Dissertations / Theses on the topic 'Direct numerical simulation'

A direct numerical simulation of a Batchelor vortex has been carried out in the presence of freely decaying turbulence, using both periodic and symmetric boundary conditions; the latter most closely approximates typical experimental conditions, while the former is often used in computational simulations for numerical convenience. A recently developed numerical method, based on compact schemes combined with three stage Runge-Kutta method for time integration, with projection method for enforcing continuity is used for numerical simulations. The Poisson solver used is a direct solver in spectral space. The higher-order velocity statistics were shown to be strongly dependent upon the boundary conditions, but the dependence could be mostly eliminated by correcting for the random, Gaussian modulation of the vortex trajectory, commonly referred to as wandering, using a technique often employed in the analysis of experimental data. Once this wandering had been corrected for, the strong peaks in the Reynolds stresses normally observed at the vortex centre were replaced by smaller local extrema located within the core region but away from the centre. Analysis of the budgets of turbulent kinetic energy and normal Reynolds stress suggest that the production budget during the growth phase of vortex development, resembles turbulent boundary layer type budgets. The analysis of the budgets of turbulent shear stresses shows that the formation and organization of `hairpin' (secondary) structures within the core is the main mechanism for turbulent production and the budget of TKE and radial tangential shear stress shows a turbulent boundary layer type budget.

The work described in the present thesis is related to a series of projects that I worked on toward the better understanding of fragmentation phenomena. In the past decades, the science of fragmentation has attracted many attentions within the researchers due to its wide range of applications. However, because of the complexity of the subject, even its basic concepts need more investigations. This thesis starts with an introduction to fragmentation of droplets using experimental or numerical approaches. It is discussed that the current mathematical and experimental tools are not able to describe all the details. Thus, high performance numerical simulations are the best alternatives to study the breakup of droplets. The introduction is followed by a discussion on the numerical method and the ranges of the non-dimensional groups. It is described that an adaptive, volume of fluid (VOF) method based on octree meshing is used, providing a notable reduction of computational cost. The rest of the thesis basically discusses the obtained results using direct numerical simulations. Two main geometries are investigated: falling droplets and droplets in a stream. For the case of falling droplets, three simulations with different Eötvös numbers are performed. For the case of droplets in a stream, two-dimensional and three-dimensional simulations are performed for a range of Weber number. The results are compared with the available mathematical theories and it is shown that the analysis presented here precisely demonstrates the mechanism of the bag breakup of falling droplets and instability growth over the droplets in an external high-speed flow. The outcomes can significantly assist the development of the secondary atomization and turbulent two-phase flows modelling.

Direct numerical simulation of Hz - O2 in the context of a temporally evolving mixing layer has been performed. Real molecular properties as well as the effects of the species differential diffusion were incorporated into an existing 3D parallel FORTRAN code. The geometry is a box with streamwise and spanwise directions being periodic whereas non-periodic boundaries were set up in transverse (vertical) directions which leads to inhomogeneity for the turbulent field in these directions. Initialisation were performed by error function distributions for streamwise velocity component, scalar mass fraction and temperature along the vertical axis of the domain, Initial pressure is set to be uniform and density Willi calculated based on ideal-gas law for the mixture. Disturbances were introduced by generating spanwise and streamwise vorticity in the middle of the mixing layer to enable transition from laminar to turbulent.

Le présent travail se concentre sur les propriétés statistiques des scalaires réactifs subissant des réactions chimiques réversibles en turbulence incompressible. Une analyse théorique des propriétés statistiques des scalaires à différents ordres de moments a été réalisée sur la base d'approximations et de modèles convenablement proposés. Les résultats théoriquement dérivés ont ensuite été comparés aux résultats numériques obtenus par simulation numérique directe (DNS). Dans la simulation numérique directe, les dérivés spatiales ont été principalement approximées en utilisant une méthode pseudi-spectrale, car la vitesse turbulente et les champs scalaires sont généralement des conditions aux limites périodiques. Pour les configurations spéciales dans lesquelles la condition aux limites n'est pas périodique, une méthode aux différences finies avec des schémas fins a été utilisée pour approximer les dérivées spatiales. L'intégration temporelle numérique a été mise en oeuvre par un schéma Runge-Kutta du troisième ordre. Tous les travaux menés dans cette thèse sont consacrés aux explorations numériques et théoriques des scalaires réactifs en turbulence incompressible de différentes configurations. Nos résultats suggèrent de nouvelles idées pour de futures études, qui sont discutées dans les conclusions
The present work focuses on the statistical properties of reactive scalars undergoing reversible chemical reactions in incompressible turbulence. Theoretical analysis about the statistical properties of scalars at different order of moments were carried out based on appropriately proposed approximations and models. The theoretically derived results were then compared with numerical results obtained by direct numerical simulation (DNS). In the direct numerical simulation, the spatial derivatives were mainly approximated by using a pseudo-spectral method, since the turbulent velocity and scalar fields are generally of periodic boundary conditions. For the special configurations in which the boundary condition is not periodic, a finite difference method with fine schemes was used to approximate the spatial derivatives. The numerical time integration was implemented by a third order Runge-Kutta scheme. All the works carried out in this thesis are devoted to the numerical and theoretical explorations about reactive scalars is incompressible turbulence of different configurations. Our finding suggest new ideas for future studies, which are discussed in the conclusions

The aim of this work is to extend an existing CFD solver, named Shock/Boundary-Layer Interaction (SBLI) code, to include a fully 3D curvilinear capability in order to perform direct numerical simulation (DNS) of turbulent flows over complex geometries. The SBLI code solves the compressible Navier-Stokes equations by the finite difference method and uses the body-fitted curvilinear coordinate system approach to treat complex geometries. The extended version of the code has been used to perform a DNS of a channel flow with longitudinally ridged walls and a DNS of a turbulent flow over an axisymmetric hill geometry. Validation and comparison with previous experimental data and numerical results are also presented. In the first part of the work, the Navier-Stokes equations are presented in a strong conservation form and test validations of the code extension have been carried out such as free stream flow preservation on a wavy grid and a laminar plane channel flow on a skewed mesh. The free stream preservation test consists of a uniform flow computation on a cosinusoidal mesh and the objective is to evaluate the velocity components changes from their initial values due to the effect of a highly skewed mesh. The maximum discrepancy found is around 10-16. For the laminar plane channel flow simulation on a skewed mesh, the purpose is to verify the symmetrical propriety of numerical errors obtained in the velocity components while the main flow direction and the position of the walls are altered in rotation around the three physical coordinates. The symmetry of the numerical error is found to be well preserved as expected. The second part of the work contains DNS of laminar and turbulent flows in a channel with longitudinally ridged walls at different Reynolds numbers. The goal is to investigate the effect of ridged walls on the turbulent flow behavior and to provide quality DNS data-for assessing other numerical simulations, such as Large Eddy Simu-lation (LES) and Reynolds-Averaged Navier Stokes (RANS) modeling. Two Reynolds numbers have been simulated (ReT = 150 and ReT = 360, based on a reference velocity UT = vol Pb( -dPldx), the bulk density and the wall viscosity) on a domain of 1.257r0 x 20 X 0.3757r0 in the streamwise, wall normal and spanwise directions, respectively. This domain is similar to the minimal flow unit for a turbulent plane channel flow. Comparisons with previous experimental data and numerical prediction have show good agreement for the ReT = 150 case and a similar flow dynamics for the ReT = 360 case. In general, the effects of ridged walls on the turbulent flow, like the reduction of the normal Reynolds stress peak values, seems to be smaller when the Reynolds number increases. The third part of this work describes the main simulation of this thesis. DNS of a turbulent flow around an axisymmetric hill is carried out in order to investigate the three-dimensional boundary-layer flow separation which occurs behind the hill. Different domain sizes and grid resolutions have been tested up to a maximum of about 54 million points. A methodology for generating inflow conditions has been implemented and tested. Results are compared with previous experimental and numerical studies. Due to a low Reynolds number used (Reo* = 500, only 5% of an experimental simulation), the time averaged separation bubbles is much bigger and the flow seems to have a laminarisation process due to a strong adverse pressure gradient presented. A small recirculation bubble detected on the top of the hill seems to be the cause of the earlier separation of the turbulent boundary layer and, then, the bigger separation observed. However, similar to the full Reynolds number experiment, same flow dynamics, consisting in the formation of a counter rotating vortex pair merging in the streamwise current, have been captured well. The final part of the work presents an extension of the single-block SBLI code to a multiblock version. A pre-processor program has been developed in order to simplify the treatment of the interface between different blocks and a description of the algorithm is also given. As a demonstration study, DNS of a square jet in a turbulent cross flow has been performed at two Reynolds numbers (Reo* = 1000 and Res- = 2000) and different jet to cross flow velocity ratios. Compared with the available data, the results are in good agree, despite the lower Reynolds number used (half of value simulated in the available data). In conclusion, a fully 3D version of the SBLI code has been successfully derived and tested for various flow configurations. The 3D curvilinear capability has also been implemented and tested by simple, but not trivial, test cases. An option for simplified treatment of Cartesian mesh has been implemented and tests have shown a factor of 2 speedup in overall performance. Two main simulations have been carried out and for the turbulent flow in a ridged channel, the results are in good agreement with published data, while, for the flow over an axisymmetric hill case, simulation is compared qualitatively well and the noticeable discrepancies are primarily due to a reduced Reynolds number conditions. The code has also been successfully extended to a multiblock version and demonstrated on a two-block domain for a jet in cross flow case. Future works includes simulations of the hill problem at higher Reynolds number and LES extension of the SBLI code to fully 3D curvilinear capability.

