Large Eddy Simulation (LES) is carried out using the Rolls-Royce Hydra CFD code in order to investigate and give further insight into highly turbulent, unsteady flow structures for automotive applications. LES resolves time dependent eddies that are modelled in the steady-state by Reynolds-Averaged Navier-Stokes (RANS) turbulence models. A standard Smagorinsky subgrid scale model is used to model the energy transfer between large and subgrid scales. Since Hydra is an unstructured algorithm, a variety of unstructured hexahedral, tetrahedral and hybrid grids are used for the different cases investigated. Due to the computational requirements of LES, the cases in this study replicate and analyse generic flow problems through simplified geometry, rather than modelling accurate race car geometry which would lead to infeasible calculations. The first case investigates the flow around a diffuser-equipped bluff body at an experimental Reynolds number of 1.01 times 10 to the power 6 based on model height and inlet velocity. LES is carried out on unstructured hexahedral grids of 10 million and 20 million nodes, with the latter showing improved surface pressure when compared to the experiments. Comparisons of velocity and vorticity between the LES and experiments at the diffuser exit plane show a good level of agreement. Flow visualisation of the vortices in the diffuser region and behind the model from the mean and instantaneous flow attempts to explain the relation or otherwise between the two. The main weakness of the simulation was the late laminar to turbulent transition in the underbody region. The size of the domain and high experimental Reynolds number make this case very challenging. After the challenges faced by the diffuser-equipped bluff body, the underbody region is isolated so that increased grid refinement can be achieved in this region and the calculation is run at a Reynolds number of 220, 000, reducing the computational requirement from the previous case. A vortex generator mounted onto a flat underbody at an onset angle to the flow is modelled to generate vortices that extend along the length of the underbody and its interaction with the ground is analysed. Since the vortex generator resembles a slender wing with an incidence to the flow, a delta wing study is presented as a preliminary step since literature on automotive vortex generators in ground effect is scarce. Results from the delta wing study which is run at an experimental Reynolds number of 1.56 times 10 to the power 6 are in very good agreement with previous experiments and Detached Eddy Simulation (DES) studies, giving improved detail and understanding. Axial velocity and vorticity contours at several chordwise stations show that the leading edge vortices are predicted very well by a 20 million node tetrahedral grid. Sub-structures that originate from the leading edge of the wing and form around the core of the leading edge vortex are also captured. Large Eddy Simulation for the flow around an underbody vortex generator over a smooth ground and a rough ground is presented. A hexahedral grid of 40 million nodes is used for the smooth ground case, whilst a 48 million node hybrid grid was generated for the rough ground case so that the detailed geometry near the ground could be captured by tetrahedral cells. The geometry for the rough surface is modelled by scanning a tarmac surface to capture the cavities and protrusions in the ground. This is the first time that a rough surface representing a tarmac road has been computed in a CFD simulation, so that its effect on vortex decay can be studied. Flow visualisation of the instantaneous flow has shown strong interaction with the ground and the results from this study have given an initial understanding in this area.

