Les orages tropicaux jouent un rôle incertain dans le transport de l'air troposphérique dans la stratosphère limitant notre capacité à prévoir le climat futur. Le transport par les orages pourrait en effet être sous-estimé dans les modèles de climat aux résolutions trop grossières. L'efficacité de ce transport est analysée à partir de simulations numériques de l'orage Hector the Convector jusqu'à une résolution de 100 m, la plus fine jamais utilisée pour un cas de convection très profonde. Les percées nuageuses, qui avaient été observées à son sommet à 18 km d'altitude, sont reproduites et l'hydratation nette de la stratosphère est quantifiée. La contribution des orages tropicaux au flux d'eau de la troposphère à la stratosphère est ainsi estimée à près de 20 %. La quasi-convergence aux résolutions de 200 m et 100 m suggère que de telles résolutions sont nécessaires pour représenter correctement les ascendances. L'analyse individuelle des ascendances indique que les deux plus grandes contribuent à plus de 90 % du flux de masse vers la basse stratosphère. Elles sont plus larges, plus puissantes et contiennent plus d'eau que les plus grandes ascendances une heure avant et une heure après, et leur cœur convectif apparaît très peu dilué. L'alimentation en surface par des lignes de convergence intensifiées par des poches froides et la faible dilution des deux plus grandes ascendances sont déterminantes dans l'apparition de la convection très profonde. L'analyse isentropique de la circulation générale dans Hector confirme le flux de masse calculé par l'analyse des ascendances. Elle le corrige dans les basses couches en prenant en compte les flux turbulents, et en haute troposphère en filtrant les ondes de gravité. Elle met en évidence l'importance du dégagement de chaleur latente dû à la congélation dans les plus grandes ascendances pendant la phase de percée en stratosphère. / The tropical thunderstorms play an uncertain role in the transport of tropospheric air into the stratosphere, limiting our capability to predict the future climate. The transport by the thunderstorms may be underestimated by the climate models, due to their coarse resolutions. The efficiency of this transport is analysed using numerical simulations of the thunderstorm Hector the Convector with resolutions down to 100 m, the finest ever used for a case of very deep convection. The overshoots, that were observed at its top at 18 km altitude, are captured and the net hydration of the stratosphere is quantified. The contribution of the tropical thunderstorms to the water flux from the troposphere to the stratosphere is then estimated to about 20 %. The almost convergence at 200 m and 100 m suggests that such resolutions are necessary to correctly represent the updafts. The individual analysis of the updrafts indicates that the two tallest contribute beyond 90 % of the mass flux into the stratosphere. They are larger, more vigorous and contain more water than the tallest updrafts one hour before and one hour after, and their convective core was weakly diluted. The supply from the surface by the convergence lines, intensified by the cold pools, and the weak dilution of the two tallest updrafts are determinant for the development of very deep convection. The isentropic analysis of the overturning inside Hector confirms the mass flux computed with the updrafts analysis. It corrects the estimate in the lower troposphere by taking into account the turbulent flux, and in the upper troposphere by filtering out the gravity waves. It highlights the importance of the latent heating due to freezing in the two tallest updrafts during the phase of overshoot in the stratosphere.

Turbulence is an open and challenging problem for mathematical approaches, physical modeling and numerical simulations. Numerical solutions contribute significantly to the understand of the nature and effects of turbulence. The focus of this thesis is the development of appropriate numerical methods for the computer simulation of turbulent flows. Many of the existing approaches to turbulence utilize analogies from kinetic theory. Degond & Lemou (J. Math. Fluid Mech., 4, 257-284, 2002) derived a k-✏ type turbulence model completely from kinetic theoretic framework. In the first part of this thesis, a numerical method is developed for the computer simulation based on this model. The Boltzmann equation used in the model has an isotropic, relaxation collision operator. The relaxation time in the collision operator depends on the microscopic turbulent energy, making it difficult to construct an efficient numerical scheme. In order to achieve the desired numerical efficiency, an appropriate change of frame is applied. This introduces a stiff relaxation source term in the equations and the concept of asymptotic preserving schemes is then applied to tackle the stiffness. Some simple numerical tests are introduced to validate the new scheme. In the second part of this thesis, alternative approaches are sought for more efficient numerical techniques. The Lattice Boltzmann Relaxation Scheme (LBRS) is a novel method developed recently by Rohan Deshmukh and S.V. Raghuram Rao for simulating compressible flows. Two different approaches for the construction of implicit sub grid scale -like models as Implicit Large Eddy Simulation (ILES) methods, based on LBRS, are proposed and are tested for Burgers turbulence, or Burgulence. The test cases are solved over a largely varying Reynolds number, demonstrating the efficiency of this new ILES-LBRS approach. In the third part of the thesis, as an approach towards the extension of ILES-LBRS to incompressible flows, an artificial compressibility model of LBRS is proposed. The modified framework, LBRS-ACM is then tested for standard viscous incompressible flow test cases.

