Cette thèse est dédiée à l'étude d'ondes non linéaires dans des écoulements en rotation. Dans une première partie, je me suis intéressé aux ondes de Rossby apparaissant par instabilités de thermoconvection dans une coquille sphérique en rotation représentant un modèle simplifié de noyau planétaire tellurique, et ce pour deux types de forçage: un chauffage interne correspondant à une activité radioactive du noyau et un chauffage différentiel lié à la différence de température entre les frontières interne et externe. Selon le théorème de Proudman-Taylor, l'écoulement possède une faible dépendance en la coordonnée axiale à cause de la rotation rapide. Cela permet de simplifier les modèles 3D en des modèles quasi géostrophiques 2D reposant sur une intégration axiale. Cette thèse présente la première comparaison systématique entre modèles 2D et 3D (Simitev, U-Glasgow) concernant des ondes de Rossby faiblement non linéaires. En 2D l'équation de Landau régissant l'amplitude de l'onde critique est calculée; l'amplitude de la convection et celle des écoulements zonaux ainsi prédites se comparent assez bien aux résultats 3D. L'existence d'une bifurcation sous-critique est établie à très bas nombre d'Ekman en chauffage interne et en chauffage différentiel, à condition dans ce dernier cas que le nombre de Prandtl soit petit. La seconde partie est une étude expérimentale de l'écoulement d'eau et de ses premières instabilités dans un canal annulaire creusé dans un plateau éventuellement en rotation surmonté d'un couvercle tournant. Trois cas sont étudiés: le cisaillement pur correspondant à la rotation du couvercle seul, la corotation rapide et la contrarotation pure. Le seuil d'instabilité détecté par mesures globales (visualisations par caméra vidéo) et locales par Vélocimétrie Laser Doppler se caractérise par des ondes spiralées. Dans le cas de la contrarotation pure, des structures localisées dans l'espace-temps peuvent coexister avec les ondes. Une comparaison est effectuée avec des calculs numériques (Serre, CNRS-Marseille). Un accord relativement bon est obtenu pour l'écoulement de base (vitesse azimutale) et la première instabilité (nombre de Reynolds, nombre d'onde et fréquence angulaire critiques) / A first part is devoted to the study of the Rossby waves that appear in a rotating spherical shell representing the core of a terrestrial planet by thermal instabilities for two heating types. Internal heating is driven by radioactive sources and differential heating is driven by a difference of temperature between the internal and external frontiers. According to the Proudman-Taylor theorem, the flow depends only weakly on the axial coordinate because of the high rotation rate. Thus the 3D models can be simplified into quasi-geostrophic 2D models \textit{via} an axial integration. I present the first systematic comparison between 2D and 3D models (Simitev, U-Glasgow) for weakly nonlinear Rossby waves. In 2D the Landau equation that controls the amplitude of the critical wave is calculated. Predicted convection' amplitude and zonal flows agree rather well with the 3D results. The existence of a subcritical bifurcation is established at very low Ekman numbers with internal and differential heating; in this latter case, the Prandtl number also has to be small for the bifurcation to be subcritical. The second part is an experimental study of water flows and its first instabilities in an annular channel digged in a plate which may rotate, and which is sheared by a rotating lid. Three cases are studied: a pure shear where only the lid turns, a rapid corotation and a pure contrarotation. The onset of instability is studied with global measurements (using a video camera) and local ones (Laser Doppler Anemometry) and is characterized by spiralling waves. In the case of contrarotation, patterns localized in space and time may coexist with the waves. The comparison of these results with numerical ones (Serre, CNRS-Marseille) is done and shows a rather good agreement for the basic azimutal flow and the first instability (critical Reynolds number, wavenumber and angular frequency)

This thesis investigates large scale flow rotation in two configurations. In the first, the effect of flow rotation on a laminar flame is investigated. The flame is anchored in the wake of a cylindrical bluff body. The flow rotation is introduced by turning the cylinder along its axis. It is shown by Direct Numerical Simulation (DNS), that the cylinder rotation breaks the symmetry of both flame branches. Flame Transfer Function (FTF) measurements performed by the Wiener-Hopf Inversion suggest, that low rotation rates lead to deep gaps in the gain and the flame becomes almost insensitive to acoustic perturbation at a specific frequency. It furthermore is demonstrated that this decrease in gain of the FTF is due to destructive interference of the heat release signals caused by the two flame branches. The frequency at which the gain becomes almost zero can be adjusted by tuning the cylinder rotation rate. The study suggests that controlling the symmetry of the flame could be a tool of open-loop control of thermoacoustic instabilities.