Les dynamiques de l'ébullition nucléée sont intimement influencées par les interactions entre les domaines fluide et solide, en particulier dans des conditions de petits angles de contact et de phénomènes d'interface complexes. Cette thèse présente le développement et l'application de méthodes numériques avancées pour simuler ces interactions avec une précision accrue. Au cœur de ce travail se trouve la mise en œuvre d'une méthode de convection Volume de Fluide (VOF) conservatrice de masse, intégrée de manière fluide avec une approche de domaine solide intégré. Cette technique est spécifiquement conçue pour relever les défis de la capture précise des dynamiques d'ébullition à de petits angles de contact.Le cadre numérique est construit sur la plateforme Basilisk, développée par Stéphane Popinet, en utilisant un modèle à fluide unique où la méthode VOF capture efficacement les dynamiques d'interface fluide. Un modèle de force de surface continue est employé pour représenter avec précision les effets de tension de surface, tandis que le modèle solide intégré assure un couplage robuste entre les domaines fluide et solide. Pour améliorer encore la fidélité de la simulation, le modèle de changement de phase de Leon Malan est intégré, incorporant une méthode d'interface de convection en deux étapes et une équation conservatrice d'énergie pour gérer les complexités de la transition de phase. De plus, un modèle de résistance thermique interfaciale développé par Lubomír Moravcík est implémenté, quantifiant la résistance thermique à l'interface fluide-solide.Un exercice de validation rigoureux est réalisé, démontrant une forte concordance avec les données expérimentales de référence et les modèles théoriques établis. Les contributions clés de ce travail incluent l'amélioration des techniques de modélisation du changement de phase, une compréhension approfondie des dynamiques des microlayers, et des insights sur l'interaction entre la tension de surface, les forces visqueuses, et le transfert de chaleur dans l'ébullition nucléée. Cette recherche constitue une base solide pour les études futures, y compris les simulations tridimensionnelles et l'investigation des effets de la rugosité de surface et de la distribution initiale de la température sur les dynamiques de l'ébullition
Nucleate boiling dynamics are intricately influenced by the interactions between fluid and solid domains, particularly under conditions of small contact angles and complex interface phenomena. This thesis presents the development and application of advanced numerical methods to simulate these interactions with enhanced precision. Central to this work is the implementation of a mass-conservative Volume of Fluid (VOF) advection method, seamlessly integrated with an embedded solid domain approach. This technique is specifically designed to address the challenges of accurately capturing the dynamics of boiling processes at small contact angles.The numerical framework is constructed within the Basilisk platform, developed by Stéphane Popinet, utilizing a one-fluid model where the VOF method efficiently captures fluid interface dynamics. A continuous surface force model is employed to accurately represent surface tension effects, while the embedded solid model ensures robust coupling between the fluid and solid domains. To further enhance the simulation's fidelity, Leon Malan's phase-change model is integrated, incorporating a two-step advection interface method and an energy-conservative equation to handle the complexities of phase transition. Additionally, an interfacial heat resistance model by Lubomír Moravcík is implemented, quantifying the thermal resistance at the fluid-solid interface.A rigorous validation exercise is performed, demonstrating strong agreement with reference experimental data and established theoretical models. Key contributions of this work include the refinement of phase-change modeling techniques, a deeper understanding of microlayer dynamics, and insights into the interplay between surface tension, viscous forces, and heat transfer in nucleate boiling. This research provides a solid foundation for future studies, including three-dimensional simulations and the investigation of surface roughness and initial temperature distribution effects on boiling dynamics

Direct numerical simulation (DNS) is used to study some aspects of the dynamics of vortex rings in viscous, incompressible ow at Reynolds numbers (defined as the ratio of the initial circulation to the kinematic viscosity) in the range of 103 to 104. Firstly, the effect of the particular initial core azimuthal vorticity profile of a vortex ring on its subsequent evolution in unbounded ow is studied. Vortex rings with a wide range of initial core vorticity profiles are shown to relax to a common equilibrium state. Additionally the behaviour of the equilibrium vortex ring at large times is studied. When the slenderness ratio of the vortex rings increases beyond a particular limit, the vortex rings diverge from the common equilibrium state and follow paths determined by the viscosity of the uid. Secondly, the interaction of a laminar vortex ring with a non-deformable, free-slip surface at an oblique angle of incidence leading to the phenomenon of vortex reconnection is investigated. Specifically the effect of Reynolds number on the dynamics of the reconnection process is studied. The scaling of the reconnection timescale with the Reynolds number is obtained. At high Reynolds numbers the reconnection process leads to a breakdown of the entire vortex ring structure to a turbulent-like ow. This phenomenon is shown to be related to the mechanics of the reconnection process. Finally, the dynamics of vortex rings with swirl in unbounded ow is studied. Two different types of vortex rings with swirl were considered: i) Vortex rings with Gaussian distributions of core azimuthal vorticity and core azimuthal velocity and ii) Steady state solutions of the Euler equations for vortex rings with swirl. Both types of vortex rings develop an elongated axial vortex after initialisation. The existence of a maximum limit for the swirl on a vortex ring is shown above which the vortex rings undergo a rapid de-swirling readjustment. A helical instability occurring in vortex rings due to swirl at high Reynolds numbers is presented. A relation is shown to exist between one of the modes of the helical instability and the geometric parameters of the vortex ring.

Existing numerical techniques for modeling saturated deformable porous media are based on homogenization techniques and thus are incapable of performing micro-mechanical investigations, such as the effect of micro-structure on the deformational characteristics of the media. In this research work, a numerical scheme is developed based on the parallelized hybrid lattice-Boltzmann finite-element method, that is capable of performing micro-mechanical investigations through direct numerical simulations. The method has been used to simulate compression of model saturated porous media made of spheres and cylinders in regular arrangements. Through these simulations it is found that in the limit of small Reynolds number, Capillary number and strain, the deformational behaviour of a real porous media can be recovered through model porous media when the parameters porosity, permeability and bulk compressive modulus are matched between the two media. This finding motivated research in using model porous geometries to represent more complex real porous geometries in order to perform investigations of deformation on the latter. An attempt has been made to apply this technique to the complex geometries of ªfeltº, (a fibrous mat used in paper industries). These investigations lead to new understanding on the effect of fiber diameter on the bulk properties of a fibrous media and subsequently on the deformational behaviour of the media. Further the method has been used to investigate the constitutive relationships in deformable porous media. Particularly the relationship between permeability and porosity during the deformation of the media is investigated. Results show the need of geometry specific investigations.

Numerical simulations of liquid flow in a micro-channel between two horizontal plates are performed. The channel is infinite in streamwise and spanwise directions and its height is taken as m, which falls within the dimension ranges of microchannels. The Navier-Stokes equations with the addition of Brinkman number (Br) to the energy equation are used as the governing equations and spectral methods based approach is applied to obtain the required accuracy to handle liquid flow in the microchannel. It is known for microchannels that Br combines the effects of conduction and viscous dissipation in liquids and is also a way of comparing the importance of latter relative to former. The present study aims to simulate the unusual behavior of decreasing of Nu with increasing Re in the laminar regime of microchannels and to show that Br can be introduced to explain this unexpected behavior. Consequently, it is seen at the end of the results that secondary effect of the Br is observed for the single-phase convective heat transfer. Therefore, a laminar flow of a liquid in a microchannel shows different characteristics compared to a similar flow in a macrochannel. To observe the differences, three different cases are run over each of a range of Reynolds numbers: one with no axial conduction assumption that corresponds to a case similar to macrochannel flow, another case with axial conduction included in the energy equation to simulate one of the main differences and lastly a case with the inclusion of Br number in the governing equations. A similar study is made for natural convection with the same numerical set-up for the same three cases. Formation of Rayleigh-Benard cells are observed for the critical numbers widely accepted in the literature. The results are compared with each other to see the effects of axial conduction and Br inclusion, in addition to Ra for natural convection.

