Thermal cracking is an industrial process sensitive to both temperature and pressure operating conditions. The use of internally ribbed reactors is a passive method to enhance the chemical selectivity of the process, thanks to a significant increase of heat transfer. However, this method also induces an increase in pressure loss, which is damageable to the chemical yield and must be quantified. Because of the complexity of turbulence and chemical kinetics, and as detailed experimental measurements are difficult to conduct, the real advantage of such geometries in terms of selectivity is however poorly known and difficult to assess. This work aims both at evaluating the real benefits of internally ribbed reactors in terms of chemical yields and at proposing innovative and optimized reactor designs. This is made possible using the Large Eddy Simulation (LES) approach, which allows to study in detail the reactive flow inside several reactor geometries. The AVBP code, which solves the Navier-Stokes compressible equations for turbulent flows, is used in order to simulate thermal cracking thanks to a dedicated numerical methodology. In particular, the effect of pressure loss and heat transfer on chemical conversion is compared for both a smooth and a ribbed reactor in order to conclude about the impact of wall roughness in industrial operating conditions. An optimization methodology, based on series of LES and Gaussian process, is finally developed and an innovative reactor design for thermal cracking applications, which maximizes the chemical yield, is proposed

Cette thèse est consacrée à l'étude des propriétés aéroacoustiques de jets supersoniques impactant une paroi par simulation des grandes échelles. Ces simulations sont réalisées à partir des équations de Navier-Stokes 3-D instationnaires compressibles exprimées pour des coordonnées cartésiennes ou cylindriques. Afin de résoudre ces équations, des schémas numériques de différenciation spatiale et d'intégration temporelle peu dispersifs et peu dissipatifs sont utilisés. Les écoulements étudiés étant supersoniques, une procédure de capture de choc est également implémentée afin de supprimer les oscillations de Gibbs de part et d'autre des chocs.Dans un premier temps, un jet rond libre et quatre jets ronds impactant une paroi avec un angle de 90 degrés sont simulés sur des maillages cylindriques. Ces jets sont supersoniques, sous-détendus, et sont caractérisés par un nombre de Reynolds calculé à partir du diamètre du jet de Re=60.000, et par un nombre de Mach parfaitement détendu de Mj=1.56. Les résultats du jet libre sont tout d'abord présentés. Ils sont comparés aux résultats de plusieurs études expérimentales et de modèles afin de valider l'approche numérique utilisée. Notamment, les différentes composantes acoustiques spécifiques aux jets sous-détendus comme le bruit de choc large-bande et le bruit de screech sont observées et analysées. Les résultats obtenus pour les quatre jets impactant une paroi sont ensuite examinés. Dans ce cas, la présence d'une boucle de rétroaction aéroacoustique entre les lèvres de la buse et la paroi est montrée. Pour finir, le comportement aérodynamique et aéroacoustique des jets est étudié, et comparé à différentes études numériques et expérimentales de la littérature. Quatre jets plans supersoniques idéalement détendus impactant une paroi avec un angle de 90 degrés sont ensuite calculés. Ils ont un nombre de Reynolds évalué à partir de la hauteur de la buse de Re=50.000 et un nombre de Mach de Mj=1.28. Une boucle de rétroaction aéroacoustique entre la buse et la paroi est de nouveau mise en évidence. Une combinaison de modèles associant un modèle d'onde stationnaire aérodynamique-acoustique et un modèle de stabilité de jet plan 2-D avec des couches de mélange infiniment minces est alors proposée. Ce modèle permet de déterminer à la fois les fréquences les plus probables de la boucle de rétroaction aéroacoustique et leurs natures plane ou sinueuse.Enfin, les simulations de deux jets plans supersoniques impactant une paroi avec des angles de 60 et 75 degrés sont réalisées grâce à l'utilisation de deux maillages cartésiens, par une méthode de recouvrement de maillages. Les modifications des propriétés de la boucle de rétroaction aéroacoustique lorsque l'angle d'impact dévie de 90 degrés sont ainsi étudiées. / In this PhD work, supersonic impinging jets are simulated using large-eddy simulation in order to investigate their aerodynamic and acoustic fields. In practice, the unsteady compressible Navier-Stokes equations are solved on Cartesian or cylindrical meshes. Low-dissipation and low-dispersion numerical methods are used for spatial differentiation and time integration. As the jets are supersonic, a shock-capturing filtering is also applied in order to avoid Gibbs oscillations near shocks.A free round jet and four round jets impinging normally on a flat plate are