The aerodynamics of modern rotorcraft is highly complex and has proven to be an arduous challenge for computational fluid dynamics (CFD). Flow features such as massively separated boundary layers or transition to turbulence are common in engineering applications and need to be accurately captured in order to predict the vehicle performance. The recent advances in numerical methods and turbulence modeling have resolved each of these issues independent of the other. First, state-of-the-art hybrid RANS-LES turbulence closures have shown great promise in capturing the unsteady flow details and integrated performance quantities for stalled flows. Similarly, the correlation-based transition model of Langtry and Menter has been successfully applied to a wide range of applications involving attached or mildly separated flows. However, there still lacks a unified approach that can tackle massively separated flows in the transitional flow region. In this effort, the two approaches have been combined and expended to yield a methodology capable of accurately predicting the features in these highly complex unsteady turbulent flows at a reasonable computational cost. Comparisons are evaluated on several cases, including a transitional flat plate, circular cylinder in crossflow and NACA 63-415 wing. Cost and accuracy correlations with URANS and prior hybrid URANS-LES approaches with and without transition modeling indicate that this new method can capture both separation and transition more accurately and cost effectively. This new turbulence approach has been applied to the study of wings in the reverse flow regime. The flight envelope of modern helicopters has increased significantly over the last few decades, with design concepts now reaching advance ratios up to μ = 1. In these extreme conditions, the freestream velocity exceeds the rotational speed of the blades, and a large region of the retreating side of the rotor disk experiences reverse flow. For a conventional airfoil with a sharp trailing edge, the reverse flow regime is generally characterized by massive boundary layer separation and bluff body vortex shedding. This complex aerodynamic environment has been utilized to evaluate the new hybrid transitional approach. The assessment has proven the efficiency of the new hybrid model, and it has provided a transformative advancement to the modeling of dynamic stall.

Turbulent flows affected by buoyancy lie at the basis of many applications, both within engineering and the atmospheric sciences. A prominent example of such an application is the atmospheric boundary layer, the lowest layer of the atmosphere, in which many physical processes are heavily influenced by both stably stratified and convective turbulent transport. Modelling these turbulent flows correctly, especially in the presence of stable stratification, has proven to be a great challenge and forms an important problem in the context of climate models. In this thesis, we address this issue considering an advanced class of turbulence models, the so-called explicit algebraic models.In the presence of buoyancy forces, a mutual coupling between the Reynolds stresses and the turbulent heat flux exists, which makes it difficult to derive a fully explicit turbulence model. A method to overcome this problem is presented based on earlier studies for cases without buoyancy. Fully explicit and robust models are derived for turbulence in two-dimensional mean flows with buoyancy and shown to give good predictions compared with various data from direct numerical simulations (DNS), most notably in the case of stably stratified turbulent channel flow. Special attention is given to the problem of determining the production-to-dissipation ratio of turbulent kinetic energy, for which the exact equation cannot be solved analytically. A robust approximative method is presented to calculate this quantity, which is important for obtaining a consistent formulation of the model.The turbulence model derived in this way is applied to the atmospheric boundary layer in the form of two idealized test cases. First, we consider a purely stably stratified boundary layer in the context of the well-known GABLS1 study. The model is shown to give good predictions in this case compared to data from large-eddy simulation (LES). The second test case represents a full diurnal cycle containing both stable stratification and convective motions. In this case, the current model yields interesting dynamical features that cannot be captured by simpler models. These results are meant as a first step towards a more thorough investigation of the pros and cons of explicit algebraic models in the context of the atmospheric boundary layer, for which additional LES data are required. / <p>QC 20150522</p>

