Fluids

Research

26 pages, 1523 KiB

Open AccessArticle

Double-Multiple Streamtube Analysis of a Flexible Vertical Axis Wind Turbine

by Lalit Roy, Kellis Kincaid, Roohany Mahmud and David W. MacPhee

Fluids 2021, 6(3), 118; https://doi.org/10.3390/fluids6030118 - 13 Mar 2021

Cited by 8 | Viewed by 4288

Vertical-axis wind turbines (VAWTs) have drawn increased attention for off-grid and off-shore power generation due to inherent advantages over the more popular horizontal-axis wind turbines (HAWTs). Among these advantages are generator locale, omni-directionality and simplistic design. However, one major disadvantage is lower efficiency, [...] Read more.

Vertical-axis wind turbines (VAWTs) have drawn increased attention for off-grid and off-shore power generation due to inherent advantages over the more popular horizontal-axis wind turbines (HAWTs). Among these advantages are generator locale, omni-directionality and simplistic design. However, one major disadvantage is lower efficiency, which can be alleviated through blade pitching. Since each blade must transit both up- and down-stream each revolution, VAWT blade pitching techniques are not yet commonplace due to increased complexity and cost. Utilizing passively-morphing flexible blades can offer similar results as active pitching, requiring no sensors or actuators, and has shown promise in increasing VAWT performance in select cases. In this study, wind tunnel tests have been conducted with flexible and rigid-bladed NACA 0012 airfoils, in order to provide necessary input data for a Double-Multiple Stream-Tube (DMST) model. The results from this study indicate that a passively-morphing VAWT can achieve a maximum power coefficient (

C_{p}

) far exceeding that for a rigid-bladed VAWT

C_{P}

(18.9% vs. 10%) with reduced normal force fluctuations as much as 6.9%. Operational range of tip-speed ratio also is observed to increase by a maximum of 40.3%. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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19 pages, 1310 KiB

Open AccessArticle

Strategic Operation of Hydroelectric Power Plants in Energy Markets: A Model and a Study on the Hydro-Wind Balance

by Hugo Algarvio, Fernando Lopes and João Santana

Fluids 2020, 5(4), 209; https://doi.org/10.3390/fluids5040209 - 16 Nov 2020

Cited by 10 | Viewed by 3265

Abstract

The European Union defined ambitious targets for the production of energy from renewable energy sources. Most European markets trade now high levels of variable renewable energy (VRE). Renewable generation increases the variability and uncertainty of the net-load (i.e., demand minus VRE). To a [...] Read more.

The European Union defined ambitious targets for the production of energy from renewable energy sources. Most European markets trade now high levels of variable renewable energy (VRE). Renewable generation increases the variability and uncertainty of the net-load (i.e., demand minus VRE). To a large extent, this variability and uncertainty can be compensated by hydroelectric power plants. Typically, hydro power producers (HPPs) consider the periods of time with low market prices (and normally low demand and/or high VRE production) to pump, and the periods with high market prices (and normally high demand and/or low VRE production) to produce energy. This article presents a model for hydro power plants and a study to analyse the hydro-wind balance in a real-world setting, namely a simplified version of the Portuguese power system, involving a significant penetration of hydro and wind power (more than 50%). The study is conducted with the help of the multi-agent system MATREM. The results confirm (and rebut) the typical behavior of hydroelectric power plants (to produce energy, to pump water or to stay idle). Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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31 pages, 7251 KiB

Open AccessArticle

Hydro-Servo-Aero-Elastic Analysis of Floating Offshore Wind Turbines

by Dimitris I. Manolas, Vasilis A. Riziotis, George P. Papadakis and Spyros G. Voutsinas

Fluids 2020, 5(4), 200; https://doi.org/10.3390/fluids5040200 - 5 Nov 2020

Cited by 17 | Viewed by 5111

Abstract

A fully coupled hydro-servo-aero-elastic simulator for the analysis of floating offshore wind turbines (FOWTs) is presented. All physical aspects are addressed, and the corresponding equations are concurrently solved within the same computational framework, taking into account the wind and wave excitations, the aerodynamic [...] Read more.

