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Numerical simulation of the wake of marine current turbines with a particle method
Abstract
This paper presents numerical results for the wake behind a three bladed horizontal axis turbine in a uniform free upstream current. A three-dimensional software, initially issuing from Pinon et al. [6], taking into account the non stationary evolution of the wake emitted by turbine blades is developed in order to assess the disturbances generated on the sea-bed and on the free surface. So an unsteady Lagrangian method is considered for these simulations using Vortex Method. The vortex flow is discretised with particles carrying vorticity, which are advected in a Lagrangian frame.
... Various authors used the VPM to simulate lifting bodies. In that respect, propellers and other marine turbines were simulated using actuator disks[7], panel methods [8,9] and Lifting-Lines[3, 10, 11]. The wide range of application of the VPM makes it all the more interesting to couple it with other solvers to account for the presence of solid boundaries in the flow. ...
... However, there is no particle in the Eulerian Domain near the solid boundaries to account for the near-wall flow. The contribution of this Eulerian flow to the total velocity is accounted for by the surface integral in Eq. (8). This velocity is evaluated similarly to the previous one, ...
... The main difficulty in hybridizing the Lagrangian and Eulerian Domains lies in the generation of the hybrid particles. The VPM can be used alongside Lifting-Lines [10,11], Panel methods [8,9] or the BEM [16,34,35] to generate particles from solid boundaries, either by shedding vortex sheets or diffusing the vorticity flux from the solid boundaries. Nevertheless, these methods would not guarantee the continuity of the flow field between the Eulerian and Lagrangian Domains. ...
... These preliminary results were really promising, even though a better account of the blade was already invoked. This motivated us to enhance the software [18] so that the computations could be improved : larger array, finer discretisation, account of ambient turbulence, etc. ...
... Several developments have been undertaken at the LOMC in Le Havre for the past few decades, based on the Vortex Particle Method applied to Navier-Stokes' equations for an incompressible fluid, and later on combined with a singularity method for the account of obstacle surfaces. They have resulted in the simulation of vortex rings [54], marine propellers and wind turbines [55], boat sails [56,57], transversal jets [58,59], and now finally three-bladed horizontal axis tidal turbines [8,18]. This latest iteration has lead to a collaboration with IFREMER, resulting in the creation of a dedicated simulation code named Dorothy. ...
... These preliminary results aim to demonstrate and validate the outcome of the Vortex Particle Method described in this Chapter, by comparison with experimental flume tank measurements. The experimental results presented hereafter were gathered at the IFREMER flume tank in Boulogne-sur-Mer, and presented already in various publications [18,78,79]. ...
In the current context of diversification of renewable energies, tidal turbines are set to occupy an important niche, and numerical simulation is a crucial tool for their investigation. The in-house simulation code DOROTHY developed in collaboration between IFREMER and LOMC uses the Vortex Particle Method offering a good compromise between physical realism and and computational time. Some additional developments are required in order to make of this software a fully rounded numerical tool able to mimic advanced realistic configurations. Firstly, an important overhaul of its computation of loads has been undertaken, including a new framework to represent the previously simplified and now fully-rendered turbine blades. This endeavour includes the mathematical justification, investigation, and preliminary validation of additional integral methods accounting for the turbine body. Secondly, the importance of the impact of ambient turbulence on the wake interaction and power output within a turbine farm cannot be ignored. This element is introduced using a Synthetic Eddy Method uniquely adapted to the present Lagrangian framework. All aspects of this method as well as a promising alternative are closely examined, culminating in the demonstration of its capabilities for the simulation of the flow and prediction of detrimental interaction effects throughout a projected four turbine pilot farm configuration.
... These preliminary results were really promising, even though a better account of the blade was already invoked. This motivated us to enhance the software [9] so that the computations could be improved: larger array, finer discretisation, account of ambient turbulence, etc. ...
... The Vortex method is an unsteady Lagrangian method, based on a discretisation of the flow into vorticity carrying particles [9,[37][38][39]. The governing equations for this unsteady and incompressible flow are the Navier-Stokes equations in their velocity/vorticity (u, ω) formulation: ...
... There are many applications for the use of such a component, such as the simulation of the nozzle of jet [49], of a sail [50], or of a monofin [51]. In the present study, the potential velocity component represents the influence of the rotor of a turbine [9], as will be demonstrated in the following sections. This velocity component u φ derives from a scalar potential φ, which must satisfy: ...
This study investigates the wake interaction of four full-scale three-bladed tidal turbines with different ambient turbulence conditions, in straight and yawed flows. A three-dimensional unsteady Lagrangian Vortex Blob software is used for the numerical simulations of the turbines’ wakes. In order to model the ambient turbulence in the Lagrangian Vortex Method formalism, a Synthetic Eddy Method is used. With this method, turbulent structures are added in the computational domain to generate a velocity field which statistically reproduces any ambient turbulence intensity and integral length scale. The influence of the size of the structures and their density (within the study volume) on the wake of a single turbine is studied. Good agreement is obtained between numerical and experimental results for a high turbulence intensity but too many structures can increase the numerical dissipation and reduce the wake extension. Numerical simulations of the four turbine array with the layout initially proposed for the NEPTHYD pilot farm are then presented. Two ambient turbulence intensities encountered in the Alderney Race and two integral length scales are tested with a straight flow. Finally, the wakes obtained for yawed flows with different angles are presented, highlighting turbine interactions.
... This potential method, first introduced by Rosenhead in 1931 [13], aims to represent continuous unsteady flows through the use of singular particles. Since then various authors managed to use this method to simulate: propeller-on-wing interactions [4], rotors in hover configuration with a Lifting Line [6], rotor wake transition and coplanar rotors wake mixing [1], marine turbines with a panel method [12]... This incompressible method is particularly well suited to represent wake decay and mixing, and viscous effects where other panel or filament methods fail to do so, making it a possible candidate to simulate the interactions between the wakes of several different rotors. ...
... These two terms contribute to the generation of vorticity because of the temporal rate of change and the spanwise variation of the circulation on the solid. To respect this, particles are shed along the LL and are set free in the wake [12][16] [15] [1]. The temporal contribution generates shedding particles, parallel to the LL while the spatial variations contribute to the generation of trailing particles, perpendicular to the LL. ...
The study detailed in this paper aims to provide a solver able to simulate of a complex flows with multiple unsteady interactions. The numerical application of vortex methods has been studied intensively during the second half of the 20 th century. In particular, the Vortex Particle Method (VPM), aims to represent an incompressible flow through singular particles. However, its computational cost curtailed its use until recently. Since each particle influences one another, the VPM cost is O(N 2 p), with N p the number of particles. Fortunately enough, recent methods can be used to accelerate such problems to make it a O(N) problem. After introducing the VPM, its implementation and acceleration are detailed. A validation of the algorithm is conducted on several test cases. In addition, a comparison with other similar studies is carried out to validate its robustness and accuracy. Eventually a lifting line is added to study the reaction of the VPM when confronted with a wing.
... DOROTHY is code based on the Lagrangian Vortex Particle Method developed by ULHN [36], [37]. The Vortex Particle Method uses a Lagrangian resolution of the fluid domain, discretised into fluid particles representing the vortex perturbation of the flow. ...
... In earlier versions, DOROTHY used a panel method for a simplified zero-thickness representation of the turbine blades [36]. However the computation of blade loads with this method proved difficult. ...
Tidal energy projects require numerical modelling for the assessment of tidal site conditions and turbine/array performance. The Interreg TIGER project has offered a unique opportunity to implement a wide range of numerical models. This paper provides an overview and comparison of the different numerical models developed by academic partners in the TIGER project. The models cover a variety of spatial and temporal scales. The largest scale models provide long-term climatic studies covering the entire English Channel region, at relatively low resolution, whilst the highest-resolution models provide detailed information about short-term and small-scale turbulent flow and its interaction with tidal turbines. The models are used for various purposes. At one end of the scale, the models have been used to inform the large-scale techno-economic assessment of tidal energy and its impact on the energy mix in the UK and France. At the other end of the scale, the numerical models provide information that feeds into detailed engineering design of tidal turbines at particular sites, and assessment of the energy yield. The models showcase the range of computational tools available to aid the development of the tidal energy industry. This paper will be useful for investors, technology developers and project stakeholders to help identify suitable numerical models to support and develop ongoing and future tidal stream projects.
... The Vortex method is an unsteady Lagrangian method, based on a discretisation of the flow into vorticity carrying particles [22,[25][26][27] . The governing equations for this unsteady and incompressible flow are the Navier-Stokes equations in their velocity/vorticity ( u , ω) formulation: ...
... There are many applications for the use of such a component, such as the simulation of the nozzle of jet [31] , of a sail [32] , or of a monofin [33] . In the present study, the potential velocity component represents the influence of the rotor of a turbine [27] , as will be demonstrated in Section 4 . This velocity component U φ derives from a scalar potential φ, which must satisfy: ...
This paper describes a detailed implementation of the Synthetic Eddy Method (SEM) initially presented in Jarrin et al. (2006) applied to the Lagrangian Vortex simulation. While the treatment of turbulent diffusion is already extensively covered in scientific literature, this is one of the first attempts to represent ambient turbulence in a fully Lagrangian framework. This implementation is well suited to the integration of PSE (Particle Strength Exchange) or DVM (Diffusion Velocity Method), often used to account for molecular and turbulent diffusion in Lagrangian simulations. The adaptation and implementation of the SEM into a Lagrangian method using the PSE diffusion model is presented, and the turbulent velocity fields produced by this method are then analysed. In this adaptation, SEM turbulent structures are simply advected, without stretching or diffusion of their own, over the flow domain. This implementation proves its ability to produce turbulent velocity fields in accordance with any desired turbulent flow parameters. As the SEM is a purely mathematical and stochastic model, turbulent spectra and turbulent length scales are also investigated. With the addition of variation in the turbulent structures sizes, a satisfying representation of turbulent spectra is recovered, and a linear relation is obtained between the turbulent structures sizes and the Taylor macroscale. Lastly, the model is applied to the computation of a tidal turbine wake for different ambient turbulence levels, demonstrating the ability of this new implementation to emulate experimentally observed tendencies.
... The design of TECs can be optimized in response to results revealed during trials. Until recently, the technology optimization, quality and reliability improvements of TECs were obtained by both experimental research in flume tanks (e.g., [2,20,6]) and modeling (e.g., [3,4,22,14,7]). For example, the most sophisticated Large Eddy Simulations, performed by Churchfield et al. [7], yield a detailed time-dependent structure of the turbulent flow and showed that the way in which the turbulent flow is simulated greatly affects the predicted power production by the array of turbines. ...
... Important results characterizing the functioning of a horizontal axis turbine and an array of turbines under a range of flow conditions have been obtained from device testing in flume tanks. These tests have provided valuable data at the small experimental scale and, in particular, provided indications on how current speed and current/wave interaction can affect the power production by the turbine (e.g., [22,25]). Other experimental works highlighted the impact of turbulence on turbine performance (e.g., [1,5,8]). ...
