Suzuki, Marcel Zijlema and Peter Troch

Water 2019, 11(5), 986; https://doi.org/10.3390/w11050986 - 10 May 2019

Cited by 9 | Viewed by 4875

Abstract

In this work, internal wave generation techniques are developed in an open source non-hydrostatic wave model (Simulating WAves till SHore, SWASH) for accurate generation of regular and irregular long-crested waves. Two different internal wave generation techniques are examined: a source term addition method where additional surface elevation is added to the calculated surface elevation in a specific location in the domain and a spatially distributed source function where a spatially distributed mass is added in the continuity equation. These internal wave generation techniques in combination with numerical wave absorbing sponge layers are proposed as an alternative to the weakly reflective wave generation boundary to avoid re-reflections in case of dispersive and directional waves. The implemented techniques are validated against analytical solutions and experimental data including water surface elevations, orbital velocities, frequency spectra and wave heights. The numerical results show a very good agreement with the analytical solution and the experimental data indicating that SWASH with the addition of the proposed internal wave generation technique can be used to study coastal areas and wave energy converter (WEC) farms even under highly dispersive and directional waves without any spurious reflection from the wave generator. Full article

(This article belongs to the Special Issue Numerical and Experimental Modelling of Wave Field Variations around Arrays of Wave Energy Converters)

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