Les procédés de trempe sont largement répandus dans l'industrie en particulier dans le domaine de l'automobile, du nucléaire et de l'aérospatiale car ils ont un impact direct sur la microstructure, les propriétés mécaniques et les contraintes résiduelles de pièces critiques. La trempe est un processus fortement non-linéaire à cause des couplages forts entre la mécanique des fluides, les transferts thermiques aux différentes interfaces, les transformations de phase du solide et l'ébullition du milieu de trempe. Malgré les progrès effectués par la simulation numérique, ce procédé reste extrêmement difficile à modéliser.Dans ce travail, nous proposons le développement d'outils numériques permettant la simulation réaliste de ce procédé à l'échelle industrielle. La mécanique des fluides est simulée en utilisant une méthode d'Elements Finis stabilisés permettant de considérer des écoulements à haut nombre de Reynolds. Les transferts thermiques sont calculés directement sans l'utilisation de coefficients de transferts empiriques, en utilisant le couplage fort entre le fluide et le solide. Nous avons développé un modèle de changement de phase pour l'eau permettant de considérer les différents régimes d'ébullition. Une formulation unifiée des équations de Navier-Stokes, considérant une phase compressible et une phase incompressible a été développée afin de prendre en compte plus précisément la dynamique de la vapeur et de l'eau. Une procédure dynamique d'adaptation anisotrope de maillage, permettant une description plus fine des interfaces et une prise en compte plus précise des caractéristiques des écoulements est utilisée.Des exemples numériques exigeants ainsi qu'une validation expérimentale permettent d'évaluer la précision et la robustesse des outils proposés.Les outils développés permettent ainsi l'optimisation du mode opératoire du procédé, des ressources consommées et servent ainsi d'outils prospectifs pour la conception de produits. / Quenching processes of metals are widely adopted procedures in the industry, in particular automotive, nuclear and aerospace industries, since they have direct impacts on changing mechanical properties, controlling microstructure and releasing residual stresses of critical parts. Quenching is a highly nonlinear process because of the strong coupling between the fluid mechanics, heat transfer at the interface solid-fluid, phase transformation in the metal and boiling. In spite of the maturity and the popularity of numerical formulations, several involved mechanisms are still not well resolved.Therefore we propose a Direct Numerical Simulation of quenching processes at the industrial scale dealing with these phenomena. The fluid mechanics is simulated using a Finite Element Method adapted for high convective flows allowing the use of high stirring velocity in the quenching bath. Heat transfers are computed directly without the use of heat transfer coefficients but using the strong coupling between the fluid and the solid. We use a phase change model for the water that models all boiling regimes. A unified formulation of the Navier-Stokes equations, taking into account a compressible gas and an incompressible liquid is developed to model more accurately the vapor-water dynamics. A dynamic mesh adaptation procedure is used, increasing the resolution in the description of the interfaces and capturing more accurately the features of the flows.We assess the behavior and the accuracy of the proposed formulation in the simulation of time-dependent challenging numerical examples and experimental results.These recent developments enable the optimization of the process in terms of operating conditions, resources consumed and products conception.

