Recent increases in computing power and memory have created renewed interest in alternative grid schemes such as unstructured grids, which facilitate rapid grid generation by relaxing restrictions on grid structure. Three rotor models have been incorporated into a popular fixed-wing unstructured computational fluid dynamics (CFD) solver to increase its capability and facilitate availability to the rotorcraft community. The benefit of unstructured grid methods is demonstrated through rapid generation of high fidelity configuration models. The simplest rotor model is the steady state actuator disk approximation. By transforming the unsteady rotor problem into a steady state one, the actuator disk can provide rapid predictions of performance parameters such as lift and drag. The actuator blade and overset blade models provide a depiction of the unsteady rotor wake, but incur a larger computational cost than the actuator disk. The actuator blade model is convenient when the unsteady aerodynamic behavior needs to be investigated, but the computational cost of the overset approach is too large. The overset or chimera method allows the blades loads to be computed from first-principles and therefore provides the most accurate prediction of the rotor wake for the models investigated. The physics of the flow fields of these models for rotor/fuselage interaction are explored, along with efficiencies and limitations of each methodology.

Self propulsion modelling is important in order to accurately simulate ships and submarinesusing Computational Fluid Dynamics (CFD). However, fully resolved simulations of hull andpropeller geometries are computationally heavy and time consuming. As such there is a greatinterest in lower order CFD models of propellers. This work investigates three lower ordermodels of a non-cavitating generic submarine propeller (INSEAN E1619) in OpenFOAM. Themodels investigated are Actuator Disk (AD). Rotor Disk (RD) and Actuator Line Model (ALM).The AD model applies a momentum change based on propeller performance coefficients overa disc cell set. The RD uses Blade Element Method (BEM) to calculate a more realistic thrustdistribution over the disk. Finally the ALM applies BEM over seven rotating lines within the cellset disc. The source code to the RD model was modified according to suggestions provided fromearlier studies on the model. The ALM used was originally designed for turbines which wasrectified by changing the force projection vectors in the source code to model propellers instead.There was not enough published data to directly utilize BEM on the E1619 propeller, thus thedata was generated by conducting 2D simulations on every element. The simulations were setup to replicate results provided in earlier works with higher order models in order to compareboth quantitative and qualitative results. It was found the ALM matched the reference databest out of the models tested in this work. The RD was qualitatively similar to the time averageof the ALM fields but numerically inaccurate. The AD results were poor, both quantitativelyand qualitatively.

Because compressibility effects arise in real applications of propellers and turbines, the Actuator Disk Theory or Froude’s Momentum Theory was established for compressible, subsonic flow using the three laws of conservation and isentropic thermodynamics. The compressible Actuator Disk Theory was established for the unducted (bare) and ducted cases in which the disk was treated as the only assembly within the flow stream in the bare case and enclosed by a duct having a constant cross-sectional area equal to the disk area in the ducted case. The primary motivation of the current thesis was to predict the ideal performance of a small ram-air turbine (microRAT), operating at high subsonic Mach numbers, that would power an autonomous Boundary Layer Data System during test flights. The compressible-flow governing equations were applied to a propeller and a turbine for both the bare and ducted cases. The solutions to the resulting system of coupled, non-linear, algebraic equations were obtained using an iterative approach. The results showed that the power extraction efficiency and the total drag coefficient of the bare turbine are slightly higher for compressible flow than for incompressible flow. As the free-stream Mach increases, the Betz limit of the compressible bare turbine slightly increases from the incompressible value of 0.593 and occurs at a velocity ratio between the far downstream and the free-stream that is lower than the incompressible value of 0.333. From incompressible to a free-stream Mach number of 0.8, the Betz limit increases by 0.021 while its corresponding velocity ratio decreases by 0.036. The Betz limit and its corresponding velocity ratio for the ducted turbine are not affected by the free-stream Mach and are the same for both incompressible and compressible flow. The total drag coefficient of the ducted turbine is also the same regardless of the free-stream Mach number and the compressibility of the flow; but, the individual contributions of the turbine drag and the lip thrust to the total drag differs between compressible and incompressible flow and between varying free-stream Mach numbers. It was concluded that overall compressibility has little influence on the ideal performance of an actuator disk.