Electron densities form field-aligned structured regions in the natural ionosphere and after a high altitude nuclear explosion (HANE). These electron densities, known as plumes, are made up of many smaller individual field-aligned regions called striations. Striation modeling for systems effects has traditionally been done use a statistical approach. This statistical approach evolves different moments of the electron density. Due to lack of test data it has never been validated. The purpose of this project was to use a direct numerical simulation to solve equations governing the differential motion of individual striations. It was done in five steps: 1) Transport a single striation, 2) solve potential equation, 3) combine transport and potential equations, 4) optimize combined solver, and 4) simulate a fully-striated plume for comparison with the statistical model.

It has been over 50 years since Toms [B.A. Toms, Proc. 1^st Int. Congr. Rheol., Sec. II, 135, North Holland (1949)] discovered that adding small quantities of long-chain polymer to turbulent pipe flow could drastically reduce the amount of turbulent drag. Since then a substantial amount of research has gone into examining dilute polymer solutions and their turbulent drag reducing properties, yet the precise mechanism by which the polymers interact with the turbulence to produce this affect is still unclear. From a theoretical standpoint, the difficulty lies in the analysis of accurate models of the fluid dynamics. The combination of the complex mathematical description required for turbulence and the intricate constitutive equations of polymer motion poses significant problems. However, recent developments in computing have resulted in machines powerful enough to simulate such flows using direct numerical simulation (DNS); a technique whereby all the important scales of turbulent fluid motion are fully resolved using an algorithm derived from the full momentum conservation equations. DNS has been successfully employed in previous studies of dilute polymer solutions in computational domains similar to experimental apparatus such as pipe flow [J. M. J. den Toonder, M. A. Hulsen, G. D. C. Kuiken and F. T. M. Nieuwstadt, J. Fluid Mech., 337, 193 (1997)] and channel flow, [R. Sureshkumar, A. N. Beris and R. A. Handler, Phys. Fluids, 9(3), 743 (1996)]. It was our aim to identify the changes within the turbulent dynamics produced by the presence of polymers in homogeneous isotropic turbulence - without an dependence on solid boundary conditions. We decided to simulate our solutions within a infinitely repeating cube subject to periodic boundary conditions, a well established technique for Newtonian fluids. In this thesis we examine a range of spectral measures and integral parameters for various non-Newtonian fluid models in statistically stationary homogeneous isotropic turbulence and compare them to an equivalent Newtonian flow using DNS. We begin by outlining the general theory of homogeneous isotropic non-Newtonian turbulence in the Fourier space domain. We then demonstrate how the general non-Newtonian momentum conservation equations are adapted for use in our DNS. This is followed by a literature review of turbulent drag reduction by long-chain polymer additives. The remainder of the thesis is concerned with the new work. We outline the four non-Newtonian fluid models we applied and present the results obtained from the DNS calculations. Each model embodies a particular characteristic of polymer solutions. The first is of our own construction and is based on the ability of polymers to increase the viscosity of the solution at small scales. Second, we simulate the nonlinear model of McComb [W. D. McComb, Int. J. Engng. Sci., 14, 239 (1976)] where the stress exhibits a nonlinear dependence on the rate of strain. For both of these we were able to obtain an analytical expression for the energy spectra and compared these to the DNS results. The third model is the viscous anisotropic model of den Toonder et al. (see above reference) which introduces a directional element based on the orientation of the polymers in the flow, by assuming they are of constant length and align with the instantaneous velocity. Finally we model a fully coupled FENE-P fluid [L. E. Wedgwood and R. B. Bird, Ind. Eng. Chem. Res., 27, 1313 (1988)] in which the polymers are finitely extensible, elastic and have their own equation of motion giving their orientation. In this way we have identified changes within the structure of turbulence itself which may be related to the drag reduction phenomenon.

In numerous applications, two-phase liquid-gas transport at sub-millimeter length scales plays a substantial role in the determination of the behavior of the system at hand. As its main application, the present work focuses on the polymer electrolyte membrane (PEM) fuel cells. Desirable performance and operational life-time of this class of high-throughput energy conversion devices requires an effective water management, which per se relies on proper prediction of the water-air transport mechanisms. Such two-phase flow involves interfacial forces and phenomena, like hysteresis, that are associated with the physicochemical properties the liquid, gas, and if present, the solid substrate. In this context, numerical modeling is a viable means to obtain valuable predictive understanding of the transport mechanisms, specially for cases that experimental analyses are complicated and/or prohibitively expensive. In this work, an efficient finite element/level-set framework is developed for three-dimensional simulation of two-phase flow. In order to achieve a robust solver for practical applications, the physical complexities are consistently included and the involved numerical issues are properly tackled; the pressure discontinuity at the liquid-gas interface is consistently captured by utilizing an enriched finite element space. The method is stabilized within the framework of variational multiscale stabilization technique. A novel treatment is further proposed for the small-cut instability problem. It is shown that the proposed model can provide accurate results minimizing the spurious currents. A robust technique is also developed in order to filter out the possible noises in the level-set field. It is shown that it is a key to prevent irregularities caused by the persistent remnant of the spurious currents. It is shown how the well-established contact-line models can be incorporated into the variational formulation. The importance of the inclusion of the sub-elemental hydrodynamics is also elaborated. The results presented in the present work rely on the combination of the linearized molecular kinetic and the hydrodynamic theories. Recalling the realistic behavior of liquids in contact with solid substrates, the contact--angle hysteresis phenomenon is taken into account by imposing a consistent pinning/unpinning mechanism developed within the framework of the level-set method. Aside from the main developments, a novel technique is also proposed to significantly improve the accuracy and minimize the the loss in the geometrical features of the interface during the level-set convection based on the back and forth error compensation correction (BFECC) algorithm. Within the context of this thesis, the numerical model is validated for various cases of gas bubble in a liquid and liquid droplets in a gas. For the latter scenario, besides free droplets, the accuracy of the proposed numerical method is assessed for capturing the dynamics droplets spreading on solid substrates. The performance of the model is then analyzed for the capturing the configuration of a water droplet on an inclined substrate in the presence the contact--angle hysteresis. The proposed method is finally employed to simulate the dynamics of a water droplet confined in a gas channel and exposed to air-flow.
Existen numerosas aplicaciones industriales en las que transporte bifásico (líquido-gas) a escalas submilimétricas resulta crucial para la determinación del comportamiento del sistema en cuestión. Entre todas ellas, el presente trabajo se centra en las pilas de combustible con membrana de electrolito polimérico (PEMFC). El rendimiento deseable y la vida útil operativa de esta clase de dispositivos de conversión de energía de alto rendimiento requieren una gestión eficaz del agua (conocida como “water management”), que per se depende de la predicción adecuada de los mecanismos de transporte de agua y aire. Así pues, el análisis del flujo microfluídico de dos fases obliga considerar fuerzas y fenómenos interfaciales, tales como la histéresis, que están asociados con las propiedades fisicoquímicas del líquido, el gas y, si está presente, el sustrato sólido. En este contexto, la modelización numérica es una alternativa viable para obtener una predicción precisa de los mecanismos de transporte, especialmente en aquellos casos en los que los análisis experimentales son prohibitivos, ya sea por su complejidad o coste económico. En este trabajo, se desarrolla un marco eficiente, basado en la combinación del método de elementos finitos y el método de “level-set”, para la simulación tridimensional de flujos bifásicos. Con el fin de lograr una herramienta numérica robusta para aplicaciones prácticas, las complejidades físicas se incluyen consistentemente y los problemas numéricos involucrados se abordan adecuadamente. Concretamente, la discontinuidad de la presión en la interfaz líquido-gas se captura consistentemente utilizando un espacio de elementos finitos enriquecido. La estabilización del método se consigue mediante la introducción de la técnica de multiescalas variacionales. Asimismo, se propone también un tratamiento novedoso para el problema de la inestabilidad de tipo “small-cut”. Se muestra que el modelo propuesto puede proporcionar resultados precisos minimizando las corrientes espurias en la interfaz liquido-gas. Complementariamente, se presenta una nueva metodología para filtrar el ruido en el campo de “level-set”. Esta metodología resulta ser crucial para prevenir las irregularidades provocadas por el remanente persistente de las corrientes espurias. El comportamiento de la línea de contacto es considerado a través de la inclusión los modelos correspondientes en la formulación variacional. A este respecto, el presente trabajo aborda la importancia de la inclusión de la hidrodinámica subelemental. Los resultados presentados se basan en la combinación de la cinética molecular linealizada y las teorías hidrodinámicas. Para representación del comportamiento realista de los líquidos en contacto con sustratos sólidos, el fenómeno de histéresis del ángulo de contacto se tiene en cuenta imponiendo un mecanismo de anclado / desanclado consistente desarrollado en el marco del método de level-set. Aparte de los desarrollos principales, también se propone una técnica novedosa para la convección de la función ”level-set”. Ésta permite mejorar significativamente la precisión, minimizando a su vez la pérdida en las características geométricas de la interfaz asociadas al transporte. Esta nueva metodología está basada en el algoritmo de corrección de compensación de errores (BFECC). La herramienta numérica desarrollada en esta tesis es validada para varios casos que involucran burbujas de gas en un líquido y pequeñas gotas de líquido en un gas. Para el último escenario, además de las gotas libres, se evalúa la precisión de la herramienta propuesta para capturar la dinámica de las gotas sobre sustratos sólidos. A continuación, se analiza el rendimiento del modelo para capturar la configuración de una gota de agua sobre un sustrato inclinado en presencia de la histéresis del ángulo de contacto. El método propuesto finalmente se aplica
Enginyeria civil

