Tidal energy represents a promising resource for the future energy mix. For harnessing tidal currents free stream horizontal axis turbines have been investigated for some years. The acting physics is very similar to the one of horizontal axis wind turbines, with the additional phenomenon of cavitation, which causes performance reduction, flow induced noise and severe damages to the turbine blade and downstream structures. The paper presents an enhanced semi-analytical model that allows the prediction of the performance characteristics including cavitation inception of horizontal axis tidal turbines. A central component is the well-known blade element momentum theory which is refined by various submodels for hydrofoil section lift and drag as a function Reynolds number and angle of attack, turbine thrust coefficient, blade hub and tip losses and cavitation. Moreover, the model is validated by comparison with comprehensive experimental data from two different turbines. Predicted power and thrust coefficient characteristics were found to agree well with the experimental results for a wide operational range and different inflow velocities. Discrepancies were observed only at low tip speed ratios where major parts of the blades operate under stall conditions. The predicted critical cavitation number is somewhat larger than the measured, i.e. the prediction is conservative. As an overall conclusion the semi-analytical model developed seems to be so fast, accurate and robust that it can be integrated in a future workflow for optimizing tidal turbines.

La raréfaction des ressources fossiles non renouvelables et le dérèglement climatique font de la question énergétique un enjeu d’envergure mondiale. L’exploitation de nouvelles sources d’énergie renouvelable devient alors un objectif de première importance. L’énergie produite à partir des courants marins suscite depuis quelques années un intérêt particulier. Le concept de turbine sous-marine, appelée hydrolienne, désigne le dispositif permettant de convertir l’énergie cinétique des courants marins en énergie électrique. Ce travail de recherche traite les problématiques que pose la conception des hydroliennes à axe horizontal. Il sera mis en évidence que le monde des hélices marines présente une piste intéressante pour l’étude du comportement hydrodynamique des hydroliennes. Certains concepteurs d’hydroliennes avancent que l’ajout d’un système de carénage est favorable pour améliorer le rendement hydrodynamique. L’étude du gain hydrodynamique à encombrement constant que pourrait procurer l’ajout d’un carénage a donc été choisie comme point de départ de ce travail de recherche. Pour répondre au besoin des industriels lié à une problématique de gain de masse, les matériaux composites présentent un atout considérable en raison de leurs excellents rapports «masse/résistance» et «masse/rigidité». Une réalisation d’un carénage en matériaux composites présentant le meilleur ratio «puissance/masse» a été obtenue. Un carénage d’hydrolienne est de par sa position particulièrement confronté à des chocs. Ceci peut s’avérer très délicat car la structure composite en question est soumise à des sollicitations sévères liées à l’environnement marin. L’impact sur un carénage d’hydrolienne a été traité en détail dans ce travail de recherche. / Against the backdrop of the increasing scarcity of non-renewable fossil resources and climate change, the energy problem has become a worldwide issue. Hence, the exploitation of new renewable energy sources becomes a worldwide goal of primary importance. The concept of the underwater turbine, called tidal current turbine, designates the device which allows the conversion of the kinetic energy produced by marine currents in electric energy. This research study examines the problems related to the design of horizontal axis tidal current turbines. The present study shows that the world of marine propellers, sometimes entirely left out by the designers of tidal current turbines, presents an interesting avenue of research with regard to the hydrodynamic behaviour of tidal current turbines. Certain designers of tidal current turbines use a duct and hold that the addition of the duct contributes to the improvement of the hydrodynamic performance. Therefore, the study of the hydrodynamic benefits of ducted turbine using a constant overall cross-section than the bare turbine was the starting point of the present research work. In order to meet the needs of the manufacturers of tidal current turbines, which is generally linked to a problem of mass gain, composite materials present a considerable asset on account of their excellent «mass/resistance» and «mass/rigidity» relations. A structural design of ducted tidal current turbines using composite materials has therefore been examined. Hence, the design of a composite duct which yields the best «power/mass» ratio has been proposed. The duct of the tidal current turbine is especially confronted by the impacts due to its particular position. The impact damage aspect has also been examined in detail in the present research study.