A fully coupled hydro-servo-aero-elastic simulator for the analysis of floating offshore wind turbines (FOWTs) is presented. All physical aspects are addressed, and the corresponding equations are concurrently solved within the same computational framework, taking into account the wind and wave excitations, the aerodynamic response of the rotor, the hydrodynamic response of the floater, the structural dynamics of the turbine-floater-mooring lines assembly and finally the control system of the wind turbine. The components of the complex multi-physics system of a FOWT interact with each other in an implicitly coupled manner leading to a holistic type of modeling. Different modeling options, of varying fidelity and computational cost, are made available with respect to rotor aerodynamics, hydrodynamic loading of the floater and mooring system dynamics that allow for timely routine certification simulations, but also for computationally intense simulations of less conventional operating states. Structural dynamics is based on nonlinear multibody analysis that allows reproducing the large rigid body motions undergone by the FOWT, as well as large deflections and rotations of the highly flexible blades. The paper includes the description of the main physical models, of the interaction and solution strategy and representative results. Verification is carried out by comparing with other state-of-art tools that participated in the Offshore Code Comparison Collaboration Continuation (OC4) IEA Annex, while the advanced simulation capabilities are demonstrated in the case of half-wake interaction of floating wind turbines by employing the free-wake aerodynamic method. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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28 pages, 1978 KiB

Open AccessEditor’s ChoiceArticle

Hydrodynamic Responses of a 6 MW Spar-Type Floating Offshore Wind Turbine in Regular Waves and Uniform Current

by Zhiping Zheng, Jikang Chen, Hui Liang, Yongsheng Zhao and Yanlin Shao

Fluids 2020, 5(4), 187; https://doi.org/10.3390/fluids5040187 - 21 Oct 2020

Cited by 21 | Viewed by 3848

Abstract

In order to improve the understanding of hydrodynamic performances of spar-type Floating Offshore Wind Turbines (FOWTs), in particular the effect of wave-current-structure interaction, a moored 6MW spar-type FOWT in regular waves and uniform current is considered. The wind loads are not considered at [...] Read more.

In order to improve the understanding of hydrodynamic performances of spar-type Floating Offshore Wind Turbines (FOWTs), in particular the effect of wave-current-structure interaction, a moored 6MW spar-type FOWT in regular waves and uniform current is considered. The wind loads are not considered at this stage. We apply the potential-flow theory and perturbation method to solve the weakly-nonlinear problem up to the second order. Unlike the conventional formulations in the inertial frame of reference, which involve higher derivatives on the body surface, the present method based on the perturbation method in the non-inertial body-fixed coordinate system can potentially avoid theoretical inconsistency at sharp edges and associated numerical difficulties. A cubic Boundary Element Method (BEM) is employed to solve the resulting boundary-value problems (BVPs) in the time domain. The convective terms in the free-surface conditions are dealt with using a newly developed conditionally stable explicit scheme, which is an approximation of the implicit Crank–Nicolson scheme. The numerical model is firstly verified against three reference cases, where benchmark results are available, showing excellent agreement. Numerical results are also compared with a recent model test, with a fairly good agreement though differences are witnessed. Drag loads based on Morison’s equation and relative velocities are also applied to quantify the influence of the viscous loads. To account for nonlinear restoring forces from the mooring system, a catenary line model is implemented and coupled with the time-domain hydrodynamic solver. For the considered spar-type FOWT in regular-wave and current conditions, the current has non-negligible effects on the motions at low frequencies, and a strong influence on the mean wave-drift forces. The second-order sum-frequency responses are found to be negligibly small compared with their corresponding linear components. The viscous drag loads do not show a strong influence on the motions responses, while their contribution to the wave-drift forces being notable, which increases with increasing wave steepness. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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Graphical abstract

19 pages, 6831 KiB

Open AccessArticle

Development of a Flexible Lightweight Hydraulic-Pneumatic Flywheel System for Wind Turbine Rotors

by Laurence Alhrshy, Clemens Jauch and Peter Kloft

Fluids 2020, 5(4), 162; https://doi.org/10.3390/fluids5040162 - 23 Sep 2020

Cited by 5 | Viewed by 2483

Abstract

In this paper, the design of a flexible piston accumulator for application in a hydraulic-pneumatic flywheel system in a wind turbine rotor is presented. The flywheel system enables a wind turbine to vary the inertia of its rotor blades to control the power [...] Read more.