[ES] La comprensión de los fenómenos físicos que acontecen en la región densa (también conocida como campo cercano) durante la atomización de los sprays ha sido una de las mayores incógnitas a la hora de estudiar sus aplicaciones. En el sector industrial, el rango de interés abarca desde toberas en aplicaciones propulsivas a sprays en aplicaciones médicas, agrícolas o culinarias. Esta evidente falta de conocimiento obliga a realizar simplificaciones en la modelización, provocando resultados poco precisos y la necesidad de grandes caracterizaciones experimentales en la fase de diseño. De esta manera, los procesos de rotura del spray y atomización primaria se consideran problemas físicos fundamentales, cuya complejidad viene dada como resultado de un flujo multifásico en un régimen altamente turbulento, originando escenarios caóticos. El análisis de este problema es extremadamente complejo debido a la ausencia sustancial de teorías validadas referentes a los fenómenos físicos involucrados como son la turbulencia y la atomización. Además, la combinación de la naturaleza multifásica del flujo y su comportamiento turbulento resultan en una gran dificultad para afrontar el problema. Durante los últimos 10 años, las técnicas experimentales han sido finalmente capaces de visualizar la región densa, pero la confianza, análisis y efectividad de dichos experimentos en esta región del spray todavía requiere de mejoras sustanciales. En este contexto, esta tesis trata de contribuir al entendimiento de estos procesos físicos y de proporcionar herramientas de análisis para estos flujos tan complejos. Para ello, mediante Direct Numerical Simulations se ha afrontado el problema resolviendo las escalas de movimiento más pequeñas, y capturando todas las escalas de turbulencia y eventos de rotura. Uno de los objetivos de la tesis ha sido evaluar la influencia de las condiciones de contorno del flujo entrante en la atomización primaria y en el comportamiento turbulento del spray. Para ello, se han empleado dos condiciones de contorno diferentes. En primer lugar se ha empleado una condición de contorno sintética para producir turbulencia homogenea a la entrada, simulando el comporamiento de la tobera. Una de las características más interesantes de este método es la posibilidad de retocar los parámetros dentro del algoritmo. En particular, la escala de longitud integral se ha variado para evaluar la influencia de las estructuras mas grandes de la tobera en la atomización primaria. El análisis de la condición de contorno sintética también ha permitido el diseño óptimo de simulaciones de las cuales se han derivado estadísticas turbulentas significativas. En este escenario, se han llevado a cabo estudios más profundos sobre la influencia de propiedades de las estructuras turbulentas como la homogeneidad y la anisotropía tanto en el espectro de los flujos como en las estadísticas de las gotas. Para tal fin, se han desarrollado metodologías novedosas para computar el análisis espectral y la estadística de las gotas Entre los resultados de este análisis destaca la independencia de la condición de contorno de entrada en las estadísticas de las gotas, mientras que por otra parte, recalca que las características turbulentas desarrolladas en el interior de la tobera afectan a la cantidad total de masa atomizada. Estas consideraciones se encuentran respaldadas por el análisis espectral realizado, mediante el cuál se concluye que la turbulencia multifásica comparte el comportamiento universal descrito por las teorías de Kolmogorov.
[CAT] La comprensió dels fenòmens físics que succeïxen en la regió densa (també coneguda com a camp pròxim) durant l'atomització dels sprays ha sigut una de les majors incògnites a l'hora d'estudiar les seues aplicacions. En el sector industrial, el rang d'interés comprén des de toveres en aplicacions propulsives a sprays en aplicacions mèdiques, agrícoles o culinàries. Esta evident falta de coneixement obliga a realitzar simplificacions en la modelització, provocant resultats poc precisos i la necessitat de grans caracteritzacions experimentals en la fase de disseny. D'esta manera, els processos de ruptura del spray i atomització primària es consideren problemes físics fonamentals, la complexitat dels quals ve donada com resultat d'un flux multifàsic en un règim altament turbulent, originant escenaris caòtics. L'anàlisi d'este problema és extremadament complex a causa de l'absència substancial de teories validades dels fenòmens físics involucrats com són la turbulència i l'atomització. A més, la combinació de la naturalesa multifàsica del flux i el seu comportament turbulent resulten en una gran dificultat per a afrontar el problema. Durant els últims 10 anys les tècniques experimentals han sigut finalment capaces de visualitzar la regió densa, però la confiança, anàlisi i efectivitat dels experiments en esta regió del spray encara requerix de millores substancials. En este context, esta tesi tracta de contribuir en l'enteniment d'estos processos físics i de proporcionar ferramentes d'anàlisi per a estos fluxos tan complexos. Per a això, per mitjà de Direct Numerical Simulations s'ha afrontat el problema resolent les escales de moviment més menudes, al mateix temps que es capturen totes les escales de turbulència i esdeveniments de ruptura. Un dels objectius de la tesi ha sigut avaluar la influència que les condicions de contorn del flux entrant tenen en l'atomització primària i en el comportament turbulent del spray. Per a això, s'han empleat dos condicions de contorn diferents. En primer lloc s'ha empleat una condició de contorn sintètica per a produir turbulència homogènia a l'entrada, simulant el comportament de la tovera. Una de les característiques més interessants d'este mètod és la possibilitat de retocar els paràmetres dins de l'algoritme. En particular, l'escala de longitud integral s'ha variat per a avaluar la influència de les estructures mes grans de la tovera en l'atomització primària. L'anàlisi de la condició de contorn sintètica també ha permés el disseny òptim de simulacions de les quals s'han derivat estadístiques turbulentes significatives. En este escenari, s'han dut a terme estudis més profunds sobre la influència de propietats de les estructures turbulentes com l'homogeneïtat i l'anisotropia tant en l'espectre dels fluxos com en les estadístiques de les gotes. Per a tal fi, s'han desenrotllat metodologies noves per a computar l'anàlisi espectral i l'estadística de les gotes. Entre els resultats d'esta anàlisi destaca la independència de la condició de contorn d'entrada en les estadístiques de les gotes, mentres que d'altra banda, es recalca que les característiques turbulentes desenrotllades en l'interior de la tovera afecten a la quantitat total de massa atomitzada. Estes consideracions es troben recolzades per l'anàlisi espectral realitzat, per mitjà del qual es conclou que la turbulència multifásica compartix el comportament universal descrit per les teories de Kolmogorov.
[EN] The understanding of the physical phenomena occurring in the dense region (also known as near field) of atomizing sprays has been long seen as one of the biggest unknown when studying sprays applications. The industrial range of interest goes from nozzles in combustion and propulsion applications to medical sprays, agricultural and food process applications. This substantial lack of knowledge is responsible for some important simplification in modeling, that often result to be inaccurate or simply partial, leading to the evident need of large experimental characterization during the design phase. In fact, the spray breakup and primary atomization processes are indeed fundamental problems of physics, which complexity results from the combination of a multiphase flow in a highly turbulent regime that leads to chaotic scenarios. The analysis of this problem is extremely problematic, due to a substantial lack of definitive theories about the physical phenomena involved, namely turbulence and atomization. Furthermore, the combination of the multiphase nature of the flow and its turbulent behavior makes substantially difficult to address the problem. Only within the last 10 years, experimental techniques have been capable of visualizing the dense region, but the experiments reliability, analysis and effectiveness in this region still requires vast improvements. In this scenario, this thesis aims to contribute in the understanding of these physical process and to provide analysis tools for these complex flows. In order to do so, Direct Numerical Simulations have been used for addressing the problem at its smallest scale of motion, while reliably capturing all turbulence scales and breakup events. The multiphase nature of the flow is accounted for by using the Volume of Fluid method. One of the goal of the thesis was to assess the influence of the inflow boundary conditions on the primary atomization and on the spray's turbulence behavior. In order to do so, two different boundary conditions were used. In a first place, a synthetic inflow boundary condition was used in order to produce a homogeneous turbulence inflow, simulating the nozzle behavior. One of the interesting features of this method was the possibility of tweaking the parameters within the algorithm. In particular, the integral length scale was varied in order to assess the influence of nozzle larger turbulent structures on the primary atomization. The analysis on the synthetic boundary condition also allowed to optimally design simulations from which derive meaningful turbulence statistics. On this framework, further studies were carried over on the influence of turbulent structures properties, namely homogeneity and anisotropy, on both the flows spectra and droplets statistics. In order to achieve this goal, novel procedures for both computing the flow spectra and analyzing droplets were developed and are carefully addressed in the thesis. The results of the analysis highlight the independence of droplets statistics from the inflow boundary condition, while, on the other hand, remarking how the total quantity of atomized mass is significantly affected by the turbulence features developed within the nozzle. This considerations are supported by the spectrum analysis performed, which also highlighted how multiphase turbulence shares the universal features described in Kolmogorov theories.
Crialesi Esposito, M. (2019). Analysis of primary atomization in sprays using Direct Numerical Simulation [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/133975
TESIS

