• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 94
  • 54
  • 36
  • 20
  • 11
  • 11
  • 7
  • 5
  • 4
  • 2
  • 2
  • 1
  • 1
  • Tagged with
  • 265
  • 101
  • 59
  • 49
  • 36
  • 33
  • 31
  • 29
  • 28
  • 28
  • 21
  • 20
  • 20
  • 19
  • 18
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
71

Modélisation et simulation de la turbulence compressible en milieu diphasique : application aux écoulements cavitants instationnaires / Modeling and simulation of compressible turbulence in two-phase : application to the cavitating unsteady flow

Decaix, Jean 11 October 2012 (has links)
La simulation des écoulements cavitants est confrontée à des difficultés de modélisation et de résolution numérique provenant des caractéristiques particulières de ces écoulements : changement de phase, gradient de masse volumique important, variation du nombre de Mach, turbulence diphasique, instationnarités. Dans cette thèse, nous nous sommes appliqués à dériver proprement le modèle de mélange homogène 1-fluide couplé à une modélisation RANS de la turbulence. A partir des termes contenus dans ces équations et de la nature des écoulements cavitants étudiés, plusieurs modèles de turbulence basés sur la notion de viscosité turbulente ont été testés : modèles faiblement non-linéaires (corrections SST et de réalisabilité), ajout des termes de turbulence compressible, application de la correction de Reboud, modèles hybrides RANS/LES (DES, SAS). Ces modèles ont été incorporés dans un code compressible qui fait appel à une résolution implicite en pas de temps dual des équations de conservation avec une technique de pré-conditionnement bas-Mach pour traiter les zones incompressibles. Les simulations 2D et 3D ont porté sur deux géométries de type Venturi caractérisées par la présence d’une poche de cavitation instationnaire due à l’existence d’un jet rentrant liquide/vapeur le long de la paroi. Elles montrent que l’ensemble des modèles sont capables de capturer le jet rentrant. En revanche, la dynamique de la poche varie entre les modèles et le manque de données expérimentales ne permet pas de discriminer les modèles entre eux. Il apparaît à la vue des résultats que les approches avec la correction de Reboud ou les modèles SAS améliorent la simulation des écoulements. / The computation of cavitating flows is a challenging issue due to the characteristics of these flows : phase transition, large density gradient, Mach number variation, interaction between phases and turbulent flow, unsteadiness. In the present study, we performed a derivation of the one-fluid compressible homogenous model coupled with a RANS approach for the turbulent flow. From these equations and the nature of the cavitating flows, several models based on the eddy viscosity assumption have been tested : weakly non-linear models (SST and realisability corrections), compressible turbulence models, hybrid RANS/LES turbulence models (SAS, DES) and the Reboud correction. All the models are implemented in a compressible code, which solves the equations using an implicit dual-time stepping method coupling with a pre-conditionning technique for the incompressible area. 2D and 3D computations are performed on two Venturi geometries characterized by an unsteady cavitation sheet with a liquid/vapor re-entrant jet. All the models are able to capture the re-entrant jet. Nevertheless, the dynamic behaviour differs from one model to another and the lack of experimental data prevents to discriminate the models between them. From the results, the computations with the SAS model and the Reboud correction improve the prediction of the flow.
72

Implementação de sensores ópticos integrados para aplicações em análises químicas e ambientais. / Implementation of integrated optical sensors for chemical analysis and environment aplications.