L’étude effectuée au cours de cette thèse a permis la caractérisation expérimentale des instabilités aérodynamiques se développant dans un compresseur centrifuge et une première évaluation de l’efficacité d’une stratégie de contrôle par aspiration de couche limite. Le compresseur, développé par Safran Helicopter Engines et dénommé Turbocel, est composé d’une roue directrice d’entrée, d’un rouet centrifuge splitté, d’un diffuseur radial aubé et splitté et d’un redresseur axial. Des travaux numériques antérieurs réalisés au Laboratoire de Mécanique des Fluides et d’Acoustique ont montré, aux bas régimes de rotation, un comportement singulier caractérisé par une structure d’écoulement dite « alternée » impliquant deux canaux adjacents du diffuseur radial. L’étude stationnaire réalisée sur l’ensemble des régimes de rotation du compresseur a conduit à une ségrégation des vitesses de rotation suite à l’établissement d’une variable– le taux d’asymétrie - caractérisant l’asymétrie de l’aérodynamique du diffuseur. Ce taux, quasi nul à très basse vitesse de rotation, croît jusqu’à atteindre un maximum à vitesse de rotation intermédiaire, puis s’effondre pour ré-augmenter légèrement. Une analyse fine des données instationnaires acquises à bas régimes a permis la description de deux modes de fonctionnement du compresseur associés à des structures de décollements différentes dans le diffuseur. Le premier mode est caractérisé par l’oscillation à une fréquence de l’ordre de 42 Hz d’un décollement localisé sur la face en dépression des aubes principales du diffuseur. Le second mode, à 12Hz, associé au pompage modéré du compresseur, correspond à la mise en place d’un schéma alterné et à son oscillation sur deux canaux adjacents du diffuseur.Les origines probables de ces différents modes de fonctionnement sont discutées à partir de considérations • aérodynamiques -- la mise en place d’une recirculation en tête de rouet est suspectée d’influer sur le taux d’asymétrie en modifiant l’incidence en entrée de diffuseur,• géométriques -- le nombre et le calage des aubes du diffuseur radial ainsi que la distance inter-roue indiquent une prédisposition du diffuseur à fonctionner en régime alterné sous certaines conditions d’incidence,• aéro-acoustiques -- un accrochage des fréquences aérodynamiques avec les fréquences des ondes acoustiques du banc d’essai semble se produire. Enfin, les résultats sur le contrôle d’écoulement par aspiration de couche limite à régime partiel sont présentés. Une amélioration du rendement est observée à certains points de fonctionnement, mais aucune extension de la plage de fonctionnement du compresseur n’est mesurée. Sans l’atténuer, l’aspiration permet de contrôler sur quels canaux s’établit le régime alterné. / This thesis presents an experimental characterization of the evolution of aerodynamic instabilities in a centrifugal compressor, and a first evaluation of the effectiveness of boundary layer suction as a control strategy. The compressor used in this study is Turbocel, a centrifugal compressor developed by Safran Helicopter Engines, featuring inlet guide vanes, a backswept splittered unshrouded impeller, a splittered vaned radial diffuser and axial outlet guide vanes.Previous numerical work, conducted at the Laboratoire de Mécanique des Fluides et d’Acoustique de Lyon, revealed a unusual behaviour of the compressor at low rotational speeds characterized by a distinctive alternate flow structure in the radial diffuser that develops across two adjacent blade channels. The steady analysis, which was conducted over the full operating range of rotational speeds, led to the distinction of different operating zones, following the establishment of a new indicator variable - the asymmetry rate - characterizing the asymmetry of the diffuser aerodynamics. This rate, which is close to zero at very low rotation speed, increases until it reaches a maximum value at intermediate rotational speed, before collapsing and slightly increasing again near the nominal rotational speed.Analysis of the unsteady data acquired at low speeds allowed for the characterization of two compressor operating modes, associated with different flow phenomena in the stalled diffuser. The first mode is characterized by the oscillation of a separation at 42 Hz, on the suction side of the main blades in the diffuser. The second mode, at 12Hz, associated with mild surge of the compressor, corresponds to the emergence of an alternate pattern of unsteady flow separation that occurs across two adjacent channels of the diffuser.The probable causes for these different operating modes are discussed in the context of different considerations:• aerodynamic -- the formation of a recirculation near the tip of the impeller is suspected to affect the asymmetry rate by changing the incidence angle at the diffuser inlet.