An experiment was performed to study the power production by a Darrieus type turbine of the Dutch company Water2Energy in a tidal estuary. Advanced instrumentation packages, including mechanical sensors, acoustic Doppler current profiler (ADCP), and velocimeter (ADV), were implemented to measure the tidal current velocities in the approaching flow, to estimate the turbine performance and to assess the effect of turbulence on power production. The optimal performance was found to be relatively high (Cp ∼ 0.4). Analysis of the power time history revealed a large increase in magnitude of power fluctuations caused by turbulence as the flow velocity increases between 1 and 1.2 m/s. Turbulence intensity does not alone capture quantitative changes in the turbulent regime of the real flow. The standard deviation of velocity fluctuations was preferred in assessing the effect of turbulence on power production. Assessing the scaling properties of the turbulence, such as dissipation rate, ε, the integral lengthscale, L, helped to understand how the turbulence is spatially organized with respect to turbine dimensions. The magnitude of power fluctuations was found to be proportional to L and the strongest impact of turbulence on power generation is achieved when the size of turbulent eddies matches the turbine size.
... The full rotor simulations require high quality grids and large computational resources, so in calculations, the generalized actuator models are usually used to simplify grids and reduce the computational costs. Another less common and less studied numerical method is the vortex particle method [23,24]. Just like CFD, it is a universal method with large computational requirements. ...
... Rðj; zÞ ¼ Rðj þ 2p; zÞ; (23) Initial conditions: ...
... In an effort to expand renewable energy extraction to tidal and fluvial environments, in-stream river turbines have been designed and tested in recent years both at the prototype scale [1][2][3] and at the laboratory scale in straight [4][5][6] and meandering channels [7]. The devices, usually referred to as marine hydrokinetic (MHK) turbines or current energy converters, have various shapes, efficiency, deployment strategies, and anchoring systems (see, e.g., [8][9][10][11][12], among others). ...
... Expressing Eq. (12) in terms of U a and using the definitions y t = k t D and U d = (1 − a)U leads to the following: ...
A modeling framework is derived to predict the scour induced by marine hydrokinetic turbines installed on fluvial or tidal erodible bed surfaces. Following recent advances in bridge scour formulation, the phenomenological theory of turbulence is applied to describe the flow structures that dictate the equilibrium scour depth condition at the turbine base. Using scaling arguments, we link the turbine operating conditions to the flow structures and scour depth through the drag force exerted by the device on the flow. The resulting theoretical model predicts scour depth using dimensionless parameters and considers two potential scenarios depending on the proximity of the turbine rotor to the erodible bed. The model is validated at the laboratory scale with experimental data comprising the two sediment mobility regimes (clear water and live bed), different turbine configurations, hydraulic settings, bed material compositions, and migrating bedform types. The present work provides future developers of flow energy conversion technologies with a physics-based predictive formula for local scour depth beneficial to feasibility studies and anchoring system design. A potential prototype-scale deployment in a large sandy river is also considered with our model to quantify how the expected scour depth varies as a function of the flow discharge and rotor diameter.
... Experimental and numerical approaches have been undertaken to study the wake development of turbine models [7,8] to ensure a good way of installing tidal farms. Ref. [9] displayed the influence of the turbulent intensity on Horizontal Axis Tidal Turbine (HATT) behaviour. ...
Horizontal-axis turbines have been well-studied; however, there is a serious lack of information on the behaviour of vertical-axis turbines under unsteady operating conditions. Among unsteady flows, waves can cause significant mechanical fatigue and modify the flow downstream of the tidal turbines. Consequently, this paper aims to characterize the effects of waves on the hydrodynamic performance and wake development of a 1/20 scale model of a ducted twin vertical axis 1 MW-rated demonstrator. Power measurements were taken from the turbine and the velocity measurements downstream of the machine using a three-component Laser Doppler Velocimeter. The results show that, in the presence of waves, the mean wake characteristics present greater average height and width compared to the current-only condition. Moreover, the wake recovery happens faster downstream due to the sheared wake region homogenization, induced by the presence of higher intensity vortices. Through the Turbulence Kinetic Energy estimation, we also observe some increased fluctuations around the turbine and close to the free surface due to the presence of waves.
... LOMC-Vortex contributed by LOMC employs a 3D unsteady Lagrangian Vortex Particle Method code, Dorothy, which represents the flow as a system of vorticity carrying particles [38]. The code can account for turbulent inflow and multi-wake interaction [39]. ...
This paper presents the first blind prediction stage of the Tidal Turbine Benchmarking Project being conducted and funded by the UK's EPSRC and Supergen ORE Hub. In this first stage, only steady flow conditions, at low and elevated turbulence (3.1%) levels, were considered. Prior to the blind prediction stage, a large laboratory scale experiment was conducted in which a highly instrumented 1.6m diameter tidal rotor was towed through a large towing tank in well-defined flow conditions with and without an upstream turbulence grid. Details of the test campaign and rotor design were released as part of this community blind prediction exercise. Participants were invited to use a range of engineering modelling approaches to simulate the performance and loads of the turbine. 26 submissions were received from 12 groups from across academia and industry using solution techniques ranging from blade resolved computational fluid dynamics through actuator line, boundary integral element methods, vortex methods to engineering Blade Element Momentum methods. The comparisons between experiments and blind predictions were extremely positive helping to provide validation and uncertainty estimates for the models, but also validating the experimental tests themselves. The exercise demonstrated that the experimental turbine data provides a robust data set against which researchers and design engineers can test their models and implementations to ensure robustness in their processes, helping to reduce uncertainty and provide increased confidence in engineering processes. Furthermore, the data set provides the basis by which modellers can evaluate and refine approaches.
... Researchers use simulation models to predictively evaluate and validate marine turbine design and performance in site-specific locations (Nelson et al. 2018), such as simulating the turbulence flows in the river-, estuary-, coastal-, and ocean-scale systems (Peng et al. 2011;Tuckey et al. 2006;Ji et al. 2001). The simulation outcomes require validation using laboratory experiments, analytical solutions, and real flow systems (James et al. 2006(James et al. , 2013Pinon et al. 2012). ...
Generating sustainable energy from marine currents using marine turbines garners much attention in recent years. Assessments of marine turbine arrays require computationally expensive and very large domain simulations. This paper proposes a framework based on a surrogate model approach paired with optimization algorithms to calibrate the adjustable parameters value of the simulator and minimize the deviation between the simulation outputs and the physical experiment results. We find that the application of more advanced surrogate models and optimization techniques improved performance by 16.97% compared to the previous approach. We identify an easy-to-implement opportunity to further improve the performance. Based on descriptive statistics, we design a visual tool that evaluates the quality of sample data quickly and easily.
... where u is the velocity field, ω = ∇ ∧ u is the vorticity field and ν is the kinematic viscosity. A first adaptation of this numerical method to the tidal energy sector is presented in [9] using a panel integral method to represent the turbine blades. For the present work, a lifting line will be preferred as a representation of the blades in order to have a better evaluation of the turbine torque and thrust force together with a more rapid calculation cost. ...
This paper presents computations of a four tidal turbine array, with a row of three upstream devices and a downstream turbine. The studied configuration is based on the layout initially proposed in the framework of the NEPTHYD project. The simulations were carried out with a three-dimensional unsteady Lagrangian Vor-tex software. A synthetic eddy method is used to take into account the ambient turbulence encountered in tidal energetic sites. The loads perceived by the turbines are estimated with a lifting line approach, recently added in the software. To study the interaction between the upstream and downstream turbines, numerical velocity maps, wake lines, unsteady velocity variations obtained with numerical probes as well as fluctuations of power and thrust coefficients are presented.
... Souza de Cursi) ence of obstacles or other turbines. To avoid this problem, other approaches have been developed such as the Vortex Lattice Method (VLM) in Pinon et al. (2012). Their focus is the wake of the turbine to study the interaction between two or more turbines (for example in Mycek et al. (2013)). ...
In this article, we investigate the movement and vibrations of a blade due to the presence of the mast. When the blade passes in front of the mast, a sudden pressure spike induces vibrations in the blade. To study the influence of stiffness, two different structures were studied. We present our numerical schemes concerning the resolution of the flow, the behavior of the structure and the coupling of the two systems. Then, we validate two methods against an experiment (Bahaj et al., 2007). In a third section, we present cases of fluid-structure interaction. Several structures are setup by modifying the stiffness of the material. Their steady open-water (without a mast) behaviors are compared. And finally, two dynamic fluid-structure computations are performed to compare the behavior of an elastic blade passing next to a mast. For all the cases, we use K–FSI developed by K-Epsilon to solve the fluid-structure interaction (FSI).
... However, this method does not include any wake effect from the freestream or bounded flow [56]. Vortex-lattice and vortex-particle methods [57,58] may be used to describe the wake vorticity in concentrated sheets of particles. Computational Fluid Dynamics (CFD) simulations make use of generalized actuator disk (AD) or direct geometry modelling techniques to model complex external effects and more accurately retain fluid structure and wall effects [23]. ...
Computational fluid dynamics is employed for detailed prediction of the hydrokinetic turbine performance and wake modelling. Of these, Reynolds-averaged Navier-Stokes (RANS) models are most widely used due to their ability to resolve power performance and detailed flow features at relatively low computational costs and acceptable accuracy. The limitations of these models are often not well understood when applied to complex turbine and wake dynamics which could lead to potential inaccurate and inappropriate conclusions. This paper focuses on the prediction of the wake generation, dissipation and flow recovery using commercially available modelling software. The approach and findings of previous numerical investigations on this matter are reviewed and compared to experimental measurements reported for a dual-rotor reference turbine. The shortcomings of these models are discussed and appropriate modelling techniques for the preliminary design or analysis of hydrokinetic turbines and inland energy generation schemes are identified. Commercially available RANS models show a good correlation of turbine performance. However, prediction of the wake behaviour is improved by using a virtual disk model with the blade element momentum theory, employing Reynolds stress closure models. These models allow for modelling the anisotropic conditions in the wake unlike the more popular eddy viscosity models. In addition, simplified rotor geometry models using blade element momentum theory are found to adequately model wake development and dissipation at a modest computational expense. The shortcomings of other approaches in terms of wake dissipation prediction and the effect of boundary and inflow conditions are analysed, emphasizing the importance of correct prescriptions of model parameters.
... Numerical models are therefore useful to estimate array efficiency under more realistic flow conditions. This can be achieved with sophisticated blade-resolved models [8], blade element momentum theory [9], or vortex models [10], but also with more simplified turbine representation based on the actuator disk (AD) concept. This modelling approach consists of representing individual turbines as disks that apply the thrust experienced by turbines on the fluid. ...
Assessing the efficiency of a tidal turbine array is necessary for adequate device positioning and the reliable evaluation of annual energy production. Array efficiency depends on hydrodynamic characteristics, operating conditions, and blockage effects, and is commonly evaluated by relying on analytical models or more complex numerical simulations. By applying the conservations of mass, momentum, and energy in an idealized flow field, analytical models derive formulations of turbines’ thrust and power as a function of the induction factor (change in the current velocity induced by turbines). This simplified approach also gives a preliminary characterization of the influence of blockage on array efficiency. Numerical models with turbines represented as actuator disks also enable the assessment of the efficiency of a tidal array. We compare here these two approaches, considering the numerical model as a reference as it includes more physics than the analytical models. The actuator disk approach is applied to the three-dimensional model Telemac3D in realistic flow conditions and for different operating scenarios. Reference results are compared to those obtained from three analytical models that permit the investigation of the flow within tidal farm integrating or excluding processes such as the deformation of the free surface or the effects of global blockage. The comparison is applied to the deployment of a fence of turbines in the Alderney Race (macro-tidal conditions of the English Channel, northwest European shelf). Efficiency estimates are found to vary significantly from one model to another. The main result is that analytical models predict lower efficiency as they fail to approach realistically the flow structure in the vicinity of turbines, especially because they neglect the three-dimensional effects and turbulent mixing. This finding implies that the tidal energy yield potential could be larger than previously estimated (with analytical models).