Acácio Luiz Siarkowski 04 May 2007 (has links)
Este trabalho tem como objetivo o desenvolvimento de sensores químicos ópticos baseados no Interferômetro Mach-Zehnder (MZI) para aplicações em análises químicas e ambientais. Tais dispositivos foram implementados a partir da fabricação de guias de ondas sobre um substrato de silício monocristalino. Os guias de onda foram obtidos através da deposição de filmes finos de nitreto de silício (Si3N4) e óxido de silício (SiO2) obtidos por técnicas de deposição química a vapor (CVD) e pela oxidação térmica do substrato de silício. A definição destes guias de ondas para a implementação de sensores é feita por processos convencionais de litografia e corrosão. Os sensores químicos deste trabalho baseiam-se na interação do campo evanescente do modo que propaga-se no guia de onda exposto a algum composto químico na fase líquida ou gasosa. Tais sensores foram projetados, simulados e implementados para as seguintes aplicações: Medida da concentração de glicose em soluções; Medida de umidade por efeito de espalhamento; Detecção de vapores de compostos orgânicos voláteis (VOC\'s). As etapas de processo para a fabricação dos sensores químicos ópticos foram definidas a partir das simulações em óptica integrada e, após os processos de fabricação, os filmes obtidos foram caracterizados. Os filmes de Si3N4 SiO2 utilizados na fabricação do sensor óptico possuem uniformidade em espessura e homogeneidade quanto aos valores de índice de refração, baixa rugosidade superficial e baixos níveis de incorporação de água e ligações OH. No entanto, os valores de rugosidade e desuniformidade aumentaram significativamente após as etapas de corrosão úmida para a definição das estruturas dos sensores ópticos. Os guias de ondas implementados apresentaram um guiamento monomodo (TE0) e os valores de atenuação experimentais ficaram entre 2 e 20 dB/cm e dependem dos valores de rugosidade superficial e lateral dos guias de ondas e da desuniformidade dos filmes empregados. Com as medidas de concentração de glicose, verificou-se o funcionamento e a sensibilidade dos sensores químicos ópticos implementados, levantando-se curvas de variação da intensidade de saída do MZI em função do índice de refração de soluções de glicose padronizadas (dn/dc). Realizou-se medidas de umidade através da implementação de um sensor óptico simplificado que tem como princípio o efeito de espalhamento óptico. Como o espalhamento óptico depende da rugosidade e do contraste de índices de refração na interface exposta ao ambiente, foi possível detectar a quantidade de água presente no dispositivo. Posteriormente, analisou-se a aplicação de HMDS (Hexametildissilazana) como filme de adsorção de compostos orgânicos voláteis (VOC\'s). Os VOC\'s foram aplicados sobre os sensores químicos ópticos em forma de vapor e os resultados obtidos para n-Hexano e 2-Propanol mostraram uma boa reprodutibilidade e alta sensibilidade. / The aim of this work is the development of optical chemical sensors based on Mach-Zehnder Interferometer (MZI) for chemical and environmental analysis applications. Such devices were implemented with waveguides fabricated on a silicon substrate. The waveguides were obtained using silicon nitride (Si3N4) and silicon oxide (SiO2) films obtained by CVD (Chemical Vapor Deposition) and thermal oxidation of the silicon substrate. The waveguides and the optical sensors were fabricated using conventional lithograph and etching processes. The developed chemical sensors are based on the evanescent field interaction in a waveguide exposed to a liquid or gaseous phase of a chemical compound. Such optical sensor were designed, simulated and implemented for the following applications: Glucose concentration; Humidity measurement by scattering effect; Volatile organic compounds (VOC\'s) vapors detection. The process steps for the optical chemical sensor fabrication were defined during the integrated optical simulations and the films were characterized after the fabrication process steps. The Si3N4 and SiO2 films used in the optical sensor fabrication are uniform and homogenous (thickness and refractive index), with low surface roughness and low levels of water incorporation and OH bonds. However, the roughness values increased significantly after the etching process for the optical sensor structures definition. The implemented waveguides were singlemoded (TE0), with experimental attenuation values between 2 and 20 dB/cm, depending on the surface and sidewall roughness of the waveguide and of the films uniformity. The operation principle and the sensitivity of the optical chemical sensor were verified by glucose concentration measurements, by measuring the MZI output variation intensity as a function of the refractive index of the glucose standard solutions (dn/dc). As the light scattering depends of the roughness and the of refractive index difference between the exposed interface and the atmosphere, it\'s possible to measure the humidity or the amount of water in the interaction window. Afterwards, the detection of volatile organic compounds (VOC\'s) was done using a HMDS (Hexamethyldissilazane) layer to adsorb the VOC\'s. The VOC\'s vapor was applied on the optical chemical sensors and the results obtained for n-Hexane and 2-Propanol showed a good reproducibility and high sensitivity.
73

Produção de Interferômetros Mach-Zehnder utilizando guias de onda do tipo pedestal e filmes finos de Bi₂O₃-WO₃-TeO₂ para aplicações em sensores ópticos integrados. / Production of Mach-Zehnder Interferometer using pedestal type optical waveguides and Bi₂O₃-WO₃-TeO₂ thin films for applications in integrated optical sensors.