• geometric -- the number and the stagger angle of the radial diffuser blades as well as the distance between the impeller and the diffuser may result in a predisposition of the diffuser to operate in an alternating mode, under certain conditions of incidence.• aero-acoustic -- as there is evidence of a lock-in of the aerodynamic frequencies with the acoustic modes of the test rig.Finally, boundary layer suction is explored as a means of flow control at partial rotational speed. Improvements in performance were observed for some operating points, however no extension of the compressor operating range was measured. Although boundary layer suction did not allow for the intensity of the oscillating separation pattern in the diffuser to be reduced, it was found to be an effective means of controlling the location of the alternate flow structure in the diffuser.

This thesis provides a numerical and theoretical investigation of transitional and turbulent enclosed rotating flows, with a focus on the formation of macroscopic coherent flow structures. The underlying processes are strongly threedimensional due to the presence of boundary layers on the discs and on the walls of the outer (resp. inner) cylindrical shroud (resp. shaft). The complexity of these flows poses a great challenge in fundamental research however the present work is also of importance for industrial rotating machinery, from hard-drives to space engines turbopumps - the design issues of the latter being behind the motivation for this thesis. The present work consists of two major investigations. First, industrial cavities are modeled by smooth rotor/stator cavities and therein the dominant flow dynamics is investigated. For the experimental campaigns on industrial machinery revealed dangerous unsteady phenomena within the cavities, the emphasis is put on the reproduction and monitoring of unsteady pressure fluctuations within the smooth cavities. Then, the LES of three configurations of real industrial turbines are conducted to study in situ the pressure fluctuations and apply the diagnostics already vetted on academic problems.

It has been recognized for some time that the very high fluence of fast (14.1MeV) neutrons produced by deuterium-tritium fusion will represent a major materials challenge for the development of next-generation fusion energy projects such as a fusion component test facility and demonstration fusion power reactor. The best-understood and most promising solutions presently available are a family of low-activation steels originally developed for use in fission reactors, but the ferromagnetic properties of these steels represent a danger to plasma confinement through enhancement of magnetohydrodynamic instabilities and increased susceptibility to error fields. At present, experimental research into the effects of ferromagnetic materials on MHD stability in toroidal geometry has been confined to demonstrating that it is still possible to operate an advanced tokamak in the presence of ferromagnetic components. In order to better quantify the effects of ferromagnetic materials on tokamak plasma stability, a new ferritic wall has been installated in the High Beta Tokamak—Extended Pulse (HBT-EP) device. The development, assembly, installation, and testing of this wall as a modular upgrade is described, and the effect of the wall on machine performance is characterized. Comparative studies of plasma dynamics with the ferritic wall close-fitting against similar plasmas with the ferritic wall retracted demonstrate substantial effects on plasma stability. Resonant magnetic perturbations (RMPs) are applied, demonstrating a 50% increase in n = 1 plasma response amplitude when the ferritic wall is near the plasma. Susceptibility of plasmas to disruption events increases by a factor of 2 or more with the ferritic wall inserted, as disruptions are observed earlier with greater frequency. Growth rates of external kink instabilities are observed to be twice as large in the presence of a close-fitting ferritic wall. Initial studies are made of the influence of mode rotation frequency on the ferritic effect, as well as observations of the effect of the ferritic wall on disruption halo currents.

