Cette thèse de doctorat est consacrée à la modélisation des écoulements turbulents réactifs pré-mélangées. Une analyse des flammes monodimensionnelles est tout d'abord conduite pour examiner les caractéristiques propagatives du front de flamme analytiquement et numériquement. Nous introduisons ensuite, dans le régime des flammes plissées(flammelettes), une nouvelle définition de l'épaisseur de flamme qui peut être différente de l'épaisseur caractéristique de la flamme laminaire. Une forme généralisée de la fonction de densité probabilité (PDF) présumée est alors proposée pour tenir compte des écarts possibles avec la limite de flamme mince. Les paramètres qui caractérisent la forme de cette PDF dépendent du rapport de l'échelle de longueur de Kolmogorov à l'épaisseur dela flamme laminaire. Le cadre de travail correspondant peut être associé à une description de type couche limite appliquée à la flamme turbulente pour laquelle les épaisseurs des différentes sous-couches sont déterminées à partir de la seule connaissance des nombres de Reynolds turbulent Re, et de Karlovitz Ka. Cette description conduit à une proposition de modélisation (i) pour le taux de conversion des réactifs en produits ainsi que (ii) pour le flux turbulent scalaire. Les valeurs des caractéristiques chimiques s'appuient sur des calculs de flammes monodimensionnelles utilisant le logiciel Mathematica TM ou reposant sur l'emploi d'une description détaillée de la cinétique chimique.Les modèles proposés ici sont finalement implantés dans le logiciel de calcul numérique Code-Saturne et validés sur des configurations expérimentales de référence : flammes en V turbulentes et flamme stabilisée dans un écoulement divergent. Les simulations numériques de ces deux configurations conduisent à des résultats satisfaisants. / The present study is devoted to the modeling of turbulent reactive flows. In a first step of the analysis, one-dimensional flames are considered to scrutinize analytically and numerically the characteristics of the propagating flame front. Then, we introduce a new definition for the flamelet thickness that may differ from the laminar flame thickness. Accordingly a generalized and consistent form of the presumed probability density function(PDF) is proposed. The parameters of this PDF depend on the ratio of the Kolmo-gorov length scale to laminar flame thickness. The corresponding developments can be associated with a boundary layer description applied to the turbulent flame in which the thicknesses of the different sub-layers are determined from the sole knowledge of the turbulent Reynolds number Re and Karlovitz number Ka. This description leads to amodeling proposal (i) for the mean reaction rate as well as (ii) for the turbulent scalar flux. The values of all chemical characteristics are calculated from one-dimensional flame susing either the MathematicaTM software or detailed chemical kinetics computations.Numerical simulations are finally performed with the CFD code Code-Saturne and the model is validated through comparisons with reference experimental configurations :turbulent V-shaped flames and impinging flames. Satisfactory results are obtained for these two distinct configurations.

No description available.