Mauricio Eiji Camilo 06 June 2014 (has links)
Neste trabalho foram produzidos Interferômetros Mach-Zehnder (IMZ) a partir de guias de onda do tipo pedestal com filmes finos de Bi₂O₃-WO₃-TeO₂ (BWT) como camada de núcleo para aplicações em sensores ópticos integrados. A influência dos parâmetros e dos materiais utilizados nas etapas de processo foi verificada. Os valores de índice de refração efetivo e coeficiente de absorção em função do comprimento de onda foram obtidos para os filmes finos BWT. Os guias de onda pedestais foram caracterizados por Microscopia Eletrônica de Varredura, medidas de perda por propagação e perfis de campo próximo em 633 nm e 1050 nm. Os valores mínimos obtidos nas perdas por propagação foram de ~1,5 dB/cm em 633 nm e 3,0 dB/cm em 1050 nm. As medidas de perfis de campo próximo mostraram que guias de onda com larguras superiores a 7 m apresentaram comportamento multimodo. Foram obtidos IMZs que apresentaram guiamento de luz por toda a estrutura, com comportamento multimodo. Sensores ópticos de pressão e temperatura foram produzidos. A fabricação de diafragmas através do processo de corrosão úmida do silício é apresentada no sensor de pressão. Sensores ópticos de temperatura foram produzidos com filamentos metálicos. As cavidades ópticas não foram obtidas nesse sensor. A potência de luz na saída dos sensores de temperatura foi medida em função da diferença de potencial aplicada no filamento metálico. Os resultados apresentados mostram que guias de onda do tipo pedestal produzidos com núcleo de BWT são promissores para aplicações em sensores ópticos integrados. / In this work Mach-Zehnder Interferometers (MZI) were produced from the pedestal-type waveguides with Bi₂O₃-WO₃-TeO₂ (BWT) thin films as the core layer for applications in integrated optical sensors. The influence of the parameters and materials used in the process steps was verified. The values of the effective refractive index and absorption coefficient as a function of wavelength were obtained for the BWT thin films. The pedestal waveguides were characterized by Scanning Electron Microscopy, by propagation loss measurements and near-field profiles at 633 nm and 1050 nm. The minimum values obtained by propagation losses were ~ 1.5 dB/cm at 633 nm and 3.0 dB/cm at 1050 nm. The measures of near-field profiles showed that waveguides with widths larger than 7 m presented multimode behavior. The MZIs obtained presented guiding light through the structure, with multimode behavior. Optical pressure sensors and temperature sensors were produced. The production of diaphragms using the wet etching process of silicon is presented on the pressure sensor. Optical temperature sensors were made with metallic filaments. Optical cavities were not obtained in this sensor. The light power in the output of the temperature sensors was measured as the voltage applied to the metal filament. The results show that the waveguides produced with pedestal-type and BWT thin films as core layer are promising for applications in integrated optical sensors.
74

Fundamentos de física quântica na formação de professores : uma análise de interações discursivas em atividades centradas no uso de um interferômetro virtual de Mach-Zehnder

Pereira, Alexsandro Pereira de January 2008 (has links)
Neste trabalho apresentamos uma investigação sobre o ensino de física quântica na formação inicial de professores. Essa pesquisa foi desenvolvida junto a uma disciplina da sétima etapa do curso de Licenciatura em Física da Universidade Federal do Rio Grande do Sul, durante o segundo semestre de 2007, através de uma atividade de ensino centrada na exploração de um software que simula o interferômetro de Mach-Zehnder. Inspirados nos trabalhos de Müller e Wiesner (2002) e Pessoa Jr. (2005), utilizamos o conceito de dualidade onda-partícula como o eixo central das discussões em sala de aula. O objetivo desse estudo consiste em analisar as tensões nos enunciados de estudantes, à luz do referencial sociocultural, e avaliar em que medida os enunciados dos estudantes se articulam à internalização de conceitos de física quântica no uso de instrumentos semióticos. Os resultados mostraram que as ações mediadas pelo uso do interferômetro virtual de Mach-Zehnder, como ferramenta cultural, auxiliaram os processos de compreensão, por parte dos alunos, viabilizando a negociação de significados em sala de aula que são aceitos e compartilhados pela comunidade científica. / In this paper, we present a research of quantum physics teaching in the context of a teachers preparation course. This investigation has been developed in a seventh level discipline of a Bachelor Degree Course in Physics Education, in the Federal University of Rio Grande do Sul, during the second semester of 2006, through a didatical activity based on the use of a computer program that simulate the Mach-Zehnder interferometer. Ispired by Müller and Wiesner (2002) and Pessoa Jr. (2005), we used the concept of wave-particle duality as the central espect of discussions in the classroom. The aim of the present study was to analyze the student’s utterences, in the light of sociocultural theories, and to investigate how the speech of the studants articulates it self to the internalization of quantum physics concepts, in the use of semiotics instruments. The results showed that the actions mediated by the use of virtual Mach-Zehnder interferometer, as a cultural tool, supported the comprehendig process in studants, feasing the negotiation of meanings that are accepted and shared in scientific community.
75