A Large Eddy-Simulation code, based on a mesh transparent algorithm, for hybrid unstructured meshes is presented to deal with complex geometries that are often found in engineering flow problems. While tetrahedral elements are very effective in dealing with complex geometry, excessive numerical diffusion often affects results. Thus, prismatic or hexahedral elements are preferable in regions where turbulence structures are important. A second order reconstruction methodology is used since an investigation of a higher order method based upon Lele's compact scheme has shown this to be impractical on general unstructured meshes. The convective fluxes are treated with the Roe scheme that has been modified by introducing a variable scaling to the dissipation matrix to obtain a nearly second order accurate centred scheme in statistically smooth flow, whilst retaining the high resolution TVD behaviour across a shock discontinuity. The code has been parallelised using MPI to ensure portability. The base numerical scheme has been validated for steady flow computations over complex geometries using inviscid and RANS forms of the governing equations. The extension of the numerical scheme to unsteady turbulent flows and the complete LES code have been validated for the interaction of a shock with a laminar mixing layer, a Mach 0.9 turbulent round jet and a fully developed turbulent pipe flow. The mixing layer and round jet computations indicate that, for similar mesh resolution of the shear layer, the present code exhibits results comparable to previously published work using a higher order scheme on a structured mesh. The unstructured meshes have a significantly smaller total number of nodes since tetrahedral elements are used to fill to the far field region. The pipe flow results show that the present code is capable of producing the correct flow features. Finally, the code has been applied to the LES computation of the impingement of a highly under-expanded jet that produces plate shock oscillation. Comparison with other workers' experiments indicates good qualitative agreement for the major features of the flow. However, in this preliminary computation the computed frequency is somewhat lower than that of experimental measurements.

In this study, Computational Fluid Dynamics (CFD) simulations have been carried out in order to investigate and improve the performance of the Standard Cirrus glider, using different Navier-Stokes methods and solving the equations for the steady flow. The work has been divided in two parts: First, a study is performed to test the quality of the transition model (Gamma-ReTheta). The two dimensional results of the glider´s airfoil are compared against the results from panel’s methods and the open-source CFD codes: SU2 and OpenFoam. In addition, three dimensional glider´s models are simulated using the transition model with the purpose of creating a validated reference model of the glider’s performance in steady level flight. The simulations are carried out in two dimensions for the outer wing airfoil for a 1.5 e+06 Reynolds number and in three dimensions for the Wing &amp; Fuselage model and Tail &amp; Fuselage model under a range of velocities. Both simulations are validated against experimental data. In the second part of the study, the validated model is used to developed possible improvements in the glider´s external geometry that could produce possible benefits in the performance and handling qualities of the glider.

The usage of ground-mounted photovoltaic solar panels is increasing, and it is essential to fully understand the wind behavior and loading on the panels, since there is no specific code or guideline for their design in the present. In addition, the University of Ottawa and Carleton University recently developed a new facility, called the Wind Damage Simulator (WDS). This study will allow to understand the flow behavior in the facility, along with the effects of different blower rpm settings on the flow. A CFD study was also conducted, in order to examine the methodology and turbulence models suitable for the flow replicated in this facility, for future research. The newly developed Wind Damage Simulator (WDS) facility was used to examine wind-induced effects on two solar panels attached to a frame. The mean pressure coefficient distribution on the photovoltaic panels was examined for several wind angles of incidence (AOI) and wind speeds. The wind AOI considered were the 0°, 30°, 45°, 180°, 210° and 225°, along with wind speeds ranging from 14 m/s to 42 m/s, with increments of around 2 m/s. The experimental results showed fluctuations in the Cp distribution on the panels, due to the WDS wind flow being highly unstable. The 180° wind AOI led to the highest uplift mean Cp equivalent (Cpeq) value on the panels. The 45° and 180° wind AOI were found to induce a Cpeq of significantly higher magnitude on the first panel compared to the second panel, whereas the 0° and 210° wind AOI induced a significantly higher Cpeq magnitude on the second panel compared to the first panel Cpeq. Moreover, the CFD study provided pressure coefficient distribution on the panels and flow visualization when interacting with the panels surfaces. The LES Dynamic Smagorinsky subgrid scale model was found to be more suitable for the WDS numerical replication than the RANS shear stress transport k-ω turbulence model. The LES model showed the fluctuating pressure coefficients on the panels’ surfaces, induced by the swirls that were formed post interaction of the wind with the panels.