... Therefore, it is used in various fluid mechanics domains as animal locomotion modelization or aerodynamic flight [3]. This model is also used in marine renewable energy simulations by [8] and [9]. ...
The undulating membrane tidal energy converter is a device that uses the flutter instabilities occurring from the interaction between a slender body and a fluid flow. A new numerical model has been developed using a 2D corotational finite element method to represent the structure and the unsteady point-vortex method to compute the flow. These methods as well as the interaction process are presented. Trajectory and frequency of the undulating motion, hydrodynamic forces on the structure and velocity field in the wake are presented. Comparison shows a good agreement with experimental results obtained from a 1/20th scale prototype without power take off tested in flume tank.
... These turbine blades are considered as infinitely thin blades represented by normal dipoles where a free slip velocity condition is applied. Further details of this vortex particle formulation are given in Pinon et al. [15,14]. These computations were run with a mesh discretisation of dh/R = 0.072, an inter-particle distance of approximately dh and a smoothing parameter for the particle-particle interactions of 1.5 dh. ...
This paper presents possible techniques for modelling ambient turbulence in the Lagrangian Vortex Method formalism. Due to the fact that regular Synthetic Eddy Method (SEM) already presented in previous studies is not divergence free by definition; improvements were necessary to develop a similar SEM method with such a divergence free property. The recent improvements formulated by R. Poletto give the way to such a possibility. This new Divergence Free Synthetic Eddy Method (DFSEM) is presented here in comparison with the regular SEM. Obtained numerical velocity fields are compared in terms of convergence properties, Power Spectral Density and also Taylor macro-scale. Finally, turbine wakes are computed with both the recent Poletto’s DFSEM and the regular Jarrin’s SEM to highlight differences. At this stage of development, the DFSEM seems very promising even though some improvements are still necessary.
... The synthetic eddy method (SEM) has also been used to develop turbulent conditions for LES [66]. This was integrated into 3D Vortex Method software [67], [68], and demonstrated to represent wake behaviour. ...
Enabling Future Arrays in Tidal (EnFAIT) is an EU Horizon 2020 flagship tidal energy project. It aims to demonstrate the development, operation and decommissioning of the world’s largest tidal array (six turbines), over a five-year period, to prove a cost reduction pathway for tidal energy and confirm that it can be cost competitive with other forms of renewable energy. To determine the optimal site layout and spacing between turbines within a tidal array, it is essential to accurately characterise tidal turbine wakes and their effects. This paper presents a state-of-the-art review of tidal turbine wake modelling methods, with an overview of the relevant fundamental theories. Numerical and physical modelling research completed by both academia and industry are considered to provide an overview of the contemporary understanding in this area. The scalability of single device modelling techniques to an array situation is discussed, particularly with respect to wake interactions.
... BEMT is a good approach to assess the open water performance of one turbine, but it fails to accurately simulate the performance in the presence of obstacles or other turbines. To avoid this problem, other approaches has been developed such as the Vortex Lattice Method (VLM) in Pinon et al. (2012). Their focus is the wake of the turbine to study the interaction between two or more turbines (for example in Mycek et al. (2013)). ...
In this article, we investigate the movement and vibrations of a blade due to the presence of the mast. When the blade goes past the mast, a sudden pressure spike induces vibrations in the blade. To study the influence of the stiffness, two different structures have been investigated. We present our numerical methodology concerning solving the flow, the structure behavior and the coupling of the two systems. Then, we validate two methods against an experiment (Bahaj et al., 2007). In a third section, we present fluid-structure interaction cases. Several structures are setup by modifying the stiffness of the material. Their steady open-water (without a mast) behaviors are compared. And finally, two dynamic fluid-structure are performed to compare the behavior of an elastic blade passing next to a mast. For all the cases, we use K-FSI developed by K-Epsilon to solve the fluid-structure interaction (FSI) effects.
... L'approche lagrangienne est en général utilisée pour calculer le déplacement de ces tourbillons. La simulation 3D instationnaire réalisée par Pinon et al. [41] pour une hydrolienne à axe horizontal donne des résultats très encourageants. ...
Des projets d'installation de parcs de plusieurs hydroliennes rapprochées en rivières ou dans les océans ont été récemment démarrés, afin de développer cette source d'énergie renouvelable. Dans ces parcs, les interactions de sillage entre les hydroliennes doivent être calculées puisqu'elles peuvent affecter leur puissance produite. Un modèle CFD stationnaire de type disque d'action couplé aux équations RANS est développé dans ce travail pour calculer la puissance produite et l'écoulement au sein d'un parc d'hydroliennes Darrieus. Ce modèle utilise des répartitions détaillées de force dont l'intensité dépend de la position sur la turbine. Elles sont obtenues par des calculs préliminaires URANS de l'écoulement sur la géométrie de la turbine en rotation. De nouvelles lois sont obtenues pour les coefficients de puissance et de force en utilisant la vitesse locale (vitesse au niveau de la turbine) au lieu de la vitesse amont dans leur définition. Ces coefficients deviennent alors indépendants du confinement de la turbine. Ces lois servent à construire un modèle qui calcule les distributions de force représentant chaque turbine du parc en fonction de la vitesse locale du fluide, pour simuler chaque turbine fonctionnant proche de son point de maximum d'efficacité. Une validation du modèle est réalisée par comparaison à de nouvelles expériences d'une turbine Darrieus à échelle réduite. Différentes configurations de parcs sont ensuite simulées par le modèle 3D, ainsi que par une version 2D du modèle. Les distances entre turbines qui permettent d'obtenir une puissance produite par le parc maximale sont notamment recherchées.
... During the last decade, numerous studies were conducted to investigate turbulence and hydrodynamic interactions between turbines and the flow. However, much of the research has been carried out numerically (e.g., [2,10,11]) or at the laboratory scale under simplified and controlled conditions (e.g., [7,12,13]). Turbulence characterization in a real flow is considered to be of prime importance for, (i), tidal energy site assessment and, (ii), improving the ability, and thus the confidence, of shelf sea circulation models in reconstructing the flow dynamics. ...
Velocity measurements collected by an upward-looking acoustic Doppler current profiler were used to provide the first study of ambient turbulence in Alderney Race. Turbulence metrics were estimated at mid-depth during peak flooding and ebbing tidal conditions. The dissipation rate ε and the integral lengthscale (L) were estimated using two independent methods: the spectral method and the structure function method. The spectral method provided ε and (L) estimates with standard deviations twice lower than that obtained from the structure function method. Removal of wave and Doppler noise-induced bias when estimating the dissipation rate was shown to be a crucial step in turbulence characterization. It allowed for a significant refining in (L) estimates derived from the spectral and structure function methods of 35% and 20% respectively. The integral lengthscale was found to be 2–3 times the local water depth. It is considered that these findings could be valuable for current turbine designers, helping them optimizing their designs as well as improving loading prediction through the lifetime of the machines.
... Son principal avantage est qu'elle ne nécessite pas un maillage de l'écoulement. Elle est utilisée pour la simulation LES d'écoulements turbulents dans les domaines hydrolien [46], [47] et éolien [48], [49]. ...
Le développement des énergies renouvelables passe par l’exploitation de nouvelles sources d’énergie. La filière hydrolienne, dédiée à la récupération de l’énergie des courants de marée, est proche de l’industrialisation. Cependant, les conditions hydrodynamiques turbulentes des sites hydroliens sont encore mal connues. Cette thèse propose d’examiner à l’échelle locale l’effet des rugosités du fond marin sur la génération de tourbillons hautement énergétiques par la simulation numérique en mécanique des fluides de type méthode de Boltzmann sur réseau. Cette méthode est particulièrement adaptée à la simulation d’écoulements instationnaires sur un domaine de simulation complexe. Dans un premier temps, les phénomènes physiques de détachements tourbillonnaires sur des macro-rugosités canoniques sont décrits. L’appariement de structures tourbillonnaires est mis en évidence dans le processus de formation de tourbillons hautement énergétiques. Dans un deuxième temps, la simulation permet d’observer de tels phénomènes dans le cas d’écoulements environnementaux intégrant une bathymétrie réelle. Ces simulations, validées par rapport à des mesures in situ, mènent à une meilleure compréhension des effets du fond marin sur la turbulence en milieu hydrolien. En particulier, l’importance des failles géologiques dans la génération de turbulence dans la zone d’étude est mise en évidence.
... The Vorticity equation was solved with the finite volume technique. Additionally, in the study carried out in [84] the wake vorticity downstream an HAHT in a uniform flow was represented by a Lagrangian approach. Summarizing, the main facts regarding the dynamics of vorticity found in these two investigations are the following: blade tip vortex generates an helical structure behind the rotor which propagates downstream which, eventually, disintegrate in pieces; such vortical fragments interact with the flow near the bed and are lifted resulting in an inclined helical structure further downstream; such fragmentation is responsible for the appearance of small scale unsteady structures beyond the rotor near wake in an area formerly considered not influenced by the turbine. ...
Hydrokinetic energy conversion devices provide the facility to capture energy from water flow without the need of large dams, impoundments, channels or deviation of the water as in conventional hydroelectric centrals. Hydrokinetic systems are intended to be used in streams, either natural (rivers, estuaries, marine currents) or artificially built channels. This article reviews the advances made over the last 10–15 years regarding the three-dimensional computational fluid dynamics modeling and simulation of this type of turbines. Technical aspects of model design, employed boundary conditions, solution of the governing equations of the water flow through the hydrokinetic turbine and assumptions made during the simulations are thoroughly described. We hope that this review will encourage new computational investigations about hydrokinetic turbines that contribute to their continuous improvement, development and implementation aimed to sustainable use of water resources and addressed to solve the problem of lack of electricity supply in small, isolated populations.
... Myers and Bahaj [14] demonstrated that increased velocity deficits and turbulence exist along the rotor centerline. Pinon et al. [15] used "vortex method" to simulate the flow field of horizontal-axis tidal current turbine. The "vortex method" was a velocity-vorticity numerical implementation of the Navier-Stokes equations to compute an unsteady evolution of the turbine wake by some threedimensional software. ...
The temporal evolution of seabed scour was investigated to prevent damage around a monopile foundation for Darrieus-type tidal current turbine. Temporal scour depths and profiles at various turbine radius and tip clearances were studied by using the experimental measurements. Experiments were carried out in a purpose-built recirculating water flume associated with 3D printed turbines. The scour hole was developed rapidly in the initial process and grew gradually. The ultimate equilibrium of scour hole was reached after 180 min. The scour speed increased with the existence of a rotating turbine on top of the monopile. The findings suggested that monopile foundation and the rotating turbine are two significant considerations for the temporal evolution of scour. The scour depth is inversely correlated to the tip-bed clearance between the turbine and seabed. Empirical equations were proposed to predict the temporal scour depth around turbine. These equations were in good agreement with the experimental data.
... Certaines méthodes vortex ne nécessitent pas de connaissanceparticulì ere des performances des pales. Par exemple, Pinon et al. (2012 [12] ont réalisé la modélisation LES d'une hydrolienné echelle 1 : 1 de 18.3 m de diamètre avec de la turbulence amont, figure 1.14. Cette modélisation est validée avec le coefficient de puissance et permet d'accéder accéder`accéderà des informations comme le moment de flexion des pales. ...