Le transport de particules par des écoulements turbulents est un phénomène présent dans de nombreux écoulements naturels et industriels, tels que la dispersion de polluants dans l'atmosphère ou du phytoplancton et plastiques dans et à la surface des océans. Les modèles prédictifs classiques ne peuvent prévoir avec précision la formation de larges fluctuations de concentrations. La première partie de cette thèse concerne une étude de la dispersion turbulente de traceurs émis à partir d'une source ponctuelle et continue. Les fluctuations spatiales de masse sont déterminées en fonction de la distance à la source et à l'échelle d'observation. La combinaison de plusieurs phénomènes physiques à l'origine du mélange limite la validité d'une caractérisation de géométrie fractale. Une approche alternative est proposée, permettant d'interpréter les fluctuations massiques en terme des différents régimes de séparation de pair dans des écoulements turbulents. La seconde partie concerne des particules ayant une inertie finie, dont la dispersion dans l'espace des vitesses requiert de développer des techniques de modélisation adaptées. Une méthode numérique originale est proposée pour exprimer la distribution des particules dans l'espace position-vitesse. Cette méthode est ensuite utilisée pour décrire la modulation de la turbulence bi- dimensionnelle par des particules inertielles. A grand nombres de Stokes, l'effet montré est analogue à celui d'une friction effective à grande échelle. Aux petits Stokes, le spectre de l'énergie cinétique du fluide et les transferts non-linéaires sont modifiées d'une manière non triviale. / The transport of particles by turbulent flows is ubiquitous in nature and industry. It occurs in planet formation, plankton dynamics and combustion in engines. For the dispersion of atmospheric pollutants, traditional predictive models based on eddy diffusivity cannot accurately reproduce high concentration fluctuations, which are of primal importance for ecological and health issues. The first part of this thesis relates to the dispersion by turbulence of tracers continuously emitted from a point source. Mass fluctuations are characterized as a function of the distance from the source and of the observation scale. The combination of various physical mixing processes limits the use of fractal geometric tools. An alternative approach is proposed, allowing to interpret mass fluctuations in terms of the various regimes of pair separation in turbulent flows. The second part concerns particles with a finite and possibly large inertia, whose dispersion in velocity requires developing efficient modelling techniques. A novel numerical method is proposed to express inertial particles distribution in the position-velocity phase space. Its convergence is validated by comparison to Lagrangian measurements. This method is then used to describe the modulation of two-dimensional turbulence by large-Stokes-number heavy particles. At high inertia, the effect is found to be analogous to an effective large-scale friction. At small Stokes numbers, kinetic energy spectrum and nonlinear transfers are shown to be modified in a non-trivial way which relates to the development of instabilities at vortices boundaries.

L’ajout de particules d’aluminium dans le chargement des moteurs à propergol solide améliore les performances propulsives, mais peut aussi entraîner différents phénomènes néfastes, dont des oscillations de pression. Des travaux de recherche sont réalisés depuis de nombreuses années afin d’améliorer la compréhension de ces phénomènes, notamment par l’utilisation de la simulation numérique. Cependant les données d’entrée de la simulation numérique, en particulier la taille et la vitesse initiale des particules d’aluminium dans l’écoulement, sont souvent difficiles à obtenir pour des propulseurs réels. L’ONERA développe depuis plusieurs années un montage d’ombroscopie permettant de visualiser les particules d’aluminium proches de la surface de petits échantillons en combustion. La présente étude porte sur le développement d’outils pour analyser les images expérimentales du montage d’ombroscopie et améliorer l’interaction avec la simulation numérique diphasique. Une première partie concerne des échantillons de propergol contenant des particules inertes, dont l’intérêt est de permettre de valider les méthodes de mesure sur des images relativement simple et avec des données de référence. Les outils mis en œuvre portent sur la détection et le suivi des particules dans des séquences d’image, ainsi que sur la localisation de la surface du propergol. Une bonne correspondance des distributions de taille a été obtenu avec les distributions de référence. La mise en vitesse des particules quittant la surface a été confrontée à un modèle simplifié de transport de particules dans un écoulement constant. L'utilisation de ce modèle a permis de souligner l'importance de la population de pistes détectées pour bien exploiter un profil de vitesse moyen, en particulier en termes de diamètre moyen. Une simulation numérique diphasique a ensuite été réalisée pour l’expérience d’ombroscopie. Différents paramètres ont été étudiées (type et taille de maillage, paramètres thermodynamiques...) afin d'obtenir un champ stationnaire simulé pour les gaz du propergol. Le mouvement des particules inertes simulées a pu être comparé aux profils expérimentaux pour différentes stratégies d'injection, soit en utilisant un diamètre moyen, soit à partir d’une distribution lognormale. L’autre partie de l'étude est consacrée à l’analyse des images expérimentales de la combustion de particules d’aluminium. La complexité des images dans ces conditions a conduit à utiliser une approche de segmentation sémantique par apprentissage profond, visant à classer tous les pixels de l'image en