first simulated on cylindrical meshes. They are underexpanded, and have a Reynolds number based on the nozzle diameter of Re=60.000 and a fully expanded Mach number of Mj=1.56. The results for the free jet are first presented. They are compared with experimental results and predictions given by models in order to validate the numerical setup. Acoustic components specific to underexpanded jets such as broadband shock-associated noise and screech noise are obtained. The results for the four impinging jets are then examined. An aeroacoustic feedback mechanism establishing between the nozzle lips and the flat plate is found to generate tones. Finally, the flow and acoustic properties of the jets are studied and compared with numerical and experimental data.Four ideally expanded jets impinging normally on a flat plate are then simulated. They have a Reynolds number based on the nozzle height of Re=50.000 and a Mach number of Mj=1.28. An aeroacoustic feedback mechanism is again observed between the nozzle lips and the flat plate. A combination of models based on an aeroacoustic feedback model and a vortex sheet model of the jet is then proposed. The model appears able to predict the most likely tone frequencies of the feedback mechanism, and the symmetric or antisymmetric nature of the corresponding jet oscillation.Finally, two ideally expanded jets impinging on a flat plate with angles between the jet direction and the plate of 60 and 75 degrees are simulated using two Cartesian meshes. The effects of the angle of impact on the properties of the aeroacoustic feedback mechanism are finally studied.

Escoamentos turbulentos têm sido por muitos anos o objetivo de importantes estudos para descobrir sua dinâmica. Dentre as características mais significativas, destaca-se a multiplicidade de escalas que os caracterizam, desde as maiores estruturas (baixas freqüências) controladas pela geometria que as geram, até as menores estruturas (altas freqüências) limitadas pela viscosidade do fluido. Uma idéia importante é o conceito de vórtices que está ligado a melhorias nas técnicas de visualização, tanto em laboratório quanto em experimentos numéricos. Estes vórtices têm um importante papel em numerosas aplicações tecnológicas, sendo necessário entender a dinâmica da organização de seus movimentos para controlar mecanicamente sua produção ou supressão. Neste contexto, a análise de um misturador de ar em um escoamento em tê é o principal objetivo de estudo deste trabalho. A geometria em tê é bastante simples, mas propicia o aparecimento de um escoamento com passagem de vórtices. Testes experimentais do escoamento, com duas entradas de ar com temperaturas diferentes, foram realizados no Laboratório de Engenharia Térmica e Fluidos da Escola de Engenharia de São Carlos da Universidade de São Paulo (LETeF - EESC - USP). As medidas de temperatura foram obtidas com termopares instalados ao longo da tubulação. Com o software CFX foram realizados estudos com métodos numéricos LES e DES aplicados ao escoamento. Estes resultados computacionais foram comparados com os dados experimentais, através da análise tempo-freqüência. Estudos preliminares do escoamento mostram regiões com passagem de vórtices, e a habilidade da técnica de análise tempo-freqüência em caracterizar a existência e a forma destas estruturas turbulentas. / Turbulent flows have been the objective of important studies to discover its dynamics. One important characteristic of these flows is the multiplicity of scales, since the large structures (low frequencies) controlled by the geometry that generates them, until the small structures (high frequencies) limited by the fluid viscosity. An important idea is the concept of vortices that it is associated with the improvements in the visualization techniques, in laboratory or numerical experiments. These vortices have an important function in many technological applications. In each one of these fields, it is necessary to understand the dynamics of its movements to control the mechanisms for producing or suppressing these vortices. In this context, the analysis of an air mixing in a tee junction is the main objective of this work. The tee geometry is sufficiently simple, but contributes for the appearance of a flow with vortices transition. Experimental tests with two different air temperatures inlets were done at the Thermal and Fluid Engineering Laboratory of the University of São Paulo at São Carlos (LETeF - EESC - USP). The measures of temperature were acquired with thermocouples installed along the pipe. Numerical studies with LES and DES methods using CFX software were applied to the flow. These computational results were compared with the experimental data through the time-frequency analysis. Preliminary studies of the flow show vortices transition regions and the ability of time-frequency technique in describing the existence and shape of turbulent structures.