In dieser Dissertation wird das kompressible, nicht-hydrostatische und dreidimensionale Modell All Scale Atmospheric Model (ASAM) für Grobstruktur- bzw. Large-Eddy-Simulationen (LES) angewendet, um lokale Inseleffekte in der karibischen Passatwindzone zu untersuchen. Da das Modell bis dato noch keine Anwendung im Bereich von LES feuchter atmosphärischer Grenzschichten und heterogener Oberflächen fand, wurden einige Bestandteile zum Modellcode hinzugefügt oder überarbeitet. Ein Hauptaugenmerk liegt dabei auf das Einbeziehen orographischer Strukturen mittels angeschnittener Zellen (engl. cut cells). Sowohl die räumliche und zeitliche Diskretisierung der Modellgleichungen als auch die nötigen physikalischen Parameterisierungen werden in einer umfassenden Modellbeschreibung zusammengefasst. Die Robustheit und Stabilität der Modellformulierung wird durch eine Reihe von Simulationen idealisierter Testfälle bestätigt. Large-Eddy-Simulationen werden für das Gebiet der Karibikinsel Barbados zur Untersuchung von Inseleffekten bezüglich Grenzschichtmodifikation, Wolkenbildung und vertikaler Durchmischung von Aerosolen durchgeführt. Durch das Vorhandensein einer topographisch strukturierten Inseloberfläche in der Mitte des Modellgebietes muss das Modellsetup offene seitliche Randbedingungen beinhalten. Damit das einströmende Windfeld konsistent mit der Dynamik einer turbulenten, marinen Grenzschicht ist, wird eine neue Methode implementiert und angewendet, welche auf Störungen des potentiellen Temperaturfeldes mittels finiter Amplituden basiert. Beobachtungen aus der SALTRACE-Messkampagne werden benutzt, um die Modellläufe anzutreiben. Die Ergebnisse einiger Sensitivitätstests zeigen Probleme der Modellierung im Bereich der \"Terra incognita\" auf. Dabei handelt es sich um die Modellierung auf räumlichen Skalen, welche zwischen denen von LES und wolkenauflösenden Modellen liegen. Außerdem werden Auswirkungen von entweder turbulent oder laminar anströmenden Windfeldern auf die Simulationsergebnisse untersucht. Besonders die Wolkeneigenschaften im Lee von Barbados werden in diesen Simulationen merklich beeinflusst. Ergebnisse einer weiteren Simulation mit einer sehr starken Passatinversion bringt deren Einfluss auf die Dicke und Höhe der simulierten Wolkenschichten zum Vorschein. Die Veränderung von Saharastaubschichten, welche Barbados über weiträumigen Transport über den Atlantik erreichen, wird analysiert. Die Auswirkungen beinhalten sowohl eine Ausdünnung und ein Absinken dieser Schichten als auch turbulenter Transport in Richtung Erdoberfläche. Die genaue Position der beeinflussten Schichten und die Stärke des turbulenten Mischens werden hauptsächlich von der atmosphärischen Schichtung, der Inversionsstärke und Windscherung gesteuert. Vergleiche zwischen den LES-Modellergebnissen und Daten aus Doppler-Windlidarmessungen zeigen gute Übereinstimmungen in der Formierung der konvektiven Strukturen tagsüber und des Vertikalwindfeldes. / In this thesis, the fully compressible, three-dimensional, nonhydrostatic atmospheric model called All Scale Atmospheric Model (ASAM) is utilized for large eddy simulations (LES) to investigate local island effects at the Caribbean. Since the model has not been applied to LES for moist boundary layers and heterogeneous surfaces so far, several parts are added to the model code or reworked. A special focus lies on the inclusion of orographical structures via the cut cell method. Spatial and temporal discretization as well as necessary physical parameterizations are summarized in a thorough model description. The robustness of the model formulation is confirmed by a set of idealized test case simulations. Large eddy simulations are performed for the area of the Caribbean island Barbados to investigate island effects on boundary layer modification, cloud generation and vertical mixing of aerosols. Due to the presence of a topographically structured island surface in the domain center, the model setup has to be designed with open lateral boundaries. In order to generate inflow turbulence consistent with the upstream marine boundary layer forcing, the newly developed cell perturbation method based on finite amplitude perturbations is applied. Observations from the SALTRACE field campaign are used to initialize the model runs. Several numerical sensitivity tests are carried out to demonstrate the problems related to \"gray zone modeling\" beyond LES scales or when the turbulent marine boundary layer flow is replaced by laminar winds. Especially cloud properties west of Barbados (downwind) are markedly affected in these simulations. Results of an additional simulation with a strong trade-wind inversion reveal its effect on cloud layer depth and height. The modification of Saharan dust layers reaching Barbados via long-range transport over the North Atlantic is analyzed. Effects of layer thinning, subsidence and turbulent downward transport near the layer bottom become apparent. The position of these layers and strength of downward mixing is found to be mainly controlled atmospheric stability, inversion strength and wind shear. Comparisons of LES model output with wind lidar data show similarities in the formation of the daytime convective plume and the vertical wind structure.