Transpiration Cooling Analysis Including Binary Diffusion Using 2-D Navier-Stokes Equations At Hypersonic Mach Numbers

Ravi, B R 06 1900 (has links) (PDF)
No description available.
76

Fázové zobrazování pod difrakčním limitem / Phase imaging below the diffraction limit

Nečesal, Daniel January 2020 (has links)
Tato diplomová práce se zabývá konstrukcí interferenčních zařízení pro výzkum měření fáze světla na mikro-strukturách a fáze plazmonů na nano-strukturách. V první kapitole je vybudován teoretický základ pro optiku a nano-fotoniku používané v budoucích kapitolách. Následně je popsána interference vln a jejich praktické použití pro prolomení difrakčního limitu pomocí holografie plazmonů. Prvním experimentální sestavou je Machův-Zehnderův holografický mikroskop. Je popsán způsob, jak ho sestavit z běžně dostupných součástek a jak je navržen ovládací software k jeho používání. Následně jsou popsány výsledky naměřené pomocí tohoto zařízení. V poslední kapitole se zaměříme na sestavení holografického SNOM přístroje pro studování plazmonů a jejich interference. Nejdříve popíšeme základní princip a navržení softwaru pro automatizaci měření, pro zrychlení vědeckých postupů. Nakonec předložíme výsledky měření mikroskopu.
77

Application of the compressible and low-mach number approaches to large-eddy simulation of turbulent flows in aero-engines / Application de l'approche compressible et de l'approche bas-Mach pour la simulation aux grandes échelles des écoulements turbulents dans des foyers aéronautique

Kraushaar, Matthias 01 December 2011 (has links)
La Simulation aux Grandes Echelles (SGE) est de plus en plus utilisée dans les processus de développement et la conception des réacteurs aéronautiques industriels. L'une des raisons pour ce besoin résulte dans la capacité de la SGE à fournir des informations instantanées d'un écoulement turbulent augmentant la quantité des prédictions de la composition des gaz d'échappement. Ce manuscrit de thèse aborde deux sujets récurrents de la SGE. D'une part, les schémas numériques pour la SGE nécessitent certaines propriétés, notamment une précision élevée avec une diffusivité faible pour ne pas nuire aux modèles de turbulence. Afin de répondre à ce pré requis, une famille de schémas d'intégration temporelle d'ordre élevée est proposée, permettant de modifier la diffusion numérique du schéma. D'autre part, la SGE étant intrinsèquement instationnaire, elle est très consommatrice en temps CPU. De plus, une géométrie complexe prend beaucoup de temps de simulation même avec les super calculateurs d'aujourd'hui. Dans le cas particulier d'intérêt et souvent rencontré dans les applications industrielles, l'approche bas-Mach est constitue une alternative intéressante permettant de réduire le coût et le temps de retour d'une simulation LES. L'impact et la comparaison des formalismes compressible et incompressible sont toutefois rarement quantifiés, ce qui est proposé dans ce travail pour une configuration représentative d'un brûleur swirlé industriel mesuré au CORIA / Large-Eddy Simulation (LES) becomes a more and more demanded tool to improve the design of aero-engines. The main reason for this request stems from the constraints imposed on the next generation low-emission engines at the industrial development level and the ability for LES to provide information on the instantaneous turbulent flow field which greatly contributes to improving the prediction of mixing and combustion thereby offering an improved prediction of the exhaust emission. The work presented in this thesis discusses two recurring issues of LES. For one, numerical schemes for LES require certain properties, i.e. low-diffusion schemes of high order of accuracy so as not to interfere with the turbulence models. To meet this purpose in the context of fully unstructured solvers, a new family of high-order time-integration schemes is proposed. With this class of schemes, the diffusion implied by the numerical scheme become adjustable and built-in. Second, since fully unsteady by nature, LES is very consuming in terms of CPU time. Even with today's supercomputers complex problems require long simulation times. Due to the low flow velocities often occurring in industrial applications, the use of a low-Mach number solver seems suitable and can lead to large reductions in CPU time if comparable to fully compressible solvers. The impact of the incompressibility assumption and the different nature of the numerical algorithms are rarely discussed. To partly answer the question, detailed comparisons are proposed for an experimental swirled configuration representative of a real burner that is simulated by LES using a fully explicit compressible solver and an incompressible solution developed at CORIA
78