This thesis examines methods for designing and analyzing kinetic turbines based on blade element momentum (BEM) theory and computational fluid dynamics (CFD). The underlying goal of the work was to assess the potential augmentation of power production associated with enclosing the turbine in an expanding duct. Thus, a comparison of the potential performance of ducted and non-ducted turbines was carried out. This required de ning optimal turbine performance for both concepts. BEM is the typical tool used for turbine optimization and is very well established in the context of wind turbine design. BEM was suitable for conventional turbines, but could not account for the influence of ducts, and no established methodology for designing ducted turbines could be found in the literature. Thus, methods were established to design and analyze ducted turbines based on an extended version of BEM (with CFD-derived coe cients), and based on CFD simulation. Additional complications arise in designing tidal turbines because traditional techniques for kinetic turbine design have been established for wind turbines, which are similar in their principle of operation but are driven by flows with inherently different boundary conditions than tidal currents. The major difference is that tidal flows are bounded by the ocean floor, the water surface and channel walls. Thus, analytical and CFD-based methods were established to account for the effects of these boundaries (called blockage effects) on the optimal design and performance of turbines. Additionally, tidal flows are driven by changes in the water surface height in the ocean and their velocity is limited by viscous effects. Turbines introduced into a tidal flow increase the total drag in the system and reduce the total flow in a region (e.g. a tidal channel). An analytical method to account for this was taken from the eld of tidal resource assessment, and along with the methods to account for ducts and blockage effects, was incorporated into a rotor optimization framework. It was found that the non-ducted turbine can produce more power per installed device frontal area and can be operated to induce a lesser reduction to the flow through a given tidal channel for a given level of power production. It was also found that by optimizing turbines for array con gurations that occupy a large portion of the cross sectional area of a given tidal channel (i.e. tidal fences), the per-device power can be improved signi cantly compared to a sparse-array scenario. For turbines occupying 50% of a channel cross section, the predicted power improves is by a factor of three. Thus, it has been recommended that future work focus on analyzing such a strategy in more detail. / Graduate

Les matériaux composites sont utilisés dans de nombreuses structures marines et de nouvelles applications sont en cours de développement telles que les pales d’hydroliennes. La fiabilité de ces composants dans un environnement très sévère est cruciale pour la rentabilité de ces systèmes récupérateurs d’énergie des courants marins. Ces structures sont sujettes à de nombreuses forces, telles que les courants marins, les vagues, tempêtes mais également diverses agressions marines telles que l’eau de mer et la corrosion. Une compréhension approfondie du comportement au long terme de ces parties mobiles est donc essentielle. La majorité des développeurs d’hydroliennes ont préféré des pales en carbone. Ainsi il est nécessaire de comprendre comment une longue immersion dans l’océan affecte ces composites. Dans cette étude, le comportement au long terme de différents composites carbone/époxy a été étudié en utilisant des essais de vieillissement accéléré. Une diminution significative des résistances des composites a été observée après saturation en eau de mer. Pour des temps d’immersion plus longs, seulement peu de changements des propriétés apparaissent. Peu d’effets significatifs ont été observés tant sur les modules que sur la ténacité. Ces changements de propriétés sont initialement dus à la plastification de la matrice, suivis par un affaiblissement de l’interface fibre/matrice. L’endommagement peut affecter le comportement au long terme des structures composites et créer de nouveaux chemins préférentiels pour la diffusion de l’eau. En conséquence un modèle basé sur un critère couplé résistance/ténacité a été proposé pour décrire le seuil d’endommagement et basé sur un critère en ténacité pour décrire la cinétique d’endommagement. Il permet de reproduire d’une manière correcte le seuil et la cinétique d’endommagement pour des matériaux vieillis et non vieillis. L’évolution de l’entrée d’eau dans les composites a été suivie dans le but de développer un modèle de diffusion prenant en compte le nature anisotrope des composites. Ainsi le modèle de diffusion a été utilisé sur pale d’hydrolienne. Finalement des premières investigations sur le couplage entre le modèle de diffusion et l’endommagement ont été réalisées. Cette étude a contribué au développement d’outils pour quantifier la durabilité au long terme des pales d’hydroliennes en composites. / Composite materials are used in many marine structures and new applications are being developed such as tidal turbine blades. The reliability of these components, in a very severe environment, is crucial to the profitability of tidal current energy systems. These structures are subject to many forces such as ocean tides, waves, storms but also to various marine aggressions, such as sea water and corrosion. A thorough understanding of the long term behavior of the moving parts is therefore essential. The majority of tidal turbine developers have preferred carbon blades, so there is a need to understand how long immersion in the ocean affects these composites. In this study the long term behavior of different carbon/epoxy composites has been studied using accelerated ageing tests. A significant reduction of composite strengths has been observed after saturation of the material in seawater. For longer immersions only small further changes in these properties occur. No significant changes have been observed for moduli nor for composite toughness. Changes in properties are initially due to matrix plasticization, followed by reductions due to fibre/matrix interface changes. Damage can affect the long term behavior of composites structures and create new pathways for water diffusion. As a consequence a damage model has been proposed based on a coupled strength/toughness criterion to describe the threshold of damage and on a toughness criterion to describe the crack development kinetics. It describes in a correct manner the damage threshold and kinetics for the as-received material and for material after sea water ageing. The evolution of the rate of water ingress into composite materials has been followed, in order to develop a diffusion model taking into account the anisotropic nature of composites. Then the diffusion model has been applied on a tidal turbine blade. Finally a first investigation of the coupling between the diffusion model and damage has been performed. This study has contributed to the development of tools to quantify long term durability of composite tidal turbine blades.