différentes classes, en particulier goutte d'aluminium et flamme d'aluminium. L’apprentissage a été mené avec une base restreinte d’images annotées en utilisant le réseau U-Net, diverses adaptations pour le traitement des images d’ombroscopie ont été étudiées. Les résultats sont comparés à une technique de référence basée sur une détection d’objets MSER. Ils montrent un net gain à l’utilisation de techniques neuronales pour la ségrégation des gouttes d'aluminium de la flamme. Cette première démonstration de l'utilisation de réseau de neurones convolutifs sur des images d'ombroscopie propergol est très prometteuse. Enfin nous traçons des perspectives côté analyse d’image expérimentales et simulation numériques pour améliorer l’utilisation conjointe de ces deux outils dans l’étude des propergols solides. / The addition of aluminum particles in the solid propellant loading improves propulsive performance, but can also lead to various adverse phenomena, including pressure oscillations. Research has been carried out for many years to improve the understanding of these phenomena, particularly through the use of numerical simulation. However, the input data of the numerical simulation, especially the size and the initial velocity of the aluminum particles in the flow, are often difficult to obtain for real rocket motors. ONERA has been developing a shadowgraphy set-up for several years to visualize aluminum particles near the surface of propellant samples in combustion. The present study deals with the development of tools to analyze the experimental images of the shadowgraphy set-up and to improve the interaction with the two-phase digital simulation. A first part concerns propellant samples containing inert particles, which interest is to make it possible to validate the measurement methods on relatively simple images and with reference data. The implemented tools concern the detection and the tracking of particles in image sequences, as well as the location of the surface of the propellant. Good correspondence of size distributions was obtained with reference distributions. The velocity of particles leaving the surface has been confronted with a simplified model of particle transport in a constant flow. The use of this model has made it possible to emphasize the importance of the population of detected tracks in order to make good use of an average velocity profile, particularly in terms of average diameter. A two-phase flow simulation was then carried out for the shadowgraphy experiment. Different parameters were studied (type and size of mesh, thermodynamic parameters ...) in order to obtain a simulated stationary field for propellant flow. The movement of the simulated inert particles could be compared to the experimental profiles for different injection strategies, either using a mean diameter or using a lognormal distribution. The other part of the study is devoted to the analysis of experimental images of the combustion of aluminum particles. The complexity of the images under these conditions has led to the use of a deep learning semantic segmentation approach, aiming to classify all the pixels of the image into different classes, in particular aluminum droplet and flame. The learning was conducted with a restricted base of annotated images using the U-Net neural network, with various adaptations on the processing of the experimental images were studied. The results are compared to a reference technique based on MSER object detection. They show a clear gain in the use of neural techniques for the segregation of aluminum drops of the flame. This first demonstration of the use of convolutional neuronal network on propellant shadowgraphy images is very promising. Finally, we draw perspectives on experimental image analysis and numerical simulation to improve the joint use of these two tools in the study of solid propellants.

Multiphase flows are a common phenomenon. Rains, sediment transport in rivers, snow and dust storms, mud slides and avalanches are examples of multiphase flows occurring in nature. Blood flow is an example of multiphase flow in the human body, which is of vital importance for survival. Multiphase flows occur widely in industrial applications from hydrocarbon extrac-tion to fuel combustion in engines, from spray painting to spray drying, evaporators, pumps and pneumatic conveying. Predicting multiphase flows is of vital importance to understand natural phenomenon and to design and improve industrial processes. Separated flows and dispersed flows are two types of multiphase flows, which occur together in many industrial applications. Physical features of these two classes are different and the transition from one to another involves complex flow physics. Experimental studies of multiphase flows are not easy, as most real world phenomenon cannot be scaled down to laboratory models. Even for those phenomenon that can be demonstrated at lab-oratory scale, rescaling to real world applications requires mathematical models. There are many challenges in experimental measurements of multiphase flows as well. Measurement techniques well suited for single phase flows have constraints when measuring multiphase phenomenon. Un-certainty in experimental measurements poses considerable difficulties in validating numerical models developed for predicting these flows. Owing to the computational effort required, direct simulation of multiphase flows, even for small scale real world applications is out of present scope. Numerical methods have been developed for dealing with each class of flow separately, that in-volves use of models for phenomenon that is computationally demanding. Reynolds Averaged Navier-Stokes (RANS) methods for predicting multiphase flows place strong requirements on turbulence models, as information about fluctuating