In this paper, the design of a flexible piston accumulator for application in a hydraulic-pneumatic flywheel system in a wind turbine rotor is presented. The flywheel system enables a wind turbine to vary the inertia of its rotor blades to control the power output and, most importantly, to influence the vibratory behaviour of wind turbine components. The method used for designing the flexible accumulator is based on the one hand on test results of a flexible piston accumulator prototype, and on the other hand, on simulation results of a model of a flexible piston accumulator. As a result, a design of flexible piston accumulators for application in the flywheel system is implemented and compared with the design of conventional steel accumulators. Due to the proposed design of the flywheel system, the impact on the mechanical loads of a wind turbine is analysed. The simulation results show that the new design of the piston accumulators causes a lower impact on the mechanical loads of the wind turbine than a previously published design of piston accumulators. It is further shown that the considered wind turbine can take on the flywheel system without the need for reinforcements in the rotor blades. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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Graphical abstract

23 pages, 5279 KiB

Open AccessArticle

Mean Drift Forces on Vertical Cylindrical Bodies Placed in Front of a Breakwater

by Dimitrios Konispoliatis and Spyridon Mavrakos

Fluids 2020, 5(3), 148; https://doi.org/10.3390/fluids5030148 - 31 Aug 2020

Cited by 3 | Viewed by 3533

Abstract

This paper presents a numerical and experimental investigation of the second-order steady horizontal and vertical drift forces acting on cylindrical bodies in regular waves. The examined bodies are either kept restrained in front of a vertical breakwater or are considered free- floating when [...] Read more.

This paper presents a numerical and experimental investigation of the second-order steady horizontal and vertical drift forces acting on cylindrical bodies in regular waves. The examined bodies are either kept restrained in front of a vertical breakwater or are considered free- floating when alone in the wave field. Two principally different approaches for mean drift forces determination are described: the momentum conservation principle and the direct integration of all pressure contributions upon the instantaneous wetted surface of the bodies, whereas, for the solution of the associated diffraction and motion radiation problems, analytical and panel methodologies are applied. The hydrodynamic interaction phenomenon between the bodies and the adjacent breakwater are taken into account by using the method of images. Theoretical and numerical results, concerning the horizontal and the vertical drift forces, are presented and compared with each other. Furthermore, additional comparisons are made with experimental data obtained during an experimental campaign at French research institute for exploitation of the sea (IFREMER), in France. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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19 pages, 9850 KiB

Open AccessArticle

Three-Dimensional Wind Field Construction and Wind Turbine Siting in an Urban Environment

by Mingrui Liu and Xiuling Wang

Fluids 2020, 5(3), 137; https://doi.org/10.3390/fluids5030137 - 16 Aug 2020

Cited by 8 | Viewed by 2856

Abstract

Three-dimensional urban wind field construction plays an important role not only in the analysis of pedestrian levels of comfort but also in the effectiveness of harnessing wind energy in an urban environment. However, it is challenging to accurately simulate urban wind flow due [...] Read more.

Three-dimensional urban wind field construction plays an important role not only in the analysis of pedestrian levels of comfort but also in the effectiveness of harnessing wind energy in an urban environment. However, it is challenging to accurately simulate urban wind flow due to the complex land use in urban environments. In this study, a three-dimensional numerical model was developed for urban wind flow construction. To obtain an accurate urban wind field, various turbulence models, including the Reynolds stress model (RSM), k-ω shear stress transport (SST), realizable k-ε, and (Re-Normalisation Group (RNG) k-ε models were tested. Simulation results were compared with experimental data in the literature. The RSM model showed promising potential in simulating urban wind flow. The model was then adopted to simulate urban wind flow for Purdue University Northwest, which is located in the Northwest Indiana urban region. Based on the simulation results, the optimal location was identified for urban wind turbine siting. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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22 pages, 5359 KiB

Open AccessArticle

Theoretical Analysis of a Vertical Cylindrical Floater in Front of an Orthogonal Breakwater

by Dimitrios N. Konispoliatis and Spyridon A. Mavrakos

Fluids 2020, 5(3), 135; https://doi.org/10.3390/fluids5030135 - 13 Aug 2020

Cited by 7 | Viewed by 2408

Abstract

This study investigates the effect of an orthogonal-shaped reflecting breakwater on the hydrodynamic characteristics of a vertical cylindrical body. The reflecting walls are placed behind the body, which can be conceived as a floater for wave energy absorption. Linear potential theory is assumed, [...] Read more.