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. / New projects have recently been launched to build farms of several tidal or river turbines, which are part of the renewable energy systems. The turbine wake interactions in the farm must be considered, as they can affect the power production of the turbines. A steady-state Actuator force model using the RANS equations is developed in the present work to calculate the power production and the flow through arrays of tidal or river Darrieus turbines. It uses detailed three dimensional force distributions depending on the position on the turbine, obtained beforehand by a set of blade-resolved URANS simulations of the turbine. New power coefficient and force coefficient laws depending on the local velocity (flow velocity at the machine position) instead of the upstream velocity are established and appear to be independent from the local turbine blockage in an array. Those laws are used to construct a model that adapt the Actuator force distributions to the local velocity of the flow reaching each turbine, in order to simulate each turbine functioning close to its maximum efficiency point. The model is validated against experimental measurements on a reduced-scale Darrieus turbine. Different farm configurations are simulated and compared to results of the same model adapted in two dimensions. The distances between turbines that are optimizing the farm power production are especially investigated.

No description available.

Dans un contexte de développement des énergies renouvelables, les énergies marines suscitent un grand intérêt. Parmi elles, les courants de marée paraissent constituer une ressource intéressante du fait de la densité de l'eau de mer et de la possibilité de prévoir les oscillations de marée à un endroit donné. Pour une turbine à axe vertical et en accord avec le partenaire industriel, les contraintes à l'échelle de la section de pale incluent la bidirectionnalité de l'écoulement, l'état de surface ainsi que la turbulence amont. La première partie du travail présentée ici s'est donc attachée à étudier deux solutions permettant de répondre à la bidirectionnalité de l'écoulement à l'échelle d'une section de pale. Un profil bidirectionnel spécifique a ainsi été comparé à un NACA 0015 en écoulement directe et inversé. La seconde partie s'est attachée à caractériser l'effet de la rugosité de surface et de la turbulence amont sur les propriétés d'un profil unidirectionnel spécifiquement développé pour les turbines à axe horizontal. Les deux sujets ont été abordés sur des profils académiques 2D, au travers d'une approche expérimentale originale et d'étude numériques. Des calculs tout turbulents et avec prise en compte de la transition ont été comparés à des mesures d'effort par balance, couplés à des observations de l'écoulement par PIV. Le foil bidirectionnel ainsi que le foil NACA en écoulement direct et inversé ont montrés des comportements singuliers qui pénalisent leurs performances dans l'optique d'une utilisation en tant que section de pale. A partir d'une valeur seuil, la hauteur de la rugosité de surface a montré engendrer un changement profond de la nature de l'écoulement autour du foil unidirectionnel. Finalement, il a été observé que la turbulence amont modifiait modérément les propriétés de ce type de foils, mais de façon moins significative à l'échelle de la pale. / In a context of development of renewable energies, there is a growing interest in marine energies. Among them, tidal currents are promising due to the density of seawater and the predictability of tidal oscillations at a given location. For horizontal axis tidal turbines and according to the industrial partner, constraints at the blade section scale include bi-directionality of the flow, surface roughness and upstream turbulence. The first part of the present work studied two solutions to achieve bi-directionality of the flow at the blade section scale. A specific bi-directional hydrofoil was compared to a NACA 0015 in forward and reversed flow. The second part focussed on the effect of surface roughness and upstream turbulence on a unidirectional blade section designed for current turbines. Both studies were carried out on academic two-dimensional hydrofoils, using both numerical investigation and a specifically developed experimental approach. Computations using fully turbulent and transition models were compared to balance force measurements coupled with PIV flow observations. The bidirectional foil as well as the NACA foil in forward and reversed flow, showed specific behaviours that could considerably reduce their performances for a use as a tidal turbine rotor. Roughness height was also observed to deeply change the foil properties, beyond a critical height. Finally, upstream turbulence resulted in moderate performance changes, less significant at the machine scale.