O presente trabalho versa sobre a modelagem de uma chama turbulenta difusiva usando a técnica de simulação de grandes estruturas turbulentas (LES), juntamente com o modelo termo-químico de folha de chama (flame sheet model) e o conceito de fração de mistura como escalar conservativo. Este trabalho também é pioneiro de utilização de LES com reação química no Brasil, podendo colaborar para o desenvolvimento desta técnica na área de combustão. O trabalho consiste na construção e validação das rotinas computacionais de um código CFD, baseado em LES e com flexibilidade para uma futura utilização de cinética química detalhada de combustão (EDC/ISAT), para casos complexos onde modelos mais simples, como a fração de mistura, são falhos. O programa será validado em uma chama de difusão turbulenta não-confinada de metano (CH4), para a qual existem dados experimentais na literatura [61,62] e utilizados pela comunidade acadêmica em excelência (Stanford, TU-Darmstadt, Imperial College, Cornell University etc). As características da implementação numérica do código permitirão sua expansão futura para outras aplicações em: queima de combustíveis líquidos, combustão em câmaras fechadas e fornalhas com a inclusão de modelo de radiação. / The present work is about modeling a diffusive turbulent flame using the Large-Eddy Simulation approach (LES) and the Flame Sheet model as the chemical model with the mixture fraction concept as the conservative scalar. This work is pioneer in the sense of using LES and reactive flow in Brazil, making possible the development for LES techniques in the combustion area. The work is intended to construct and validate a CFD code based on LES and with future flexibility for a more detailed combustion chemical model (EDC/ISAT) for complex flows, where simple models are failed, like the mixture fraction. The program will be validated for a turbulent diffusion methane (CH4) flame not confined, which there are some experimental data on the specialized literature [61,62], and commonly used by the academic community (Stanford, TU-Darmstadt, Imperial College, Cornell University etc). The features of the numerical code implementation will make possible future expansion of its use in other applications: liquid fuel burning, combustion chambers and ovens with the radiation model inclusion.

Escoamentos turbulentos têm sido por muitos anos o objetivo de importantes estudos para descobrir sua dinâmica. Dentre as características mais significativas, destaca-se a multiplicidade de escalas que os caracterizam, desde as maiores estruturas (baixas freqüências) controladas pela geometria que as geram, até as menores estruturas (altas freqüências) limitadas pela viscosidade do fluido. Uma idéia importante é o conceito de vórtices que está ligado a melhorias nas técnicas de visualização, tanto em laboratório quanto em experimentos numéricos. Estes vórtices têm um importante papel em numerosas aplicações tecnológicas, sendo necessário entender a dinâmica da organização de seus movimentos para controlar mecanicamente sua produção ou supressão. Neste contexto, a análise de um misturador de ar em um escoamento em tê é o principal objetivo de estudo deste trabalho. A geometria em tê é bastante simples, mas propicia o aparecimento de um escoamento com passagem de vórtices. Testes experimentais do escoamento, com duas entradas de ar com temperaturas diferentes, foram realizados no Laboratório de Engenharia Térmica e Fluidos da Escola de Engenharia de São Carlos da Universidade de São Paulo (LETeF - EESC - USP). As medidas de temperatura foram obtidas com termopares instalados ao longo da tubulação. Com o software CFX foram realizados estudos com métodos numéricos LES e DES aplicados ao escoamento. Estes resultados computacionais foram comparados com os dados experimentais, através da análise tempo-freqüência. Estudos preliminares do escoamento mostram regiões com passagem de vórtices, e a