Despite the high cost of memory and CPU time required to resolve the boundary layer, a viscous unstructured grid solver has many advantages over a structured grid solver such as the convenience in automated grid generation and shock or vortex capturing by solution adaption. Since the geometry and flow phenomenon of a helicopter are very complex, unstructured grid-based methods are well-suited to model properly the rotor-fuselage interaction than the structured grid solver. In present study, an unstructured Cartesian grid solver is developed on the basis of the existing solver, NASCART-GT. Instead of cut-cell approach, immersed boundary approach is applied with ghost cell boundary condition, which increases the accuracy and minimizes unphysical fluctuations of the flow properties. The standard k-epsilon model by Launder and Spalding is employed for the turbulence modeling, and a new wall function approach is devised for the unstructured Cartesian grid solver. It is quite challenging and has never done before to apply wall function approach to immersed Cartesian grid. The difficulty lies in the inability to acquire smooth variation of y+ in the desired range due to the non-body-fitted cells near the solid wall. The wall function boundary condition developed in this work yields stable and reasonable solution within the accuracy of the turbulence model. The grid efficiency is also improved with respect to the conventional method. The turbulence modeling is validated and the efficiency of the developed boundary condition is tested in 2-D flow field around a flat plate, NACA0012 airfoil, axisymmetric hemispheroid, and rotorcraft applications. For rotor modeling, an actuator disk model is chosen, since it is efficient and is widely verified in the study of the rotor-fuselage interaction. This model considers the rotor as an infinitely thin disk, which carries pressure jump across the disk and allows flow to pass through it. The full three dimensional calculations of Euler and RANS equations are performed for the GT rotor model and ROBIN configuration to test implemented actuator disk model along with the developed turbulence modeling. Finally, the characteristics of the rotor-fuselage interaction are investigated by comparing the numerical solutions with the experiments.

<p>This dissertation investigates the optimal aerodynamic performance and design of conventional and coaxial helicopters in hover and forward flight using conventional and higher harmonic blade pitch control. First, we describe a method for determining the blade geometry, azimuthal blade pitch inputs, optimal shaft angle (rotor angle of attack), and division of propulsive and lifting forces among the components that minimize the total power for a given forward flight condition. The optimal design problem is cast as a variational statement that is discretized using a vortex lattice wake to model inviscid forces, combined with two-dimensional drag polars to model profile losses. The resulting nonlinear constrained optimization problem is solved via Newton iteration. We investigate the optimal design of a compound vehicle in forward flight comprised of a coaxial rotor system, a propeller, and optionally, a fixed wing. We show that higher harmonic control substantially reduces required power, and that both rotor and propeller efficiencies play an important role in determining the optimal shaft angle, which in turn affects the optimal design of each component. Second, we present a variational approach for determining the optimal (minimum power) torque-balanced coaxial hovering rotor using Blade Element Momentum Theory including swirl. We show that the optimal hovering coaxial rotor generates only a small percentage of its total thrust on the portion of the lower rotor operating in the upper rotor's contracted wake, resulting in an optimal design with very different upper and lower rotor twist and chord distributions. We also show that the swirl component of induced velocity has a relatively small effect on rotor performance at the disk loadings typical of helicopter rotors. Third, we describe a more refined model of the wake of a hovering conventional or coaxial rotor. We approximate the rotor or coaxial rotors as actuator disks (though not necessarily uniformly loaded) and the wake as contracting cylindrical vortex sheets that we represent as discrete vortex rings. We assume the system is axisymmetric and steady in time, and solve for the wake position that results in all vortex sheets being aligned with the streamlines of the flow field via Newton iteration. We show that the singularity that occurs where the vortex sheet terminates at the edge of the actuator disk is resolved through the formation of a 45 degree logarithmic spiral in hover, which results in a non-uniform inflow, particularly near the edge of the disk where the flow is entirely reversed, as originally hypothesized by previous authors. We also quantify the mutual interference of coaxial actuator disks of various axial spacing. Finally, we combine our forward flight optimization procedure and the Blade Element Momentum Theory hover optimization to form a variational approach to the multipoint aerodynamic design optimization of conventional and coaxial helicopter rotors. The resulting nonlinear constrained optimization problem may be used to map the Pareto frontier, i.e., the set of rotor designs for which it is not possible to improve upon the performance in one flight condition without degrading performance in the other. We show that for both conventional and coaxial rotors analyzed in hover and high speed flight, a substantial tradeoff in performance must be made between the two flight conditions. Finally, computational results demonstrate that higher harmonic control is able to improve the Pareto efficiency for both conventional and coaxial rotors.</p> / Dissertation