A three dimensional computational model of a full scale axial flow tidal turbine has been used to investigate the effects of a range of realistic environmental conditions on turbine performance. The model, which is based on the Reynolds averaged Navier-Stokes equations, has been developed using the commercial flow solver ANSYS Fluent. A 1:30 scale tidal turbine is simulated in an open channel for comparison to existing experimental data. The rotor blades are directly resolved using a body-fitted, unstructured computational grid. Rotor motion is enabled through a sliding mesh interface between the rotor and the channel boundaries. Reasonably good agreement in thrust and power is observed. The computed performance curves are offset from the measured performance curves by a small increment in rotor speed. Subsequently, a full scale axial flow turbine is modelled in a variety of conditions representative of tidal channel flows. A parametric study is carried out to investigate the effects of flow shear, confinement and alignment on turbine performance, structural loading, and wake recovery. Mean power and thrust are found to be higher in sheared flow, relative to uniform flow of equivalent volumetric flow rate. Large fluctuations in blade thrust and torque occur in sheared flow as the blade passes through the high velocity freestream flow in the upper portion of the profile and the lower velocity flow near the channel bed. A stronger shear layer is formed around the upper portion of the wake in sheared flow, leading to enhanced wake mixing. Mean power and thrust are reduced when the turbine is simulated at a lower position in a sheared velocity profile. However, fluctuations in blade loading are increased due to the higher velocity gradient. The opposite effects are observed when the turbine operates at greater heights in sheared flow. Flow misalignment has a negative impact on mean rotor thrust and power, as well as on unsteady blade loading. Although the range of unsteady loading is not increased significantly, additional perturbations are introduced due to interactions between the blade and the nacelle. A deforming surface is introduced using the volume-of-fluid method. Linear wave theory is combined with the existing free surface model to develop an unsteady inflow boundary condition prescribing combined sheared flow and free surface waves. The relative effects of the sheared profile and wave-induced velocities on turbine loading are identified through frequency analysis. Rotor and blade load fluctuations are found to increase with wave height and wave length. In a separate study, the performance of bi-directional ducted tidal turbines is investigated through a parametric study of a range of duct profiles. A two dimensional axi-symmetric computational model is developed to compare the ducted geometries with an unducted device under consistent blockage conditions. The best-performing ducted device achieves a peak power coefficient of approximately 45% of that of the unducted device. Comparisons of streamtube area, velocity and pressure for the flow through the ducted device shows that the duct limits the pressure drop across the rotor and the mass flow through the rotor, resulting in lower device power.