habilidade da técnica de análise tempo-freqüência em caracterizar a existência e a forma destas estruturas turbulentas. / Turbulent flows have been the objective of important studies to discover its dynamics. One important characteristic of these flows is the multiplicity of scales, since the large structures (low frequencies) controlled by the geometry that generates them, until the small structures (high frequencies) limited by the fluid viscosity. An important idea is the concept of vortices that it is associated with the improvements in the visualization techniques, in laboratory or numerical experiments. These vortices have an important function in many technological applications. In each one of these fields, it is necessary to understand the dynamics of its movements to control the mechanisms for producing or suppressing these vortices. In this context, the analysis of an air mixing in a tee junction is the main objective of this work. The tee geometry is sufficiently simple, but contributes for the appearance of a flow with vortices transition. Experimental tests with two different air temperatures inlets were done at the Thermal and Fluid Engineering Laboratory of the University of São Paulo at São Carlos (LETeF - EESC - USP). The measures of temperature were acquired with thermocouples installed along the pipe. Numerical studies with LES and DES methods using CFX software were applied to the flow. These computational results were compared with the experimental data through the time-frequency analysis. Preliminary studies of the flow show vortices transition regions and the ability of time-frequency technique in describing the existence and shape of turbulent structures.

Avec l’accroissement considérable de la puissance de calcul, les simulations aux grandes échelles (SGE) sont maintenant utilisées de façon routinière dans de nombreuses applications d’ingénierie. Les modèles de combustion usuels utilisés dans les SGE sont le plus souvent basés sur une hypothèse d’équilibre entre le mouvement des structures turbulentes et le plissement de la surface de la flamme. Ils s’écrivent alors sous forme d’expressions algébriques fonctions de grandeurs connues aux échelles résolues ainsi que de paramètres dont l’ajustement est à la charge de l’utilisateur selon la configuration étudiée et les conditions opératoires. Le modèle dynamique récemment développé ajuste automatiquement au cours du calcul les paramètres de modélisation qui peuvent alors dépendre du temps et de l’espace. Cette thèse présente une étude détaillée d’un modèle dynamique pour la simulation aux grandes échelles de la combustion turbulente prémélangée. L’objectif est de caractériser, explorer les avantages et les inconvénients, appliquer et valider le modèle dynamique dans plusieurs configurations. / Large eddy simulation (LES) is currently applied in a wide range of engineering applications. Classical LES combustion models are based on algebraic expressions and assume equilibrium between turbulence and flame wrinkling which is generally not verified in many circumstances as the flame is laminar at early stages and progressively wrinkled by turbulent motions. In practice, this conceptual drawback has a strong consequence: every computation needs its own set of constants, i.e. any small change in the operating conditions or in the geometry requires an adjustment of model parameters. The dynamic model recently developed adjust automatically the flame wrinkling factor from the knowledge of resolved scales. Widely used to describe the unresolved turbulent transport, the dynamic approach remains underexplored in combustion despite its interesting potential. This thesis presents a detailed study of a dynamic wrinkling factor model for large eddy simulation of turbulent premixed combustion. The goal of this thesis is to characterize, unveil pros and cons, apply and validate the dynamic modeling in different flow configurations.