Quelques résultats mathématiques sur les gaz à faible nombre de Mach / Some mathematical results on gases with small Mach number

Liao, Xian 24 April 2013 (has links)
Cette thèse est consacrée à l'étude de la dynamique des gaz à faible nombre de Mach. Le modèle étudié provient des équations de Navier-Stokes complètes lorsque le nombre de Mach tend vers zéro. On cherche à montrer que le problème de Cauchy correspondant est bien posé. Les cas visqueux et non visqueux sont tous deux considérés. Les coefficients physiques peuvent dépendre de la densité (ou de la température) inconnue. En particulier, nous prenons en compte les effets de conductivité thermique et on autorise de grandes variations d'entropie. Rappelons qu'en absence de diffusion thermique, la limite à faible nombre de Mach implique la condition d'incompressibilité. Dans le cadre étudié ici, en introduisant un nouveau champ de vitesses à divergence nulle, le système devient un couplage non linéaire entre une équation quasi-parabolique pour la densité et un système de type Navier-Stokes (ou Euler) pour la vitesse et la pression. Pour le cas avec viscosité, on établit le résultat classique, à savoir qu'il existe une solution forte existant localement (resp. globalement) en temps pour des données initiales grandes (resp. petites). On considère ici le problème de Cauchy avec données initiales dans des espaces de Besov critiques. Lorsque les coefficients physiques du système vérifient une relation spéciale, le système se simplifie considérablement, et on peut alors établir qu'il existe des solutions faibles globales en temps à énergie finie. Par un argument d'unicité fort-faible, on en déduit que les solutions fortes à énergie finie existent pour tous les temps positifs en dimension deux. Pour le cas sans viscosité, on montre d'abord le caractère bien posé dans des espaces de Besov limites, qui s'injectent dans l'espace des fonctions lipschitziennes. Des critères de prolongement et des estimations du temps de vie sont établis. Si l'on suppose la donnée initiale à énergie finie dans l'espace de Besov limite à exposant de Lebesgue infini, on a également un résultat d'existence locale. En dimension deux, le temps de vie tend vers l'infini quand la densité tend vers une constante positive. Des estimations de produits et de commutateurs, ainsi que des estimations a priori pour les équations paraboliques et pour le système de Stokes (ou d'Euler) à coefficients variables, se trouvent dans l'annexe. Ces estimations reposent sur la théorie de Littlewood-Paley et le calcul paradifférentiel / This thesis is devoted to the study of the dynamics of the gases with small Mach number. The model comes from the complete Navier-Stokes equations when the Mach number goes to zero, and we aim at showing that it is well-posed. The viscous and inviscid cases are both considered. The physical coefficients may depend on the unknown density (or on the unknown temperature).In particular, we consider the effects of the thermal conductivity and hence large variations of entropy are allowed. Recall that if there is no thermal diffusion, then the low Mach number limit just implies the incompressibility condition. In the framework considered here, by introducing a new solenoidal velocity field, the system becomes a nonlinear coupling between a quasi-parabolic equation for the density and an evolutionary Stokes (or Euler) system for the velocity and the pressure. For the case with viscosity, we establish classical results, namely the strong solutions exist locally (resp. globally) in time for big (resp. small) initial data. We consider the Cauchy problem in the critical Besov spaces with the lowest regularity. Under a special relationship between the two physical coefficients, the system recasts in a simpler form and one may prove that there exist weak solutions with finite energy. In dimension two, this implies that strong solutions with finite energy exist for all positive times. In the inviscid case, we first prove the well-posedness result in endpoint Besov spaces, which can be embedded into the set of Lipschitzian functions. Continuation criterions and estimates for the lifespan are both established.If we suppose the initial data to be in the borderline Besov spaces with infinite Lebesgue exponent and to be of finite energy, we also have a local existence result. In dimension two, the lifespan goes to infinity when the density tends to a positive constant. Estimates for products and commutators, together with a priori estimates for the parabolic equations and the Stokes (or Euler) system with variable coefficients, are postponed in the appendix. These estimates are based on the Littlewood-Paley theory and the paradifferential calculus
79

