Homogenization of the Schrodinger equation with a time oscillating potential

We study the homogenization of a Schrodinger equation in a periodic medium with a time dependent potential. This is a model for semiconductors excited by an external electromagnetic wave. We prove that, for a suitable choice of oscillating (both in time and space) potential, one can partially transfer electrons from one Bloch band to another. This justifies the famous "Fermi golden rule" for the transition probability between two such states which is at the basis of various optical properties of semiconductors. Our method is based on a combination of classical homogenization techniques (two-scale convergence and suitable oscillating test functions) and of Bloch waves theory

Diffraction of Bloch Wave Packets for Maxwell's Equations

We study, for times of order 1/h, solutions of Maxwell's equations in an O(h^2) modulation of an h-periodic medium. The solutions are of slowly varying amplitude type built on Bloch plane waves with wavelength of order h. We construct accurate approximate solutions of three scale WKB type. The leading profile is both transported at the group velocity and dispersed by a Schr\"odinger equation given by the quadratic approximation of the Bloch dispersion relation. A weak ray average hypothesis guarantees stability. Compared to earlier work on scalar wave equations, the generator is no longer elliptic. Coercivity holds only on the complement of an infinite dimensional kernel. The system structure requires many innovations

Instability thresholds for flexible rotors in hydrodynamic bearings

Two types of fixed pad hydrodynamic bearings (multilobe and pressure dam) were considered. Optimum and nonoptimum geometric configurations were tested. The optimum geometric configurations were determined by using a theoretical analysis and then the bearings were constructed for a flexible rotor test rig. It was found that optimizing bearings using this technique produces a 100% or greater increase in rotor stability. It is shown that this increase in rotor stability is carried out in the absence of certain types of instability mechanisms such as aerodynamic crosscoupling. However, the increase in rotor stability should greatly improve rotating machinery performance in the presence of such forces as well

Aerodynamic stiffness of an unbound eccentric whirling centrifugal impeller with an infinite number of blades

An unbounded eccentric centrifugal impeller with an infinite number of log spiral blades undergoing synchronous whirling in an incompressible fluid is considered. The forces acting on it due to coriolis forces, centripetal forces, changes in linear momentum, changes in pressure due to rotating and changes in pressure due to changes in linear momentum are evaluated

Oil seal effects and subsynchronous vibrations in high-speed compressors

Oil seals are commonly used in high speed multistage compressors. If the oil seal ring becomes locked up against the fixed portion of the seal, high oil film crosscoupled stiffnesses can result. A method of analysis for determining if the oil seals are locked up or not is discussed. The method is then applied to an oil seal in a compressor with subsynchronous vibration problems

Le discours asynchrone d’une communauté d’apprentissage en réseau : source reconnue de l’action en classe par des stagiaires en enseignement

Les communautés d’apprenants en réseau, incluant celles en formation initiale à l’enseignement, offrent un contexte propice à la réflexion sur l’action. Cette étude descriptive documente en quoi le discours asynchrone élaboré collectivement est réinvesti par les stagiaires en enseignement lorsqu’ils sont de retour en classe. Les variables de l’intervention en classe favorisant l’apprentissage des élèves ont servi de cadre de référence. La méthodologie combine des analyses qualitatives et quantitatives. Les résultats indiquent que les futurs enseignants estiment que les idées partagées au sein de la communauté en réseau aiguillent leur action en classe dans une proportion appréciable. Networked learning communities, including those devoted to pre-service teacher training, are a good backdrop for reflection-on-action. This descriptive study documents what exactly, of collectively elaborated asynchronous discourse, is integrated into student teachers’ practices when they are back in the classroom. The variables of classroom intervention have served as a framework to this study. Our methodology combines both quantitative and qualitative analyses. Results show that student teachers say that the ideas shared in the networked community considerably influence their teaching practice

Un photographe chez les automatistes. Entretien avec Maurice Perron

Maurice Perron évoque ici pour la première fois les conditions liées à sa pratique de la photographie lors des différentes manifestations du groupe automatiste. Il explicite les principes qui le guidaient dans ses prises de vues, ses affinités avec l'esthétique automatiste, mais aussi le statut problématique que la photo, non reconnue comme art, avait au sein du groupe à l'époque.Maurice Perron evokes here for the first time the conditions of his photographic work at the time of the different activities of the automatist group. He explains the principles which guided him in his taking of photographs, his affinities with the automatist aesthetics, but also the problematical status of photography as an art, which was not recognized as such by the group at that time

The influence of collective asynchronous discourse elaborated online by pre-service teachers on their educational intervention in the classroom

Networked learning communities are growing and they offer new opportunities for reflection on practice in education. Many authors have studied the processes followed and the contents produced by such communities. On the other hand, few have observed how collective asynchronous discourse can be enacted in the classroom. This objective was pursued in the context of this exploratory research. Data was provided by asynchronous discourse written on the Knowledge Forum by five pre-service teachers during a 14-week practicum throughout the winter 2011 semester, as well as in situ observations from their university supervisor, while they were teaching students. Qualitative and descriptive quantitative analyses were conducted. In particular, results showed participants were highly concerned with quality of teaching when elaborating collectively in the electronic forum regarding ideas of interventions, and with the importance of properly handling the possible enactment of such ideas in the classroom while teaching. Les communautés d’apprentissage en réseau se multiplient et elles offrent de nouvelles avenues pour réfléchir aux pratiques en éducation. De nombreux auteurs étudient les processus utilisés et les contenus produits par ces communautés. Par contre, peu se sont penchés sur l’utilisation de discours collectifs asynchrones dans une classe. C’est l’objectif poursuivi dans cette recherche exploratoire. Les données provenaient de propos asynchrones rédigés sur le Knowledge Forum par cinq futurs enseignants au cours d’un stage de 14 semaines durant le semestre de l’hiver 2011 ainsi que d’observations in situ de leur superviseur universitaire. Des analyses quantitatives descriptives et qualitatives ont été effectuées. Les résultats ont montré notamment que les participants au forum électronique se préoccupaient au plus haut point, lors de leurs échanges sur des interventions possibles, de la qualité de l’enseignement et de l’importance de mettre en oeuvre, le cas échéant, ces idées en classe de façon adéquate