Dans un contexte mondial où l’accès à l’énergie est un problème de premier plan, l’exploitation des courants de marée avec des hydroliennes revête un intérêt certain. Les écoulements dans les zones à fort potentiel énergétique propices à l’installation d’hydroliennes sont souvent fortement turbulents. Or la turbulence ambiante impacte fortement l’hydrodynamique avoisinante et le fonctionnement de la turbine. Une prédiction fine de la turbulence et du sillage est fondamentale pour l'optimisation d'une ferme d'hydroliennes. Un modèle de simulation de l'écoulement autour de la turbine doit donc être précis et tenir compte de la turbulence ambiante. Un outil basé sur la méthode de Boltzmann sur réseau (LBM) est utilisé à ces fins, en association avec une approche de simulation des grandes échelles (LES). La LBM est une méthode instationnaire de modélisation d’écoulement fluide. Une méthode de génération de turbulence synthétique est implémentée afin de prendre en compte la turbulence ambiante des sites hydroliens. Les géométries complexes, potentiellement en mouvement, sont modélisées avec la méthode des frontières immergées (IBM). La mise en place d’un modèle de paroi est réalisée afin de réduire le cout en calcul du modèle. Ces outils sont ensuite utilisés pour modéliser en LBM-LES une hydrolienne dans un environnement turbulent. Les calculs, réalisés à deux taux de turbulence différents, sont comparés avec des résultats expérimentaux et des résultats NS-LES. Les modélisations LBM-LES sont ensuite utilisées pour analyser le sillage de l'hydrolienne. Il est notamment observé qu'un faible taux de turbulence impacte de manière significative la propagation des tourbillons de bout de pale.
... The synthetic eddy method (SEM) has also been used to develop turbulent conditions for LES [66]. This was integrated into 3D Vortex Method software [67], [68], and demonstrated to represent wake behaviour. ...
Enabling Future Arrays in Tidal (EnFAIT) is an EU Horizon 2020 flagship tidal energy project. It aims to demonstrate the development, operation and decommissioning of the world’s largest tidal array (six turbines), over a five-year period, to prove a cost reduction pathway for tidal energy and confirm that it can be cost competitive with other forms of renewable energy. To determine the optimal site layout and spacing between turbines within a tidal array, it is essential to accurately characterise tidal turbine wakes and their effects. This paper presents a state-of-the-art review of tidal turbine wake modelling methods, with an overview of the relevant fundamental theories. Numerical and physical modelling research completed by both academia and industry are considered to provide an overview of the contemporary understanding in this area. The scalability of single device modelling techniques to an array situation is discussed, particularly with respect to wake interactions.
... The first approach applies a uniform force to the flow, and it is a computationally inexpensive alternative that allows multiple devices simultaneously, but lose the representation of important elements like rotor swirl (Nguyen et al., 2016;Batten et al., 2013;Blackmore et al., 2014). The latter ALMs and models that consider the detailed geometry of the device improve the turbine representation, but require considerably more computational resources, due to computationally expensive interpolations to the grid nodes and therefore have not been used for large turbine arrays, even though they can resolve near-field features and unsteady vortical structures generated by the turbine geometry (Creech et al., 2017;Afgan et al., 2013;Lloyd et al., 2014;Churchfield et al., 1985;Pinon et al., 2012;Kang et al., 2014;Chawdhary et al., 2017). ...
As tidal and hydrokinetic energy systems develop, new tools are needed to assess quantitatively the effects of turbines on the environment and to estimate their performance. When installed in an array, turbine wakes interact, increasing the complexity of the flow and changing their performance. Experimental and numerical approaches have been employed to analyze flows with multiple turbines, but it is not yet clear which level of detail is necessary to represent the flow hydrodynamics and the details of the devices. In numerical approaches, questions remain on the selection of turbulence models and turbine representations, since more realistic but computationally expensive methodologies not necessarily produce an improvement on the understanding of these flows. In this investigation we perform simulations of turbine arrays to study the hydrodynamics of wakes and their interactions, comparing with experiments and previous simulations. We propose a methodology that couples detached-eddy simulations (DES) with Blade Element Momentum (BEM), showing that by capturing the dynamically-rich coherent structures of the flow, we improve the description of mean quantities and turbine performance. The results show that for downstream turbines, there is an accelerated wake development, increasing the temporal variability of the bed shear stress, and the power and thrust coefficients.
... Faced to difficulties in characterizing the turbulent properties of the flow in a broad scale, analysis of a possible link between the "local turbulence strength" and tidal energy conversion device performance is often done using a theoretical framework of energy multifractal cascades [2], experimental approach in a flume tank [3,4], and modelling approach [5,6]. ...
A vertical axis tidal turbine (VATT) of the Dutch company "Water2Energy", mounted on a surface platform , was tested during several weeks in real sea conditions in a tidal estuary-the Sea Scheldt, Belgium. Velocity measurements were performed to estimate the major turbulent properties of the tidal flow at the experimental site, to evaluate the tidal turbine performance, and to quantify the effect of turbulence on the output power. The optimal performance (C p) of the turbine was founded to be 0.42 in the velocity range 1.1−1.2 m/s. Results show that the dissipation rate, ε, and turbulent strength, σ u are tightly related with the magnitude of the output power variability. It was demonstrated that tidal turbine responds more actively to turbulence on scales similar to the rotor diameter.
... However, the vortex methods used at IFPEN are limited to inviscid flows or flows with artificial viscosity from empirical laws. Still, viscosity can be introduced in such methods (see Chatelain et al., [8] and Pinon et al., [24]), but it increases both computational time and complexity. In the present work, a modern approach is adopted: the Lattice Boltzmann Method. ...
This paper presents the development of a solver based on a Lattice Boltzmann Method (LBM) to perform reliable wind turbine simulations: LaBoheMe. LBM offers an interesting framework with low dissipation and high performance computing compared to usual Computational Fluid Dynamics (CFD) solvers based on Navier-Stokes equations. In order to take wind turbines into account in the LBM solver, LaBoheMe is enhanced by the addition of local force terms representing the influence of the wind turbines on the flow. These forces are computed using an Actuator-Line Model (ALM). The present solver is a 2D LBM/ALM solver allowing the simulation of "slices" of a Vertical Axis Wind Turbine (VAWT).
The Lattice Boltzmann and the Actuator Line methods are described and the coupling adopted in this work is detailed. Validation cases for pure LBM (Backward Facing Step, BFS) and coupled LBM/ALM are performed. The LBM/ALM coupling is validated against experimental data and compared with a vortex method and a large eddy simulation of the wake. Both blade forces and wake velocities are considered.
... BEMT is a good approach to assess the performance of one turbine, but it fails to perform for multiple turbines. To avoid this problem, other approaches has been developed such as the Vortex Lattice Method (VLM) in [8]. Their focus is the wake of the turbine to study the interaction between two or more turbines [9]. ...
In this article, we aim at providing results of fluid-structure interaction between water turbine blades and its mast. When the blade goes past the mast, a sudden pressure spike is recording, and the acceleration of the blade is recorded. Many results are provided, and two different structures are compared. At first, we validate our model against an experiment (Bahaj et al. [1]) as in [2]. This work is the direct continuation of [2] where many results of Computational Fluid Dynamics (CFD) were described. A methodology for estimating accurately the performance of a turbine for open water cases with CFD tools was outlined. The goal is now to validate those results for full cases (i.e. including the static parts) and including fluid-structure interaction effects. First, we validate the fluid only in dynamic mode. Then, several structures are setup by modifying the stiffness of the material and their behaviors are compared. We use K-FSI developed by K-Epsilon to solve the dynamic problem and the Quasi-Static Problem.
... On the other hand, Thiebot et al [16] developed the effects of large arrays of tidal turbines with dept-average Actuator Disks by modelling. Pinon et al [17] predicted wake of marine current turbines with a particle method by using the numerical simulation. Whereas, Elhanafi et al [18] used the numerical simulation with CFD to analyzing the offshore stationaryfloating oscillating water column-wave energy converter. ...
The paper presents validation of the numerical program that computes the distribution of marine current velocities in the Bangka strait and the kinetic energy potential in the form the distributions of available power per area in the Bangka strait. The numerical program used the RANS model where the pressure distribution in the vertical assumed to be hydrostatic. The 2D and 3D numerical program results compared with the measurement results that are observation results to the moment conditions of low and high tide currents. It found no different significant between the numerical results and the measurement results. There are 0.97-2.2 kW/m² the kinetic energy potential in the form the distributions of available power per area in the Bangka strait when low tide currents, whereas when high tide currents of 1.02-2.1 kW/m². The results show that to be enabling the installation of marine current turbines for construction of power plant in the Bangka strait, North Sulawesi, Indonesia.
... This is particularly suitable for the caudal fin-shaped blade because of the increasing concave bend and thinness at the tip and thus, should be dimensioned to have sufficient strength at the tip of the blade. The most important design variables that affect the overall efficiency of the horizontal axis tidal turbine system are chord lengths of airfoils, twist distribution, the overall height of the blade, the angle of attack, the angular velocity of the blade, blade material, and the fluid velocity acting on the blade (Afgan et al., 2013;Betz, 2014;Clancy, 1975;Mycek et al., 2013;Pinon et al., 2012). The design variables selected to move the straight blade towards the caudal finshaped blade are airfoil chord lengths, the total blade height, and twist angle distribution; however, the other important design variables that construct a HATT are outside of the scope of this paper. ...
A design study was conducted to understand the implications of bio-mimicking a curved caudal fin type horizontal axis tidal turbine blade design, using National Advisory Committee for Aeronautics (NACA) 0018 is presented. A method of transforming the traditional horizontal axis tidal turbine by defining a third order polynomial center line on which the symmetrical airfoils were stationed is also disclosed. Each of the airfoil characteristics: twist angle distribution, chord lengths, and a center line passing through the airfoil centers were automatically transformed to create the curved caudal fin-shaped blade; translating the spinal blade axis into percentage wise chord lengths, using NACA 0018 airfoil. A 3D mesh independency study of a straight blade horizontal axis tidal turbine modeled using computational fluid dynamics (CFD) was carried out. The grid convergence study was produced by employing two turbulence models, the standard k-ε model and shear stress transport (SST) in ANSYS CFX. Three parameters were investigated: mesh resolution, turbulence model, and power coefficient in the initial CFD, analysis. It was found that the mesh resolution and the turbulence model affect the power coefficient results. The power coefficients obtained from the standard k-ε model are 15% to 20% lower than the accuracy of the SST model. Further analysis was performed on both the designed blades using ANSYS CFX and SST turbulence model. The results between the straight blade designed according to literature and the caudal fin blade showed a maximum power coefficient of 0.4028%, and 0.5073% respectively for 2.5m/s inlet velocity.
... On the other hand, Thiebot et al [16] developed the effects of large arrays of tidal turbines with dept-average Actuator Disks by modelling. Pinon et al [17] predicted wake of marine current turbines with a particle method by using the numerical simulation. Whereas, Elhanafi et al [18] used the numerical simulation with CFD to analyzing the offshore stationary-floating oscillating water column-wave energy converter. ...