L'assimilation de données en tant qu'outil pour la mécanique des fluides a connu une croissance exponentielle au cours des dernières décennies. La possibilité de combiner des mesures précises mais partielles avec un modèle dynamique complet est précieuse et a de nombreuses applications dans des domaines allant de l'aérodynamique, à la géophysique et à l’aéraulique. Cependant, son utilité reste limitée en raison des contraintes imposées par l'assimilation de données notamment en termes de puissance de calcul, de besoins en mémoire et en informations préalables. Cette thèse tente de remédier aux différentes limites de la procédure d'assimilation pour faciliter plus largement son utilisation en mécanique des fluides. Un obstacle majeur à l'assimilation des données est un coût de calcul prohibitif pour les écoulements complexes. Une modélisation de la turbulence à grande échelle est intégrée à la procédure d'assimilation afin de réduire considérablement la coût de calcul et le temps requis. La nécessité d'une information volumétrique préalable pour l'assimilation est abordée à l'aide d'une nouvelle méthodologie de reconstruction développée et évaluée dans cette thèse. L'algorithme d'optimisation reconstruit les champs 3D à partir d'observations dans deux plans orthogonaux en exploitant l'homogénéité directionnelle. La méthode et ses variantes fonctionnent bien avec des ensembles de données synthétiques et expérimentaux fournissant des reconstructions précises. La méthodologie de reconstruction permet également d’estimer la matrice de covariance d’ébauche indispensable à un algorithme d’assimilation efficace. Tous les ingrédients sont combinés pour effectuer avec succès l'assimilation de données variationnelles d'un écoulement turbulent dans le sillage d'un cylindre à un nombre de Reynolds transitoire. L'algorithme d'assimilation est validé pour des observations volumétriques synthétiques et est évalué sur des observations expérimentales dans deux plans orthogonaux. / Data assimilation as a tool for fluid mechanics has grown exponentially over the last few decades. The ability to combine accurate but partial measurements with a complete dynamical model is invaluable and has numerous applications to fields ranging from aerodynamics, geophysics, and internal ventilation. However, its utility remains limited due to the restrictive requirements for performing data assimilation in the form of computing power, memory, and prior information. This thesis attempts at redressing various limitations of the assimilation procedure in order to facilitate its wider use in fluid mechanics. A major roadblock for data assimilation is the computational cost which is restrictive for all but the simplest of flows. Following along the lines of Joseph Smagorinsky, turbulence modelling through large-eddy simulation is incorporated in to the assimilation procedure to significantly reduce computing power and time required. The requirement for prior volumetric information for assimilation is tackled using a novel reconstruction methodology developed and assessed in this thesis. The snapshot optimisation algorithm reconstructs 3D fields from 2D cross- planar observations by exploiting directional homogeneity. The method and its variants work well with synthetic and experimental data-sets providing accurate reconstructions. The reconstruction methodology also provides the means to estimate the background covariance matrix which is essential for an efficient assimilation algorithm. All the ingredients are combined to perform variational data assimilation of a turbulent wake flow around a cylinder successfully at a transitional Reynolds number. The assimilation algorithm is validated with synthetic volumetric observation and assessed on 2D cross-planar observations emulating experimental data.

The launch of large stratospheric balloons is highly dependant on the meteorological conditions at ground level, including wind speed. The balloon launch base Esrange Space Center in northern Sweden currently uses forecasts delivered through the Swedish Meteorological and Hydrological Institute to predict opportunities for balloon launches. However the staff at Esrange Space Center experience that the current forecasts are not accurate enough. For that reason the Weather Research and Forecasting model is used to improve the forecast. The model performs a Large Eddy Simulation over the area closest to Esrange Space Center to predict wind speed and turbulence. During twelve hypothetical launch days the improved forecast have an overall accuracy of 93% compared to the old forecast accuracy of 69%. With some improvements and the right computational power the system is thought to be operationally viable.