The present thesis considers the flow mechanics of a natural convection boundary layer (NCBL) along an isothermally heated vertical wall. Large scale direct numerical simulations are carried out to investigate the laminar stability and the turbulent mechanics of the flow. In this study, a computationally efficient temporal framework, where periodic boundary conditions are imposed in the homogeneous directions, has been used to develop a temporally evolving (instead of a spatially evolving) flow. The stability properties of the laminar temporally developing NCBL, with Prandtl number 0.71, are numerically investigated in the configuration of a temporally evolving parallel flow. By assuming the timescales of the laminar base flow and the perturbations are separate, the instantaneous linear stability of the flow is investigated by an eigenvalue approach with a quasi-steady assumption, whereby the unsteady base flow is frozen in time. Temporal responses of the discrete perturbation modes are numerically obtained by solving the two-dimensional linearised disturbance equations using a `frozen' base flow as an initial-value problem at various 〖Gr〗_δ. The resultant amplification rates of the discrete modes are compared with the quasi-steady eigenvalue analysis, and both two-dimensional and three-dimensional direct numerical simulations (DNS) of the temporally evolving flow. The amplification rate predicted by the linear theory compares well with the direct numerical simulation solutions up to a transition point. The extent of the linear regime where the perturbations linearly interact with the base flow is thus identified. The value of the transition 〖Gr〗_δ, according to the three-dimensional DNS results, is dependent on the initial perturbation amplitude. Beyond the transition point, the DNS results diverge from the linear stability predictions as nonlinear mechanisms become important. For the turbulent NCBL flows, three-dimensional direct numerical simulations (DNS) with different initial conditions were carried out to investigate the turbulent mechanics up to 〖Gr〗_δ=1.2×10^8. The turbulent NCBL is examined in two distinct regions separately: a near-wall boundary-layer-like region and an outer bulk plume-like region. In the near-wall region, a constant heat flux layer (see also in George & Capp, 1979; Ho ̈lling & Herwig, 2005) and a constant forcing layer are identified for the turbulent NCBL. In the close vicinity of the wall (y^+<5) a laminar-like sublayer has developed, and the temperature profile follows the linear relation, consistent with the studies of spatially developing flows (Tsuji & Nagano. 1988a); whereas such a linear relation cannot be observed for the velocity profile due to the extra buoyancy. Similar to earlier studies (Ng et al., 2017), this buoyancy effect is shown to asymptotically approach zero with increasing 〖Gr〗_δ. Further away from the wall (y^+>50), there is a log-law region for the mean temperature profile as reported by Tsuji & Nagano (1988a). In this region, the turbulent length scale which characterises mixing scales linearly with distance from the wall once 〖Gr〗_δ is sufficiently large. By taking the varying buoyancy into consideration with the robust mixing length model, a modified log-law for the mean velocity profile for y^+>50 is proposed. The effect of the initialization is shown to persist until relatively high 〖Gr〗_δ as a result of slow adjustment of the buoyancy (temperature) profile. Once these differences are accounted for, our two DNS cases and the spatially developing data of Tsuji & Nagano (1988a) show excellent agreement with the modified log-law. Beyond a wall-normal distance δ_i, the NCBL can be characterised as an outer bulk plume-like region. This region is found to be well described by an self-similar integral model with profile coefficients (cf. van Reeuwijk & Craske, 2015) which are 〖Gr〗_δ-independent after 〖Gr〗_δ=10^7. The entrainment coefficient for the plume-like region is investigated by decomposing contributions from shear production, buoyancy, viscosity and boundary conditions. For the turbulent NCBL, the entrainment coefficient is found to be mainly affected by the buoyancy in the flow and appears constant beyond 〖Gr〗_δ=10^7. Solution to the self-similar integral model are analytically obtained by solving ordinary differential equations (ODE) with profile coefficients empirically obtained from the DNS results. The DNS results also suggest that the wall heat transfer of the NCBL is directly related to the top-hat scales which characterise the plume-like region. The Nusselt number of the NCBL is found to follow 〖Nu〗_δ∝〖Gr〗_δ^0.373, similar to the observations by Ng et al. (2017) for a vertical NCBL in differentially heated slot and He et al. (2012) for a Rayleigh--Be ̀nard convection. This power-law correlation is higher than the empirical 1/3-power-law correlation reported for spatially developing NCBLs at lower 〖Gr〗_δ, but appears consistent with the ultimate heat transfer with a logarithmic correction suggested by Grossmann & Lohse (2011). Using empirical correlations for the wall shear stress, it is shown that the buoyancy effect in the near-wall region would become negligible, and the near-wall mechanics of the NCBL would become similar to that of a neutrally buoyant turbulent boundary layer above 〖Gr〗_δ>2×10^9 for the present study.

In this study, which is numerical in nature, direct numerical simulation (DNS) of the pipe flow is performed. For the DNS a solenoidal spectral method is employed, this involves the expansion of the velocity using divergence free functions which also satisfy the prescribed boundary conditions, and a subsequent projection of the N-S equations onto the corresponding dual space. The solenoidal functions are formulated in Legendre polynomial space, which results in more favorable forms for the inner product integrals arising from the Petrov-Galerkin scheme employed. The developed numerical scheme is also used to investigate the effects of spanwise oscillations and phase randomization on turbulence statistics, and drag, in turbulent incompressible pipe flow for low to moderate Reynolds numbers (i.e. $mathrm{Re} sim 5000$) ).

Offshore marine applications often include configurations of cylindrical structures that produce complex three-dimensional flow features. Catenary risers, for instance, can create complex flow patterns when subjected to hydrodynamic loads. In recent published studies, the shape of a catenary riser has been approximated by a quarter segment of a ring followed by a horizontal extension, obtaining a curved circular cylinder. In the present Master thesis, Direct Numerical Simulations at Re = 100 and 500 have been conducted in order to study the flow past such geometry. The main flow direction was parallel to the plane of curvature of the cylinder and directed towards the convex face of the quarter-of-ring. Additionally, a sheared incoming flow has been considered in the analysis by imposing a linearly varying velocity profile at the inlet. The shedding mechanism observed in uniform flow was similar to that reported in previous published studies. One single shedding frequency prevailed along the entire span of the cylinder at Re = 100 and 500. Moreover, the vortex cores at Re = 100 were normal to the flow direction and exhibited slight distortions as they were convected downstream, whereas at Re = 500 the wake topology was characterized by three-dimensional structures of smaller scale. A sheared inflow, on the other hand, gave rise to an oblique and cellular vortex shedding pattern with two cells of different shedding frequencies. The strong slanting of the vortices, as well as the cellular pattern, was clearly induced by the variation of the local Reynolds number along the front stagnation point. The basic knowledge gained from this thesis appear as very promising in the context of marine structures, it is therefore expected that this work will constitute a basis for further investigations considering this type of geometry.