The paper presents validation of the numerical program that computes the distribution of marine current velocities in the Bangka strait and the kinetic energy potential in the form the distributions of available power per area in the Bangka strait. The numerical program used the RANS model where the pressure distribution in the vertical assumed to be hydrostatic. The 2D and 3D numerical program results compared with the measurement results that are observation results to the moment conditions of low and high tide currents. It found no different significant between the numerical results and the measurement results. There are 0.97-2.2 kW/m 2 the kinetic energy potential in the form the distributions of available power per area in the Bangka strait when low tide currents, whereas when high tide currents of 1.02-2.1 kW/m 2. The results show that to be enabling the installation of marine current turbines for construction of power plant in the Bangka strait, North Sulawesi, Indonesia.
... Suitable instrumentation and techniques of measuring turbulent properties optimally is not clearly identified yet. For this reason, the characterization of the turbulence in a broad scale and analysis of a possible link between the « local turbulence strength » and tidal energy conversion device performance is often done using a theoretical framework of energy multifractal cascades [2], experimental approach in a flume tank [3], [4], and modelling approach [5], [6]. ...
The aim of this paper is to compare two different computational methods that analyse the flow around wind turbine using Large-Eddy-Simulations (LES). The first method uses a three-dimensional unsteady Lagrangian Vortex Particle method (VP) associated to a lifting-line (LL) approach providing radial loads, integrated thrust and power coefficients, circulations and angle of attack across the turbine blades. The second method solves the incompressible Navier-Stokes equations with the classical Finite Volume (FV) method coupled to the Actuator Line (AL) method to model the rotor blades. Both methods are compared by means of a benchmark consisting in a single full-scale NREL5MW wind turbine and the results are thus compared to Martínez-Tossas et al (2018). The comparisons involve loads, angle of attack and velocity along the blade. Wakes downstream of the turbine are analysed via the evolution of flow fields as mean velocity and vorticity. Results are found to be in good agreement with the verification case. A comparison is also performed on the evaluation of the numerical cost, precision and efficiency of both computational methods.
The current study presents numerical results on three tidal turbine models (two in front, one downstream) interacting in a turbulent upstream flow. The numerical results come from a lifting-line (LL) embedded in a Lagrangian vortex particle (VP) solver: Dorothy LL-VP. The objective is to assess the extent to which this numerical tool is suited to reproduce accurately wakes interaction as well as fluctuating loads perceived by the downstream turbine. To this aim, the numerical set-up reproduces an experimental campaign led at IFREMER’s wave and current flume tank. The downstream turbine is placed at different positions to change the wakes interaction. Two upstream turbulence intensities (TI) experimentally tested are reproduced numerically using the synthetic eddy method (SEM). Favourable comparisons are obtained between numerical and experimental wakes, including velocity profiles. Preliminary results suggest that the downstream turbine performance decrease is numerically well captured. More investigations are needed on the loads fluctuations with longer computation time, and adding an angular velocity controller as well as hub modelling to Dorothy LL-VP.
This paper presents a lifting‐line implementation in the framework of a Lagrangian vortex particle method (LL‐VP). The novelty of the present implementation lies in the fluid particles properties definition and in the particles shedding process. In spite of mimicking a panel method, the LL‐VP needs some peculiar treatments described in the paper. The present implementation converges rapidly and efficiently during the shedding sub‐iteration process. This LL‐VP method shows good accuracy, even with moderate numbers of sections. Compared to its panel or vortex filaments counterparts, more frequently encountered in the literature, the present implementation inherently accounts for the diffusion term of the Navier‐Stokes equations, possibly with a turbulent viscosity model. Additionally, the present implementation can also account for more complex onset flows: upstream ambient turbulence and upstream turbine wakes. After validation on an analytical elliptic wing configuration, the model is tested on the Mexnext‐III wind turbine application, for three reduced velocities. Accurate results are obtained both on the analytical elliptic wing and on the New MEXICO rotor cases in comparison with other similar numerical models. A focus is made on the Mexnext‐III wake analysis. The numerical wake obtained with the present LL‐VP is close to other numerical and experimental results. Finally, a last configuration with three tidal turbines in interaction is considered based on an experimental campaign carried out at the IFREMER wave and current flume tank. Enhanced turbine‐wake interactions are highlighted, with favourable comparisons with the experiment. Hence, such turbine interactions in a farm are accessible with this LL‐VP implementation, be it wind or tidal energy field.
Hydrokinetic energy is a promising technology to harness predictable renewable energy from free-flowing water, tides and ocean currents. Many studies have been conducted by researchers and engineers to find out ways to enhance the performance of the hydrokinetic turbine. The current paper reports the experimental study of using hydrophobic coating as an alternative way to improve the performance of hydrokinetic turbine. A hydrophobic coating can lower the friction drag of a surface that is in contact with liquid. For hydrokinetic turbine blade, reduction in friction drag may allow a blade section to have a better lift/drag ratio and have its efficiency improved. In this study, a formula to predict the pattern of drag reduction over a hydrophobic surface has been derived. Two hydrophobic coatings were applied on NACA 63418 hydrofoils and their performances were tested. It was found that NACA 63418 hydrofoil with the hydrophobic coatings had its drag reduced by an average of 3%–4.0%. When the coatings were applied on a 350 mm diameter three-bladed turbine, the maximum increment of rotational speed of the turbine was found to be 2.5%. The performance of the two coatings against marine fouling was also investigated. The weight of plate with and without the coatings increased by 10% and 100%, respectively.
The tidal and river in-stream energy resource in the Shannon Estuary (W Ireland) is investigated using of high-resolution numerical modelling and spatial analysis. Although freshwater discharges are large, their influence on the available resource is found to be all but negligible, the tide being the main driver of estuarine circulation. The Tidal Stream Exploitability (TSE) index is adapted to the analysis of estuaries with non-depth-limited areas (TSEndl), such as the Shannon Estuary, and then used to select the hotspots with potential for a tidal stream farm. For this purpose, a new depth penalty-limiting function is defined to avoid overestimating the available energy potential in areas with depths greater than those required for tidal energy converter operation. Seven hotspots are identified based on the revised index. The approach followed in this study illustrates the applicability of high-resolution numerical modelling and spatial analysis for identifying the most appropriate areas for tidal stream energy conversion. Finally, the potential of tidal stream energy to contribute to the much-needed decarbonisation of the energy mix in Ireland is emphasized.
The ocean contains a variety of renewable energy resources, little of which has been exploited. Here we review both tidal range and tidal stream energy, with a focus on the resource, feedbacks, and environmental interactions. The review covers a wide range of timescales of relevance to tidal energy, from fortnightly (spring-neap) and semi-diurnal variability, down to array and device-scale turbulence. When simulating the regional tidal energy resource, and to assess environmental impacts, it is necessary to account for feedbacks between the tidal array and the resource itself. We critically review various methods for simulating energy extraction, from insights gained through theoretical studies of "tidal fences" in idealized channels, to realistic 3D model studies with complex geometry and arrays of turbines represented by momentum sinks and additional turbulence due to the presence of rotors and support structures. We discuss how variability can be reduced by developing multiple (aggregated) sites with consideration of the enhanced phase diversity offered by exploiting less energetic tidal currents. This leads to future research questions that have not yet been explored in depth at first generation tidal sites in relatively sheltered channels (e.g. the interaction of waves with currents). Such enhanced understanding of real sea conditions, including the effects of wind and waves, leads to our other identified primary future research direction -- reduced uncertainties in turbulence predictions, including the development of realistic models that simulate the interaction between ambient turbulence and the turbulence resulting from multiple wakes, and changes to system-wide hydrodynamics, water quality, and sedimentation.
Deployment of tidal array farms is the next stage of tidal energy development which concentrates on the capture of the enormous energy. Characterization into the wake of tidal turbine aids in determining the tidal farm layout and energy yields. This paper develops a framework that employs the Computational Fluid Dynamics (CFD) simulation and machine learning to analyze turbine wake with high accuracy and good efficiency. Multilayer perceptron neural network (MLP-NN) was introduced to establish the interrelation between the incoming flow conditions and the wake profiles. The Reynolds-averaged Navier-Stokes equations (RANS) are associated with a k − ε turbulence model to offer a number of datasets of wake profile for training, testing, and validation of the MLP-NN models. It was found that the MLP–NN–based model has achieved a considerably high level of accuracy by comparing it with empirical and numerical models. The reliability of the MLP-NN based model coupled with the wake combination RSS model to predict the resultant wake profile and power output of multiple turbines are also assessed. The techniques significantly enhance the efficiency and accuracy of wake predictions.
Large-Eddy Simulation is adopted to reproduce the wake of an axial-flow hydrokinetic turbine. The process of momentum recovery is correlated with the destabilization of the wake system, starting wake contraction and radial flows from the free-stream towards the wake core. All terms of the momentum balance equation are analyzed. The radial advection dominates the momentum recovery at the outer radii (in the vicinity of the radial boundary of the wake), just downstream of the instability of the system of tip vortices. Further downstream the reduction of the mean radial gradients makes advection less significant and the radial turbulent transport becomes the main source of momentum replenishment at inner radii, near the wake axis. The details of the process of momentum recovery and its correlation with the instability of the wake system, revealed by the present high-fidelity eddy-resolving computations, provide new insight on the development of the wake of axial-flow hydrokinetic turbines.
Before initiating a study on the interaction of multiple wakes, it is imperative that turbine wake hydrodynamics are studied in isolation. In this paper CFD computer simulations of downstream turbine wakes have been run using a scale-resolving hybrid turbulence model known as a detached eddy simulation. To allow validation of the CFD simulations the computer models were supported by flume measurements with a lab scale tidal stream turbine run at three tip-speed ratios and three turbulence conditions, varying both turbulence intensity and length-scale.
From the study it was demonstrated that turbulence intensity has a significant impact on the wake development for both recovery and width. The turbulence length scales of between 0.25 and 1.0 rotor diameter did not have a significant impact on the wake.
The turbine operating condition also had an impact on the resulting wakes. In the near wake, centreline velocity recovery was found to increase with increasing turbine thrust due to flow being diverted towards the turbine nacelle. For a volumetric averaged wake, greater power extraction was found to cause the greatest near-wake deficit. Wake width was found to increase with increasing tip-speed ratio (and therefore turbine thrust).
The extraction of power from the flow of water has become an important potential source of clean energy. In spite of significant interest in the interaction between energy extraction devices and water currents, comparatively little work has focused on flow asymmetry. Indeed, unusual wake behaviour and limits of turbine array efficiency have typically been attributed to boundary effects rather than the particular turbine geometry. The aim of the present study was to reveal the asymmetries in the hydrodynamic wake and the interactions with the sediment bed due to the presence of a hydrokinetic turbine. We combined: (i) computational fluid dynamics simulations; (ii) optical flow measurements from a series of flume experiments above a fixed rough bed; and (iii) acoustic measurements from a further series of flume experiments above a mobile sand bed. Results showed flow asymmetry due to the presence of the rotor which appeared to be related to the development of the wake and potentially to the gyre of the blades. Suspended sediments in the flume also exhibited asymmetrical characteristics due to the flow asymmetry. This imbalance in the flow field and sediment transport may decrease energy extraction efficiency in turbine arrays and also could have important environmental consequences.