In turbomachines, rotors and stators differ by the rotation of the former. Hence, half of each stage is directly influenced by rotation effects. The influence of rotation on the flow structure and its impact on the performance is studied through wall-resolving large Eddy simulations of a rotor with large relative tip gap size. The simulations are performed in a rotating frame with rotation accounted for through a Coriolis force term. In a first step, experimental results are used to provide validation. The main part of the study is the comparison of the results from two simulations, one representing the rotating configuration and one with the Coriolis force removed, without any other change. This setup allows a very clean assessment of the influence of rotation. The turbulence-resolving approach ensures that the turbulent flow features are well represented. The results show a significant impact of rotation on the secondary flow. In the tip region, the tip leakage vortex is enlarged and destabilized. Inside the tip gap, the flow is altered as well, with uniformization in the rotating case. At the blade midspan, no significant effects are observed on the suction side, while an earlier transition to turbulence is found on the pressure side. Near the hub, rotation effects are shown to reduce the corner separation significantly.

The contribution compares results of Large Eddy Simulations of the cavitating flow in a model oil hydraulic spool valve using an Euler-Euler and a one-way coupled Euler- Lagrange model. The impact of the choice of the empirical constants in the Kunz cavitation model is demonstrated. Provided these are chosen appropriately the approach can yield reasonable agreement with the corresponding experiment. The one-way Euler-Lagrange model yields less agreement. It is demonstrated that this is due to the lack of realistic volumetric coupling, rarely accounted for in this type of method. First results of such an algorithm are presented featuring substantially more realism.

The advance in computational fluid dynamics in recent years has provided the opportunity for many fluid dynamic problems to be analyzed numerically. One such problem concerns the modeling of plunging water into a still water body, often encountered in pump stations. Air bubbles introduced into the system by the plunging jet can be a significant problem, especially when consumed into operating pumps. The classical approach to investigate the hydrodynamics of plunging jet in pump stations is by physical model studies. This approach is time consuming, tedious and costly. The availability of computational power today, along with appropriate numerical techniques, allows such phenomenon to be studied in a greater level of detail and more cost efficient. Despite the advantages of numerical studies, little attention has been devoted to solve the plunging jet and air transport problem numerically. In this current work, a 3-dimensional finite volume, Large Eddy Simulation (LES) code is developed to simulate these flow conditions. For turbulent flow, the large scale quantities were numerically resolved while the dynamic sub-grid scale model is used to model the small scale energy dissipations. The code also has the capability to handle free surface deformation, an important aspect in simulating the impact section of an impinging jet. Modeling of the air entrainment is performed numerically utilizing the information obtained from the hydrodynamics. Migration of air bubbles is modeled using the scalar transport equation, modified to account for the buoyancy of the bubbles. Instead of the typical Lagrangian schemes, which track individual air bubbles, air bubble dynamics are modeled in the form of concentrations. Modeling air bubbles in this manner is computational efficient and simpler to implement. For the air entrainment simulations, standard numerical boundaries conditions and empirical entrainment equations are used to provide the necessary boundary conditions. The developed model is compared with the literature, producing satisfactory results, suggesting that the code has an excellent potential of extending its application to practical industry practices.

