Multiscale phenomena implicitly appear in every physical model. The understanding of the general behavior of a given model at different scales and how one can correlate the behavior at two different scales is essential and can offer new important information. This thesis describes a series of new techniques and results in the analysis of multi-scale phenomena arising in PDEs on variable geometries. In the Second Chapter of the thesis, we present a series of new error estimate results for the periodic homogenization with nonsmooth coefficients. For the case of smooth coefficients, with the help of boundary layer correctors, error estimates results have been obtained by several authors (Oleinik, Lions, Vogelius, Allaire, Sarkis). Our results answer an open problem in the case of nonsmooth coefficients. Chapter 3 is focused on the homogenization of linear elliptic problems with variable nonsmooth coefficients and variable domains. Based on the periodic unfolding method proposed by Cioranescu, Damlamian and Griso in 2002, we propose a new technique for homogenization in perforated domains. With this new technique classical results are rediscovered in a new light and a series of new results are obtained. Also, among other advantages, the method helps one prove better corrector results. Chapter 4 is dedicated to the study of the limit behavior of a class of Steklov-type spectral problems on the Neumann sieve. This is equivalent with the limit analysis for the DtN-map spectrum on the sieve and has applications in the stability analysis of the earthquake nucleation phase model studied in Chapter 5. In Chapter 5, a $Gamma$-convergence result for a class of contact problems with a slip-weakening friction law, is described. These problems are associated with the modeling of the nucleation phase in earthquakes. Through the $Gamma$-limit we obtain an homogenous friction law as a good approximation for the local friction law and this helps us better understand the global behavior of the model, making use of the micro-scale information. As to our best knowledge, this is the first result proposing a homogenous friction law for this earthquake nucleation model.

In this thesis we investigate some partial differential equations with respect to G-convergence and homogenization. We study a few monotone parabolic equations that contain periodic oscillations on several scales, and also some linear elliptic and parabolic problems where there are no periodicity assumptions. To begin with, we examine parabolic equations with multiple scales regarding the existence and uniqueness of the solution, in view of the properties of some monotone operators. We then consider G-convergence for elliptic and parabolic operators and recall some results that guarantee the existence of a well-posed limit problem. Then we proceed with some classical homogenization techniques that allow an explicit characterization of the limit operator in periodic cases. In this context, we prove G-convergence and homogenization results for a monotone parabolic problem with oscillations on two scales in the space variable. Then we consider two-scale convergence and the homogenization method based on this notion, and also its generalization to multiple scales. This is further extended to the case that allows oscillations in space as well as in time. We prove homogenization results for a monotone parabolic problem with oscillations on two spatial scales and one temporal scale, and for a linear parabolic problem where oscillations occur on one scale in space and two scales in time. Finally, we study some linear elliptic and parabolic problems where no periodicity assumptions are made and where the coefficients are created by certain integral operators. Here we prove results concerning when the G-limit may be obtained immediately and is equal to a certain weak limit of the sequence of coefficients.

HOMOGENEISATION NUMERIQUE DE PARAMETRES PETROPHYSIQUES POUR DES MAILLAGES DESTRUCTURES EN SIMULATION DE RESERVOIR : Dans cette thèse on s?intéresse aux méthodes d?homogénéisations qui nécessitent la résolution de problèmes locaux soumis à des conditions aux limites. Dans la première partie on étudie une méthode avec des conditions aux limites linéaires applicables sur des maillages déstructurés. On montre qu?en utilisant une méthode d?éléments finis mixtes [resp. conformes] on obtient une approximation numérique inférieure [resp. supérieure] du tenseur équivalent. On démontre également un résultat de stabilité pour la G-convergence parabolique. Des approximations numériques du tenseur équivalent calculées sur des domaines 2D et 3D déstructurés sont données ainsi que des simulations d?écoulements monophasiques et diphasiques permettant de valider la méthode. Dans la deuxième partie on définit une nouvelle méthode de mise à l?échelle permettant de prendre en compte des conditions aux limites quelconques appliquées aux problèmes locaux. Sur des maillages déstructurés le tenseur équivalent est déterminé en minimisant l?écart des énergies dissipées (ou. des vitesses moyennes) locales et globales. On obtient, en utilisant les techniques du contrôle optimal, un algorithme de calcul effectif qui permet de retrouver, pour les conditions aux limites classiques, les résultats bien connus. Des résultats de convergence et des estimations d?erreurs sont établis. On montre enfin que cette méthode est stable pour la G-convergence et quelques essais numériques 2D sont présentés.

This thesis mainly deals with questions concerning the convergence of some sequences of elliptic and parabolic linear and non-linear operators by means of G-convergence and homogenization. In particular, we study operators with oscillations in several spatial and temporal scales. Our main tools are multiscale techniques, developed from the method of two-scale convergence and adapted to the problems studied. For certain classes of parabolic equations we distinguish different cases of homogenization for different relations between the frequencies of oscillations in space and time by means of different sets of local problems. The features and fundamental character of two-scale convergence are discussed and some of its key properties are investigated. Moreover, results are presented concerning cases when the G-limit can be identified for some linear elliptic and parabolic problems where no periodicity assumptions are made.

<p>The present thesis is devoted to the homogenization of certain elliptic and parabolic partial differential equations by means of appropriate generalizations of the notion of two-scale convergence. Since homogenization is defined in terms of H-convergence, we desire to find the H-limits of sequences of periodic monotone parabolic operators with two spatial scales and an arbitrary number of temporal scales and the H-limits of sequences of two-dimensional possibly non-periodic linear elliptic operators by utilizing the theories for evolution-multiscale convergence and λ-scale convergence, respectively, which are generalizations of the classical two-scale convergence mode and custom-made to treat homogenization problems of the prescribed kinds. Concerning the multiscaled parabolic problems, we find that the result of the homogenization depends on the behavior of the temporal scale functions. The temporal scale functions considered in the thesis may, in the sense explained in the text, be slow or rapid and in resonance or not in resonance with respect to the spatial scale function. The homogenization for the possibly non-periodic elliptic problems gives the same result as for the corresponding periodic problems but with the exception that the local gradient operator is everywhere substituted by a differential operator consisting of a product of the local gradient operator and matrix describing the geometry and which depends, effectively, parametrically on the global variable.</p>

The present thesis is devoted to the homogenization of certain elliptic and parabolic partial differential equations by means of appropriate generalizations of the notion of two-scale convergence. Since homogenization is defined in terms of H-convergence, we desire to find the H-limits of sequences of periodic monotone parabolic operators with two spatial scales and an arbitrary number of temporal scales and the H-limits of sequences of two-dimensional possibly non-periodic linear elliptic operators by utilizing the theories for evolution-multiscale convergence and λ-scale convergence, respectively, which are generalizations of the classical two-scale convergence mode and custom-made to treat homogenization problems of the prescribed kinds. Concerning the multiscaled parabolic problems, we find that the result of the homogenization depends on the behavior of the temporal scale functions. The temporal scale functions considered in the thesis may, in the sense explained in the text, be slow or rapid and in resonance or not in resonance with respect to the spatial scale function. The homogenization for the possibly non-periodic elliptic problems gives the same result as for the corresponding periodic problems but with the exception that the local gradient operator is everywhere substituted by a differential operator consisting of a product of the local gradient operator and matrix describing the geometry and which depends, effectively, parametrically on the global variable.