Este trabalho discute a questão da interação entre o casco do navio e o propulsor em funcionamento conjunto, e sua simulação por ferramentas computacionais. O texto se concentra em descrever os principais efeitos dessa interação, as principais dificuldades em se estimar esses efeitos, os métodos tradicionalmente usados, e como ferramentas computacionais podem ser aplicadas de maneira vantajosa. No texto também pode ser encontrada uma análise crítica dos métodos mais comuns e dos resultados, baseada em trabalhos de diversos autores, publicados nacional e internacionalmente. É apresentado o método dos volumes finitos, usado nesta pesquisa, algumas de suas particularidades principais, vantagens e desvantagens, e os resultados das simulações realizadas, interpretados à luz dos valores experimentais usados para comparação e das limitações do método dos Volumes Finitos. A comparação é feita analisando-se grandezas integrais, como a resistência do casco ou o empuxo do propulsor; e também as características do escoamento, como o perfil de velocidade na esteira, presença de vórtices, e outras estruturas típicas. Os resultados obtidos têm a mesma ordem de precisão dos que tem sido obtidos por outros pesquisadores, internacionalmente, e são coerentes qualitativamente; mas algumas questões referentes aos modelos físico e numérico escolhidos ainda limitam a precisão dos resultados e restringem sua adoção em atividades de engenharia. Entretanto, diversas características observadas no escoamento contribuem para aumentar o conhecimento de alguns fenômenos envolvidos no problema. / This work is about the hull and propeller interaction in joint functioning, and its simulation by computational tools. The text concentrates in describing the main effects of such interaction, the main difficulties in the estimation of these effects, the methods traditionally adopted, and how computational tools can be applied in advantageous way. A critical analysis of the most common methods and results, based on paperworks of several different authors worldwide, can also be found. The Finite Volumes method, used in this research, is presented - its main issues, advantages and disadvantages, and the simulations outcomes, compared to the experimental values and explained by the knowledge of the Finite Volumes method limitations. The comparison is made by means of both integral values, such as the hull\'s resistance or the propeller thrust; as well as the characteristics of the flow, like the wake velocity profile, presence of vortex, and other typical structures. The results shows the same error band than the ones which has been obtained by other researchers, worldwide, and most of the typical characteristics of the flow were observed. But some issues concerned to the chosen physical and numerical models still limit the precision of the outcomes, and restrict the application of such models at engineering activities. But several insights about the flow, obtained at this study, can be useful to the understanding of some phenomena involved in the propeller operation.

Stationary gas turbines manufacturers and operators are under constant scrutiny to both reduce environmentally harmful emissions and obtain efficient combustion. Numerical simulations have become an integral part of the development and optimisation of gas turbine combustors. In this thesis work, the gas turbine combustion process is analysed in two parts, a study on air-fuel mixing and turbulent combustion. For computational fluid dynamic analysis work the open-source CFD code OpenFOAM and STAR-CCM+ are used. A fuel jet injected to cross-flowing air flow is simplified air-fuel mixing arrangement, and this problem is analysed numerically in the first part of the thesis using both Reynolds Averaged Navier Stokes (RANS) method and Large Eddy Simulation (LES) methods. Several turbulence models are compared against experimental data in this work, and the complex turbulent vortex structures their effect on mixing field prediction is observed. Furthermore, the numerical methods are extended to study twin jets in cross-flow interaction which is relevant in predicting air-fuel mixing with arrays of fuel injection nozzles. LES methods showed good results by resolving the complex turbulent structures, and the interaction of two jets is also visualised. In this work, all three turbulent combustion regimes non-premixed, premixed, partially premixed are modelled using different combustion models. Hydrogen blended fuels have drawn particular interest recently due to enhanced flame stabilisation, reduced CO2 emissions, and is an alternative method to store energy from renewable energy sources. Therefore, the well known Sydney swirl flame which uses CH4: H2 blended fuel mixture is modelled using the steady laminar flamelet model. This flame has been found challenging to model numerically by previous researchers, and in this work, this problem has been addressed with improved combustion modelling approach with tabulated chemistry. Recognizing that the current and future gas turbine combustors operate on a mixed combustion regime during its full operational cycle, combustion simulations of premixed/partially premixed flames are also performed in this thesis work. Dynamical artificially thickened flame model is implemented in OpenFOAM and validated using propagating and stationary premixed flames. Flamelet Generated Manifold (FGM) methods are used in the modelling of turbulent stratified flames which is a relatively new field of under investigation, and both experimental and numerical analysis is required to understand the physics. The recent experiments of the Cambridge stratified burner are studied using the FGM method in this thesis work, and good agreement is obtained for mixing field and temperature field predictions.