This study investigates the effect of an orthogonal-shaped reflecting breakwater on the hydrodynamic characteristics of a vertical cylindrical body. The reflecting walls are placed behind the body, which can be conceived as a floater for wave energy absorption. Linear potential theory is assumed, and the associated diffraction and motion radiation problems are solved in the frequency domain. Axisymmetric eigenfunction expansions of the velocity potential are introduced into properly defined ring-shaped fluid regions surrounding the floater. The hydrodynamic interaction phenomena between the body and the adjacent breakwaters are exactly taken into account by using the method of images. Results are presented and discussed concerning the exciting wave forces on the floater and its hydrodynamic coefficients, concluding that the hydrodynamics of a vertical cylindrical body in front of an orthogonally shaped breakwater differ from those in unbounded waters. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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29 pages, 7911 KiB

Open AccessArticle

A Probabilistic Approach to Analyze Wind Energy Curtailment in Non-Interconnected Greek Islands Based on Typical Wind Year Meteorological Data

by George Caralis, Alexandros Kontzilas, Yang Peijin, Petros Chasapogiannis, Vassiliki Kotroni, Konstantinos Lagouvardos and Arthouros Zervos

Fluids 2020, 5(3), 123; https://doi.org/10.3390/fluids5030123 - 26 Jul 2020

Cited by 1 | Viewed by 2648

Abstract

Wind energy and photovoltaic solar energy (PV) are the most mature renewable energy technologies and are widely used to increase renewable energy penetration in non-interconnected Greek islands. However, their penetration is restricted due to technical issues related to the safe operation of autonomous [...] Read more.

Wind energy and photovoltaic solar energy (PV) are the most mature renewable energy technologies and are widely used to increase renewable energy penetration in non-interconnected Greek islands. However, their penetration is restricted due to technical issues related to the safe operation of autonomous power systems, the current conventional power infrastructure and their variable power output. In this framework, renewable energy curtailment is sometimes a necessity to ensure the balance between demand and supply. The ability of autonomous power systems to absorb wind and PV power is related to the load demand profile, the type and the flexibility of conventional power plants, the size of power system and the spatial dispersion of wind farms. In this connection, a probabilistic approach for estimating wind energy curtailment is thoroughly applied in most of the autonomous power systems in Greece, using detailed information about load demand and conventional power supply. In parallel, high resolution mesoscale model-based hourly wind data for typical meteorological wind year are used to represent the wind features in all the sites of interest. Technical constraints imposed by the local power system operator, related to the commitment of conventional power plants and the load dispatch strategies are taken into account to maximize renewable energy penetration levels. Finally, application for wide ranges of wind and PV capacity and the thorough analysis of the parameters leads to the presentation of comparable results and conclusions, which could be widely used to predict wind energy curtailment in non-interconnected power systems. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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21 pages, 10405 KiB

Open AccessArticle

Validation of a CFD-Based Numerical Wave Tank Model of the 1/20th Scale Wavestar Wave Energy Converter

by Christian Windt, Nicolás Faedo, Demián García-Violini, Yerai Peña-Sanchez, Josh Davidson, Francesco Ferri and John V. Ringwood

Fluids 2020, 5(3), 112; https://doi.org/10.3390/fluids5030112 - 14 Jul 2020

Cited by 17 | Viewed by 3846

Abstract

Numerical wave tanks (NWTs) provide efficient test beds for the numerical analysis at various stages during the development of wave energy converters (WECs). To ensure the acquisition of accurate, high-fidelity data sets, validation of NWTs is a crucial step. However, using experimental data [...] Read more.

Numerical wave tanks (NWTs) provide efficient test beds for the numerical analysis at various stages during the development of wave energy converters (WECs). To ensure the acquisition of accurate, high-fidelity data sets, validation of NWTs is a crucial step. However, using experimental data as reference during model validation, exact knowledge of all system parameters is required, which may not always be available, thus making an incremental validation inevitable. The present paper documents the numerical model validation of a 1/20 scale Wavestar WEC. The validation is performed considering different test case of increasing complexity: wave-only, wave excitation force, free decay, forced oscillation, and wave-induced motion cases. The results show acceptable agreement between the numerical and experimental data so that, under the well-known modelling constraints for mechanical friction and uncertainties in the physical model properties, the developed numerical model can be declared as validated. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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30 pages, 12645 KiB

Open AccessArticle

Hydrodynamic Analysis of Surge-Type Wave Energy Devices in Variable Bathymetry by Means of BEM

by Alexandros Magkouris, Markos Bonovas and Kostas Belibassakis

Fluids 2020, 5(2), 99; https://doi.org/10.3390/fluids5020099 - 23 Jun 2020

Cited by 8 | Viewed by 3024

Abstract

A variety of devices and concepts have been proposed and thoroughly investigated for the exploitation of renewable wave energy. Many of the devices operate in nearshore and coastal regions, and thus, variable bathymetry could have significant effects on their performance. In particular, Oscillating [...] Read more.