The proposed analytical wake model which is proposed for tidal stream turbines multi-wake predictions. This analytical wake model is developed referring to the classical Jensen wake model, reanalyzing the equations used in predicting the velocity distribution of ship propeller jets, and expressing the wake decay coefficient as a function of the ambient turbulence intensity. Coupled with three types of multi-wake combination models, the reliability of the proposed wake prediction model is evaluated through comparing with experimental and numerical data of two wakes interaction. It is found that the wake deficit predicted by the proposed wake model exhibit pleasant agreement with the numerical results as well as the experimental measurements. The developed model also provides reasonable predictions of power output in the cases five aligned turbines under two different turbulence conditions.
The performance prediction of two counter-rotating vertical axis hydrokinetic turbines is presented in this paper. The flow field is governed by the 3D time-dependent incompressible Navier–Stokes equations. The system of equations is discretized using the Arbitrary Lagrangian-Eulerian Variational Multi-scale formulation for turbulence modeling on moving domains. Sliding interfaces are used to handle the rotor-stator interactions. Weak enforcement of essential boundary conditions is used to relax the requirement of boundary layers resolution. A grid convergence study based on the evaluation of the grid convergence index for the computed torque and the time-averaged axial wake velocity is performed. The grid convergence study shows good convergence with grid refinement in our formulation. Experimental validation of the averaged torque is performed for two different flow conditions and different turbine rotational velocities with good agreement. The finest mesh resolution from the grid convergence study is used for the multiple turbines simulation. The simulation shows almost 25% deficiency in the averaged torque of the downstream turbine. A multi-domain method is introduced to predict the performance of the turbine array at a lower computational cost than the full array simulation. The results from the multi-domain method show good matching on the averaged torque with the full array simulation. The initial study on the effect of struts on the torque generation is presented. The results assure the robustness of the ALE-VMS formulation and how it can be used to simulate multiple turbines at full-scale and full geometric complexity.
Tidal energy usage is continuously increasing among the countries, it offers the availability of renewable energy which is predictable and emits less amount of Greenhouse gases compared to the Thermal power plants and Diesel power plants. Speaking about India itself, Gulf of Khambhat has the potential of producing 5000MW of power. As we know the density of water is higher compared to air, which means a larger amount of Drag force will act directly on the blades of turbine and if we use the same capacity of turbine that of air in water the results will be promising with increased power generation on negligible primary cost. The aim of this project is to design a blade with combination of airfoils and to analyze. We will overcome the situation by decreasing the length of turbine blade and selecting proper hydrofoils, thus decreasing the risk of fatigue failure.
In moderate-depth and relatively shallow offshore sites, bottom-fixed platforms have been widely used for oil and gas development. There are similarities and differences between ship-shaped offshore structures and trading tankers. As for the case of the other offshore renewable energy systems namely: wave energy converters (WECs) and offshore wind turbines (OWTs), the physical modelling of the tidal current turbines is dominated by the modelling of the power take-off (PTO) mechanism of the turbine and the applicability of the similitude laws. Offshore renewable energy systems are expected to significantly contribute in the coming years to reach the energy targets worldwide; the leading technology in offshore renewable energy sector that can be considered mature is that for OWTs. In order to share ocean space, infrastructures and costs in diverse offshore activities such as energy, transport, aquaculture, protection or leisure, multi-purpose offshore structures and systems (MPOSSs) have been developed.
The understanding of interaction effects between marine energy converters
represents the next step in the research process that should eventually lead to
the deployment of such devices. Although some a priori considerations have been
suggested recently, very few real condition studies have been carried out
concerning this issue. Trials were run on 1/30th scale models of three-bladed
marine current turbine prototypes in a flume tank. The present work focuses on
the case where a turbine is placed at different locations in the wake of a
first one. The interaction effects in terms of performance and wake of the
second turbine are examined and compared to the results obtained on the case of
one single turbine. Besides, a three-dimensional software, based on a vortex
method is currently being developed, and will be used in the near future to
model more complex layouts. The experimental study shows that the second
turbine is deeply affected by the presence of an upstream device and that a
compromise between individual device performance and inter-device spacing is
necessary. Numerical results show good agreement with the experiment and are
promising for the future modelling of turbine farms.
The book Vortex Methods: Theory and Practice presents a comprehensive account of the numerical technique for solving fluid flow problems. It provides a very nice balance between the theoretical development and analysis of the various techniques and their practical implementation. In fact, the presentation of the rigorous mathematical analysis of these methods instills confidence in their implementation. The book goes into some detail on the more recent developments that attempt to account for viscous effects, in particular the presence of viscous boundary layers in some flows of interest. The presentation is very readable, with most points illustrated with well-chosen examples, some quite sophisticated. It is a very worthy reference book that should appeal to a large body of readers, from those interested in the mathematical analysis of the methods to practitioners of computational fluid dynamics. The use of the book as a text is compromised by its lack of exercises for students, but it could form the basis of a graduate special topics course. Juan Lopez
In this paper, the three-dimensional (3D) flow of a jet in a crossflow was investigated. This is a numerical study based on a vortex particle method. In order to account for the lower length scales, a crude LES turbulence model was implemented together with a regridding procedure. The numerical simulation was done with an injection ratio Rinj = Wjet/U∞ = 4.0. The code was run until an averaged steady state of the jet was reached. The resulting vortex structures were shown to be consistent with previously published analyses of the flow, from both experiments and numerical simulations. In particular, the onset of counter-rotating vortex pairs (CVP) was also investigated.
Initial findings from the Severn Tidal Fence Consortium are presented; the project was conducted as part of the UK Government's Severn Embryonic Technologies Scheme. The Severn Tidal Fence Consortium undertook a feasibility study to assess the potential for a large tidal fence system spanning the width of the Severn Estuary. The fence system would consist of a string of tidal stream energy converters spanning the estuary, with a free passage navigation gap. Tidal fences and piers (a fence connected at one end) have appreciable benefits when compared with tidal barrages, including reduced environmental impact, less disruption to shipping and lower capital investment. Tidal fences and piers can also be shown to have significant cost of energy benefits over tidal stream arrays in lower flow resources through savings in infrastructure and operation and maintenance. These initial findings aim to help justify and support the feasibility of further development of a tidal fence solution for the Severn Estuary.
Actuator discs may be used as a simple method for simulating horizontal axis tidal turbines, both in experiments and
CFD models. They produce a similar far wake to a real turbine, but eliminate some of the scaling issues which occur
in experiments, and reduce the mesh density required in CFD simulations. This paper examines methods for applying
a simple actuator disc in a commercial CFD code, Ansys CFX, and compares the wake produced with experimental
results for similar values of disk thrust coefficient (CT ). The results show that the CFD model gives reasonable
agreement with the experimental results. The main factors affecting the wake structure are the initial CT value, the
ambient turbulence levels, and potentially the disc induced turbulence. The main differences between the models and
experiments were in terms of the turbulence levels throughout the model. With further development, it is considered
that the CFD actuator disc could be an accurate and validated method for numerically modelling tidal turbines
An extension of actuator disc theory is used to describe the properties of a tidal energy device, or row of tidal energy devices, within a depth-averaged numerical model. This approach allows a direct link to be made between an actual tidal device and its equivalent momentum sink in a depth-averaged domain. Extended actuator disc theory also leads to a measure of efficiency for an energy device in a tidal stream of finite Froude number, where efficiency is defined as the ratio of power extracted by one or more tidal devices to the total power removed from the tidal stream. To demonstrate the use of actuator disc theory in a depth-averaged model, tidal flow in a simple channel is approximated using the shallow water equations and the results are compared with the published analytical solutions.
A particle method is presented for computing vortex sheet motion in three-dimensional flow. The particles representing the sheet are advected by a regularized Biot–Savart integral in which the exact singular kernel is replaced by the Rosenhead–Moore kernel. New particles are inserted to maintain resolution as the sheet rolls up. The particle velocities are evaluated by an adaptive treecode algorithm based on Taylor approximation in Cartesian coordinates, and the necessary Taylor coefficients are computed by a recurrence relation. The adaptive features include a divide-and-conquer evaluation strategy, nonuniform rectangular clusters, variable-order approximation, and a run-time choice between Taylor approximation and direct summation. Tests are performed to document the treecode's accuracy and efficiency. The method is applied to simulate the roll-up of a circular-disk vortex sheet into a vortex ring. Two examples are presented, azimuthal waves on a vortex ring and the merger of two vortex rings.
There is a growing demand for the use of renewable energy technologies to generate electricity due to concerns over climate change. The oceans provide a huge potential resource of energy. Energy extraction using marine current energy devices (MCEDs) offers a sustainable alternative to conventional sources and a predictable alternative to other renewable energy technologies. A MCED utilises the kinetic energy of the tides as opposed to the potential energy which is utilised by a tidal barrage. Over the past decade MCEDs have become an increasingly popular method of energy extraction. However, marine current energy technology is still not economically viable on a large scale due to its current stage of development. Ireland has an excellent marine current energy resource as it is an island nation and experiences excellent marine current flows. This paper reviews marine current energy devices, including a detailed up-to-date description of the current status of development. Issues such as network integration, economics, and environmental implications are addressed as well as the application and costs of MCEDs in Ireland.
This book presents and analyses vortex methods as a tool for the direct numerical simulation of incompressible viscous flows. Vortex methods have matured, offering an interesting alternative to finite difference and spectral methods for high-resolution numerical solutions of the Navier–Stokes equations. Research in the numerical analysis aspects of vortex methods has provided a solid mathematical background for understanding the accuracy and stability of the method. At the same time vortex methods retain their appealing physical character that was the motivation for their introduction. Scientists working in the areas of numerical analysis and fluid mechanics will benefit from this book, which may serve both communities as both a reference monograph and a textbook for computational fluid dynamics courses.
An experimental and theoretical investigation of the flow field around small-scale mesh disk rotor simulators is presented. The downstream wake flow field of the rotor simulators has been observed and measured in the 21m tilting flume at the Chilworth hydraulics laboratory, University of Southampton. The focus of this work is the proximity of flow boundaries (seabed and surface) to the rotor disks and the constrained nature of the flow. A three-dimensional Eddy-viscosity numerical model based on an established wind turbine wake model has been modified to account for the change in fluid and the presence of a bounding free surface. This work has shown that previous axi-symmetric modelling approaches may not hold for marine current energy technology and a novel approach is required for simulation of the downstream flow field. Such modelling solutions are discussed and resultant simulation results are given. This work has been conducted as part of a BERR-funded project to develop a numerical modelling tool which can predict the flow onto a marine current turbine within an array. The work presented in this paper feeds into this project and will eventually assist the layout design of arrays which are optimally spaced and arranged to achieve the maximum possible energy yield at a given tidal energy site.
A low order potential based panel code is used to analyze the flow around the blades of a horizontal axis marine current turbine. An empirical vortex model is assumed for the turbine wake, which includes the variation of pitch of the helicoidal vortices trailing behind the blades. The analysis is carried out for uniform inflow conditions in steady flow for a turbine with controllable pitch for two different pitch settings in a wide range of tip-speed-ratios. Grid convergence studies carried out to verify the accuracy of predicted pressure distributions and integrated forces show a fast convergence with grid refinement for this geometry. The effect of the helicoidal wake model parameters used in the analysis is found to have a strong influence in the performance curves. The results are compared with experimental data from literature and with the lifting line theory. A discussion of viscous effects is also provided to help explaining the main discrepancies with the data.