Tidal power, or the use of tides for electricity production, exists in many forms including tidal barrages, which exploit tidal head differentials, and turbines placed directly in regions with large tidal current velocities. The latter is actively being investigated in many countries around the world as a means of providing renewable and wholly predictable electricity (cf. wind, solar and wave power). The expansion of the in-stream tidal industry is hindered however by several factors including: turbine durability, deployment and maintenance costs, and the lack of abundant locations which meet the necessary current velocities for turbine start-up and economic power production. A new novel type of augmentation device, entitled the ‘Tidal Acceleration Structure’ or TAS (Canadian patent pending 2644792), has been proposed as a solution to the limited number of coastal regions which exhibit fast tidal currents. In preliminary investigations, the TAS, a simple Venturi section consisting of walls extending from the seafloor to above the high water mark in an hourglass shape, was found as able to more than double current velocities entering the device. The results indicated a significant advantage over other current channeling technologies and thus the need for more in-depth investigations. The main objective of the present study was to optimise the design of the TAS and to predict the power that a turbine placed within it could extract from flow. To do this, two principal methods were employed. Firstly, a 1:50 scale model of the TAS was tested and its shape optimised in a 1.5 m wide flume. Secondly, a 3D numerical model (ANSYS Fluent) was used for comparison with the experimental results. During the tests, a TAS configuration was found that could accelerate upstream velocities by a factor of 2.12. In separate tests, turbines were simulated using Actuator Disc Theory and porous plates. The TAS-plate combination was found to be able to extract up to 4.2 times more power from flow than the stand-alone plate, demonstrating that the TAS could provide turbines with a significant advantage in slower currents. Though further research is needed, including the testing of a larger TAS model in conjunction with a small in-stream turbine, the results of this thesis clearly demonstrate the potential of the TAS concept to unlock vast new areas for tidal energy development.

En dépit d’un développement opérationnel actuel seulement émergeant le long des côtes françaises, l’extraction de l’énergie cinétique des courants de marée par les hydroliennes apparaît, dès à présent, comme une solution prometteuse pour contribuer, localement, à la transition énergétique de territoires insulaires déconnectés du réseau électrique continental, ayant des besoins électriques limités, et situés dans des espaces naturels sensibles et/ou touristiques intégrant un patrimoine visuel à préserver. Ce travail de thèse se consacre à la modélisation numérique tridimensionnelle des impacts hydrodynamiques et sédimentaires d’hydroliennes d’axe horizontal déployées dans le site pilote d’implantation de fermes hydroliennes du Passage du Fromveur, au cœur du parc naturel marin d’Iroise, à la pointe Bretagne. Les simulations numériques se basent sur le modèle océanographique ROMS (« Regional Ocean Modelling System ») modifié pour intégrer un sous-modèle théorique de disque actuateur assimilant l’hydrolienne à un disque poreux opposant à l’écoulement une force équivalente à la poussée de la turbine. La méthode est validée avec des mesures effectuées en laboratoire dans le sillage d’un disque poreux de 10 cm de diamètre (D) immergé dans un courant permanent. Une résolution spatiale minimale égale à D/10 est nécessaire pour reproduire les observations. Après une étude de convergence numérique à échelle réelle, le modèle ROMS est appliqué, selon une approche gigogne de maillages imbriqués focalisée sur le Passage du Fromveur, pour examiner les interactions des sillages et les effets cumulés au sein d’un parc de huit turbines de 10 m de diamètre susceptible de satisfaire au besoin énergétique de l’île d’Ouessant. L’agencement des turbines suit les recommandations communément adoptées avec une disposition en quinconce et des espacements longitudinaux de 10D et latéraux de 5D. En condition de vive-eau moyenne, le désalignement du courant au pic de flot exacerbe les interactions entre sillages, réduisant la production énergétique du parc de près de 15 % par rapport à celle du pic de jusant. Ce déficit de production énergétique est limité à 2 % en ramenant l’espacement latéral des turbines à 3D. Les prédictions de ROMS sont enfin exploitées pour appréhender l’influence de cette dernière configuration de parc sur la dynamique sédimentaire locale. Les principaux effets attendus concernent (i) le dépôt de sédiments de plus de 2 mm de diamètre dans le sillage des dispositifs et (ii) la mise en mouvement de cailloutis de 5 cm de diamètre entre les sillages. / In spite of a present only emerging operational development along the coast of France, the extraction of the kinetic energy of tidal currents by turbines appears as a promising solution to contribute locally to the energetic transition of insulary territories that are not connected to the continental electricity grid, with limited power needs, and are located in protected and touristic natural areas with a visual heritage to preserve. This doctoral thesis was devoted to the three-dimensional numerical modelling of the hydrodynamic and sedimentary impacts induced by horizontal-axis turbines deployed within the pilot site of the Fromveur Strait, in the natural marine Iroise park, off western Brittany. Numerical simulations are performed with the oceanographic model ROMS (Regional Ocean Modelling System) modified to integrate a theoretical actuator-disk sub-model that assimilates the device to a porous disc opposing the flow with a force equal to the turbine’s thrust. This method was assessed against laboratory measurements within the wake of a porous disc with a diameter (D) of 0.1 m immersed in a permanent flow. A spatial resolution as small as D/10 was required to reproduce observations. After a numerical convergence study at full scale, ROMS was applied, using a nested grid approach targetted towards the Fromveur Strait, to examine wake interactions and cumulative effects within an array of height turbines of 10-m-diameter expected to fulfill the energy need of the island of Ushant. The array layout followed the commonly recommended staggered configuration with respective longitudinal and lateral spacings of 10D and 5D. During spring tidal conditions, the misalignment of the peak flood flow enhanced the wake interactions, reducing by about 15% the array power production in comparison with the peak ebb flow. This lack of power production capacity was lowered to 2% by reducing the lateral spacing to 3D. ROMS predictions were finally exploited to address the influence of this last array of turbines on the local sediment dynamics. Main effects were expected on (i) the deposition of sediments with diameter over 2 mm within the turbine wakes and (ii) the setting in motion of gravels with a diameter of 5 cm between the wakes.