quantities in the field, that have significant effects on dispersed phase, is not available. Large Eddy Simulation (LES) gives better predictions than RANS as the instantaneous field data is available and large scale unsteadiness that effects the dispersed phase can be captured. Recent LES studies of multiphase flows showed that the sub-grid-scale (SGS) model used for the continuous phase has an effect on the evolution of the dispersed phase. In this work, LES of multiphase flows is performed using Explicit Filtering Large Eddy Sim-ulation method. In this method, spatial derivatives are computed using higher order compact schemes that have spectral-like resolution. SGS modeling is provided by the use of a filter with smoothly falling transfer function. This method is mathematically consistent and converges to a DNS as the grid is refined. It has been successfully applied to combustion and aero-acoustics and this work is the first application of the method to multiphase flows. Study of dispersed multiphase flows was carried out in this work. Modeling of the dispersed phase is kept simple since the in-tention was to evaluate the capability of explicit filtering LES method in predicting multiphase flows. Continuous phase is solved using a compressible formulation with explicit filtering method. Spatial derivatives are computed using fourth and sixth order compact schemes that use derivative splitting method proposed by Hixon & Turkel (2000a) and second order Runge-Kutta (RK2) time stepping. The grid is stretched as needed. Non-reflecting boundary conditions due to Poinsot & Lele (1992) are used to avoid acoustic reflections from boundaries. Buffer zones (Bogey & Bailly (2002)) are employed at outflow and lateral boundaries to damp vortical structures. The code developed for continuous phase is evaluated by studying round jets at Re =36,000 and comparing with experimental measurements of Hussein et al. (1994) and Panchapakesan & Lumley (1993). Simulations showed excellent agreement with experimental results. Rate of decay of axial velocity and the evolution of turbulence intensities on the centerline matched very well with measurements. Radial profiles of mean and fluctuating components of velocities exhibit self-similarity. A set of studies were then performed using this code to assess the effect of numerical scheme, grid refinement & stretching and simulation times on the predictions. Results from these simulations showed good agreements with experiments and established the code for use in multiphase flows under various simulation conditions. To assess the prediction of multiphase flows using this LES method, an evaporating spray ex-periment by Chen et al. (2006) was simulated. The experiment uses a nebuliser for generating a finely atomized spray of acetone, which avoids complex breakdown phenomenon associated with air blast atomizers and provides well defined boundary conditions for model evaluation. The neb-uliser sits upstream in a pipe carrying air and droplets travel along with air for a distance of 10 diameters before exiting into a wind tunnel with co-flowing air. Droplet breakdown, if any, takes place inside the pipe and the spray is finely atomized by the time it reaches pipe exit. One of the experimental cases at Re =31,600, with a mass loading of 1.1% and a jet velocity of 56 m/s is simulated. Particle size has a χsquared distribution with a Sauter mean diameter of 18µm. In the self-similar region, decay of centerline velocity and turbulence intensities matched well with ex-perimental results. Continuous phase exhibits self-similar behavior. A series of simulations were then performed to match the initial region of the spray by altering the inflow conditions in the sim-ulation. Simulation that matched the breakdown location of the experiment revealed the presence of a relaxation zone with a higher initial spreading rate, followed by a lower asymptotic spreading rate. Studies were performed to understand the effect of various phenomenon like evaporation and droplet size on this behavior. A study of breakdown region of particle-laden jets was performed to understand the presence of relaxation zone post breakdown. Flow conditions were similar to evaporating spray experiment except that particles do not evaporate, mass loading is 2% and jet Reynolds number Re =2000. A series of grid refinements were performed and on the largest grid, gird spacing Δy =7.5η, where ηis an estimate of the Kolmogorov length scale based on flow conditions. Decay of axial velocity on the centerline showed variations with grid refinement, tending to the experimentally measured value as the grid is refined. Variation of turbulence intensities along the centerline revealed a jump in axial velocity fluctuations at the breakdown location, while radial and azimuthal velocities showed a smooth increase to their asymptotic value. This jump was resolved on grid refinement and on fine grids axial velocity fluctuations followed the other two quantities closely in their rise to asymptotic state. Comparison of these quantities with a jet without particles revealed that the flow features are same for a jet with and without particles, and at the mass loading studied, particles have negligible effect on jet breakdown. Another study performed at a higher Reynolds number of Re =11,000, under similar flow conditions showed similar behavior. To assess the ability of predicting dispersed phase, simulations of particle-laden flows at low Stokes number were performed and compared against an experiment by Lau & Nathan (2014). The experiment studies variation of velocity and particle concentration along the centerline, and half widths of a jet velocity and concentration. Particles are injected into a pipe along with air, and the two phase flow is fully developed by the time it exits the pipe into a wind tunnel along with a co-flow. Particles are mono-disperse with a density of 1200 kg/m3. Mass loading is 40% so that particles have a significant effect on the continuous phase. Two cases at particle Stokes number of 1.4, one with Re =10,000, bulk velocity of 12 m/s and particle diameter of 20µm and another with Re =22,500, bulk velocity of 36 m/s and particle diameter of 10µm were simulated. Simulations of both the cases showed good match with experimental measurements of centerline decay for the continuous phase. For the dispersed case, simulations with larger particles showed good match with experimental results, while smaller particles showed differences. This was understood to be the effect of lateral migration which is prominent in case of smaller particles, the models for which have not been used in the present simulation study.