In the last 10-15 years, large-eddy simulation (LES) has become well established for non-reacting flows, and several successful models have been developed for the transfer of momentum and kinetic energy to the subfilter-scales (SFS). However, for reacting flows, LES is still undergoing significant development. In particular, for many premixed combustion applications, the chemical reactions are confined to propagating surfaces that are significantly thinner than the computational grids used in practical LES. In these situations, the chemical kinetics and its interaction with the turbulence are not resolved and must be entirely modelled. There is, therefore, a need for accurate and robust physical modelling of combustion at the subfilter-scales. In this thesis, modelled transport equations for progress variable and flame surface density (FSD) were implemented and coupled to the Favre-filtered Navier-Stokes equations for a compressible reactive thermally perfect mixture. In order to reduce the computational costs and increase the resolution of simulating combusting flows, a parallel adaptive mesh (AMR) refinement finite-volume algorithm was extended and used for the prediction of turbulent premixed flames. The proposed LES methodology was applied to the numerical solution of freely propagating flames in decaying isotropic turbulent flow and Bunsen-type flames. Results for both stoichiometric and lean flames are presented. Comparisons are made between turbulent flame structure predictions for methane, propane, hydrogen fuels, and other available numerical results and experimental data. Details of subfilter-scale modelling, numerical solution scheme, computational results, and capabilities of the methodology for predicting premixed combustion processes are included in the discussions. The current study represents the first application of a full transport equation model for the FSD to LES of a laboratory-scale turbulent premixed flame. The comparisons of the LES results of this thesis to the experimental data provide strong support for the validity of the modelled transport equation for the FSD. While the LES predictions of turbulent burning rate are seemingly correct for flames lying within the wrinkled and corrugated flamelet regimes and for lower turbulence intensities, the findings cast doubt on the validity of the flamelet approximation for flames within the thin reaction zones regime.

This study investigates the complex flow structure generated by riparian emergent vegetation along the edge of floodplain. Detailed velocity and boundary shear stress measurements were carried out for various arrangements of emergent rigid cylindric rods of 3 mm, 6 mm and 9 mm diameters and for three different rod densities. In addition, the impact of foliage on the flow field was assessed during a series of experiments where brushes were used instead of smooth rods. The results of these new experiments are first presented. In addition to the laboratory data, field data was obtained through Acoustic Doppler Current Profiler measurements for two flood events in a stretch of the river Rhône that can be approximated to a straight compound channel with vegetated banks. The analysis of the flow structure highlights the presence of strong secondary circulation and increased vorticity on the river banks. The rods on the edge of the floodplain increase significantly flow resistance, reducing velocity and decreasing boundary shear stress. Flow rate was seen to decrease with increasing vegetative density for all cases except when foliage was added. This suggests that an optimum threshold density, for which a smaller density would lead to an increased flow rate might exist. Wakes trailing downstream of the vegetation stem, planform coherent structures advected between the main channel and the floodplain, and eddying motion in the flow due to enhanced turbulence anisotropy are among the defining patterns observed in the studied compound channel flows with one line of emergent vegetation along the edge of the floodplain. The Shiono and Knight Method (SKM) was modified in order to account for the increased turbulence activity due to the rods. The drag force term was introduced in the same way as in the work of Rameshwaran and Shiono (2007). However, a new term was added to the transverse shear stress term in the form of an Elder formulation, incorporating a friction drag coefficient which can be derived from the experimental data. In this proposed version, the advection term was set to zero. Another version of the SKM, similar to Rameshwaran and Shiono (2007), was also tested with the addition of a local drag friction only applied in the rod region. The proposed SKM version without the advection term was favored as it can be more closely related to the experimental data and to physical processes. Finally, the capabilities of Telemac-2D were tested against the experimental data for various turbulence models. The Large Eddy Simulation turbulence model highlighted some unsteady flow patterns that were observed during experiments, while satisfactorily predicting the lateral velocity and boundary shear stress distributions.