In many areas of aerodynamics the technique of Large Eddy Simulation (LES) has proved a practical way of modelling the unsteady phenomena in numerical simulations. Few applications are as dependent on such an approach as the prediction of flow within a gas turbine combustor. Like any form of Computational Fluid Dynamics (CFD), LES requires specification of the velocity field at the inflow boundary, with much evidence suggesting the specification of inlet turbulence can be critical to the resultant accuracy of the prediction. While a database of time-resolved velocity data may be obtained from a precursor LES calculation, this technique is prohibitively expensive for complex geometries. An alternative is to use synthetic inlet conditions obtained from experimental data High-speed Particle Image Velocimetry (PIV) is used here to provide planar velocity data at up to 1kHz temporal resolution in two test cases representative of gas turbine combustor flows (a vortex generator in a duct and an idealised combustor). As the data sampling rate is approaching a typical LES time-step it introduces the possibility of applying instantaneous experimental data directly as an inlet condition. However, as typical solution domain inlet regions for gas turbine combustor geometries cannot be adequately captured in a single field of PIV data, it is necessary to consider a method by which a synchronous velocity field may be obtained from multiple PIV fields that were not captured concurrently. A method is proposed that attempts to achieve this by a combined process of Linear Stochastic Estimation and high-pass filtering. The method developed can be generally applied without a priori assumptions of the flow and is demonstrated to produce a velocity field that matches very closely that of the original PIV, with no discontinuities in the velocity correlations. The fidelity and computational cost of the method compares favourably to several existing inlet condition generation methods. Finally, the proposed and existing methods for synthetic inlet condition generation are applied to LES predictions of the two test cases. There is shown to be significant differences in the resulting flow, with the proposed method showing a marked ii reduction in the adjustment period that is required to establish turbulent equilibrium downstream of the inlet. However, it is noted the presence of downstream turbulence generating features can mask any differences in the inlet condition, to the extent that the flow in the core of the combustor test case is found to be insensitive to the inlet condition applied at the entry to the feed annulus for the test conditions applied here.

Climate change is emerging as a significant challenge in terms of the response needed to mitigate or adapt to the predicted global changes. Severe impacts due to rising sea-level, seasonal shifts, increased frequency and intensity of extreme weather events such as storms, floods or droughts have become accepted by the scientific community as a real and present threat to civilisation. The most significant impacts are expected in the Arctic, the Asian mega-deltas, Small Island Developing States (SIDS) and sub-Saharan Africa (IPCC 2007). There are two approaches to global climate change either mitigation or adaptation. This dissertation aims to provide the initial design concepts for a system to mitigate methane, a significant Greenhouse Gas (GHG), emitted from coalmines by ventilation air circulated through the underground workings. The VamTurBurner©, a Ventilation Air Methane (VAM) gas turbine based methane burning system, is proposed as a method of extracting the thermal energy from the VAM. A key aspect of the problem responsible for the difficulty in extracting the energy from VAM is the low concentration of methane in the high volume ventilation airflow. This approach recasts the concepts of combustion dynamics of a premixed fuel flow to that expected for VAM to ascertain the conditions conducive to combustion or oxidation of the methane in the ventilation air. A numerical model using Large Eddy Simulation (LES) to study the combustion dynamics revealed that the temperature of the incoming ventilation air is a key variable related to the concentration of the VAM. Computational Fluid Dynamics modeling was used to study the design features needed to engineer a system capable of providing the required temperature of the incoming ventilation air. Applications for the available thermal energy are discussed in terms of the potential to generate electricity with steam turbines, provide space heating, produce hot water for many uses, and use the heat for industrial drying or as desired. The efficiency of the energy system is enhanced when the output from the amount of natural gas or electricity purchased is compared to the output enhanced by the addition of methane, considered as free. The VamTurBurner© concept, as described in this dissertation, appears to be a viable method of mitigating atmospheric methane in the pursuit greenhouse gas reduction.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / M.H. Hosni / Z.C. Zheng / In order to study the capability of computational methods in investigating the mechanisms associated with disease and contaminants transmission in aircraft cabins, the Computational Fluid Dynamics (CFD) models are used for the simulation of turbulent airflow, tracer gas diffusion, and particle dispersion in a generic aircraft cabin mockup. The CFD models are validated through comparisons of the CFD predictions with the corresponding experimental measurements. It is found that using Large Eddy Simulation (LES) with the Werner-Wengle wall function, one can predict unsteady airflow velocity field with relatively high accuracy. However in the middle region of the cabin mockup, where the recirculation of airflow takes place, the accuracy is not as good as that in other locations. By examining different k-ε models, the current study recommends the use of the RNG k-ε model with the non-equilibrium wall function as a Reynolds Averaged Navier Stokes (RANS) model for predicting the steady-state airflow