L’étude des écoulements compressibles, notamment supersoniques, traversant les chambres de combustion de certains moteurs ramjet et scramjet, requiert la prise en compte de différents dispositifs complexes pour l’amélioration du processus de combustion et en particulier de leur stabilisation. La connaissance des interactions entre la turbulence, les effets de compressibilité, et les interaction fluide-solide dans ce type d’écoulement reste imparfaite. Ce travail de thèse est dédié à l’amélioration de notre compréhension de ce type d’écoulement dans un certain nombre de configurations canoniques par la biais de la simulation numérique directe. L’ensemble des simulations conduites s’appuie sur l’emploi d’un outil de simulation numérique haute fidélité : CREAMS (Compressible REActive Multi-species Solver) développé à l’Institut Pprime. Ce code de calcul met en oeuvre des schémas numériques d’ordre élevé : schéma Runge–Kutta d’ordre 3 pour l’intégration temporelle combiné à un schéma WENO d’ordre 7 et centré d’ordre 8 pour la discrétisation spatiale. Dans un premier temps, nous présentons une nouvelle méthode de frontières immergées pour le calcul d’écoulement d’un fluide visqueux compressible dans des géométries irrégulières. La méthode développée dans le cadre de cette thèse est basée sur la combinaison de l’approche appelée "Direct forcing" et celle de "Ghost-Point-Forcing". L’originalité de cette méthode réside dans sa capacité à simuler des écoulements subsoniques et supersoniques à différents nombres de Reynolds. L’examen de précision de cette méthode a permis d’établir un ordre supérieur à deux et sa robustesse est éprouvée par l’étude d’un bon nombre de cas tests. Dans un second temps, une configuration canonique idéalisée d’interaction choc-turbulence est étudiée pour mettre en lumière les mécanismes physiques fondamentaux caractéristiques du phénomène d’interaction entre une turbulence homogène isotrope et une onde de choc droite. Cette étude est complétée par une étude d’interaction choc-mélange scalaire pour étudier l’impact du choc normal sur le processus du mélange. Ce travail permet de mettre en place une base de données de résultats susceptibles d’être confrontés ultérieurement à des calculs basés sur l’emploi de modèles de turbulence. Enfin, nous nous sommes intéressés à l’effet des propriétés de transport moléculaire, en particulier celles de la viscosité volumique, sur le développement d’une couche de mélange impactée par un choc oblique. Les simulations réalisées dans cette configuration ont permis d’étudier la validité de l’hypothèse de Stokes consistant à négliger l’effet de la viscosité volumique. / The study of compressible flows, especially supersonic, passing through the combustion chambers of ramjet and scramjet engines, requires the consideration of various complex devices for improving the combustion processand in particular its stabilization. Indeed, the knowledge of the interactions between turbulence, compressibility effects, and fluid-solid interactions in this type of flow still remains imperfect. This thesis is dedicated to improving our understanding of this type of flow in a number of canonical flow configurations through direct numerical simulation. All the simulations that have been conducted are based on the use of a high-precision numerical simulations tool, called CREAMS (Compressible REActive Multi-species Solver), developed at the Pprime Institute. This computational solver makes use of high precision numerical schemes: a 3rd order Runge–Kutta scheme for time integration combined with a 7th order WENO and 8th order centered scheme for spatial discretization. In a first step of this study, we present a new immersed boundary method for calculating the flow of compressible viscous fluids in irregular geometries. The method developed in this thesis is based on the combination of the "Directforcing" approach with the "Ghost-Point-Forcing" strategy. The originality of this method lies in its ability to simulate subsonic and supersonic flows at different Reynolds numbers. The accuracy of this method is found to be slightly larger than second order and its robustness is investigated by considering a large set of benchmarks. Ina second step, an idealized canonical configuration of shock-turbulence interaction is studied to highlight the fundamental physical mechanisms that are characteristic of the interaction between an isotropic homogeneous turbulence and a normal shock wave. This analysis is complemented by a scalar shock-mixing interaction study to investigate the impact of normal shock on the mixing process properties. Through this work, a database is made available. It can be used to assess and improve turbulence models. Finally, we investigated the effect of molecular transport properties, more specifically the volume viscosity, on the development of a mixture layer impacted by an oblique shock. The simulations performed in this configuration allow to scrutinize the validity of the Stokes hypothesis that is based on the neglection of the volume viscosity.

No description available.