The primary motivation for this work was to predict the conditions that would yield minimum drag for a small Ram-Air Turbine used to provide a specified power requirement for a small flight test instrument called the Boundary Layer Data System. Actuator Disk Theory was used to provide an analytical model for this work. Classic Actuator Disk Theory (CADT) or Froude’s Momentum Theory was initially established for quasi-one-dimensional flows and inviscid fluids to predict the power output, drag, and efficiency of energy-extracting devices as a function of wake and freestream velocities using the laws of Conservations of Mass, Momentum, and Energy. Because swirl and losses due to the effects of viscosity have real and significant impacts on existing turbines, there is a strong motivation to develop models which can provide generalized results about the performance of an energy-extractor, such as a turbine, with the inclusion of these effects. A model with swirl and a model with losses due to the effects of viscosity were incorporated into CADT which yielded equations that predicted the performance of an energy-extractor for both un-ducted and ducted cases. In both of these models, for this application, additional performance parameters were analyzed including the drag, drag coefficient, power output, power coefficient, force coefficient, and relative efficiency. For the un-ducted CADT, it is well known that the wake-to-freestream velocity ratio of 1/3 will give the maximum power extraction efficiency of 59.3%; this result is called the Betz limit. However, the present analysis shows that reduced drag for a desired power extraction will occur for wake-to-freestream velocity ratios higher than the value of 1/3 which results in maximum power extraction efficiency. This in turn means that a turbine with a larger area than the smallest possible turbine for a specified power extraction will actually experience a lower drag. The model with the inclusion of swirl made use of the Moment of Momentum Theorem applied to a single-rotor actuator disk with no stators, in addition to the laws of Conservation of Mass, Momentum, and Energy from the CADT. The results from the model w/swirl showed that drag remains unchanged while power extracted decreases with the addition of swirl, with swirl effects becoming more severe for tip speed ratios below about 5. As for CADT, reduced drag for a specified power extraction can be achieved when the wake-to-freestream velocity ratio is higher that than which provides maximum power extraction efficiency. The model w/losses due to viscosity incorporated the losses into the Conservation of Energy relationship. The results from the model w/losses showed that there is a distinct wake-to-freestream velocity ratio at which minimum drag for a specified power output is achieved, and that this velocity ratio is usually—but not always—higher than that for which the power extraction efficiency is a maximum. It was concluded that a lower drag for a specified power output of an energy-extractor can usually be achieved at a wake-to-freestream velocity ratio higher than that which produces the v maximum power extraction efficiency. The latter condition, known as the Betz limit for CADT, and which defines the minimum size for a turbine to provide a specified power extraction, is therefore not the correct target design condition to achieve lowest drag for a small Ram-Air Turbine to power BLDS.

