1,720,994 research outputs found
Asymptotic preserving numerical methods for moment models of plasma physics
No full textDans le cadre de la recherche pour la production d’énergie par fusion par confinement inertiel, le développement de nouvelles méthodes numériques est primordial. En effet, les simulations permettent non seulement d’affiner la compréhension des phénomènes physiques complexes ayant lieu sous les conditions thermodynamiques extrêmes nécessaires à la fusion, mais aussi d’aider à la conception de nouveaux dispositifs expérimentaux. Au sein des simulations numériques pour la fusion, le calcul du flux de chaleur des électrons est un enjeu central. Du fait des conditions extrêmes de la matière, la théorie usuelle de Spitzer-Härm devient insuffisante pour décrire la conduction thermique. Aussi pour restituer les effets cinétiques à moindre coût, des modèles aux moments à l’échelle mésoscopique sont utilisés à la place pour décrire le comportement des électrons. Ce manuscrit de thèse se concentre sur la résolution numérique de ces modèles aux moments ; tout d’abord dans le cadre simplifié du transport linéaire puis dans celui du transport électronique. La nature multi-échelle de ces modèles complique l’élaboration de schémas numériques, capables de correctement résoudre tous les régimes : pour ce faire, une classe particulière de méthodes, dites préservant l’asymptotique, a émergé. Une des plus récentes et des plus prometteuses est l’Unified Gas Kinetic Scheme (UGKS) : ce schéma volumes finis pour les équations cinétiques de type relaxation s’appuie sur la solution intégrale obtenue à partir de la méthode des caractéristiques. Dans ce manuscrit, une méthode est proposée pour obtenir des schémas cinétiques préservant l’asymptotique pour des modèles aux moments à partir d’UGKS. L’idée principale introduite est d’appliquer une fermeture à l’échelle numérique dans les flux d’UGKS. Dans les deux cas d’application étudiés, cette nouvelle méthode se révèle particulièrement performante et la poursuite de son développement semble pertinent dans l’objectif de résoudre des modèles physiques plus complexes. De plus, son caractère générique et sa flexibilité en font une alternative intéressante aux schémas usuels préservant l’asymptotique. Des premiers résultats de stabilité sont notamment démontrés sur ce nouveau schéma et une extension sur maillage non structuré est proposée.As part of the research on energy production through inertial confinement fusion, the development of new numerical methods is crucial. Indeed, simulations not only allow to refine the understanding of complex physical phenomena occurring under the extreme thermodynamic conditions necessary for fusion but also permit to assist in the design of new experimental devices. Within numerical simulations for fusion, the calculation of the electron heat flux is a central issue. Due to the extreme conditions of matter, the conventional Spitzer-Härm theory becomes insufficient to describe thermal conduction. Thus, to capture kinetic effects at a lower computational cost, moment models at the mesoscopic scale are used instead to describe the electron behaviour. This thesis manuscript focuses on the numerical resolution of these moment models, first in the simplified framework of linear transport and then in the context of electronic transport. The multi-scale nature of these models complicates the development of numerical schemes, which must accurately resolve all regimes : to do this, a particular class of methods, known as asymptoticpreserving methods, has emerged. One of the most recent and promising one is the Unified Gas Kinetic Scheme (UGKS) : this finite volume scheme for relaxation-type kinetic equations relies on the integral solution obtained from the method of characteristics. In this manuscript, a method is proposed to derive asymptotic-preserving kinetic schemes for moment models based on UGKS. The main idea which is introduced is to apply a closure at the numerical scale in the UGKS fluxes. In the two studied application cases, this new method reveals to be particularly effective and continuing its development seems relevant in order to solve more complex physical models. Moreover, its generic nature and flexibility make it a pertinent alternative to conventional asymptotic-preserving schemes. Moreover, some stability results are also demonstrated for this new scheme, and an extension to unstructured meshes is proposed
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Euler-Euler modelling of the interaction between a supersonic dilute flow and a detached shock
No full textLes travaux présentés dans cette thèse portent sur l’interaction d'un brouillard avec un choc en régime supersonique. L’approche eulérienne a été utilisée car elle est particulièrement adaptée aux machines fortement parallèles. Un module nommé SDFS (Supersonic Dilute Flow Solver), permettant de modéliser le comportement du brouillard, a été créé et validé au sein d’un code d’aérodynamique du CEA. Les travaux ont porté sur trois aspects de la modélisation numérique. Premièrement, l’étude et la création de nouveaux modèles. Deuxièmement, l’implémentation numérique et la validation sur des cas académiques de référence. Enfin, la confrontation entre les résultats numériques et des données expérimentales issues de la littérature.In this thesis we focus on the interaction between a supersonic dilute flow and a detached shock. The Eulerian approach has been chosen because it is particularly adapted to parallel computing. A program named SDFS (Supersonic Dilute Flow Solver), was created and validated in a CEA aerodynamic code. Three aspects of the computational simulation have been studied. First the study and creation of new models. Then the numerical implementation and the validation through academic reference cases. Finally, the comparison between numerical results and experimental data
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Contribution à la modélisation et à la simulation des écoulements complexes : application à la rentrée atmosphérique et aux interactions particules-fluide
No full textThis manuscript is devoted to the presentation of the different studies that I have done since 2003 and the beginning of my Phd at the French Atomic Agency (CEA). I will try to put in perspective all the things done since then and give guidelines of what I should explore later. The works presented here essentially deal with the physics of atmospheric reentry and fluid-particles interactions. For these two mains applications we focus on models, mathematical properties, numerical results and experimental results
Models and methods for complex flows: application to atmospheric reentry and particle / fluid interactions
No full textThis manuscript is devoted to the presentation of the different studies that I have done since 2003 and the beginning of my Phd at the French Atomic Agency (CEA). I will try to put in perspective all the things done since then and give guidelines of what I should explore later. The works presented here essentially deal with the physics of atmospheric reentry and fluid-particles interactions. For these two mains applications we focus on models, mathematical properties, numerical results and experimental results
Models and methods for complex flows: application to atmospheric reentry and particle / fluid interactions
No full textThis manuscript is devoted to the presentation of the different studies that I have done since 2003 and the beginning of my Phd at the French Atomic Agency (CEA). I will try to put in perspective all the things done since then and give guidelines of what I should explore later. The works presented here essentially deal with the physics of atmospheric reentry and fluid-particles interactions. For these two mains applications we focus on models, mathematical properties, numerical results and experimental results
Contribution à la modélisation et à la simulation des écoulements complexes : application à la rentrée atmosphérique et aux interactions particules-fluide
No full textThis manuscript is devoted to the presentation of the different studies that I have done since 2003 and the beginning of my Phd at the French Atomic Agency (CEA). I will try to put in perspective all the things done since then and give guidelines of what I should explore later. The works presented here essentially deal with the physics of atmospheric reentry and fluid-particles interactions. For these two mains applications we focus on models, mathematical properties, numerical results and experimental results
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