Further development of Level Set method (modified level set equation and its numerical assessment)

Pas de résuméThe level set method was introduced by Osher & Sethian (1988) as a general technique to capture moving interfaces. It has been used to study crystal growth, to simulate water and fire for computer graphics applications, to study two-phase flows and in many other fields. The wellknown problem of the level set method is the following: if the flow velocity is not constant, the level set scalar may become strongly distorted. Thus, the numerical integration may suffer from loss of accuracy. In level set methods, this problem is remedied by the reinitialization procedure, i.e. by reconstruction of the level set function in a way to satisfy the eikonal equation. We propose an alternative approach. We modify directly the level set equation by embedding a source term. The exact expression of this term is such that the eikonal equation is automatically satisfied. Furthermore on the interface, this term is equal to zero. In the meantime, the advantage of our approach is this: the exact expression of the source term allows for the possibility of derivation of its local approximate forms, of first-and-higher order accuracy. Compared to the extension velocity method, this may open the simplifications in realization of level set methods. Compared to the standard approach with the reinitialization procedure, this may give the economies in the number of level set re-initializations, and also, due to reduced number of reinitializations, one may expect an improvement in resolution of zero-set level. Hence, the objective of the present dissertation is to describe and to assess this approach in different test cases.LYON-Ecole Centrale (690812301) / SudocSudocFranceF

An adaptive mesh refinement method for solution of the transported PDF equation

This paper presents the results of an investigation into a possible alternative to Monte Carlo methods for solving the transported probability density function (PDF) equation for scalars (compositions). The method uses a finite‐volume approach combined with adaptive mesh refinement (AMR) in a multi‐dimensional compositional space. Comparisons are made between the new method and Monte Carlo solutions for analytical test cases involving the reaction of two or three chemical species. These tests demonstrate the potential of the new method in terms of both accuracy and run time. Additional test cases involving various models for molecular mixing were also conducted with similar conclusions

Mélange et combustion dans les petits volumes (application au micro systèmes énergétiques)

Depuis les années 90, beaucoup de laboratoires, particulièrement aux Etats-Unis et en Asie, travaillent sur divers concepts de micro-moteurs utilisant la technologie de la microélectronique. Parmi eux, le concept de la micro-turbine semble être le plus prometteur, en particulier pour l'application aux micro-drones (<15 cm). L'étude présentée dans ce mémoire propose une vue d'ensemble des difficultés liées à la combustion dans les petits volumes (~ 500 mm3). Nous cherchons à concevoir une micro-chambre favorisant une combustion stable et intense, à l'aide des divers outils numériques et expérimentaux. Dans cette étude, nous nous concentrons sur la combustion et le mélange dans les chambres de faibles dimensions. Dans une première approche, les gammes de combustion stable ainsi que les paramètres les plus influents sur l'efficacité de la combustion ont été déterminées, à l'aide des modèles 0D de réacteur parfaitement mélangé et partiellement mélangé. Le mélange entre l'air et le carburant à l'amont de l'injection dans la micro-chambre de combustion, a été caractérisé expérimentalement en utilisant la méthode de Fluorescence Induite par Laser (PLIF). Un jet de carburant est injecté perpendiculairement à un écoulement d'air dans un petit canal (hauteur 1,3 mm). L'étude paramétrique, entreprise expérimentalement, fournit une large base de données, utile pour définir les conditions d'entrée des calculs 3D de la combustion dans les micro-chambres. Des simulations numériques 3D de la combustion, dans des géométries réalistes de chambre, ont été effectuées. Les résultats de cette étude ont permis de préparer les essais dans le nouveau laboratoire de l'Onera consacré à l'étude de la combustion dans les petits volumes. Nous avons également vu que, à ces petites dimensions, le mélange turbulent est faible et le micromélange est un paramètre très important pour la stabilité et l'efficacité de la combustion. Le modèle de PaSR a été utilisé pour comparer et choisir un modèle de micromélange parmi trois. Ce modèle de micromélange sera intégrer dans un code de CFD pour affiner la modélisation 3D de la combustion dans les micro chambres.The study presented in this memory proposes an overview of difficulties related to combustion in small volumes. We seek to design a micro combustor supporting a stable and intensive combustion using the various numerical and experimental tools. In this study, we focus on the combustion and mixing in micro combustors. In a first approach, the ranges of stable combustion were determined as well as the most influential parameters on the combustion efficiency using OD models of Perfectly Stirred Reactor and Partially Stirred Reactor.The mixing between air and fuel upstream of the injection in the micro combustor was characterised experimentally using the PLIF method. A jet of fuel was injected perpendicularly to a flow of air in a small channel (1.3 mm height). The parametric study undertaken experimentally provides an extensive data base useful to define the inlet conditions in 3D calculations of micro combustors. Moreover, 3D numerical simulations of combustion in realistic geometries of micro combustors were carried out. The results of this study will allow to prepare trial runs in the ONERA new laboratory dedicated to the study of combustion in small volumes. We have also seen that, at these small dimensions, the turbulent mixing is weak and the micromixing is a very important parameter for the stability and the combustion efficiency. The model of PaSR was used to compare and to choose a micromixing model among three, in order to integrate it in a CFD code and to refine the modeling of combustion in micro combustors.POITIERS-BU Sciences (861942102) / SudocSudocFranceF