La magnétohydrodynamique (MHD) est la science et le formalisme qui décrivent les mouvements d'un fluide conducteur d'électricité. Il est possible que de tels mouvements donnent lieu à l'effet dynamo qui consiste en la génération d'un champ magnétique stable et de grande échelle. Ce phénomène est vraisemblablement à l'origine des champs magnétiques des planètes, des étoiles et des galaxies. <p><p>Il est surprenant qu'alors que les mouvements fluides à l'intérieur de ces objets célestes sont turbulents, les champs magnétiques généré soient de grande échelle spatiale et stables sur de longues périodes de temps. De plus, ils peuvent présenter une dynamique temporelle régulière comme c'est le cas pour le champ magnétique solaire dont la polarité s'inverse tous les onze ans. <p><p>Décrire et prédire les mouvements d'un fluide turbulent reste l'un des problèmes les plus difficiles de la mécanique classique. <p>%La description aussi bien analytique que numérique d'un fluide hautement turbulent est d'une effroyable complexité, si pas tout simplement impraticable. Dans cette situation, <p>Il est donc utile de construire des modèles aussi proches que possible du système de départ mais de moindre complexité de sorte que des études théoriques et numériques deviennent envisageables.<p><p>Deux approches ont été considérées ici. D'une part, nous avons développé des modèles présentant un très petit nombre de degrés de liberté (de l'ordre de la dizaine). Une étude analytique est alors possible. Ces modèles ont une dépendance en les paramètres physiques - nombres de Reynolds cinétique et magnétique et injection d'hélicité - qualitativement similaire aux dynamos célestes et expérimentales.<p><p>D'autre part, les modèles en couches permettent de caractériser les transferts d'énergie entre les structures de différentes tailles présentes au sein du champ de vitesse. Nous avons développé un nouveau formalisme qui permet d'étudier aussi les échanges avec le champ magnétique. <p><p>De plus, nous proposons une étude de la MHD dans le cadre de la décomposition hélicoïdale des champs solénoïdaux - une idée similaire à la décomposition de la lumière en composantes polarisées et que nous sommes les premiers à appliquer à la MHD. Nous avons montré comment exploiter cette approche pour déduire systématiquement des modèles simplifiés de la MHD. En particulier, nos méthodes multiplient le nombre de situations descriptibles par les modèles en couche comme par exemple le problème anisotrope de la turbulence en rotation. Elles permettent aussi de construire des modèles à basse dimension en calquant les résultats de simulations numériques directes. Ces modèles peuvent alors être étudiés à moindre coûts.<p><p><p>_______________<p><p><p><p><p>Magnetohydrodynamics (MHD) is both the science and the formalism that describe the motion of an electro-conducting fluid. Such motion may yield the dynamo effect consisting in the spontaneous generation of a large scale stationary magnetic field. This phenomenon is most likely the reason behind the existence of planetary, stellar and galactic magnetic fields. <p>\ / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Indiana University-Purdue University Indianapolis (IUPUI) / This study seeks to improve the understanding of inlet conditions of a large rotor-stator cavity in a turbofan engine, often referred to as the drive cone cavity (DCC). The inlet flow is better understood through a higher fidelity computational fluid dynamics (CFD) modeling of the inlet to the cavity, and a coupled finite element (FE) thermal to CFD fluid analysis of the cavity in order to accurately predict engine component temperatures. Accurately predicting temperature distribution in the cavity is important because temperatures directly affect the material properties including Young's modulus, yield strength, fatigue strength, creep properties. All of these properties directly affect the life of critical engine components. In addition, temperatures cause thermal expansion which changes clearances and in turn affects engine efficiency. The DCC is fed from the last stage of the high pressure compressor. One of its primary functions is to purge the air over the rotor wall to prevent it from overheating. Aero-thermal conditions within the DCC cavity are particularly challenging to predict due to the complex air flow and high heat transfer in the rotating component. Thus, in order to accurately predict metal temperatures a two-way coupled CFD-FE analysis is needed. Historically, when the cavity airflow is modeled for engine design purposes, the inlet condition has been over-simplified for the CFD analysis which impacts the results, particularly in the region around the compressor disc rim. The inlet is typically simplified by circumferentially averaging the velocity field at the inlet to the cavity which removes the effect of pressure wakes from the upstream rotor blades. The way in which these non-axisymmetric flow characteristics affect metal temperatures is not well understood. In addition, a constant air temperature scaled from a previous analysis is used as the simplified cavity inlet air temperature. Therefore, the objectives of this study are: (a) model the DCC cavity with a more physically representative inlet condition while coupling the solid thermal analysis and compressible air flow analysis that includes the fluid velocity, pressure, and temperature fields; (b) run a coupled analysis whose boundary conditions come from computational models, rather than thermocouple data; (c) validate the model using available experimental data; and (d) based on the validation, determine if the model can be used to predict air inlet and metal temperatures for new engine geometries. Verification with experimental results showed that the coupled analysis with the 3D no-bolt CFD model with predictive boundary conditions, over-predicted the HP6 offtake temperature by 16k. The maximum error was an over-prediction of 50k while the average error was 17k. The predictive model with 3D bolts also predicted cavity temperatures with an average error of 17k. For the two CFD models with predicted boundary conditions, the case without bolts performed better than the case with bolts. This is due to the flow errors caused by placing stationary bolts in a rotating reference frame. Therefore it is recommended that this type of analysis only be attempted for drive cone cavities with no bolts or shielded bolts.