Cette thèse vise à étudier les effets de la transition laminaire - turbulent et du contrôle actif de l’angle de calage des pales sur les performances de l’hydrolienne à axe transverse SHIVA (Système Hydrolien Intelligent à Variation d’Angle) développée à l’institut de Recherche de l’Ecole-Navale (IRENav). L’écoulement transitionnel autour d’un hydrofoil est, d’abord étudié en comparant des résultats expérimentaux et numériques. Les résultats expérimentaux ont été obtenus dans le tunnel hydrodynamique de l’IRENav. La transition s’effectue par un mécanisme de bulbe de séparation laminaire. Les comparaisons sont fondées sur l’analyse locale des pressions, des profils de vitesse dans la zone du bulbe de séparation laminaire et sur l’analyse des portances, traînées et moments mesurés sur un profil fixe et en mouvement de tangage forcé. Des calculs RANS 2D, avec et sans modèle de transition (ɣ— Reo), RANS 3D et LES 2.5D ont été menés afin de comparer les approches et évaluer la précision des simulations. L’étude montre que le modèle de transition ɣ — Reo améliore nettement les résultats obtenus par rapport à un modèle tout turbulent (k — w SST) dans le cas d’un écoulement transitionnel. L’influence de la transition laminaire - turbulent sur les performances de la turbine SHIVA est ensuite étudiée en comparant les résultats de calculs effectués avec et sans modèle de transition. L’approche est bidimensionnelle. L’utilisation du modèle de transition est intéressante au paramètre d’avance ʎ = 2 pour lequel les pales subissent un décrochage dynamique important. Le développement du tourbillon de bord d’attaque, favorisé par le modèle de transition, permet en effet une meilleure prédiction du décrochage. Les valeurs de ʎ supérieures sont moins concluantes du fait de la prédiction d’une tramée trop faible par le modèle de transition. Enfin, l’influence du contrôle actif du calage des pales sur les performances de la turbine est étudiée au point de fonctionnement optimal de la turbine ʎ = 3. Des lois de calage avancées sont développées, permettant d’agir indépendamment sur la moitié amont ou aval de la turbine. La meilleure loi testée permet une augmentation du coefficient de puissance de 34% tout en lissant la répartition du couple. / This work studies the laminar-turbulent transition and the pitch control effects on the performances of a vertical axis tidal turbine (SHIVA) developed at the French naval academy research institute. Firstly, experimental and numerical results are compared to study the transitional flow around a hydrofoil. The experiments were carried out in the hydrodynamic tunnel of the French naval academy research institute and the laminar-turbulent transition was triggered by a laminar separation bubble mechanism. Comparisons are based on the local analysis of pressure data and velocity profiles in the vicinity of the laminar separation bubble. Lift, drag and moment coefficients measured on a fixed hydrofoil and on a hydrofoil undergoing a pitching movement are also used for comparison. 2D RANS calculations carried out with or without a transition modal (ɣ — Reo), 3D RANS calculations and 2.5D LES calculations were run so as to assess the accuracy of each type of simulation. This study shows that the ‘y Reo transition modal clearly improves the accuracy of the results compared to a fully turbulent turbulence model (k— w SST) when considering a transitional flow. The influence of the laminar-turbulent transition on the performance of the SHIVA turbine is then studied. Results of 2D calculations run with and without transition model are compared. The use of the transition modal is relevant at the tip speed ratio value ʎ = 2 for which the blades undergo dynamic stall. The transition modal leads to a better prediction of the leading edge vortex development and then allows a better prediction of the dynamic stall. The use of the transition model at higher ʎ values is less relevant since the transition modal appears to predict a drag too low. Finally, the effect of the pitch control on the SHIVA turbine performance is .studied at ʎ = 3, for which the power coefficient is the highest. Advanced pitching laws are developed to modify the blades’ angle of attack independently on the upstream and downstream halves of the turbine. The best pitching law tested in this study leads to an improvement of the power coefficient by 34% and smooths the torque distribution.

This study presents a numerical investigation of ducted tidal turbines, employing three-dimensional Reynolds-averaged Navier-Stokes simulations. Bidirectional ducted turbines are modelled with and without aperture, referred to as ducted and open- centre turbines respectively. The work consists of two investigations. In the first, the turbine rotors are represented by actuator discs, a simplification which captures changes in linear momentum and thus the primary interaction of the turbine with the flow through and around the duct, while greatly reducing computational complexity. In the second investigation, the turbine rotors are represented through a CFD-integrated blade element momentum model, employing realistic rotor data, capturing swirl and blade drag in addition to the extraction of linear momentum. Both modelling techniques were employed to investigate the performances of bare, ducted, and open-centre turbines, relating these to the flow fields exhibited. For axial flow, substantial decreases in power generated by the ducted and open-centre turbines were found, relative to a bare turbine of equal total device diameter. For open-centre turbines, an increase in aperture size leads to a further reduction in power generated. Increased blockage was shown to positively affect the performance of all devices. Two further measures of performance were employed: power density, normalising the power by the rotor area, and basin efficiency, relating the power generated to the overall power removed from the flow. Moderate increases in power density can be achieved for the ducted and open-centre devices, while their basin efficiencies are of similar value to that of the bare turbine. For yawed inflow, the performance of the bare turbine decreases, whilst that of the ducted and open-centre turbines increases. This is due to an increased flow velocity following flow acceleration around the inlet lip of the duct and also an increase in effective blockage as ducts present greater projected frontal area when approached non-axially.