The present thesis deals with two-phase electrohydrodynamic simulations of bubble and droplet dynamics under externally applied electric fields. We used the Coupled Level-Set and Volume-of-fluid method (CLSVOF) and two different electrohydrody-namic formulations to study the process of bubble and drop formation from orifices and needles, the interactions of two conducting drops immersed in a dielectric medium, and the oscillations of sessile drops under two different ways of applying external elec-tric field. For the process of bubble formation in dielectric liquids due to the injection of air from submerged orifices and needles, we show that a non-uniform electric field pro-duces smaller bubbles while a uniform electric field changes only the bubble shape. We further explain the reason behind the bubble volume reduction under a non-uniform electric field. We show that the distribution of the electric stresses on the bubble inter-face is such that very high electric stresses act on the bubble base due to a non-uniform electric field. This causes a premature neck formation and bubble detachment lead-ing to the formation of smaller bubbles. We also observe that the non-uniform elec-tric stresses pull the bubble interface contact line inside the needle. With oscillatory electric fields, we show that a further reduction in bubble sizes is possible, but only at certain electric field oscillation frequencies. At other frequencies, bubbles bigger than those under a constant electric field of strength equal to the amplitude of the AC electric field, are produced. We further study the bubble oscillation modes under an oscillatory electric field. We implemented a Volume-of-fluid method based charge advection scheme which is charge conservative and non-diffusive. With the help of this scheme, we were able to simulate the electrohydrodynamic interactions of conducting-dielectric fluid pairs. For two conducting drops inside a dielectric fluid, we observe that they fail to coalesce when the strength of the applied electric field is beyond a critical value. We observe that the non-coalescence between the two drops occur due to the charge transfer upon drop-drop contact. The electric forces which initially bring the two drops closer, switch direction upon charge transfer and pull the drops away from each other. The factors governing the non-coalescence are the electric conductivity of the drop’s liquid which governs the time scale of charge transfer relative to the capillary time scale and the magnitude of the electric forces relative to the capillary and the viscous forces. Similar observations are recorded for the interactions of a charged conducting drop with an interface between a dielectric fluid and a conducting fluid which is the same as the drop’s liquid. For the case of a pendant conducting drop attached to a capillary and without any influx of liquid from the capillary, we observed that the drop undergoes oscillations at lower values of electric potential when subjected to a step change in the applied electric potential. At higher values of electric potential, we observed the phenomenon of cone-jet formation which results due to the accumulation of the electric charges and thus the electric forces at the drop tip. For the formation of a pendant conducting drops from a charged capillary due to liquid injection, we observed that the drops are elongated in presence of an electric field. This happens because the free charge which appears at the drop tip is attracted towards the grounded electrode. This also leads to the formation of elongated liquid threads which connect the drop to the capillary during drop detachment. We plotted the variation of total electric charge inside the drops with respect to time and found the charge increases steeply as the drop becomes elongated and moves towards the grounded electrode. For sessile drop oscillations under an alternating electric field, two different modes of operations are studied. In the so called ‘Contact mode’ case, the droplet is placed on a dielectric coated grounded electrode and the charged needle electrode remains in direct contact with the drop as it oscillates. In the ‘Non-contact mode’ case, the drop is placed directly on the grounded electrode and electric potential is applied to a needle electrode which now remains far from the drop. We show that the drop oscillations in the contact mode are caused by concentration of electric forces near the three phase contact line where the electric charge accumulates because of the repulsion from the needle. For the non-contact mode, we observe that the electric charge is attracted by the needle towards the drop apex resulting in a concentration of the electric forces in that region. So the drop oscillates due to the electric forces acting on a region near the drop tip. We also present the variation of the total electric charge inside the drop with respect to time for the two cases studied.