velocity data. It is also found that changing the cabin air-inlet nozzle height has a significant effect on the flow behavior in the middle and upper part of the cabin, while the flow pattern in the lower part is not affected as much. Through the use of LES and species transport model in simulating tracer gas diffusion, very good agreement between predicted and measured tracer gas concentration data is observed for some monitoring locations, but the agreement level is not uniform for all the sampling point locations. The reasons for the deviations between predictions and measurements for those locations are discussed. The Lagrange-Euler approach is invoked in the particle dispersion simulations. In this approach, the equation of motion for the discrete phase is coupled with the continuous phase governing equations through the calculation of drag and buoyancy forces acting on particles. The continuous phase flow is turbulent and RANS is employed in order to calculate the continuous phase velocity field. A complete study on grid dependence for RANS simulation is performed through a controllable regional mesh refinement scheme. The grid dependence study shows that using unstructured grid with tetrahedral and hybrid elements in the refinement region are more efficient than using structured grid with hexahedral elements. The effect of turbulence on the particle dispersion is taken into account by using a stochastic tracking method (Discrete Random Walk model). One of the significant features of this study is the investigation of the effect of the number of tries on the accuracy of particle concentration predictions when Discrete Random Walk is used to model turbulent distribution of particles. Subsequently, the optimum number of tries to obtain the most accurate predictions is determined. In accordance with the corresponding experimental data, the effect of particle size on particle distribution is also studied and discussed through the simulation of two different sizes of mono-disperse particles in the cabin with straight injection tube, i.e., 3µm and 10µm. Due to the low particle loading, neglecting the effect of particles motion on the continuous phase flow-field seems to be a reasonable, simplifying assumption in running the simulations. However, this assumption is verified through the comparison of the results from 1-way and 2-way coupling simulations. Eventually through the simulations for the particle injection using the cone diffuser, the effects of cabin pressure gradient as well as the particle density on particles dispersion behavior are studied and discussed. In the last part of this dissertation, the turbulent airflow in a full-scale Boeing 767 aircraft cabin mockup with eleven rows of seats and manikins is simulated using steady RANS method. The results of this simulation cannot only be used to study the airflow pattern, but also can be used as the initial condition for running the tracer gas diffusion and particle dispersion simulations in this cabin mockup.

O estudo do transporte de sedimentos, partindo da estimativa de estruturas turbulentas relevantes, constituiu o principal foco deste trabalho. Assim, a equação de transporte de massa (advecção-difusão) foi aplicada em conjunto com as equações de Navier-Stokes e da continuidade filtradas. Neste contexto, houve a necessidade de uma descrição conveniente da turbulência, o que ocorreu mediante a aplicação da simulação de grandes escalas acoplada a modelos de viscosidade turbulenta sub-malha. O método de fronteira imersa foi utilizado na modelação da interface sólido/fluido, representada pela geometria de fundo dos canais. As equações de Navier-Stokes filtradas e da continuidade foram resolvidas numericamente pelo método de passos fracionados, o qual estabeleceu o almejado acoplamento entre ambas. Na discretização das equações governantes foi utilizado o método de diferenças finitas, aplicado sobre malhas deslocadas. Os esquemas explícitos de Adams-Bashforth (de segunda e quarta ordens) foram utilizados no avanço temporal das velocidades do escoamento e das concentrações de sedimentos. Uma nova formulação para a velocidade de sedimentação foi desenvolvida analiticamente, enquanto que eventuais fluxos de ressuspensão foram impostos como condição de contorno no fundo do canal. Todos os códigos computacionais, que estabeleceram as diretrizes e a lógica de cálculo, foram criados no contexto deste trabalho. Os resultados obtidos indicam que a simulação de grandes escalas, associada ao método de fronteira imersa, considerando velocidade de sedimentação conforme aqui modelada, e ainda utilizando a equação de advecção-difusão para o transporte de massa, constituem ferramentas altamente adequadas à estimativa do transporte de sedimentos pela água. / The goal of this work is the research of sediment transport phenomena, deriving from outstanding turbulent eddies estimative. Thus, the mass transport equation (advection-diffusion) was connected with the filtered Navier-Stokes and continuity equations. In this context, the large-eddy simulation and sub-grid viscosity modeling established a convenient description of turbulence effects. The immersed boundary method was applied to model solid/fluid interface, represented here by the shapes of channel bottom. The filtered Navier-Stokes and continuity equations were solved by the fractional step method. The equations were discretized with the finite difference method, applied over staggered grids, whereas explicit Adams-Bashforth schemes (second and forth orders) were used in temporal advancement of velocities and sediment concentration fields. A new analytical formulation for settling velocity was obtained, while fortuitous re-suspension flux was applied like a boundary condition in the channel bottom. The computational code was totally developed in this work. The results of present simulations show that large-eddy simulation coupled to the immersed boundary method, considering, yet, the settling velocity of particles and the advection-diffusion equation for mass transport, constitute potential tools for sediment transport evaluation in water flows.