An experimental and theoretical investigation of the flow field around small-scale mesh disk rotor simulators is presented. The downstream wake flow field of the rotor simulators has been observed and measured in the 21m tilting flume at the Chilworth hydraulics laboratory, University of Southampton. The focus of this work is the proximity of flow boundaries (seabed and surface) to the rotor disks and the constrained nature of the flow. A three-dimensional Eddy-viscosity numerical model based on an established wind turbine wake model has been modified to account for the change in fluid and the presence of a bounding free surface. This work has shown that previous axi-symmetric modelling approaches may not hold for marine current energy technology and a novel approach is required for simulation of the downstream flow field. Such modelling solutions are discussed and resultant simulation results are given. This work has been conducted as part of a BERR-funded project to develop a numerical modelling tool which can predict the flow onto a marine current turbine within an array. The work presented in this paper feeds into this project and will eventually assist the layout design of arrays which are optimally spaced and arranged to achieve the maximum possible energy yield at a given tidal energy site.
The results of cavitation tunnel and tank tests on an 800 mm diameter model of a marine current turbine (MCT) are presented. The tests were carried out in a 2.4 m×1.2 m cavitation tunnel and the 60 m towing tank. Results for power and thrust coefficients are presented for a range of tip speed ratio and pitch settings for various conditions. The results of this investigation provided an insight into the operation of a singe turbine in straight or yawed flow, the effect on performance of changes in the tip immersion of the rotor, the interference between twin rotors and the likely areas of cavitation inception. In addition, the analysed results presented provide useful information for the hydrodynamic design of MCTs and detailed data for the validation of numerical models.
A particle method for convection-diffusion equations based on the approximation of diffusion operators by integral operators and the use of a particle method to solve integro-differential equations previously described is presented and studied. The isotropic diffusion operators are dealt with first. Two approximation possibilities are obtained, depending on whether or not the integral operator is positive. An extension of the method to anisotropic diffusion operators follows. The consistency and the accuracy of the method require much more complex conditions on the cutoff functions than in the isotropic case. After detailing these conditions, several examples of cutoff functions which can be used for practical computations are given. A detailed error analysis is then performed.
Large Eddy Simulation (LES) is an approach to compute turbulent flows based on resolving the unsteady large-scale motion of the fluid while the impact of the small-scale turbulence on the large scales is accounted for by a sub-grid scale model. This model distinguishes LES from any other method and reduces the computational demands compared with a Direct Numerical Simulation. On the other hand, the cost typically is still at least an order of magnitude larger than for steady Reynolds-averaged computations. The LES approach is attractive when statistical turbulence models fail, when insight into the vortical dynamics or unsteady forces on a body is desired, or when additional features are involved such as large-scale mixing, particle transport, sound generation etc. In recent years the rapid increase of computer power has made LES accessible to a broader scientific community, and this is reflected in an abundance of papers on the method and its applications. Still, however, some fundamental aspects of LES are not conclusively settled, a fact residing in the intricate coupling between mathematical, physical, numerical and algorithmic issues. In this situation it is of great importance to gain an overview of the available approaches and techniques.
Pierre Sagaut, in the style of a French encyclopedist, gives a very complete and exhaustive treatment of the different kinds of sub-grid scale models which have been developed so far. After discussing the separation into resolved and unresolved scales and its application to the Navier-Stokes equations, more than 140 pages are directly devoted to the description of sub-grid scale models. They are classified according to different criteria, which helps the reader to find his or her way through the arsenal of reasonings. The theoretical framework for which these models have mostly been developed is isotropic turbulence. The required notions from classical turbulence theory are summarized together with notions from EDQNM theory in two concise and helpful appendices. Further sections deal with numerical and implementational issues, boundary conditions and validation practice. A final section assembles a few key applications, cumulating in a condensed list of some general experiences gained so far.
The book very wisely concentrates on issues particular to LES, which to a large extent is sub-grid scale modelling. Classical issues of CFD, such as numerical discretization schemes, solution procedures etc, or post-processing are not addressed. Limiting himself to incompressible, non-reactive flows, the author succeeds in describing the fundamental issues in great detail, thus laying the foundations for the understanding of more complex situations. The presentation is essentially theoretical and the reader should have some prior knowledge of turbulence theory and Fourier transforms. The text itself is well written and generally very clear. A pedagogical effort is made in several places, e.g. when an overview over a group of models is given before these are described in detail. A few typing errors and technical details should be amended in a second edition, though, such as the statement that a filter which is not a projector is invertible (p 12), but this is not detrimental to the quality of the text.
Overall the book is a very relevant contribution to the field of LES and I read it with pleasure and benefit. It constitutes a worthy reference book for scientists and engineers interested in or practising LES and may serve as a textbook for a postgraduate course on the subject.
Jochen Fr?hlich
Relationships between small and large scales of motion in turbulent flows are of much interest in large-eddy simulation of turbulence, in which small scales are not explicitly resolved and must be modeled. This paper reviews models that are based on scale-invariance properties of high-Reynolds-number turbulence in the inertial range. The review starts with the Smagorinsky model, but the focus is on dynamic and similarity subgrid models and on evaluating how well these models reproduce the true impact of the small scales on large-scale physics and how they perform in numerical simulations. Various criteria to evaluate the model performance are discussed, including the so-called a posteriori and a priori studies based on direct numerical simulation and experimental data. Issues are addressed mainly in the context of canonical, incompressible flows, but extensions to scalar-transport, compressible, and reacting flows are also mentioned. Other recent modeling approaches are briefly introduced.
The energy available from tidal currents is substantial and considerable work has been conducted into determining the size of the resource and what the large-scale consequences of extraction might be. This paper describes the work conducted to establish a laboratory-scale model, by using the commercial computational fluid dynamics (CFD) code FLUENT™, in order to predict local-flow consequences resulting from the extraction of energy in two and three dimensions from within the water column in a tidal flow. As might be expected, a wake is formed but there is considerable localized flow acceleration around and, most especially, under an extraction zone. The wake behind the device is shown to be associated with a drop in the free surface which, in turn, is associated with the decline in the wake itself.
The conversion of the kinetic energy presented by ocean or marine currents offers an exciting proposition as it can provide regular and predictable energy resource. The majority of the proposed designs for converting this type of kinetic energy are based on the concept of the horizontal axis turbines, which has common characteristics to those being used in wind energy. Although a lot can be learnt and transferred from wind turbine technology, there are significant differences. These include the effects of the free surface and the occurrence of cavitation. Consequently, any developed numerical methods need to be verified. This study reports on the development and verification of simulation tools based on blade element momentum theory—a commercial code (GH-Tidal Bladed) and an academic in-house code (SERG-Tidal). Validation is derived from experimental measurements conducted on a model 800mm diameter turbine in a cavitation tunnel and a towing tank. The experimental data includes measurements of shaft power and thrust generated by the turbine for a series of blade pitch settings and speeds. The results derived from the two codes are compared. These indicate that the two developed codes demonstrate similar trends in the results and provide a satisfactory representation of the experimental turbine performance. Such results give the necessary confidence in the developed codes resulting in appropriate tools that can to be utilised by developers of marine current turbines.
This paper is devoted to the presentation and the analysis of a new particle method for convection-diffusion equations. The method has been presented in detail in the first part of this paper for an isotropic diffusion operator. This part is concerned with the extension of the method to anisotropic diffusion operators. The consistency and the accuracy of the method require much more complex conditions on the cutoff functions than in the isotropic case. After detailing these conditions, we give several examples of cutoff functions which can be used for practical computations. A detailed error analysis is then performed.
Tidal stream turbines are exploited in regions of high tidal currents. Such energy extraction will alter the hydrodynamics of a tidal region, analogous to increasing the bed friction in the region of extraction. In addition, this study demonstrates that energy extracted with respect to tidal asymmetries due to interactions between quarter (M4) and semi-diurnal (M2) currents will have important implications for large-scale sediment dynamics. Model simulations show that energy extracted from regions of strong tidal asymmetry will have a much more pronounced effect on sediment dynamics than energy extracted from regions of tidal symmetry. The results show that energy extracted from regions of strong tidal asymmetry led to a 20% increase in the magnitude of bed level change averaged over the length of a large estuarine system, compared with energy extracted from regions of tidal symmetry. However, regardless of the location of a tidal stream farm within a tidal system, energy extraction reduces the overall magnitude of bed level change in comparison with non-extraction cases. This has practical application to many areas surrounding the UK, including the Irish Sea and the Bristol Channel, that exhibit strong tidal currents suitable for exploitation of the tidal stream resource, but where large variations in tidal asymmetry occur.
A unified approach to approximating spatial derivatives in particle methods using integral operators is presented. The approach is an extension of particle strength exchange, originally developed for treating the Laplacian in advection–diffusion problems. Kernels of high order of accuracy are constructed that can be used to approximate derivatives of any degree. A new treatment for computing derivatives near the edge of particle coverage is introduced, using “one-sided” integrals that only look for information where it is available. The use of these integral approximations in wave propagation applications is considered and their error is analyzed in this context using Fourier methods. Finally, simple tests are performed to demonstrate the characteristics of the treatment, including an assessment of the effects of particle dispersion, and their results are discussed.
The world has been suffering dramatically increasing energy consumption during recent years. As the biggest developing country, China has a more urgent situation. To highlight the promising potential of renewable energy may be the only solution. Due to the vast sea area and continuous coast line of 18,000Â km, China has an excellent tidal current energy resource, which has good prospect for development. In this paper, the development of tidal current in China is briefly reviewed. The description is focused on the tidal current energy resource and the status of conversion technologies in China. Finally, the opportunities of the development of tidal current in China, including the urgent energy situation, the increasing pressure on reducing emissions, national policies for tidal current energy development and the increase of investment, are discussed in detail. A conclusion is made that the tidal current energy ought to be an important option for China in terms of renewable energy.
Experimental results of tests carried out to investigate the hydrodynamics of marine current turbines are presented. The objective is to build an experimental database in order to validate the numerical developments conducted to characterise the flow perturbations induced by marine current turbines. For that purpose, we used a tri-bladed horizontal axis turbine. The work is dedicated to measuring the behaviour of the system and to characterising the wake generated by the turbine. The efficiency of the device is quantified by the measurement of the thrust and the amount of power generated by the rotor for various inflow conditions, whereas the wake is characterised by Laser Doppler Velocimetry. Particular attention is paid to the flow characteristic effects on the performance of a 0.70 m diameter turbine. The load predictions on the structure and the measured performance of the turbine over its working range of currents and rotational speeds are presented. The results showed that this kind of turbine is sensitive to the quality of the incoming flow. The turbulence intensity effects on turbine behaviour and on its wake are also characterised in order to study how the far wake decays downstream and to estimate the effect produced in downstream turbines.
The present study deals with the efficiency of cross flow water current turbine for free stream conditions versus power farm conditions. In the first part, a single turbine for free fluid flow conditions is considered. The simulations are carried out with a new in house code which couples a Navier–Stokes computation of the outer flow field with a description of the inner flow field around the turbine. The latter is based on experimental results of a Darrieus wind turbine in an unbounded domain. This code is applied for the description of a hydraulic turbine. In the second part, the interest of piling up several turbines on the same axis of rotation to make a tower is investigated. Not only is it profitable because only one alternator is needed but the simulations demonstrate the advantage of the tower configuration for the efficiency. The tower is then inserted into a cluster of several lined up towers which makes a barge. Simulations show that the average barge efficiency rises as the distance between towers is decreased and as the number of towers is increased within the row. Thereby, the efficiency of a single isolated turbine is greatly increased when set both into a tower and into a cluster of several towers corresponding to possible power farm arrangements.