Le phénomène connu sous l'appellation "méandrement" (ou meandering) désigne les variations aléatoires de la trajectoire du sillage aérodynamique d'une éolienne. Ce phénomène est responsable de contraintes mécaniques particulières sur les éoliennes positionnées dans le sillage d'autres éoliennes et joue donc rôle dans la conception et dans la prévision de production des parcs éoliens.Ce travail propose d'étudier le méandrement par des moyens expérimentaux et numériques. La problématique est traitée en deux parties, la première portant sur la modélisation de l'écoulement de couche limite atmosphérique, avec une attention particulière portée à la modélisation des grandes échelles de la turbulence atmosphérique. La seconde partie porte sur l'étude du sillage d'un disque actuateur soumis à un écoulement atmosphérique. Chacune de ces parties comporte un volet expérimental et un volet numérique. La modélisation numérique instationnaire de l'écoulement atmosphérique fait intervenir une technique de génération stochastique de champs de vitesse turbulente avec évolution temporelle, spécialement développée au cours de la présente thèse et à laquelle un chapitre spécifique est dédié.L'un des principaux résultats est que le méandrement du sillage est fortement corrélé avec les grandes échelles de la turbulence atmosphérique. / The phenomenon known as meandering describes the unsteady trajectory variations of the wake of a wind turbine. This phenomenon is responsible for specific mechanical stresses on turbines positioned in the wake of other turbines. As such, this phenomenon must be accounted for in the design and operation of wind turbine plants.This work uses numerical fluid simulation and wind tunnel testing in order to study the meandering of the wake of a wind turbine. The subject is discussed in two parts. The first part discusses the modeling of the atmospheric boundary layer, with a focus on the large scales of the atmospheric turbulence. The second part is a study of the behavior of the wake of an actuator disc model in atmospheric wind conditions.Both parts include experimental and numerical work. The numerical simulation of the atmospheric boundary layer involves the generation of synthetic turbulent velocity time series by mean of a stochastic technique developed during this thesis, to which a chapter is dedicated.One of the main results of this work is that the meandering is highly correlated with the large scales of the atmospheric turbulence.

Cette étude se propose d’analyser le comportement du double rotor contra-rotatif caréné dans lecadre des échelles réduites des microdrones, pour exploiter le potentiel d’amélioration desperformances stationnaires des rotors libres. La demande d’une performance propulsive de hautniveau, alors que les échelles sont très réduites constitue un véritable défi scientifique. De façongénérale, par rapport au rotor libre, l’ajout de la carène permet de piloter la contraction del’écoulement et offre un potentiel de poussée de carène. La tuyère par sa condition d’adaptationpilote le débit entrant à puissance donnée. L’augmentation du débit massique, par comparaison ausystème de rotor libre, amplifie la poussée à travers la dépression distribuée sur toute la surface decaptation. Pour comprendre les lois de fonctionnement d’un système propulsif caréné, il a d’abord été proposé un modèle théorique simplifié basé sur une extension de la théorie de Froude pour les rotors libres: le système rotor est assimilé à un disque actuateur, générateur de débit dans une conduite à section variable. Une simulation Navier Stokes 2D axisymétrique a permis d’optimiser les paramètres de forme du carénage. Les simulations ont confirmé l’influence déterminante des sections d’entrée et de sortie, et relativisé l’impact des formes possibles, pourvu que les variations de sections limitent le décollement de la couche limite. Après conception d’un banc d’essai utilisant un doublet de rotor coaxial placé dans cette carène optimisée, l’étude expérimentale complète et confirme les performances globales du système et qualifie l’écoulement méridien. Enfin, une simulation 3D instationnaire a été entreprise pour compléter l’analyse de l’écoulement autour des rotors. / This study aims to analyze the behavior of shrouded, contrarotating coaxial rotor in the reducedMAVs’ scale in order to exploit its potential to improve the free rotor steady performance. The highhover ability under low operational Reynolds number is therefore, a scientific challenge. Generally,comparing with free rotor, the addition of the shroud decreases the flow contraction and gives thepotential to generate an extra thrust. A suitable nozzle can control the mass flow for a given power.The increased mass flow, comparing with free rotor, amplifies the thrust offered by the lowpressure formed at the air entrance. To understand the principals of shrouded propulsion system, a simplified theory model was first proposed through the extension of Froude theory for free rotors: the double rotor is initially treated as an actuator disk, generating the flow at varied sections through the shroud passage. A 2D simulation which accounts for an axial flow of viscous effects within the actual shroud profile, confirmed effects of all defined geometrical parameters. It further demonstrated that within the non-stalling region of the different crosssections, shroud shape and inlet shape do not have asignificant impact on performance. The experimental study, carried out with coaxial rotor, contributed to the confirmation of the overall performance and the approximation of the flow field through the shroud. Meanwhile, the 3D simulation, developed to better model the actual coaxial rotor in counter rotation, was validated to well solve the steady performance. It was applied to complement the analysis of the flow around the coaxial rotor.