This thesis discusses the importance of tidal turbine support structures through analytical and computational modelling. A head-driven analytical channel model was first developed to determine the sensitivity of the flow to the presence and type of support structures. It showed that there was a significant potential reduction in farm power output even when only considering approximate force coefficients for rotor and support structure. To confirm these findings, computational simulations were performed on a full-scale turbine to obtain more accurate force coefficients considering full rotor-support structure interactions. The flow interaction effects between the rotor and its support structure were studied using Computational Fluid Dynamics (CFD) for different support structure shapes for a range of tidal velocities including the power-capping zone. The integrated rotor force coefficients were higher in the presence of the cylindrical support structure than the elliptical support due to the higher opposing thrust from the cylinder in the channel redirecting the flow and increasing the flow velocity over the top half of the rotor. The presence of rotor caused a drop in the stream-wise forces on the support structure. The amplitude of the stream-wise sectional forces along the support structure height was lower in the case of an elliptical than a circular cylinder due to more streamlined shape of the ellipse. At device scale, the computational model was used to study the turbine performance in the power-capping zone by pitching the blades to feather. The influence of pitch-to- feather power-capping strategy was examined by studying the forces and angle of attack on the turbine blades, and the wake at three different blade pitch angles. Increasing blade pitch angle resulted in a significant drop in the average load on the blade. Also since the tidal channel flow has a shear in its velocity profile, the influence of shear on turbine performance was studied by comparing it to the same turbine in a uniform flow. The analytical channel flow model was used to investigate the characteristics of tidal stream energy extraction for large tidal farms deployed in tidal channels with specific focus on the limitations to realizable farm power due to turbine support structure drag and constraints on volume flow rate reduction. The force coefficients dataset from computational modelling was used to obtain a better estimate of the farm power output. Support structures were seen to contribute significantly to the overall resistive force in the channel and thus reduce the overall flow rates in the channel, leading to losses in realizable power. Over a wide range of channel characteristics, realistic levels of support structure drag lead to up to a 10% reduction in realizable power, and an associated reduction in the number of turbines that can be economically installed.

Many environmental free surface flows involve water and sediment transport. The net changes to the surface level of an erodible bed by sediment entrainment and deposition processes have a feedback effect on the local ow hydrodynamics. Bed morphological change is of great socio-economic and environmental importance in that it affects navigation, flood risk management, water quality, species diversity, and overall river sustainability. This thesis describes a mathematical model of the depth-averaged shallow water-sediment equations based on mass and momentum conservation laws. A 2D numerical model is then presented of the fully coupled, variable-density governing equations, which are solved using a Godunov-type HLLC scheme. Dependent variables are specially selected in the numerical model to handle the presence of the variable-density mixture in the mathematical formulation. The model includes suspended sediment, bedload transport, and bed morphological change. The numerical model is verified against benchmark analytical and semi-analytical solutions for complicated, clear water flows, bedload transport and suspended sediment transport. The well-balanced property of the governing equations is verified for a variable-density dam break flow over a bed step. Simulations of an idealised dam-break flow over an erodible bed, in excellent agreement with previously published results, validate the ability of the model to capture complex water-sediment interactions under rapidly-varying flow conditions and a mobile bed, and validate the eigenstructure of the system of variable-density governing equations. The model is then further validated against laboratory based data for complex 2D partial dam breaks over fixed and mobile beds, respectively. The simulations of 2D dam break flows over mobile beds highlight the sensitivity of the results to the choice of closure relationships for sediment transport. To investigate this further, a parameter study is carried out using a variety of commonly used empirical formulae for suspended sediment transport. The numerical model is also used to inform a theoretical model that predicts the flow through and around a porous obstruction in a shallow channel. This problem is relevant to several practical applications, including flow through aquatic vegetation and the performance of arrays of tidal turbines in a finite-width tidal channel. The theoretical model is used to reinterpret the core flow velocities in laboratory-based data for an array of emergent cylinders in a shallow channel. Comparison with experimental data indicates the maximum obstacle resistance for which the theoretical model is valid. In a final application, the theoretical model examines the optimum arrangement of tidal turbines to generate power in a tidal channel, confirming that natural bed resistance increases the power extraction potential for a partial tidal fence.