Ce travail porte sur la simulation numérique des écoulements multiphasiques compressibles en déséquilibre de vitesses. Un solveur de Riemann diphasique de type HLLC, à la fois robuste, simple et précis est développé et validé à partir de solutions exactes et de données expérimentales. Cette méthode numérique est étendue au cas 3D non-structuré. Par ailleurs, la construction d’une technique numérique pour la répartition de l’énergie d’une onde de choc dans les différentes phases constituant le milieu est établie et permet le respect des conditions de choc multiphasiques. L’extension multiphasique du solveur de Riemann de type HLLC est réalisée, permettant ainsi la simulation d’une plus large gamme d’applications. Enfin, un modèle de transfert de chaleur et de masse dans un brouillard de gouttes ou nuage de bulles, en présence d’effets couplés de diffusion thermique et massiques, est proposé et dévoile des résultats intéressants. / This work deals with the numerical simulation of compressible multiphase flows in velocity disequilibrium. A HLLC-type two-phase Riemann solver is developed and validated against exact solutions and experimental data. This solver is robust, simple, accurate and entropy preserving. The numerical method is then implemented in 3D unstructured meshes. Furthermore, a numerical technique consisting in enforcing the correct energy partition at a discrete level in agreement with the multiphase shock relations is built. The multiphase extension of the HLLC-type Riemann solver is realized and allows the simulation of a wide range of applications. Finally, a droplet heat and mass transfer model with large range of validity is derived. It is valid in any situation: evaporation, flashing and condensation. It accounts for coupled heat and mass diffusion in the gas phase, thermodynamics of the multi-component gas mixture and heat diffusion inside the liquid droplet, enabling in this way consideration of both droplets heating and cooling phenomena.

Ce travail porte sur la modélisation de la formation et la dispersion d'un nuage de gouttes, par déconfinement d'un liquide: agression extérieure ou situation accidentelle. Le but est la construction d'un modèle apte à reproduire simultanément les conditions génératrices de la formation du nuage et l'évolution de ce nuage dans le temps (dispersion). La principale difficulté réside en la différence des modèles adaptés à la description d'écoulements caractérisant chaque étape du phénomène global : modèle d'écoulement multiphasique à phases compressibles (milieux continus) initialement, puis fragmentation et formation du nuage de gouttes dispersées dans une phase porteuse (modèle d'écoulements dilués). En l'absence de modèle analytique unique apte à décrire l'ensemble de ces processus, on propose une approche originale pour réaliser un couplage effectif entre ces deux modèles. La problématique de formation et de dispersion de liquide implique la prise en compte de plusieurs phénomènes physiques: fragmentation, transferts de chaleur et de masse ainsi que la traînée entre les phases. Ces différents phénomènes sont introduits dans le modèle global via des termes d'interactions présents dans les systèmes d'équations. La construction de ce modèle complet à permis la réalisation de calculs décrivant la formation et la dispersion d'un nuage de gouttes pouvant intervenir lors de situations accidentelles sur des sites industriels par exemple. / This work focuses on modeling the formation and the dispersion of a cloud of droplets, induced by ejection of a liquid, resulting from an external aggression or an accidental situation. The goal is to build a model able to reproduce simultaneously the conditions which generate the cloud formation and the cloud evolution in time (dispersion). The main difficulty lies in the differences between the already existing models adapted to the description of flows which are able to characterize each stage of the global phenomenon: initially a multiphase flow model with compressible phases (Continuum), then the atomization and the formation of a cloud of droplets dispersed in a carrier phase (dilute flow model). We propose a new approach to achieve an effective coupling between these two models. The problem of the formation and the dispersion of the liquid requires to take into account several physical phenomena: atomization, heat and mass transfers and drag between phases. These phenomena are included in the global model through interaction terms involved in the systems of equations. The construction of this model has permited the realization of calculations describing the formation and dispersion of a cloud of droplets which may occur during, for axample, in accidental situations at industrial sites.

