One approach to examining the stability of a fluid flow is to linearize the evolution equation at an equilibrium and determine (if possible) the stability of the resulting linear evolution equation. In this dissertation, the space of perturbations of the equilibrium flow is split into two classes and growth of the linear evolution operator on each class is analyzed. Our classification of perturbations is most naturally described in V.I. Arnold’s geometric view of fluid dynamics. The first class of perturbations we examine are those that preserve the topology of vortex lines and the second class is the factor space corresponding to the first class. In this dissertation we establish lower bounds for the essential spectral radius of the linear evolution operator restricted to each class of perturbations. / text

Cette thèse présente des travaux originaux dans le domaine des méthodes de moyennisation d'ordre élevé. On s'intéresse notamment à des procédures de moyennisation dite stroboscopique ou quasi-stroboscopique dans des espaces de Banach ou de Hilbert. Ces procédures sont ensuite appliquées à des exemples concrets: des équations d'évolutions hautement oscillantes. Plus précisément, on montre dans un premier temps un résultat de moyennisation stroboscopique dans un espace de Banach où l'on obtient des estimations d'erreurs exponentielles. Ce théorème est ensuite appliqué sur deux équations des ondes semi-linéaire hautement oscillantes. On montre également que la Stroboscopic Averaging Method s'applique à une équation des ondes semi-linéaire avec conditions de Dirichlet. On trouve enfin numériquement, une dynamique intéressante de l'équation des ondes semi-linéaire mise en lumière par la procédure de moyennisation. Dans un second temps, on présente un théorème de moyennisation quasi-stroboscopique dans un espace de Hilbert quelconque avec des estimations d'erreurs exponentielles. Ce théorème est alors appliqué de façon indirecte à une équation de Schrödinger semi-linéaire oscillante. Cette équation est d'abord projeté dans un espace de dimension finie pour qu'on puisse lui appliquer le théorème de moyennisation quasi-stroboscopique. On écrit alors un résultat de moyennisation quasi-stroboscopique pour l'équation de Schrödinger semi-linéaire avec des estimations d'erreur polynomiales. / This thesis presents some original work in the field of high order averaging procedure. In particular, we are interested in stroboscopic and quasi-stroboscopic averaging procedure in abstract Banach or Hilbert spaces. This procedures is applied to concrete examples: some highly oscillatory evolution equations. More precisely, we first show a theorem of stroboscopic averaging in a Banach space where we obtain exponential error estimates. This theorem is then applied on two semi-linear and highly oscillatory wave equations. We also put in evidence that the {\it Stroboscopic Averaging Method} works fine with a semi-linear wave equation with Dirichlet conditions. Finally, the averaging procedure puts in evidence, numerically, an interesting dynamics regarding the semi-linear wave equation with Dirichlet conditions. In a second part, we present a quasi-stroboscopic averaging theorem in a Hilbert space with exponential error estimates. This theorem is applied on a semi-linear Schrödinger equation. This equation has first, to be project in a finite dimensional space in order to fit in the hypotheses of the theorem. We then write a quasi-stroboscopic averaging theorem for a semi-linear Schrödinger equation with polynomial error estimates.

Abstract in English / In this dissertation we apply linear evolution equations to the Newtonian derivative, Caputo time fractional derivative and $-time fractional derivative. It is notable that the most utilized fractional order derivatives for modelling true life challenges are Riemann- Liouville and Caputo fractional derivatives, however these fractional derivatives have the same weakness of not satisfying the chain rule, which is one of the most important elements of the match asymptotic method [2, 3, 16]. Furthermore the classical bounded perturbation theorem associated with Riemann-Liouville and Caputo fractional derivatives has con rmed not to be in general truthful for these models, particularly for solution operators of evolution systems of a derivative with fractional parameter ' that is less than one (0 < ' < 1) [29]. To solve this problem, we introduce the derivative with new parameter, which is de ned as a local derivative but has a fractional order called $-derivative and apply this derivative to linear evolution equation and to support what we have done in the theory, we utilize application to population dynamics and we provide the numerical simulations for particular cases. / Mathematical Sciences / M.Sc. (Applied Mathematics)