Les courants marins sont aujourd’hui considérés comme une source d’énergie renouvelable prometteuse. De nombreux projets internationaux consistent à installer différents types de convertisseurs d’énergie des courants marins. La caractérisation des ressources marines est alors essentielle pour optimiser cette production d’énergie. En particulier, les zones à fort potentiel hydrolien sont sujettes à une turbulence multi-échelles, allant de petits tourbillons capables de solliciter les pales en fatigue aux gros tourbillons pouvant perturber la production de la turbine. Une meilleure connaissance de la génération de ces tourbillons et de leur propagation est essentielle. C’est l’objet du projet ANR/FEM THYMOTE (Turbulence, Hydrolienne, Modélisation, Observations et TEsts en bassin) avec comme site d’étude le Raz Blanchard : l’un des sites les plus prometteurs d’Europe. L’une des questions posées concerne la capacité des grandes structures morphologiques du fond marin à produire des tourbillons. La méthode utilisée est l’emploi d’un modèle régional 3D pour couvrir la zone occupée par ces reliefs.Les modèles régionaux tels que TELEMAC-3D utilisent une fermeture turbulente de type URANS (Unsteady Reynolds Averaged Navier Stokes), avec par exemple le modèle $k-\varepsilon$. Cette approche ne permet pas une description fine des instationnarités de la turbulence. Cependant, grâce à l’augmentation des performances de calcul, la méthode Large Eddy Simulation (LES) devient envisageable. Celle-ci s’appuie sur un filtrage de l’écoulement, et consiste à simuler uniquement les plus grandes échelles de turbulence. Les plus petites, elles, sont modélisées. Le code TELEMAC-3D a été modifié durant cette thèse de manière à introduire cette fermeture turbulente. Le code développé permet de simuler des écoulements à surface libre en tenant compte d'une large gamme d'échelles allant de la turbulence à la propagation de la marée. Le code TELEMAC-LES a été validé sur la base de résultats expérimentaux issus de la littérature. Il est ensuite utilisé pour étudier les écoulements turbulents dans le Raz Blanchard grâce à une stratégie par emboîtement. La méthode LES permet alors une description fine de la turbulence de ces milieux, conduisant à l’identification de structures tourbillonnaires énergétiques, et donc la définition des zones les plus appropriées pour l’installation d'hydroliennes. / Nowadays tidal currents are considered a promising renewable energy source. Many worldwide projects involve the installation of different types of marine current energy converters. The characterisation of marine resources is therefore essential to increase efficiency of energy production. Areas with high hydroturbine potential are particularly subject to multi-scale turbulence, ranging from small vortices able to cause large fatigue loads, to large vortices capable of disrupting turbine production. A better knowledge of the generation of these eddies and their propagation is essential. This is the purpose of the ANR/FEM THYMOTE project (Turbulence, Hydrolienne, Modélisation, Observations et TEsts en bassin) studying one of the most promising sites in Europe: the Alderney Race. One of the questions raised concerns the ability of large morphological structures on the seabed to produce eddies. The adopted method uses a 3D regional model to cover the area occupied by these bedforms.Regional models such as TELEMAC-3D use a turbulent URANS (Unsteady Reynolds Averaged Navier Stokes) closure, with for example the $k-\varepsilon$. This approach does not allow a detailed description of the instability of turbulence. However, thanks to the increase in computing resources, the large scale method (LES) becomes feasible. This is based on flow filtering, and consists of simulating only the largest turbulence scales, whereas the smaller ones are modeled. The TELEMAC-3D code was modified during this thesis in order to introduce this turbulent closure. The code developed allows free surface flows to be simulated over a wide range of scales from turbulence to tidal propagation. The TELEMAC-LES code has been validated on the basis of experimental results from the literature. It is then used to study turbulent flows in the Alderney Race using a nesting strategy. The LES method allows a detailed description of the turbulence of these environments. It finally leads to the identification of energetic vortex structures, and thus the definition the most appropriate zones for the installation of tidal turbines.