The unsteady behavior of a tip leakage flow downstream of a simulated axial compressor rotor has been studied. The Virginia Tech low speed linear cascade wind tunnel was adapted to model the unsteady tip leakage flow produced by a rotor operating in the vortical wakes of a set of stator vanes. The cascade, consisting of 8 GE rotor B blades, has adjustable tip gap, inlet angle of 65.1 degrees, turning angle of 11.8 degrees and solidity of 1.076. The cascade Reynolds number, based on blade chord, was 393,000. A moving end wall was used to simulate the relative motion between rotor and casing, and vortex generators attached to the moving end wall were used to produce an idealized periodic unsteady vortical inflow similar to that shed by the junction of a row of inlet guide vanes. Measurements of the vortical inflow to the cascade produced by the generators and of the mean blade loading at the mid span are presented. The periodic and aperiodic behavior of the tip leakage flow downstream of the cascade, produced by this vortical disturbance, is also presented using phase and time averaged 3-component turbulence and pressure fluctuation measurements. These measurements are made for tip gap from 0.83% to 3.3% chord and streamwise locations from 0.772% to 1.117% blade spacing axially downstream of the cascade. The phase averaged inflow measurements reveal that the inflow produced by the vortex generators consists of a pair asymmetric counter-rotating vortices embedded in a thin (4.6% chord) endwall boundary layer. The vortices extend some 7.4% chord from the end wall. Their strength is about two orders smaller than the typical circulation of the tip leakage vortices produced by the cascade. Phase averaged single point three component hot-wire measurements downstream of the cascade reveal that the vortical inflow is, however, capable of producing significant large scale fluctuations in the size, strength, structure and position of the tip leakage vortex. These effects increase in magnitude with increase of tip gap. For small tip gaps these effects appear to be due to simple superposition between the inflow vortices and the tip leakage vortex. However for larger tip gaps these effects appear primarily a consequence of the inflow vortices interfering with the shedding of circulation from the blade tip. The fact that the circulation fluctuation is consistent with the inviscid unsteady loading prediction suggests that the inviscid response may be a major mechanism for generating the tip leakage unsteadiness. Although there is large periodic fluctuation in the tip leakage flow disturbed by the inflow, there is a larger aperiodic component. Two point correlation measurements and linear stochastic estimation are used to reveal the structure of this aperiodic part for a tip gap of 3.3% chord. The aperiodic fluctuation, containing most of the turbulence energy, is found appearing to be organized structures in large scale, and making the estimated instantaneous velocity field significantly different from the phase averaged periodic velocity field. Phase averaged pressure fluctuation measurements made using a microphone in the tip leakage vortex downstream of the cascade reveal that there are significant periodic fluctuating pressure waves and intense mean square fluctuation of the aperiodic fluctuating pressure. They are consistent with the measured periodic flow and aperiodic flow field respectively. These microphone measurements are validated using fluctuating pressure gradient estimates determined from the hot-wire measurements. / Ph. D.

The effects of serrated trailing edge designs, designed for noise reduction, on the flow-field downstream of an idealized aircraft engine fan blade row were investigated in detail. The measurements were performed in the Virginia Tech low speed linear cascade tunnel on one set of baseline GE-Rotor-B blades and four sets of GE-Rotor-B blades with serrated trailing edges. The four serrated blade sets consisted of two different serration sizes (1.27 cm and 2.54 cm) and for each different serration size a second set of blades with added trailing edge camber. The cascade row consisted of 8 GE-Rotor-B blades and 7-passages with adjustable tip gap settings. It had an inlet angle of 65.1º, stagger angle of 56.9º and a turning angle of 11.8º. The tunnel was operated with a tip gap setting of 1.65% chord, with a Reynolds number based on the chord of 390,000. Blade loading measurements performed on each set of blades showed that it was slightly dependent on the serration shape. As the serration size was increased the blade loading decreased, but adding droop increased the blade loading. The Pitot-static cross-sections showed that flow-fields near the upper and lower endwalls cascade tunnel were similar with the baseline or the serrated blade downstream of the blade row. In the wake region, the individual trailing edge serrations tips and valleys could be seen. As the wake convected downstream, the individual tips and valleys became less visible and the wake was more uniform in profile. The tip leakage vortex was only minimally affected by the trailing edge serrations. This conclusion was further reinforced by the three-component hot-wire cross-sectional measurements that were performed from the lower endwall to the mid-span of the blade. These showed that the mean streamwise velocity, turbulence kinetic energy and turbulence kinetic energy production in the tip leakage region were nearly the same for all four serrated blades as well as the baseline. The vorticity in this region was a more dependent on the serration shape and as a result increased with serration size compared to the baseline. Mid-span measurements performed with the three-component hot-wire showed the spreading rate of the wake and the decay rate of the wake centerline velocity deficit increased with serration size compared to the baseline case. Drooping of the trailing edge only minimally improved the spreading and decay rates. This improvement in these rates was predicted to reduce the tonal noise at the leading edge of the downstream stator vane because the periodic fluctuation associated with the sweeping of the rotor blade wakes across it, was due to the pitchwise variation in the mean streamwise velocity. The wakes were further compared to the mean velocity and turbulence profiles of plane wakes, which the baseline and the smallest serration size agreed the best. As the serration size was increased and drooping was added, the wakes became less like plane wakes. Spectral plots at the wake centerline in all three velocity directions showed some evidence of coherent motion in the wake as a result of vortex shedding. / Master of Science