The performance of an inducer with the integration of an inlet cover bleed system known as a stability control device (SCD) is investigated using computational fluid dynamics. Inducers are the first stage of high suction performance pumps and are designed to operate under cavitating conditions. Improvements in design have allowed inducers to operate stably with low inlet head conditions, however, cavitation instabilities ultimately lead to pump failure. It has been shown that inducers that employ an SCD fully suppress cavitation instabilities.The performance of an inducer is explored at both on- and off-design flow coefficients, where the flow coefficient is a normalized flow rate through the inducer. Both the cavitating and non-cavitating performance of the inducer are analyzed. Improved stability is observed when the SCD is implemented, particularly at flow coefficients below the design value. The stabilizing effect of the SCD allows the inducer to operate stably at much lower flow coefficients, which allows for significant improvements in the pumps ability to operate with minimal inlet head. Cavitation instabilities, such as rotating cavitation, are also suppressed when the SCD is implemented.As part of this work, the design space created by the SCD is explored. Variations in the SCD geometry as well as the inlet blade angle of the inducer are explored. High suction performance pumps are required to operate at very low flow coefficients in order to have the best suction performance. Traditionally, only inducers with small inlet blade angles can maintain stable operation at very low flow coefficient. Because of the stabilizing effect of an SCD, inducers with larger inlet blade angles can now operate stably at the low flow rates require for high suction performance pumps. The influence of varying the inlet blade angle is explored in inducers that employ an SCD. This provides a better understanding of the flow physics in inducers that employ an SCD and help to define their design criteria. Stable operation at low flow coefficients is achieved with the larger inlet blade angles, confirming that inducers with larger inlet blade angles that employ an SCD can be used in high suction performance pumps. Modifications to the SCD geometry are considered to better optimize the design. Variations in the SCD geometry have almost no effect on the cavitation breakdown curve for each inducer, however, the stability of the pumps is greatly influenced by the SCD geometry. Some cavitation instabilities are observed in inducers that operate with an SCD. The physics that leads to the generation of these instabilities is unique to an inducer with an SCD. Modifications to the SCD geometry can allow inducers that employ an SCD to suppress traditional cavitation instabilities that occur without an SCD as well as the new instabilities that are observed when an SCD is implemented.