A variety of devices and concepts have been proposed and thoroughly investigated for the exploitation of renewable wave energy. Many of the devices operate in nearshore and coastal regions, and thus, variable bathymetry could have significant effects on their performance. In particular, Oscillating Wave Surge Converters (OWSCs) exploit the horizontal motion of water waves interacting with the flap of the device. In this work, a Boundary Element Method (BEM) is developed, and applied to the investigation of variable bathymetry effects on the performance of a simplified 2D model of a surge-type wave energy converter excited by harmonic incident waves. Numerical results, illustrating the effects of depth variation in conjunction with other parameters, like inertia and power-take-off, on the performance of the device, are presented. Finally, a comparative evaluation of the present simplified surge-type WEC model and point absorbers is presented for a case study in a selected coastal site on the Greek nearshore area, characterized by relatively increased wave energy potential. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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11 pages, 3468 KiB

Open AccessArticle

The Method of Image Singularities Employed for Oscillating Oblate Spheroids under a Free Surface

by Eirini I. Anastasiou, Eva Loukogeorgaki and Ioannis K. Chatjigeorgiou

Fluids 2020, 5(2), 75; https://doi.org/10.3390/fluids5020075 - 17 May 2020

Cited by 2 | Viewed by 2640

Abstract

The main objective of this study is to develop a semi-analytical formulation for the radiation problem of a fully immersed spheroid in a liquid field of infinite depth. The term “spheroid” refers herein to the oblate geometry of arbitrary eccentricity and to the [...] Read more.

The main objective of this study is to develop a semi-analytical formulation for the radiation problem of a fully immersed spheroid in a liquid field of infinite depth. The term “spheroid” refers herein to the oblate geometry of arbitrary eccentricity and to the axisymmetric case, where the axis of symmetry is normal to the free surface. The proposed numerical approach is based on the method of image singularities, and it enables the accurate and fast calculation of the hydrodynamic coefficients for the translational degrees of freedom of the oblate spheroid. The excellent agreement of the results, with those of other investigators for the limiting case of the sphere and with those obtained using a respected boundary integral equation code, demonstrates the accuracy of the proposed methodology. Finally, extensive calculations are presented, illustrating the direct impact of the immersion depth and the slenderness of the spheroid on the hydrodynamic coefficients. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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14 pages, 2486 KiB

Open AccessFeature PaperArticle

An Analytical Approach for the Two-Dimensional Plunging Breaking Wave Impact on a Vertical Wall with Air Entrapment

by Theodosis D. Tsaousis and Ioannis K. Chatjigeorgiou

Fluids 2020, 5(2), 58; https://doi.org/10.3390/fluids5020058 - 24 Apr 2020

Cited by 2 | Viewed by 3311

Abstract

This study investigates an idealized formulation of the two-dimensional impact of a breaking wave on a vertical impermeable wall. An overturning-like wave is assumed, which is close to the concept of a plunging breaker. It is assumed that during the collision an air [...] Read more.

This study investigates an idealized formulation of the two-dimensional impact of a breaking wave on a vertical impermeable wall. An overturning-like wave is assumed, which is close to the concept of a plunging breaker. It is assumed that during the collision an air pocket is entrapped between the wave and the wall. The air pocket width is assumed to be negligible and the compression effects are omitted. The problem is considered in the two-dimensional space (2D) using linear potential theory along with the small-time approximation. We use a perturbation method to cope with the linearized free-surface kinematic and dynamic boundary conditions. We impose the complete mixed boundary value problem (bvp) and we solve for the leading order of the velocity potential. The problem derived involves dual trigonometrical series and is treated analytically. The main assumption made is that, within the air pocket, the pressure is zero. Results are presented for the velocity potential on the wall, the velocity, and the free-surface elevation. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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