Modélisation et simulation numérique d'écoulements diphasiques pour la microfluidique / Modeling and numerical simulation of multiphase flow for microfluidics

Prigent, Guillaume 24 January 2013 (has links)
Ce travail de thèse est consacré à la modélisation et simulation numérique d'écoulements diphasiques liquide-gaz mettant en jeu des transferts de chaleur. La simulation de configurations où la prise en compte des effets de compressibilité de la phase gazeuse est indispensable (micropompes, microactionneurs, etc...) a nécessité l'utilisation d'un modèle original, considérant le liquide incompressible et le gaz compressible sous l'hypothèse faible Mach. Lors de cette thèse, ce modèle a été implémenté dans un code diphasique prenant en compte l'interface à l'aide d'une méthode de front-tracking. Des cas tests ont été développés spécifiquement afin de vérifier la conservation de l'énergie pour des configurations de complexité croissante. Les résultats des cas tests ont permis de mettre en évidence la difficulté à assurer la conservation de l'énergie lorsque l'interface n'est pas discontinue mais lissée, comme c'est le cas dans la méthode de front-tracking standard. Une méthode de traitement d'interface hybride a été proposée, rétablissant le caractère discontinu de l'interface avec la reconstruction d'une fonction indicatrice de phase échelon, tandis que le déplacement de l'interface est assuré d'un pas de temps à l'autre à l'aide du front-tracking. Les résultats obtenus avec cette nouvelle méthode hybride sont très satisfaisants, la méthode hybride permettant d'assurer la conservation de l'énergie et de la masse avec précision dans les simulations. / This thesis is devoted to the modeling and the numerical simulation of liquid-gas flows in non isothermal micro-cavities or micro-channels. The objective is to describe two-phase flows in which compressibility of the gaseous phase plays a key role (as for instance in micropumps, microactuators, etc...). An original model is developed, considering in the same computational domain, an incompressible liquid and a compressible gas under the low Mach approximation. This model has been implemented in a code using the front-tracking method for the interface description. In order to check the proper satisfaction of the energy balance, specific test cases have been developed considering several configurations of increasing complexity. It has been shown from these test cases that energy conservation can hardly be satisfied when the interface is described by the means of a smooth function, which is done in the front-tracking method. An hybrid method has been proposed, restoring the discontinous nature of the interface. It makes use of a step function combined with the front tracking method. Results obtained with this new hybrid method show that mass conservation and energy balance are very properly enforced during the computations. This thesis is devoted to the modelling and numerical simulation of liqui-gas flow envolving heat transfer. Simulation of configurations where it is essential to take into account the compressibility nature of the gaseous phase(for instance micropumps, microactuators, etc...) require the use of an original model, considering an incompressible liquid and a compressible gas under the low Mach assuption. During this thesis, the model has been implemented in a multiphase flow code using the front-tracking method to handle the interface. Test cases have been developped specifically to check the energy conservation for differents configurations of an increasing complexity. Numerical results highlighted difficulties encountered to ensure the energy conservation while using smooth description of the interface, as it is the case in the standard front-tracking method. An hybrid method has been proposed, restoring the discontinous character of the interface by reconstructing a step maker function, whereas the front displacement from a time step to the next, is still handled with the front-tracking. Results obtained using this new hybride method are very satisfactory, the hybrid method allowing the code to ensure accurately the energy and mass conservation during the computations.
80

Simulação computacional de escoamentos reativos com baixo número Mach aplicando técnicas de refinamento adaptativo de malhas / Computational simulation of low Mach number reacting flows applying adaptive mesh refinement techniques.