A large eddy simulation (LES) model has been developed and validated for turbulent non-premixed and partially premixed combustion systems. LES based combustion modelling strategy has the ability to capture the detailed structure of turbulent flames and account for the effects of radiation heat loss. Effects of radiation heat loss is modelled by employing an enthalpy-defect based non-adiabatic flamelet model (NAFM) in conjunction with a steady non-adiabatic flamelet approach. The steady laminar flamelet model (SLFM) is used with multiple flamelet solutions through the development of pre-integrated look up tables. The performance of the non-adiabatic model is assessed against experimental measurements of turbulent CH4/H2 bluff-body stabilized and swirl stabilized jet flames carried out by the University of Sydney combustion group. Significant enhancements in the predictions of mean thermal structure have been observed with both bluff body and swirl stabilized flames by the consideration of radiation heat loss through the non-adiabatic flamelet model. In particular, mass fractions of product species like CO2 and H2O have been improved with the consideration of radiation heat loss. From the Sydney University data the HM3e flame was also investigated with SLFM using multiple flamelet strategy and reasonably fair amount of success has been achieved. In this work, unsteady flamelet/progress variable (UFPV) approach based combustion model which has the potential to describe both non-premixed and partially premixed combustion, has been developed and incorporated in an in-house LES code. The probability density function (PDF) for reaction progress variable and scalar dissipation rate is assumed to follow a delta distribution while mixture fraction takes the shape of a beta PDF. The performance of the developed model in predicting the thermal structure of a partially premixed lifted turbulent jet flame in vitiated co-flow has been evaluated. The UFPV model has been found to successfully predict the flame lift-off, in contrast SLFM results in a false attached flame. The mean lift-off height is however over-predicted by UFPV-δ function model by ~20% for methane based flame and under-predicted by ~50% for hydrogen based flame. The form of the PDF for the reaction progress variable and inclusion of a scalar dissipation rate thus seems to have a strong influence on the predictions of gross characteristics of the flame. Inclusion of scalar dissipation rate in the calculations appears to be successful in predicting the flame extinction and re-ignition phenomena. The beta PDF distribution for the reaction progress variable would be a true prospect for extending the current simulation to predict the flame characteristics to a higher degree.

The periods of time-varying turbulence in the atmospheric boundary layer, i.e.\ the morning and evening transitions, are often overlooked or highly idealised by dispersion models. These transitions make up a significant portion of the diurnal cycle and are known to affect the spread of pollution due to the properties of turbulence in the residual and stable layers, resulting in phenomena such as lofting, trapping, and fumigation.\\ Two main simulation techniques are presented for the purpose of modelling the dispersion of passive tracers in both convective and evening transitional boundary layers: Lagrangian stochastic (LS) modelling for 1D, inhomogeneous, non-stationary turbulence; and large-eddy simulation (LES) with a particle model tracing pollutant paths using a combination of the resolved flow velocities and a random displacement model to represent sub-grid scale motions.\\ In the convective boundary layer, LS models more accurately representing the state of turbulence, and including the effect of skewness, are shown to produce dispersion results in closer agreement with LES. By considering individual particle trajectories, a reflective top boundary in LS models is shown to produce un-physical, sharp changes in velocity and position. By applying a correction to the vertical velocity variance based on representing the stable potential temperature gradient above the boundary layer, particles are contained within the boundary layer in a physically accurate way. \\ An LS model for predicting dispersion in time-varying, skewed turbulence is developed and tested for various particle releases in transitional boundary layers with different rates of decay, showing an improvement in accuracy compared with previous LS models. Further improvement is made by applying a correction to the vertical velocity variance to represent the effect of a positive potential temperature gradient developing over the course of the transition. Finally, a developing stable boundary layer is shown to have a significant trapping effect on particles released near the surface. \\