The master's thesis contains 97 pages, 29 tables, 33 figures and 43 sources of literature. The paper presents a study on increasing the efficiency of the power supply of remote offshore oil and gas facilities through the use of horizontal-axial tidal stream turbines. A general theoretical description of floating semi-submersible drilling rigs and a more detailed description of semi-submersible drilling unit 6000/200 type "Shelf" are presented. A theoretical description of various devices that transform the kinetic energy of water into electric power is also presented, and horizontal-axial tidal stream turbines are considered in more detail. The technical calculation for determining the parameters of turbines is carried out. The calculation was used for the case where one or several turbines are used. Economic analysis was carried out with different indicators, namely: the price of diesel fuel, the price of equipment, the load factor of hydro generators, the discount rate and the productivity of the drilling unit. An analysis of the reduction of carbon dioxide emissions was also carried out. Conclusions were presented on the effectiveness of the introduction of this method of energy supply. / Magisteruppsatsen innehåller 97 sidor, 29 tabeller, 33 figurer och 43 litteraturkällor. I papperet presenteras en studie om att effektiviteten i kraftförsörjningen av avlägsna offshore olje- och gasanläggningar ökar genom användning av horisontella axiella tidvattenströmsturbiner. En generell teoretisk beskrivning av flytande halvdämpbara borriggar och en mer detaljerad beskrivning av semi-submersible borrningsenhet 6000/200 typ "Hylla" presenteras. En teoretisk beskrivning av olika anordningar som omvandlar den kinetiska energin hos vatten till elkraft presenteras också, och horisontella axiella tidvattenströmsturbiner behandlas mer detaljerat. Den tekniska beräkningen för att bestämma parametrarna för turbiner utförs. Beräkningen användes för det fall där en eller flera turbiner används. Ekonomisk analys utfördes med olika indikatorer, nämligen: priset på dieselbränsle, priset på utrustning, belastningsfaktorn för hydrageneratorer, diskonteringsräntan och borrningens produktivitet. En analys av minskningen av koldioxidutsläpp gjordes också. Slutsatser presenterades om effektiviteten av införandet av denna metod för energiförsörjning.

The tidal stream industry has seen large growth in recent years, and the number of pre-commercial scale devices currently being tested reflects this development. However, commercialising this technology whilst showing that their environmental impacts is minimal remains a challenge. The impact on benthic communities is not considered to be a key strategic consenting issue, yet it is anticipated that the benthic habitat will change as a result of the presence of tidal turbines. To date, only single tidal turbine devices have been installed to demonstrate the application of tidal stream technology but despite successful tests there are still uncertainties surrounding the quantitative impacts these turbines have on local benthic communities. Unlike the wind industry, where physical effects of wind turbines have been catalogued through deployment of thousands of turbines, the tidal stream industry lacks these array scale quantitative data. Local impacts are known, but understanding the scale of the impacts and their relative significance of large arrays remains unknown. Tidal turbines (both single and arrays) interact with the hydrodynamics by decreasing the near field current flow directly in its wake through energy extraction and the drag caused by the physical structure. However, turbines may also affect the far field hydrodynamics, altering bed characteristics, sediment transport regimes and suspended sediment concentrations. As benthic habitats are closely linked to the physical seabed composition and the hydrodynamic conditions, the benthic environment is affected by to changes in the current flow. This thesis presents a series of studies investigating the interaction between tidal turbines and the benthic environment. Based on the hydrodynamic modelling software, TELEMAC2D, a numerical model has been developed to investigate the hydrodynamic impact of a single tidal array at Ramsey Sound, Pembrokeshire as well as the cumulative impact of multiple tidal developments in the Irish Sea. Based on the results of the models, the hydrodynamic outputs were used as inputs to drive a species distribution model, based on the software MaxEnt, to investigate how the distribution of benthic species altered in the presence of a 10MW tidal array at Ramsey Sound. Results of the study showed the development would have a minimal negative impact on the benthic environment.

Ce mémoire traite de la problématique d’hybridation éolienne-hydrolienne. Elle pose d’abord l’hypothèse d’une éolienne basée sur une Machine Synchrone à Aimants Permanents (MSAP) et une hydrolienne utilisant une Machine Asynchrone à Double Alimentation (MADA). Puis, elle présente la modélisation des différents éléments qui composent chacune des chaines de conversion, de la turbine à la connexion au réseau en passant par la machine électrique et les convertisseurs statiques. Des stratégies de commande y sont aussi développées. Celles-ci permettent d’extraire le maximum d’énergie tout en tenant compte des limites des systèmes. La thèse étudie également deux possibilités de couplage d’une éolienne et hydrolienne qui toutes deux utilisent une MSAP. Les résultats de simulation obtenus des modèles que nous avons développés dans l’environnement Matlab/Simulink/ SimpowerSystem permettent de valider les stratégies de commande utilisées et de conclure qu’un bon choix serait d’opter pour le couplage au niveau du bus continu. / AThis thesis addresses the problem of wind-tidal turbines hybridization. It first raises the hypothesis of a wind turbine based on Synchronous Permanent Magnet Machine (PMSM) and a tidal using a Double-Fed Induction Generator (DFIG). So, it presents the modeling of different elements that make up each system studied, from the turbine to the network connection through the electric machine and static converters. Control strategies are also developed. These are used to extract the maximum energy while taking into account the limitations of the systems. The thesis also examines two possible coupling of wind and tidal turbines which both use a PMSM. The simulation results of the models that we have developed in Matlab / Simulink / SimpowerSystem allow to validate the control strategies and conclude that a good choice would be to opt for coupling to the DC bus.