La motivation principale de cette thèse est d’être capable de prédire précisément la rupture d’une tôle d’aluminium anisotrope avec un chemin de déformation linéaire et non-linéaire. Dans le cas présent, le matériau utilisé est l’AA6016 et le chemin de déformation considéré est traction uniaxiale suivie de pliage jusqu’à rupture. Deux approches sont appliquées et comparées, l’une utilise la plasticité associée (AFR) et l’autre la plasticité non-associée (NAFR). Dans le but d’obtenir une bonne représentation de l’anisotropie en AFR, un critère de plasticité très flexible est utilisé : Yld2004-18p. L’identification des paramètres est faite avec une approche inverse qui consiste à minimiser itérativement l’écart entre les résultats numériques et expérimentaux. Une fois que l’écart arrête d’évoluer (minimum local) ou atteint une valeur prédéterminé e assez faible, l’optimisation s’arrête et les derniers paramètres mis à jour sont enregistrés. En corrélation avec des travaux de recherche plus récents, un modèle NAFR est utilisé pour modéliser l’anisotropie du matériau. Il combine deux critères de plasticité qui sont utilisés pour la surface d’écrouissage et le potentiel plastique. Leurs paramètres sont identifiés grâce aux ratios de contraintes et aux valeurs r obtenues expérimentalement. Concernant la rupture, des modèles découplés macroscopiques sont étudiés : un critère Hosford-Coulomb modifié et un critère basé sur DF2014. Ces deux critères prennent en compte les trois invariants du tenseur des contraintes pour prédire la déformation équivalente à rupture mais sont identifiés avec différentes méthodes pour prendre en compte l’anisotropie de la rupture. Enfin, des résultats sur des instabilités plastiques obtenus avec un modèle NAFR sont présentés dans le but de prouver les possibilités de cette approche comparée à une approche AFR. / The main motivation of this thesis is to be able to predict accurately the fracture of an anisotropic aluminium alloy thin sheet under linear and non-linear strain paths. In the studied case, the material used is the AA6016 and the non-linear strain path considered is uniaxial tension followed by free bending until fracture. Two approaches are considered and compared which respectively use the associated flow rule (AFR) and the non-associated flow rule (NAFR). In order to obtain a good representation of the high anisotropy of the material in AFR, a very flexible yield criterion is used: Yld2004-18p. The identification of its parameters is done with an inverse approach consisting of iteratively minimizing the gap between numerical and experimental results. Once this gap stops evolving (local minimum) or reaches a low enough pre-determined value, the optimization stops and the last updated parameters are saved. In correlation with more recent research work, a NAFR model is used to model the anisotropy of the material. It combines two different yield functions that are used for the yield surface and the plastic potential. Their parameters are identified by using stress ratios and rvalues measured experimentally. Concerning the fracture, uncoupled macroscopic models are studied: a modified Hosford-Coulomb and a DF2014 based criteria. Both these criteria take into account the three invariants of the stress tensor to predict the equivalent strain to fracture but their parameters are identified with different methods to take into account the anisotropy of the fracture. Finally, results on plastic instabilities obtained with a NAFR model are presented in order to prove the possibilities of this approach compared to AFR.

Doctor of Philosophy / We investigate the linear growth and vertical structure of the magnetorotational instability (MRI) in weakly ionised, stratified accretion discs. The magnetic field is initially vertical and perturbations have vertical wavevectors only. Solutions are obtained at representative radial locations from the central protostar for different choices of the initial magnetic field strength, sources of ionisation, disc structure and configuration of the conductivity tensor. The MRI is active over a wide range of magnetic field strengths and fluid conditions in low conductivity discs. For the minimum-mass solar nebula model, incorporating cosmic ray and x-ray ionisation and assuming that charges are carried by ions and electrons only, perturbations grow at 1 AU for B < 8G. For a significant subset of these strengths (200mG < B < 5 G), the growth rate is of order the ideal MHD rate (0.75 Omega). Hall conductivity modifies the structure and growth rate of global unstable modes at 1 AU for all magnetic field strengths that support MRI. As a result, at this radius, modes obtained with a full conductivity tensor grow faster and are active over a more extended cross-section of the disc, than perturbations in the ambipolar diffusion limit. For relatively strong fields (e.g. B > 200 mG), ambipolar diffusion alters the envelope shapes of the unstable modes, which peak at an intermediate height, instead of being mostly flat as modes in the Hall limit are in this region of parameter space. Similarly, when cosmic rays are assumed to be excluded from the disc by the winds emitted by the magnetically active protostar, unstable modes grow at this radius for B < 2 G. For strong fields, perturbations exhibit a kink at the height where x-ray ionisation becomes active. Finally, for R = 5 AU (10 AU), unstable modes exist for B < 800 mG (B < 250 mG) and the maximum growth rate is close to the ideal-MHD rate for 20 mG < B < 500 mG (2 mG < B < 50 mG). Similarly, perturbations incorporating Hall conductivity have a higher wavenumber and grow faster than solutions in the ambipolar diffusion limit for B < 100 mG (B < 10 mG). Unstable modes grow even at the midplane for B > 100 mG (B ~ 1 mG), but for weaker fields, a small dead region exists. When a population of 0.1 um grains is assumed to be present, perturbations grow at 10 AU for B < 10 mG. We estimate that the figure for R = 1 AU would be of order 400 mG. We conclude that, despite the low magnetic coupling, the magnetic field is dynamically important for a large range of fluid conditions and field strengths in protostellar discs. An example of such magnetic activity is the generation of MRI unstable modes, which are supported at 1 AU for field strengths up to a few gauss. Hall diffusion largely determines the structure and growth rate of these perturbations for all studied radii. At radii of order 1 AU, in particular, it is crucial to incorporate the full conductivity tensor in the analysis of this instability, and more generally, in studies of the dynamics of astrophysical discs.