Fossil fuel reserves are projected to be decreasing, and emission regulations are becoming more stringent due to increasing atmospheric pollution. Alternative fuels for power generation in industrial gas turbines are thus required able to meet the above demands. Examples of such fuels are synthetic gas, blast furnace gas and coke oven gas. A common characteristic of these fuels is that they are multi-component fuels, whose composition varies greatly depending on their production process. This implies that their combustion characteristics will also vary significantly. Thus, accurate and yet flexible enough combustion sub-models are required for such fuels, which are used during the design stage, to ensure optimum performance during practical operating conditions. Most combustion sub-model development and validation is based on Direct Numerical Simulation (DNS) studies. DNS however is computationally expensive. This, has so far limited DNS to single-component fuels such as methane and hydrogen. Furthermore, the majority of DNS conducted to date used one-step chemistry in 3D, and skeletal chemistry in 2D only. The need for 3D DNS using skeletal chemistry is thus apparent. In this study, an accurate reduced chemical mechanism suitable for multi-component fuel-air combustion is developed from a skeletal mechanism. Three-dimensional DNS of a freely propagating turbulent premixed flame is then conducted using both mechanisms to shed some light into the flame structure and turbulence-scalar interaction of such multi-component fuel flames. It is found that for the multi-component fuel flame heat is released over a wider temperature range contrary to a methane flame. This, results from the presence of individual species reactions zones which do not all overlap. The performance of the reduced mechanism is also validated using the DNS data. Results suggest it to be a good substitute of the skeletal mechanism, resulting in significant time and memory savings. The flame markers commonly used to visualize heat release rate in laser diagnostics are found to be inadequate for the multi-component fuel flame, and alternative markers are proposed. Finally, some popular mean reaction rate closures are tested for the multi-component fuel flame. Significant differences are observed between the models’ performance at the highest turbulence level considered in this study. These arise from the chemical complexity of the fuel, and further parametric studies using skeletal chemistry DNS would be useful for the refinement of the models.

Thesis (Ph.D. in Mechanical Engineering)--S.M.U.
Title from PDF title page (viewed July 20, 2007). Source: Dissertation Abstracts International, Volume: 67-02, Section: B, page: 1125. Adviser: Radovan Kovasevic. Includes bibliographical references.

Reynolds averaged Navier Stokes (RANS) solvers have become the workhorse for simulating turbulent flows for most practical purposes. While the incompressible turbulence models used with RANS equations have improved considerably in their predictive capability, significant breakthrough has not been achieved for their compressible counterparts. With the advancement in computing power, high resolution direct numerical simulation (DNS) of low Reynolds number turbulent flows has become feasible. DNS of simple turbulent flows provides a detailed database which can be used for developing and testing turbulence models. In this work, we perform DNS of compressible homogeneous turbulence—decaying isotropic turbulence and homogeneous shear flow—for a range of initial turbulent Mach numbers, (M_{t 0} = 0.05–0.4) using the more natural initial conditions. Simulations were performed on grids with 128³ and 256³ points. Compressibility effects on the evolution of turbulent kinetic energy were studied. We found negligible compressibility effects for decaying isotropic turbulence, while homogeneous shear flow demonstrated compressibility effects in the growth rate of turbulent kinetic energy. Compressibility corrections to turbulence models in the form of the ratio &epsis;_d/&epsis; _s, have been tested with the results from the simulations. For decaying isotropic turbulence a [special characters omitted] scaling was found to be better than [special characters omitted] while for homogeneous shear flow it was the opposite. The small value of the ratio &epsis;_d/&epsis;_s in decaying isotropic turbulence makes the [special characters omitted] scaling less relevant. Based on the DNS results of homogeneous shear flow, a new correction parameterized by the gradient Mach number, M_g, is proposed. The parameter C_μ, which is assumed constant for incompressible two equation eddy viscosity models, is computed explicitly from the DNS data. An M_g, dependence of the parameter, C_μ, is proposed.

A three-dimensional, turbulent flow in a channel with a sudden expansion was studied by direct numerical simulation of the incompressible Navier-Stokes equations. The objective of this study was to provide statistical data of backwardfacing step flow for turbulence modelling. Additionally, analysis of the statistical and dynamical properties of the flow is performed. The Reynolds number of the main simulation was Reh = 9000, based on the step height and mean inlet velocity, with the expansion ratio ER = 2:0. The discretisation is performed using the spectral/hp element method with stiffly-stable velocity correction scheme for time integration. The inlet boundary condition is a fully turbulent velocity and pressure field regenerated from a plane downstream of the inlet. A constant flowrate was ensured by applying Stokes flow correction in the inlet regeneration area. Time and spanwise averaged results revealed, apart from the primary recirculation bubble, secondary and tertiary corner eddies. Streamlines show an additional small eddy at the downstream tip of the secondary corner eddy, with the same circulation direction as the secondary vortex. The analysis of the 3D, timeonly average shows the wavy spanwise structure of both primary and secondary recirculation bubble, that results in spanwise variations of the mean reattachment location. The visualisation of spanwise averaged pressure uctuations and streamwise velocity showed that the interaction of vortices with the recirculation bubble is responsible for the apping of the reattachment position. The characteristic frequency St = 0:078 was found. The analysis of small-scale energy transfer was performed to reveal large backscatter regions in strong Reynolds stress areas in the mixing layer. High correlation of small-scale transfer with non-linear interaction of large-scale velocity and small-scale vorticity was found. The data of the flow fields was archived. It contains the averages for velocities, pressure and Reynolds stress tensor, as well as 3D instantaneous pressure and velocity history.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2001.
Includes bibliographical references (p. 129-136).
Our understanding of solid-fluid dynamics has been severely limited by the nonexistence of a high-fidelity modeling capability for these multiphase systems. Continuum modeling approaches overlook the microscale solid-fluid interactions from which macroscopic system properties emerge, while experimental inquiries have been plagued by high costs and limited resolution. One promising numerical alternative is to simulate solid-fluid systems at the grain-scale, fully resolving the interaction of individual solid particles with other solid particles and the surrounding fluid. Until recently, the direct simulation of these systems has proven computationally intractable. In this thesis an accurate, efficient, and robust modeling capability for the direct simulation of solid-fluid systems is formulated and implemented. The coupled equations of motion governing both the fluid phase and the individual particles comprising the solid phase are solved using a highly efficient numerical scheme based on the discrete-element (DEM) and the lattice-Boltzmann (LB) methods. Particle forcing mechanisms represented in the model to at least the first order include dynamic fluid-induced forces, buoyancy forces, and intergranular forces from particle collisions, static formation stresses, and intergranular bonding. Coupling is realized with an immersed moving boundary scheme that has been thoroughly validated.
(cont.) For N solid bodies under simulation, the coupled DEM-LB numerical scheme scales roughly as O(N), and is highly parallelizable due to the local and explicit nature of the underlying algorithms. The coupled method has been implemented into a generalized modeling environment for the seamless definition, simulation, and analysis of two-dimensional solid-fluid physics. Extensive numerical testing of the model has demonstrated its accuracy and robustness over a wide range of dynamical regimes. Various fundamental phenomena have been reproduced in simulations, including drafting-kissing-tumbling interactions between settling particles, and the saltating transport regime of bed erosion.
by Benjamin Koger Cook.
Sc.D.