The current standard of simulating flood flow in natural river reaches is based on solving the 1-D or 2-D St. Venant equations or using hybrid 1-D/2-D models based on the same equations. These models are not always able to accurately predict floodwave propagation, especially around and downstream of regions where 3-D effects become important, or at times when the main assumptions associated with these models are violated (e.g. flow becomes pressurized due to presence of a hydraulic structure like a bridge or a culvert). A 3-D modeling approach, though computationally much more expensive, is not subject to such limitations and should be able to predict accurately predict floodwave propagation even in regions where 3-D effects are expected to be significant. This dissertation describes the development and validation of a 3-D time-accurate RANS-based model to study flood-related problems in natural environments. It also discusses how results from these 3-D simulations can be used to better calibrate lower order models. Applications are included where the flow becomes pressurized during high flow conditions and the sediment entrainment potential of the flow during the flooding event is estimated. Another important category of applications discussed in the present study are floodwave propagation induced by a sudden dam break failure. Results show that 2-D models show fairly large differences with 3-D model predictions especially in regions where 3-D effects are expected to be significant (e.g. near channel-floodplain transition, in highly curved channels, near hydraulic structures). The study also discusses the use of the validated 3-D model as an engineering design tool to identify the optimum solution for flood protection measures intended to reduce flooding in the Iowa River near Iowa City. 3-D simulation results are also used to discuss hysteresis effects in the relationship between bed shear stress and the stage/discharge. Such effects need to be taken into consideration to accurately estimate erosion associated with the passage of a floodwave.

Ce travail est consacré à la simulation numérique des écoulements inertes et réactifs stabilisés en aval d'un élargissement brusque symétrique, alimenté par deux écoulements de canal pleinement développés d'air ou air+propane.<br />L'objectif est, d'une part, de tester sur une géométrie "simple", une modélisation de la combustion en écoulements turbulents et en régime de prémélange et, d'autre part, d'estimer le niveau de précision obtenue afin de pouvoir quantifier une éventuelle amélioration liée à l'utilisation de modèles plus sophistiqués. Le modèle de turbulence k-epsilon sous sa forme standard avec la loi de paroi, et le modèle de combustion du type PDF présumé, modèle CLE, ont été retenus pour nos simulations.<br />Une étude préliminaire du type KPP nous a permis de déduire les caractéristiques propagatives des zones de réactions moyennes à richesse constante, calculées sur la base du modèle de combustion retenu.<br />Dans ce travail, ont été simulés : trois écoulements inertes et trois écoulements réactifs à richesse constante pour nombres de Reynolds égaux à 25000, 50000 et 75000 et deux écoulements réactifs à richesse variable pour nombre de Reynolds 25000.<br />Concernant les écoulements inertes, on retrouve bien la dissymétrie observée expérimentalement des deux zones de recirculation moyennes. Pour les écoulements réactifs à richesse constante, on retrouve également la resymétrisation de l'écoulement moyen observée expérimentalement.<br />En revanche, la longueur des zones de recirculation moyenne est fortement sous-estimée par le calcul.<br />Un comportement auto-semblable dans la zone proche de l'élargissement est également retrouvé. Pour les écoulements à richesse variable, la perte de symétrie de l'écoulement moyen est bien retrouvée, mais les longueurs des zones de recirculation moyennes sont toujours sous-estimées par le calcul. Des indicateurs quantitatifs d'écart entre les résultats numériques et expérimentaux sont fournis pour les écoulements inertes et réactifs considérés, permettant à l'avenir, d'estimer le gain apporté par le recours à des simulations basées sur une évolution ou sur une amélioration de modèles physiques retenus de le cadre de cette étude.

En situation industrielle, le calcul des caractéristiques instationnaires et tridimensionnelles d' écoulements complexes, est souvent nécessaire. La simulation des grandes échelles requiert un coût de calcul prohibitif surtout près de parois. L'un des objectifs des méthodes hybrides est d'optimiser le coût de calcul, en simulant certaines zones d'un écoulement en mode RANS et d'autres en mode LES. Cette dernière s'articule en géné́ral autour du filtrage spatial, alors que dans la plupart des écoulements, l'opérateur RANS correspond à une moyenne temporelle. L'approche PITM (Partially Integrated Transport Model ), conçue en turbulence homogène, est une méthode hybride justifié́e thé́oriquement. Sa transposition au contexte temporel (turbulence stationnaire) a déjà été explorée précédemment, montrant que, sous certaines hypothèses, les versions spatiale et temporelle sont formellement identiques. La méthode PITM présente toutefois une certaine difficulté à piloter le niveau de résolution. La présente thèse propose une approche dynamique pour corriger ce point. Dans un second temps, la version temporelle du PITM, le T -PITM, est comparé à la DES (Detached Eddy Simulation), une méthode hybride populaire, mais empirique. Il est montré que les deux méthodes produisent des résultats similaires, conférant une justification théorique par procuration, à la DES. Le modèle RANS sous-jacent est la pondération elliptique, permettant la prise en compte des effets induits par une paroi, sans utiliser de fonctions d'amortissement ni de lois de paroi.