The determination of the leakage and consequently the friction factor is an important part of analyzing the flow through a seal. This is done experimentally by means of a flat plate tester, which allows for the simplified representation of the seal pattern on a flat plate surface tested under a range of clearances and pressure drops. The setup mounts a smooth plate opposite a second plate which may be smooth or have a roughened surface while the separation between plates is held constant. The present study analyzes the phenomenon of friction factor 'upset' ? wherein it was seen that as the pressure drop across the parallel plates is increased, there is a sudden increase in the friction factor (i.e. a decrease in flow rate) at a certain Reynolds number and for any further increase in the pressure differential, the friction factor shows the expected trend and decreases slowly. This phenomenon was initially believed to be an anomaly in the rig and was attributed to choking at an upstream flow control valve. The present author differs from that view and hypothesized that the reason for the abrupt change is linked to the flow mechanics of the system and the current study analyzes the same. Preliminary analysis of available data has established that the cause for the 'upset' was not related to the switch from a normal mode resonance driven by the Helmholtz frequency of the cavities on the stator to a shear layer instability, as was seen earlier by Ha. The friction factor jump for this case is therefore proposed to be due to a change of the instability modes as the fluid passes over the cavities in the plate. A detailed analysis of the physics of the flow will be carried out via a numerical simulation using a Large Eddy Simulation (LES) model from ANSYS Fluent. Results will be validated through comparisons with experimental data from the flat plate test rig.

Natural gas is the primary fuel used in industrial gas turbines for power generation. Hydrocarbon blends of methane, ethane, and propane make up a large portion of natural gas and it has been shown that dimethyl ether can be used as a supplement or in its pure form for gas turbine combustion. Because of this, a fundamental understanding of the physical characteristics such as the laminar flame speed is necessary, especially at elevated pressures to have the most relevance to the gas turbine industry. This thesis discusses the equations governing premixed laminar flames, historical methods used to measure the laminar flame speed, the experimental device used in this study, the procedure for converting the measured data into the flame speed, the results of the measurements, and a discussion of the results. The results presented in this thesis include the flame speeds for binary blends of methane, ethane, propane, and dimethyl ether performed at elevated pressures, up to 10-atm initial pressure, using a spherically expanding flame in a constant-volume vessel. Also included in this thesis is a comparison between the experimental measurements and four chemical kinetic models. The C4 mechanism, developed in part through collaboration between the National University of Ireland Galway and Texas A&M, was improved using the data presented herein, showing good agreement for all cases. The effect of blending ethane, propane, and dimethyl ether with methane in binary form is emphasized in this study, with the resulting Markstein length, Lewis number (Le), and flame stability characterized and discussed. It was noticed in this study, as well as in other studies, that the critical radius of the flame typically decreased as the Le decreased, and that the critical radius of the flame increased as the Le increased. Also, a rigorous uncertainty analysis has been performed, showing a range of 0.3 cm/s to 3.5 cm/s depending on equivalence ratio and initial pressure.