Direct numerical simulations (DNS) of turbulent pipe flows are carried out to investigate the suppression of previously-identified internal noise sources with an acoustic liner using a time-domain acoustic liner model developed by Tam and Auriault (AIAA Journal, 34(5), 917-923, 1996). The liner model is implemented and tested in an in-house DNS code. Validation tests are conducted to show its correct implementation in the DNS solver. In order to study the liner model capability a number of tests are carried out with different liner parameters and flow Mach Numbers. To understand the effect of the liner on the acoustic and turbulent components of the unsteady wall pressure, an azimuthal/axial Fourier transform is applied and the acoustic and turbulent wavenumber regimes are clearly identified. It is found that the spectral component occupying the turbulent wavenumber range is unaffected by the liner, whereas the acoustic wavenumber components are strongly attenuated, with individual radial modes evident as each cuts on with increasing Strouhal number. The acoustic wavenumber analysis shows that the acoustic component of the wall pressure prevails over the hydrodynamic wall pressure. This allows the acoustic liner model to dissipate the acoustic field only, leaving the hydrodynamic component statistically unchanged. Furthermore, a DNS of a pipe/jet configuration is computed to study the effects of the acoustic liner model on the far-field noise. Noise prediction is performed using the Ffowcs Williams-Hawkings (FWH) method. The FWH method has also been tested to identify the best configuration of the FWH surface. A conical-shaped surface proved to be a better surface. Furthermore, results show far-field noise reduction when the liner model is present.

Flow through a sinuous stenosis with varying degrees of shape asymmetry andat Reynolds number ranging from 250 up to 800 is investigated using direct numericalsimulation (DNS), global linear stability analysis and sensitivity analysis.The shape asymmetry consists of an offset of the stenosis throat, quantifiedas the eccentricity parameter, E. At low Reynolds numbers in a symmetricgeometry, the flow is steady and symmetric. Our results show that when Reynoldsnumber is increased, the flow obtains two simultaneous linearly stablesteady states through a subcritical Pitchfork bifurcation: a symmetric stateand an asymmetric state. The critical Reynolds number for transition betweenthe states are found to be very sensitive to asymmetric shape variations, thusbifurcation can also occur with respect to eccentricity for a given Reynoldsnumber. The final state observed in the DNS can be either nearly symmetricor strongly asymmetric, depending on the initial condition. When eccentricityis increased from zero, the symmetric state becomes slightly asymmetric,flow asymmetry varying nearly linearly with eccentricity. When eccentricityis increased further, the nearly symmetric state becomes linearly unstable. Alinear global stability analysis shows that the eigenvalue sensitivity to eccentricityis of the second order, this is also confirmed by preliminary sensitivityanalysis. For higher Reynolds numbers, the asymmetric solution branch displaysregimes of periodic oscillations as well as intermittency. Comparisons aremade to earlier studies and a theory that attempts to explain and unite thedifferent numerical and experimental results within the field is presented.

Rapid surface ablation by a turbulent flow creates complex flow and surface phenomena arising from the evolving boundary topography and its interaction with a turbulent flow that transports the ablative agent onto the surface. The dynamic nature of ablative flow boundaries generate unsteady flow dynamics and thermodynamics occurring over a wide range of scales. The non-equilibrium nature of these phenomena pose a major challenge to the current fundamental understanding of turbulence, which is mostly derived from equilibrium flows, and to Computational Fluid Dynamics (CFD). The simulation of moving boundaries is a necessary tradeoff between computational speed and accuracy. The most accurate methods use surface-conforming grids, forcing the grid to move and deform in time at a high computational cost. The technique used in this study, immersed boundary methods, removes the need for a surface-conforming grid, typically at the expense of numerical accuracy. The objectives of the present study are (i) to develop an Energy Immersed Boundary Method (EIBM) to simulate conjugate heat transfer and phase change with a spatial order of accuracy larger than one, and (ii) use the EIBM to study the dynamics of ablative flows. A generalized finite volume (FV) flow solver with second-order accuracy in time and space and energy conserving schemes is the basis of the EIBM algorithm development. The EIBM com- bines level-set method for the definition and transport of the fluid/solid interface with an immersed boundary method, i.e. a modification of the transport equation to enforce the proper boundary conditions at the solid surface. The proposed algorithm is shown to be second order accurate in space in the simulation of conjugate heat transfer flows. The validation also included comparison with phase-change (melting) experiments where it was shown to correlate very well to previous ex- periments of a rectangular slab of gallium melted from one side. As well as showing second order convergence for the mass loss and the ablated shape of a cylinder in a melting cross flow. The EIBM is applied to an investigation of the interactions between turbulence and an erodible surface. The study first focuses on the response of a turbulent flow over a receding wall, with constant recession velocity. It is found that wall recession velocities, near the small scale, the Kolmorgorov microscale, velocity of the buffer layer, produce minute shear free layers near the wall which both enhanced and stretched out the low and high velocity streaks near the wall. The larger streak area produced larger turbulent intensities on the dynamic boundary side of the channel, and far more semi-streamwise vortices. In the Second study the EIBM is applied to the ablation of a generic slab in a turbulent channel heated from one side in the absence of gravity. The study focuses on the characterization of the surface topography in relation to the evolution of coherent structures in the flow as ablation proceeds. The produced surface topology is linked to the flow topology and the turbulent generating and dissipating forces inside the turbulent flow. It is shown that the streaks for stefan numbers producing average ablation velocities slightly smaller than the Kolmorgorov microscale create groves in which the high speed buffer layer streaks sit, and their sinus motion in the spanwise direction is reduced.

We study the use of lattice Boltzmann (LB) methods for simulation of turbulent fluid flows motivated by their high computational throughput and amenability to highly parallel platforms such as graphics processing units (GPUs). Several algorithmic improvements are unearthed including work on non-unit Courant numbers, the force operator, use of alternative topologies based on face and body centered cubic lattices and a new formulation using a generalized eigendecomposition that allows a new freedom in tuning the eigenvectors of the linearised collision operator. Applications include a variable bulk viscosity and the use of a stretched grid, our implementation of which reduces errors present in previous efforts. We present details for numerous lattices including all required matrices, their moments the procedures and programs used to generate these and perform linear stability analysis. Small Mach number flows where density variations are negligible except in the buoyancy force term allow the use of a highly accurate finite volume solver to simulate the evolution of the buoyancy field which is coupled to the LB simulation as an external force. We use a multidimensional flux limited third order flux integral based advection scheme. The simplified algorithm we have devised is easier to implement, has higher performance and does not sacrifice any accuracy compared to the leading alternative. Our algorithm is particularly suited to an outflow based implementation which furthers the stated benefits. We present numerical experiments confirming the third order accuracy of our scheme when applied to multidimensional advection. The coupled solver is implemented in a new code that runs in parallel across multiple machines using GPUs. Our code achieves high computational throughput and accuracy and is used to simulate a range of turbulent flows. Details regarding turbulent channel flow and sheared convective boundary layer simulations are presented including some new insight into the scaling properties of the latter flow.

L'objectif principal de ce travail est d'analyser les effets d'un gradient de pression défavorable modéré sur la dynamique d'écoulement d'une couche limite turbulente. Dans ce contexte, une simulation numérique directe (DNS) de la couche limite turbulente (TBL) soumise à un gradient de pression défavorable modéré (APG) hors équilibre a été réalisée jusqu'à un Reynolds de 8000 en utilisant le code open-source Incompact3d. Une large base de données résolues en temps et en espace a été collectée et utilisée pour analyser les statistiques de la turbulence. Une attention particulière a été consacrée à l'existence et à l'évolution du pic de contraintes de Reynolds observé dans la zone externe de la couche limite. Différentes échelles de vitesse ont été étudiées, testées et confrontées aux résultats numériques. L'échelle de vitesse basée sur la contrainte de cisaillement permet de mettre à l'échelle tous les profils de contraintes de Reynolds pour plusieurs nombres de Reynolds, ce qui indique que toutes les contraintes de Reynolds sont associées à une dynamique unique des structures turbulentes.Les structures cohérentes à grande échelle des fluctuations de vitesse longitudinales ont été étudiées en utilisant la corrélation spatiale en deux points. Une comparaison avec un cas sans gradient de pression à un nombre de Reynolds équivalent nous permet d'étudier l'effet du gradient de pression sur la taille et l'inclinaison des structures cohérentes attachées. Une étude approfondie sur les structures cohérentes a également été réalisée, où chaque structure a été détectée séparément en utilisant une méthode de seuillage afin de distinguer les effets des grandes et petites échelles et de mieux comprendre les mécanismes qui contrôlent la dynamique de ces structures. La contribution des mouvements de grande échelle (LSM) sur les contraintes de Reynolds en comparaison avec le cas ZPG a également été analysée
The main objective of this work is to analyze the effects of a moderate adverse pressure gradient on the dynamics of turbulent boundary layer flows. For that purpose, a direct numerical simulation (DNS) of the turbulent boundary layer (TBL) subjected to a moderate adverse pressure gradient (APG) out of equilibrium has been performed using the open-source code Incompact3d up to a Reynolds number of 8000 based on momentum thickness. A large database resolved in time and space was collected and used to analyze the turbulence statistics. Special attention has been paid to the existence and evolution of the outer peak of Reynolds stresses observed in APG wall-bounded flows. Different velocity scalings have been investigated and tested against the numerical results. The velocity scale based on the shear stress is shown to scale all the Reynolds stresses profiles for different Reynolds numbers, indicating that all Reynolds stresses are associated with a single dynamics of turbulent structures.The large-scale coherent structures of the streamwise velocity fluctuations have been investigated using two-point spatial correlation. A comparison with a zero pressure gradient case at an equivalent Reynolds number allows us to further investigate the effect of the pressure gradient on the size and inclination of attached coherent structures. A deeper investigation of the coherent structures was also performed, where each structure was detected separately based on a thresholding method to distinguish between the effects of large and small scales and to better understand the mechanisms controlling the dynamics of these structures. The contribution of large-scale motions (LSM) on the Reynolds stresses comparing with ZPG case was also analyzed