Understanding the flow field around horizontal axis marine current turbines is important if this new energy generation technology is to advance. The aim of this work is to identify and provide an understanding of the principal parameters that govern the downstream wake structure and its recovery to the free-stream velocity profile. This will allow large farms or arrays of devices to be installed whilst maximising device and array efficiency. Wake characteristics of small-scale mesh disk rotor simulators have been measured in a 21 m tilting flume at the University of Southampton. The results indicate that wake velocities are reduced in the near wake region (close behind the rotor disk) for increasing levels of disk thrust. Further downstream all normalised wake velocity values converge, enforcing that, as for wind turbines, far wake recovery is a function of the ambient flow turbulence. Varying the disk proximity to the water surface/bed introduces differential mass flow rates above and below the rotor disk that can cause the wake to persist much further downstream. Finally, the introduction of increased sea bed roughness whilst increasing the depth-averaged ambient turbulence actually decreases downstream wake velocities. Results presented demonstrate that there are a number of interdependent variables that affect the rate of wake recovery and will have a significant impact on the spacing of marine current turbines within an array.
The development of a blade element momentum (BEM) model for the hydrodynamic design of marine current turbines is presented. The model includes routines for interpolation of 2D section data and extrapolation for stall delay. The numerical model is compared with experimental data obtained from tests of an 800 mm diameter model rotor carried out in a cavitation tunnel. The theoretical predictions are in good agreement with the experiments. Using this validated model, a typical 3D rotor is used to demonstrate parametric variations of the design parameters. The effect of tip immersion on possible cavitation is assessed for this rotor. The model is then used to solve the dynamic effects of a tidal profile. The effect of an increase in blade roughness is presented, indicating a relatively small reduction in power. This work demonstrates that the numerical model developed can provide a useful tool for the investigation of the hydrodynamic design and operation of marine current turbines.
Facing great pressure of economic growth and energy crisis, China pays much attention to the renewable energy. An overview of policy and legislation of renewable energy as well as status of development of renewable energy in China was given in this article. By analysis, the authors believe that ocean energy is a necessary addition to existent renewable energy to meet the energy demand of the areas and islands where traditional forms of energy are not applicable and it is of great importance in adjusting energy structure of China. In the article, resources distribution and technology status of tidal energy, wave energy, marine current energy, ocean thermal energy and salinity gradient energy in China was reviewed, and assessment and advices were given for each category. Some suggestions for future development of ocean energy were also given.
In this paper, we present a new method for solving the Navier-Stokes equations. This method is based on a particle discretisation of the flow. This viscous terms are accounted for by using an elementary solution of the diffusion equation with a Dirac'function for the initial data. The algorithm is especially designed to conserve the total vorticity. Numerical results concerning the shear layer problem are presented and the influence of the Reynolds number on the numerical solution is explored.
As interest in renewable energy sources is steadily on the rise, tidal current energy is receiving more and more attention from politicans, industrialists, and academics. In this article, the conditions for and potential of tidal currents as an energy resource in Norway are reviewed. There having been a relatively small amount of academic work published in this particular field, closely related topics such as the energy situation in Norway in general, the oceanography of the Norwegian coastline, and numerical models of tidal currents in Norwegian waters are also examined. Two published tidal energy resource assessments are reviewed and compared to a desktop study made specifically for this review based on available data in pilot books. The argument is made that tidal current energy ought to be an important option for Norway in terms of renewable energy.
Portland Bill (Dorset, UK) is a promising site for tidal stream energy exploitation; it combines high tidal stream velocities around the headland with a location closer to population centres than other proposed sites. To better estimate available energy resources at the site, a two-dimensional tidally driven hydrodynamic numerical model of Portland Bill was developed using the TÉLÉMAC system, with validation using tidal elevation measurements and tidal stream diamonds from Admiralty charts. The results of the model were used to produce a time series of the tidal stream velocity over the simulation period and may be used in future work to optimize the location of turbine arrays at the site.
This work aims at the development of a Lagrangian large eddy simulation (LES) scheme. The scheme is based on the filtered vorticity transport equation and on modeling the effects of subfilter scale (SFS) velocity and vorticity fluctuations using a dynamic eddy diffusivity model. The dynamic implementation of the model relies on multiple filtering in order to determine model coefficients from the resolved data. The performance of the dynamic SFS model is examined usinga prioritests that are based on direct numerical simulations of forced, homogeneous, isotropic turbulence. The tests show a fair correlation of the model with SFS convection of vorticity. In addition, the computed value of the dynamic model coefficient is in good agreement with predictions based on enstrophy balances. Finally, the direct numerical simulation data is used to compare a three-dimensional particle representation of the model with spectral evaluations. The tests show that when the particle representation is sufficiently resolved, the Lagrangian model predictions are in good agreement with spectral results.
Although a lot can be learnt from technology transfer from wind turbines and ship propellers, there have been a few experiments investigating marine current turbines. As a result, a study has been carried out on the power, thrust and cavitation characteristics of 1/20th scale model of a possible 16 m diameter horizontal axis tidal turbine. Cavitation tunnel experiments for different blade pitch settings have been compared with simulations based on a developed blade element-momentum theory. This theory has been shown to provide a satisfactory representation of the experimental turbine performance characteristics. As an example application, the developed theory has been used to design possible horizontal axis tidal turbines for the tidal flows around Portland Bill. The results show that there is a clear balance between design loads and optimisation of energy yields.
Energy from marine currents offers the promise of regular and predictable electrical generation at higher power densities than other renewables. The marine current resource is potentially large but mainly concentrated in a number of sites around the world. The power density for a horizontal axis turbine operating in such currents has a similar form to that of a wind turbine and is dependent on the cube of the velocity and the fluid density which for water is about 1000 times that of air. These two factors imply that the power density for marine current energy converters will be appreciably higher than that of wind generators resulting in smaller and hence more manageable size turbines.
The results of cavitation tunnel and tank tests on an 800 mm diameter model of a marine current turbine (MCT) are presented. The tests were carried out in a 2.4 m×1.2 m cavitation tunnel and the 60 m towing tank. Results for power and thrust coefficients are presented for a range of tip speed ratio and pitch settings for various conditions. The results of this investigation provided an insight into the operation of a singe turbine in straight or yawed flow, the effect on performance of changes in the tip immersion of the rotor, the interference between twin rotors and the likely areas of cavitation inception. In addition, the analysed results presented provide useful information for the hydrodynamic design of MCTs and detailed data for the validation of numerical models.
The hydrodynamics of a three bladed horizontal axis turbine is investigated. The performance of a turbine of 0.7 m of diameter and its wake were obtained from experiments and numerical simulations. A six components load cells is used for thrust and hydrodynamic power measurements, and Laser Doppler Velocimetry technics for wake characterisation. A three-dimensional software, taking into account the non stationary evolution of the wake generated by turbine blades is developed. The flow is discretised with particles carrying vorticity, which are advected in a Lagrangian frame. Both numerical and experimental power coefficient Cp and velocity maps were compared. Experiments and numerical data showed a satisfactory agreement in term of the shape of the wake and of Cp, whereas the turbine was only modelled as a rotor composed of three blades. The results showed that the maximum decay of the wake velocity deficit is obtained in the near wake and the extension of velocity deficit until 10 D downstream. These studies highlight the problems that can appear during the configuration of turbines in a farm. The main aim of this work is to understand how the wake decays downstream, in order to estimate the effects produced in the lee side of turbines and more generally on its close environment.
The Pentland Firth, located between the north coast of mainland Scotland (UK) and the Orkney Islands, is recognised as an excellent location for the utilisation of tidal stream technology. Potential ecological impacts associated with tidal stream technology may ultimately depend on device design, array size and deployment location. Available ecological data for the Pentland Firth is summarised and strategic priorities for assessing ecological impacts are provided. Baseline data on marine species and habitats in the Pentland Firth is severely lacking and consequently the integrity of any environmental impact assessment could be compromised by this lack of data.
The three dimensional, time dependent (incompressible) vorticity equations were used to simulate numerically the decay of isotropic box turbulence and time developing mixing layers. The vorticity equations were spatially filtered to define the large scale turbulence field, and the subgrid scale turbulence was modeled. A general method was developed to show numerical conservation of momentum, vorticity, and energy. The terms that arise from filtering the equations were treated (for both periodic boundary conditions and no stress boundary conditions) in a fast and accurate way by using fast Fourier transforms. Use of vorticity as the principal variable is shown to produce results equivalent to those obtained by use of the primitive variable equations.
Bay of fundy tidal energy development -opportunities and challenges
- G Trowse
- R Krasten
G. Trowse, R. Krasten, Bay of fundy tidal energy development -opportunities and challenges, Proceeding of the 3rd International Conference on Ocean
Energy, Bilbao, Spain, October 6-8.
Inuence of ow connement on the performance of a crossow turbine
- C Consul
- R Willden
C. Consul, R. Willden, Inuence of ow connement on the performance of
a crossow turbine, Proceeding of the 3rd International Conference on Ocean
Energy, Bilbao, Spain, October 6-8.
Optimisation des méthodes de calculs d'écoulements tourbillonnaires instationnaires
- F Hauville
F. Hauville, Optimisation des méthodes de calculs d'écoulements tourbillonnaires instationnaires, Ph.D. thesis, Université du Havre (1996).
Numerical calculation af an incompressible, inviscid three-dimensional ow about a wind turbine with partial span pitch control
- B Marichal
- F Hauville
B. Marichal, F. Hauville, Numerical calculation af an incompressible, inviscid three-dimensional ow about a wind turbine with partial span pitch control, Société Roumaine de Mathématique appliquées et Industrielles -Oraéda,
Roumanie.
Les jets d'Inverseurs de poussée d'un avion en phase d'atterrissage
- G Pinon
G. Pinon, Les jets d'Inverseurs de poussée d'un avion en phase d'atterrissage,
Broché -Éditions Universitaires Europeénnes -ISBN-10: 6131508658 -ISBN-13: 978-6131508653, 2010.
Résolution numérique de problèmes non linéaires de l'hydrodynamique navale : manoeuvrabilité et tenueà la mer des navires
- F Rouffi
F. Rouffi. Résolution numérique de
problèmes non linéaires de l'hydrodynamique
navale : manoeuvrabilité et tenueà la mer
des navires. PhD thesis, Université Pierre et
Marie Curie, 1992.
Flow characteristic eects on marine current turbine behaviour and on its wake properties
- F Maganga
- G Germain
- J King
- G Pinon
- E Rivoalen
F. Maganga, G. Germain, J. King, G. Pinon, E. Rivoalen, Flow characteristic
eects on marine current turbine behaviour and on its wake properties, IET
Renew. Power Gener. Vol. 4 (6) (2010) doi:10.1049/ietrpg.2009.0205 p. 498
509.
An innovative tidal fence development for severn estuary
- J Giles
- I Godfrey
- I Bryden
- L Myers
- J O'nians
- A Bahaj
- J Griths
J. Giles, I. Godfrey, I. Bryden, L. Myers, J. O'Nians, A. Bahaj, J. Griths,
An innovative tidal fence development for severn estuary, Proceeding of the 3rd
International Conference on Ocean Energy, Bilbao, Spain, October 6-8.