The increasing use of Intelligent Transport System (ITS) can enable very close vehicle spacings which generally results in a net drag reduction for the resulting convoys. The majority of vehicle development has, to date, been for vehicles in isolation, thus the study of interaction effects is becoming increasingly important. The main objective of this research is to investigate the use of Computational Fluid Dynamics (CFD) for understanding convoy aerodynamics and to further the understanding of airflow interaction between vehicles via CFD. In this study, time-averaged characteristics of a simplified, generic passenger vehicle, called the Ahmed car model, after Ahmed et.al (1984) is investigated computationally using the available commercial CFD code, Fluent version 6.1.22. Three different platoon combinations were analysed for the current study which includes a two, three and six model platoons for various rear end configurations of the Ahmed model geometry. Experiments were conducted in RMIT University Industrial Wind Tunnel for analysing the effects of drafting on drag coefficients using two different scales of Ahmed car models. This is an extension to the previous study performed on two 100% scales of Ahmed models (Vino and Watkins, 2004) and the results for both the current and previous experiments were compared using CFD. The CFD proved to be a useful technique since its results compared reasonably well for both the current and the previous experiments on drafting, using Ahmed models of identical (30°) rear slant configurations. However, near critical rear slant angles (~30°) for isolated vehicles some discrepancies were noted. The reasonable validation of experimental results enabled the study to be extended further computationally using CFD, to analyse the effects of inter-vehicle spacing on a platoon of 3 and 6 models for various rear end configurations (between 0° and 40°), in an attempt to provide useful information on vehicle-wake interaction for the Future Generation Intelligent Transport System (FGITS). Critical gaps were identified via CFD for the case of a two, three and six model platoons and the simulations clearly exposed the reasons for these critical gaps. At extremely close proximity, the models experienced more pressure recovery at their rear vertical base, which reduced the drag coefficient. Surprisingly, at some of the close vehicle spacings, the drag coefficients reached values that were higher than that of a vehicle in isolation. This was found due to the high momentum flow impingement to the fore body of the model and was similar to results found in physical experiments. Thus the current CFD analysis revealed that rear slant angle of the model and the inter-vehicle spacing greatly influences the wake structures and ultimately the vehicles aerodynamic drag coefficients in platoons. Even though the current CFD model (Realizable k-B turbulence model) predicted the basic flow structures such as the C-pillar vortices from the rear slant and 2D horse shoe vortices in the model's vertical rear base, the separation bubble on the rear slant that supplies energy to the strong C-pillar vortices was not replicated accurately, which is evidenced from the flow structure analysis. Hence it is recommended for further work, that the study should be extended using the Reynold's stress models or the Large Eddy Simulation (LES) turbulence models for flow structure observation and analysing vortex interactions between the models.

<p>A new large eddy simulation (LES) code is developed and used to investigate non-local features of turbulent planetary boundary layers (PBLs). The LES code is based on filtered Navier-Stokes equations, which describe motions of incompressible, Boussinesq fluid at high Reynolds numbers. The code computes directly large-scale, non-universal turbulence in the PBL whereas small-scale, universal turbulence is parameterized by a dynamic mixed subgrid closure. The LES code is thoroughly tested against high quality laboratory and field data. </p><p>This study addresses non-local properties of turbulence which emphasis on the stable stratification. Its basic results are as follows. The flow stability in PBLs is generally caused by two mechanisms: the negative buoyancy force (in the stable density stratification) and the Coriolis force (in the rotating system). The latter stabilizes the flow if the earth’s vorticity and the turbulent vorticity are anti-parallel. The Coriolis force stability suppresses large-scale turbulence and makes large eddies asymmetric. The density stratification suppresses vertical scales of turbulence. Joint actions of the Coriolis and the buoyancy forces result in a more complex behavior of turbulence. Particularly, the layers of vigorous turbulence may appear in the course of development of low-level jets in baroclinic atmosphere. </p><p>Non-local effects determine integral measures of PBLs, first of all the PBL depth. This study clearly demonstrates its pronounced dependences on the Coriolis parameter, the Kazanski-Monin internal stability parameter, and newly introduced imposed-stability and baroclinicity parameters. An LES database is created and used to validate an advanced PBL-depth formulation. LES support the idea that PBLs interact with the stably stratified free flow through the radiation of gravity waves, excited by large turbulent eddies at the interface. </p>