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.

Anthropogenic climate change is approaching predicted tipping points and there is an urgent need to de-carbonize energy systems on a global scale. Generation technologies that do not emit greenhouse gas need to be rapidly deployed, and energy grids need to be updated to accommodate an intermittent fluctuating supply. Rapidly advancing battery technology, cost reduction of solar and wind power and other emerging generation technologies are making the needed changes technically and economically feasible. Extracting energy from fast-flowing tidal currents using turbines akin to those used in wind farms, offers a reliable and predictable source of GHG free energy. The tidal power industry has established the technical feasibility of tidal turbines, and is presently up-scaling deployments from single isolated units to large tidal farms containing many turbines. However there remains significant economic uncertainty in financing such projects, partially due to uncertainty in predicting the long-term energy yield. Since energy yield is used in calculating the project revenue, it is of critical importance. Predicting yield for a prospective farm has not received sufficient attention in the tidal power literature. this task has been the primary motivation for this thesis work, which focuses on establishing and validating simulation-based procedures to predict flows through large tidal farms with many turbines, including the back effects of the turbines. This is a challenging problem because large tidal farms may alter tidal flows on large scales, and the slow-moving wake downstream of each rotor influences the inflow to other rotors, influencing their performance and loading. Additionally, tidal flow variation on diurnal and monthly timescales requires long-duration analysis to obtain meaningful statistics that can be used for forecasting. This thesis presents a hybrid simulation method that uses 2D coastal flow simulations to predict tidal flows over long durations, including the influence of turbines, combined with higher-resolution 3D simulations to predict how wakes and local bathymetry influence the power of each turbine in a tidal farm. The two simulation types are coupled using a method of bins to reduce the computational cost within reasonable limits. The method can be used to compute detailed 3D flow fields, power and loading on each turbine in the farm, energy yield and the impact of the farm on tidal amplitude and phase. The method is demonstrated to be computationally tractable with modest high-performance computing resources and therefore are of immediate value for informing turbine placement, comparing turbine farm-layout cases and forecasting yield, and may be implemented in future automated layout optimization algorithms. / Graduate

This thesis details the development, verification and validation of an unsteady unstructured high order (≥ 3) h/p Discontinuous Galerkin - Fourier solver for the incompressible Navier-Stokes equations on static and rotating meshes in two and three dimensions. This general purpose solver is used to provide insight into cross-flow (wind or tidal) turbine physical phenomena. Simulation of this type of turbine for renewable energy generation needs to account for the rotational motion of the blades with respect to the fixed environment. This rotational motion implies azimuthal changes in blade aero/hydro-dynamics that result in complex flow phenomena such as stalled flows, vortex shedding and blade-vortex interactions. Simulation of these flow features necessitates the use of a high order code exhibiting low numerical errors. This thesis presents the development of such a high order solver, which has been conceived and implemented from scratch by the author during his doctoral work. To account for the relative mesh motion, the incompressible Navier-Stokes equations are written in arbitrary Lagrangian-Eulerian form and a non-conformal Discontinuous Galerkin (DG) formulation (i.e. Symmetric Interior Penalty Galerkin) is used for spatial discretisation. The DG method, together with a novel sliding mesh technique, allows direct linking of rotating and static meshes through the numerical fluxes. This technique shows spectral accuracy and no degradation of temporal convergence rates if rotational motion is applied to a region of the mesh. In addition, analytical mappings are introduced to account for curved external boundaries representing circular shapes and NACA foils. To simulate 3D flows, the 2D DG solver is parallelised and extended using Fourier series. This extension allows for laminar and turbulent regimes to be simulated through Direct Numerical Simulation and Large Eddy Simulation (LES) type approaches. Two LES methodologies are proposed. Various 2D and 3D cases are presented for laminar and turbulent regimes. Among others, solutions for: Stokes flows, the Taylor vortex problem, flows around square and circular cylinders, flows around static and rotating NACA foils and flows through rotating cross-flow turbines, are presented.