A system able to simultaneously separate and pump a gas-liquid mixture was developed.<p>It works efficiently and can be used in many applications (nuclear power plants,<p>pulp and paper processing, petroleum extraction, etc.). However, this pump and separator<p>system (PASS) was especially designed to handle air-oil mixture generated in<p>aero-engine lubrication systems. The PASS combines three important functions of the<p>scavenge part of the lubrication system: the deaeration and deoiling of the air-oil mixture<p>generated in the bearing and gearbox sumps and the pumping of the oil towards<p>the tank. These are critical functions for the engine. Indeed, a poor deoiling efficiency<p>leads to a high oil consumption. This reduces the flight endurance, increases the size<p>and weight of the oil tank and has a negative impact on the environment. Poor deaeration<p>and pumping characteristics lead to problems in the cooling and the lubrication of<p>the engine bearings.<p><p>Integrating a PASS into the lubrication system allows considerable improvements<p>(and simplification) to the lubrication system architecture. An important number of<p>components are suppressed: the vent lines, the deoiler, the cyclone deaerator and the<p>scavenge pumps. This reduces the size and the weight of the lubrication system and<p>increases its reliability. Furthermore, an important part of this PhD thesis focuses on<p>reducing the oil consumption in the PASS. This improves the flight endurance, reduces<p>engine maintenance and working costs and is profitable to the environment.<p><p>In addition to the development of an advanced PASS design system, the objective of<p>this thesis was to obtain a good understanding of the separation processes occurring in<p>the PASS and to develop theoretical models able to predict the separation performance<p>for every working condition encountered in a typical aircraft flight. To achieve this<p>goal, three main tasks were performed: the development of different two-phase measurement<p>systems, the experimental tests of four different PASS architectures and the<p>theoretical development (after an extensive literature review) of correlations predicting<p>the performance of the PASS in function of the working conditions. Five specific aspects<p>of the PASS were studied: the inlet flow, the deoiling efficiency, the deaeration efficiency,<p>the pumping efficiency and the pressure drop. Finally, the models that have been developed<p>with the help of the measurement systems and of the experiments have been<p>integrated in a complete model of the lubrication system (under the EcosimPro modelling<p>environment). This helps to predict real in flight PASS working conditions and<p>performance. Indeed, the PASS is very sensitive to the engine working conditions and<p>an optimisation of the prototype size and performance is only feasible with an accurate<p>knowledge of these working conditions and a complete lubrication system model.<p>Finally, with the results of this PhD thesis, a new PASS design, optimised for different<p>aero-engine lubrication systems, is presented. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Ces travaux portent sur la modélisation et la simulation numérique des effets du déplacement d'un corps indéformable dans un écoulement multiphasique compressible. Ils se placent dans le cas où plusieurs objets sont en mouvement ou dans le cas où un objet est en mouvement dans un milieu aux géométries complexes. L'étude ne peut alors pas être placée dans le référentiel lié à l'objet en mouvement. Le modèle est basé sur une méthode multiphasique à interfaces diffuses où les différentes phases sont en équilibre mécanique. Le système régissant l'écoulement fluide est augmenté d'une équation d'advection. Cette dernière s'applique sur une fonction Level Set dont le niveau zéro permet de localiser le mobile dans l'espace. Des termes de couplage sont ajoutés au membre de droite des équations d'évolution de la quantité de mouvement et de l'énergie totale. Ces termes sont composés d'un facteur du type pénalisation et d'un facteur du type relaxation de vitesses. Cette nouvelle méthode permet de simuler des cas complexes où peuvent interagir des mobiles à hautes vitesses, des ondes de choc et des interfaces liquide/gaz. / This work deals with modelling and the numerical simulation of the effects of a moving rigid body on a multiphase flow. Here more than one object is moving, or an object is moving in a complex geometry domain. So the reference frame linked to the moving body can't be used. The model is build on a multiphase diffuse interface method with mechanical equilibrium. An advection equation is added. It applies on a Level Set function used to track the moving body. Coupling terms are added to the momentum equation and to the total energy equation. These terms are made of a penalization factor and a velocity relaxation factor. This new method allows to simulate complex cases where can interact high velocity objects, shock waves and liquid / gas interfaces.