Indiana University-Purdue University Indianapolis (IUPUI) / This thesis concentrates on the study of hybrid gas journal bearings (bearings with externally pressurized mass addition). It differs from most work in that it goes back to “basics” to explore the hydrodynamic phenomena in the bearing gap. The thesis compares geometrically identical bearings with 2 configurations of external pressurization, porous liners where mass-addition compensation is varied by varying the liner’s permeability, and bushings with 2 rows of 6 feedholes where the mass-addition compensation is varied by the feedhole diameter. Experimentally, prototype bearings with mass-addition compensation that spans 2 orders of magnitude with differing clearances are built and their aerostatic properties and mass addition characteristics are thoroughly tested. The fundamental equations for compressible, laminar, Poiseuille flow are used to suggest how the mass flow “compensation” should be mathematically modeled. This is back-checked against the experimental mass flow measurements and is used to determine a mass-addition compensation parameter (called Kmeas) for each prototype bushing. In so doing, the methodology of modeling and measuring the mass addition in a hybrid gas bearing is re-examined and an innovative, practical, and simple method is found that makes it possible to make an “apples-to-apples” comparison between different configurations of external pressurization. This mass addition model is used in conjunction with the Reynolds equation to perform theory-based numerical analysis of virtual hybrid gas journal bearings (CFD experiments). The first CFD experiments performed use virtual bearings modeled to be identical to the experimental prototypes and replicate the experimental work. The results are compared and the CFD model is validated. The ontological significance of appropriate dimensionless similitude parameters is re-examined and a, previously lacking, complete set of similitude factors is found for hybrid bearings. A new practical method is developed to study in unprecedented detail the aerostatic component of the hybrid bearings. It is used to definitively compare the feedhole bearings to the porous liner bearings. The hydrostatic bearing efficiency (HBE) is defined and it is determined that the maximum achievable hydrostatic bearing efficiency (MAHBE) is determined solely by the bearing’s mass addition configuration. The MAHBE of the porous liner bearings is determined to be over 5 times that of the feedhole bearings. The method also presents a means to tune the Kmeas to the clearance to achieve the MAHBE as well as giving a complete mapping of the hitherto misunderstood complex shapes of aerostatic load versus radial deflection curves. This method also rediscovers the obscure phenomenon of static instability which is called in this thesis the “near surface effect” and appears to be the first work to present a practical method to predict the range of static instability and quantify its resultant stiffness fall-off. It determines that porous liner type bearings are not subject to the phenomenon which appears for feedhole type bearings when the clearance exceeds a critical value relative to its mass-addition compensation. The standing pressure waves of hydrostatic and hybrid bearings with the 2 configurations of external pressurization as well as a geometrically identical hydrodynamic bearing are studied in detail under the methodology of the “CFD microscope”. This method is used to characterize and identify the development, growth, and movement of the pressure wave extrema with increased hydrodynamic action (either increasing speed or increasing eccentricity). This method is also used to determine the “cause” of the “near surface effect”. A gedanken experiment is performed based on these results which indicates that a bearing with a “stronger aerostatic strength” component should be more stable than one with a low aerostatic strength component. Numerical instability “speed limits” are found that are also related to the hydrostatic strength of the bearing. The local conditions in the standing waves are characterized in terms of their local Mach number, Knudsen number, Reynolds number, and Taylor Number. It is concluded that low eccentricity bearing whirl can be attributed to the off load-line orientation of the bearing load force caused by the overlay of the hydrodynamic bearing standing wave onto the hydrostatic bearing wave of the hybrid bearing, whereas it is hypothesized that aperiodic and random self-excited vibration which occurs at high eccentricity, as reported in the literature, is probably due to shock waves, turbulence, near surface effect, and slip at local areas of the standing wave.