La recherche en bruit de combustion est de nos jours majoritairement consacrée au développement d'outils de calcul du bruit rayonné par les flammes. Les méthodes actuelles de CFD telles que la LES ou la DNS sont capables de fournir le champ acoustique rayonné par des sources de bruit, mais elles sont cependant limitées à des domaines de faible taille, ceci dû à leur fort coût de calcul. Pour surmonter cette limitation, on a vu l'émergence de méthodes hybrides. Dans cette approche, les sources de bruit sont découplées du son rayonné. Les sources sont alors calculées par les méthodes de DNS et de LES tandis que l'analogie acoustique permet de calculer le son rayonné par des codes acoustiques, moins coûteux en temps de calcul.L'objet de cette étude est de développer un outil numérique sur la base de l'analogie acoustique de Phillips pour de faibles nombres de Mach. Il permet de prendre en compte l'impact des conditions limites sur le champ acoustique résultant. La LES et le code de calcul acoustique développé ont été utilisés pour évaluer le bruit produit par une flamme turbulente confinée. Les deux techniques donnent des résultats en accord tant que les bonnes quantités sont comparées: il a été observé que le signal de pression obtenu directement à partir de la LES contient une quantité non négligeable d'hydrodynamique, laquelle doit être négligée si on cherche à comparer seulement les champs acoustiques issus des deux codes.L'hypothèse d'un nombre de Mach faible est totalement réaliste si l'on considère l'écoulement présent dans une chambre de combustion. Elle conduit à des simplifications significatives lorsque les analogies acoustiques sont considérées. Cependant, cette hypothèse ne peut pas être utilisée pour l'écoulement en amont (entrée d'air, compresseur) ni en aval (turbine, tuyère) des chambres de combustion aéronautiques. Un outil numérique a été développé pour pallier ce problème. Il est basé sur les équations d'Euler Quasi-1D, qui prennent en compte des écoulements convectifs, non isentropiques et non isenthalpiques. Cet outil permet d'estimer les conditions limites acoustiques qui doivent être imposées sur les entrées/sorties d'une chambre de combustion pour prendre en compte la présence d'un écoulement de nombre de Mach non négligeable, alors que les calculs acoustiques sont eux effectués sous cette hypothèse fortement restrictive. / Today, much of the current effort in combustion noise is the development of efficient numerical tools to calculate the noise radiated by flames. Although unsteady CFD methods such as LES or DNS can directly provide the acoustic field radiated by noise sources, this evaluation is limited to small domains due to high computational costs. Hybrid methods have been developed to overcome this limitation. In these schemes, the noise sources are decoupled from the radiated sound. The sources are still calculated by DNS or LES codes whereas the radiated sound is evaluated by acoustic codes using an acoustic analogy.In the present study, a numerical tool based on the Phllips' analogy for low Mach numbers flows has been developed. This tool accounts for the role of the boundary conditions in the resulting acoustic field. Both LES and the acoustic code developed here are used to assess the noise produced by a turbulent confined flame of a turbulent swirled-stabilized staged combustor. Good agreements are obtained between both techniques as long as the good quantities are compared: the pressure signal obtained directly from LES contains a non negligible amount of hydrodynamics that must be removed when a suitable acoustics-acoustics comparison is sought. The low Mach number assumption is completely realistic when considering the flow within a combustion chamber; it also conducts to considerable simplifications when leading with acoustic analogies. However, it cannot be used for the upstream (air-intake, compressors) and downstream (turbines, nozzle) of an aeronautical combustion chamber. A numerical tool is developed based on the quasi-1D Linearized Euler Equations in order to account for convective, non-isentropic and non-isenthalpic flows. By means of this tool, it is possible to estimate the acoustic boundary conditions that should be imposed at the inlet/oultlet of a given combustion chamber when performing low-Mach number acoustic computations.

Numerical simulation allows investigation into the influence of separation distance and rotation on the performance of two vertical-axis hydrokinetic turbines. Compu- tational fluid dynamics is applied to calulate the lift and drag coefficients acting upon interacting NACA 0021 turbine blades for a Reynolds number of Red = 10, 000. To understand the effect of separation distance, large-eddy simulation of the flow around side-by-side and staggered cylinders, ReD = 3,000, and airfoils, Rec = 3,000, are also performed. Based upon the simulations, a drag reduction of 11.3% and 19.8% is determined for the downstream cylinder and airfoil, respectively. A reduction in Reynolds stresses is also observed for the staggered configuration compared to the side-by-side configuration. Due to computational resources of large-eddy simulation, the Reynolds averaged Navier-Stokes method is also applied to investigate the influence of separation distance and rotation on two vertical axis hydrokinetic turbines. The numerical simulations show that a drag reduction of 15.5% occurs when the non-dimensional spanwise and streamwise separation distances, based on turbine diameter, reach 1 and 2, respectively. / October 2016

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.