Calegari, Priscila Cardoso 12 June 2012 (has links)
O foco principal do presente trabalho é estender uma metodologia numérica embasada no uso de uma técnica de refinamento adaptativo de malha (AMR - Adaptive Mesh Refinement) e no uso de esquemas temporais multipasso implícitos-explícitos (IMEX) a aplicações envolvendo escoamentos reativos com baixo número de Mach. Originalmente desenvolvida para escoamentos incompressíveis, a formulação euleriana daquela metodologia emprega as equações de Navier-Stokes como modelo matemático para descrever a dinâmica do escoamento e o Método da Projeção, baseado no divergente nulo da velocidade do escoamento, para tratar o acoplamento pressão-velocidade presente na formulação com variáveis primitivas. Tal formulação euleriana original é estendida para acomodar novas equações agregadas ao modelo matemático da fase contínua: conservação de massa, fração de mistura (para representar as concentrações de combustível e oxidante), e energia. Além disso, uma equação termodinâmica de estado é integrada ao modelo matemático estendido e é empregada juntamente com a equação de conservação de massa para produzir uma nova restrição (não nula desta vez) ao divergente do campo de velocidade. Assume-se que o escoamento ocorre a baixo número de Mach (hipótese principal). O Método de Diferença Finita é empregado na discretização espacial das variáveis eulerianas de estado, empregando-se uma malha AMR. As vantagens e dificuldades desta extensão são cuidadosamente investigadas e reportadas. Pela importância, do ponto de vista de aplicações práticas, alguns estudos numéricos preliminares envolvendo escoamentos incompressíveis turbulentos com sprays são realizados (as gotículas compõem a fase dispersa). Num primeiro momento, apenas sprays com gotículas inertes são considerados. Embora ainda apenas iniciais, tais estudos já se mostram importantes pois identificam com clareza, em primeira instância, algumas das dificuldades inerentes a serem enfrentadas ao se tratar dentro desta nova metodologia um conjunto relativamente grande de gotículas lagrangianas. No caso de escoamentos incompressíveis turbulentos com sprays, a integração temporal se dá com métodos IMEX para a fase contínua e com o Método de Euler Modificado para a fase dispersa. A turbulência, em todos os casos que a envolvem, é tratada pelo modelo de Simulação das Grandes Escalas (LES - Large Eddy Simulation). As simulações computacionais se dão em um domínio tridimensional, um parelelepípedo, e empregam uma extensão (resultante do presente trabalho) do código AMR3D, um programa de computador sequencial implementado em Fortran90, oriundo de uma colaboração de longa data entre o IME-USP e o MFLab/FEMEC-UFU (Laboratório de Dinâmica de Fluidos da Universidade Federal de Uberlândia). O processamento foi efetuado no LabMAP (Laboratório da Matemática Aplicada do IME-USP). / It is the main goal of the present work to extend a numerical methodology based on both the use of an adaptive mesh refinement technique (AMR) and the use of a multistep, implicit-explicit time-step strategy (IMEX) to applications involving low Mach number reactive flows. Originally developed for incompressible flows, the Eulerian formulation of that methodology employs the Navier-Stokes equations to model the flow dynamics and the Projection Method, based on the vanishing divergence of the velocity field, to tackle the pressure-velocity coupling present when using primitive variables. That Eulerian formulation is extended by adding a new set of equations to the original mathematical model, describing the various properties of the continuous phase: mass conservation, mixture fraction (to represent concentrations of fuel and oxidizer) and energy. Also, a thermodynamic equation of state is included into the extended mathematical model which is employed, along with the equation for the conservation of mass, to derive a new restriction (this time, different from zero) to the divergence of the velocity field. It is assumed that one is dealing with a low Mach number flow (the main hipothesis). The discretization in space employs the Finite Difference Method for the Eulerian variables on a AMR mesh. Advantages and difficulties of such an extension of the previous methodology are carefully investigated and reported. For its importance in the real-world applications, few preliminary numerical studies involving incompressible turbulent flows with sprays are performed (the droplets form what it is called the dispersed phase). Only sprays formed by inert droplets are considered. Even though initial yet, such studies are most important because they clearly identify, first hand, certain difficulties in handling relatively large sets of Lagrangian droplets in the context of this new AMR methodology. In the context of turbulent incompressible flows with sprays, the overall time-step scheme is given by IMEX methods for the continuous phase and by the Improved Euler Method for the dispersed phase. In all the cases in which it is considered, turbulence is modeled by the Large Eddy Simulation (LES) model. The computational simulations are held in a tridimensional domain given by a paralellepiped and all of them employ the extention (resulting of the present work) of the AMR3D code, a sequencial computer program implemented in Fortran90, whose origin is the collaborative work between IMEUSP and MFLab/FEMEC-UFU (Fluid Dynamics Laboratory, Federal University of Uberlândia). Computations were performed at LabMAP (Applied Mathematics Laboratory at IME-USP).

Page generated in 0.0187 seconds