<div>This dissertation focuses on a consistent and conservative Phase-Field method for multiphase flow problems, and it includes both model and scheme development. The first general question addressed in the present study is the multiphase volume distribution problem. A consistent and conservative volume distribution algorithm is developed to solve the problem, which eliminates the production of local voids, overfilling, or fictitious phases, but follows the mass conservation of each phase. One of its applications is to determine the Lagrange multipliers that enforce the mass conservation in the Phase-Field equation, and a reduction consistent conservative Allen-Cahn Phase-Field equation is developed. Another application is to remedy the mass change due to implementing the contact angle boundary condition in the Phase-Field equations whose highest spatial derivatives are second-order. As a result, using a 2nd-order Phase-Field equation to study moving contact line problems becomes possible.</div><div><br></div><div>The second general question addressed in the present study is the coupling between a given physically admissible Phase-Field equation to the hydrodynamics. To answer this general question, the present study proposes the <i>consistency of mass conservation</i> and the <i>consistency of mass and momentum transport</i>, and they are first implemented to the Phase-Field equation written in a conservative form. The momentum equation resulting from these two consistency conditions is Galilean invariant and compatible with the kinetic energy conservation, regardless of the details of the Phase-Field equation. It is further illustrated that the 2nd law of thermodynamics and <i>consistency of reduction</i> of the entire multiphase system only rely on the properties of the Phase-Field equation. All the consistency conditions are physically supported by the control volume analysis and mixture theory. If the Phase-Field equation has terms that are not in a conservative form, those terms are treated by the proposed consistent formulation. As a result, the proposed consistency conditions can always be implemented. This is critical for large-density-ratio problems.</div><div><br></div><div>The consistent and conservative numerical framework is developed to preserve the physical properties of the multiphase model. Several new techniques are developed, including the gradient-based phase selection procedure, the momentum conservative method for the surface force, the boundedness mapping resulting from the volume distribution algorithm, the "DGT" operator for the viscous force, and the correspondences of numerical operators in the discrete Phase-Field and momentum equations. With these novel techniques, numerical analyses ensure that the mass of each phase and momentum of the multiphase mixture are conserved, the order parameters are bounded in their physical interval, the summation of the volume fractions of the phases is unity, and all the consistency conditions are satisfied, on the fully discrete level and for an arbitrary number of phases. Violation of the consistency conditions results in inconsistent errors proportional to the density contrasts of the phases. All the numerical analyses are carefully validated, and various challenging multiphase flows are simulated. The results are in good agreement with the exact/asymptotic solutions and with the existing numerical/experimental data.</div><div> </div><div><br></div><div>The multiphase flow problems are extended to including mass (or heat) transfer in moving phases and solidification/melting driven by inhomogeneous temperature. These are accomplished by implementing an additional consistency condition, i.e., <i>consistency of volume fraction conservation</i>, and the diffuse domain approach. Various problems are solved robustly and accurately despite the wide range of material properties in those problems.</div>