Thesis (Ph. D.) -- University of Maryland, College Park, 2005.
Thesis research directed by: Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Particle-laden turbulent flow through a straight square duct at Reτ = 300 is studied using direct numerical simulation (DNS) and Lagrangian particle tracking. A parallelized 3-D particle tracking direct numerical simulation code has been developed to perform the large-scale turbulent particle transport computations reported in this thesis. The DNS code is validated after demonstrating good agreement with the published DNS results for the same flow and Reynolds number. Lagrangian particle transport computations are carried out using a large ensemble of passive tracers and finite-inertia particles and the assumption of one-way fluid-particle coupling. Using four different types of initial particle distributions, Lagrangian particle dispersion, concentration and deposition are studied in the turbulent straight square duct. Particles are released in a uniform distribution on a cross-sectional plane at the duct inlet, released as particle pairs in the core region of the duct, distributed randomly in the domain or distributed uniformly in planes at certain heights above the walls. One- and two-particle dispersion statistics are computed and discussed for the low Reynolds number inhomogeneous turbulence present in a straight square duct. New detailed statistics on particle number concentration and deposition are also obtained and discussed.

Dissertation (Ph.D.) -- Worcester Polytechnic Institute.
Keywords: Multiphase flow; Two-fluid modeling; Direct numerical simulation; Two fluid modeling. Includes bibliographical references (leaves 116-119).

Turbulent mixing evolving from the Richtmyer-Meshkov instability, also known as shock-induced turbulent mixing, is investigated using numerical simulations of fundamental test problems with high-resolution computational methods. An existing state-of-the-art implicit large eddy simulation algorithm for compressible multispecies flows is extended to include the effects of viscous dissipation, thermal conductivity and species diffusion by deriving a novel set of governing equations for binary mixtures. This allows for direct numerical simulations of shock-induced turbulent mixing to be performed for arbitrary gas mixtures cases where the ratio of specific heats may vary with mixture composition at much greater computational efficiency. Using direct numerical simulation, a detailed study is performed of the effects of Reynolds number on the transition to turbulence in shock-induced mixing evolving from narrowband initial conditions. Even though the turbulence in the highest Reynolds number case is not fully developed, a careful analysis shows that the high Reynolds number limit of several key quantities is able to be estimated from the present data. The mixing layer is also shown to be persistently anisotropic at all Reynolds numbers, which also has important consequences for modelling. At the time of writing, the highest Reynolds number case from this set of simulations is the highest achieved in any fully-resolved direct numerical simulations presented in the open literature for this class of problems. Implicit large eddy simulation is employed to investigate the influence of broadband initial conditions on the late-time evolution of a shock-induced turbulent mixing layer. Both the bandwidth of initial modes as well as their relative amplitudes are varied, showing that both the growth rate of the mixing layer width and the decay rate of fluctuating kinetic energy strongly depend on initial conditions. Finally, both implicit large eddy simulations and direct numerical simulations are performed of an idealised shock tube experiment to analyse the effects of additional long wavelength, low amplitude modes in the initial perturbation. These calculations represent the first direct numerical simulations performed of Richtmyer-Meshkov instability evolving from broadband initial conditions.

Cette thèse est consacrée aux écoulements homogènes de bulles, ainsi qu'à leur couplage avec le transport d'un scalaire et la turbulence. Elle s'intéresse plus spécifiquement aux effets de taille finie, des interactions hydrodynamiques et de la microstructure de la suspension qui sont étudiés à l'aide de simulations numériques directes à l'échelle d'une seule bulle. La dynamique d'une suspension laminaire de bulles induite par la seule gravité est d'abord revisitée. L'influence de la fraction volumique sur la vitesse de dérive des bulles est établie analytiquement et numériquement pour une suspension parfaitement ordonnée, puis des ressemblances entre suspensions ordonnées et suspensions désordonnées sont mises en évidence. Ces résultats sont ensuite mis à profit pour la modélisation du transport d'un scalaire passif au sein d'une suspension laminaire, tel que décrit par une diffusivité effective tensorielle, et des différences essentielles entre systèmes ordonnés et systèmes désordonnés concernant le transport de scalaire sont mises en exergue. Enfin, la turbulence est prise en compte dans les simulations et son interaction avec une bulle de taille finie est caractérisée. Il est montré que le comportement dynamique d'une bulle de taille comparable à la microéchelle de Taylor ressemble qualitativement à celui d'une microbulle, avec, notamment, une préférence pour certaines régions caractéristiques de l'écoulement. Une définition de l'écoulement vu par la bulle compatible avec les modèles standards de masse ajoutée et de portance est finalement proposée
This thesis is devoted to the study of homogeneous bubbly flows and their coupling with scalar transport and turbulence. It focuses on the effects of finite size, hydrodynamic interactions, and suspension microstructure, which are investigated using direct numerical simulations at the bubble scale. The dynamics of laminar buoyancy-driven bubbly suspensions is first revisited. More specifically, the effect of volume fraction on the bubble drift velocity is clarified by connecting numerical results to theory for dilute ordered systems, and similarities between perfectly ordered and free disordered suspensions are evidenced. These results are then used for the modeling of passive scalar transport in laminar suspensions as described by an effective diffusivity tensor, and crucial differences between ordered and disordered systems with respect to scalar transport are highlighted. Lastly, turbulence is included in the simulations, and its interaction with a finite-size bubble is characterized. The behavior of a bubble as large as Taylor microscale is shown to share a number of common features with that of a microbubble, most notably, the flow sampled by the bubble is biased. A definition of the liquid flow seen by the bubble, as it enters in usual models for the added mass and the lift forces, is finally proposed

Ce travail de thèse présente l'implémentation de la simulation d'écoulements diphasiques dans des conditions de réacteurs nucléaires à caloporteur eau, à l'échelle de bulles individuelles. Pour ce faire, nous étudions plusieurs modèles d'écoulements thermohydrauliques et nous focalisons sur une technique de capture d'interface mince entre phases liquide et vapeur. Nous passons ainsi en revue quelques techniques possibles de maillage adaptatif (AMR) et nous fournissons des outils algorithmiques et informatiques adaptés à l'AMR par patchs dont l'objectif localement la précision dans des régions d'intérêt. Plus précisément, nous introduisons un algorithme de génération de patchs conçu dans l'optique du calcul parallèle équilibré. Cette approche nous permet de capturer finement des changements situés à l'interface, comme nous le montrons pour des cas tests d'advection ainsi que pour des modèles avec couplage hyperbolique-elliptique. Les calculs que nous présentons incluent également la simulation du système de Navier-Stokes incompressible qui modélise la déformation de l'interface entre deux fluides non-miscibles
This PhD work presents the implementation of the simulation of two-phase flows in conditions of water-cooled nuclear reactors, at the scale of individual bubbles. To achieve that, we study several models for Thermal-Hydraulic flows and we focus on a technique for the capture of the thin interface between liquid and vapour phases. We thus review some possible techniques for Adaptive Mesh Refinement (AMR) and provide algorithmic and computational tools adapted to patch-based AMR, which aim is to locally improve the precision in regions of interest. More precisely, we introduce a patch-covering algorithm designed with balanced parallel computing in mind. This approach lets us finely capture changes located at the interface, as we show for advection test cases as well as for models with hyperbolic-elliptic coupling. The computations we present also include the simulation of the incompressible Navier-Stokes system, which models the shape changes of the interface between two non-miscible fluids