68 research outputs found
Simultaneous double transformations of functions depending on space and time
It is shown that performing simultaneously two transformations on functions
of space and time (for instance a Fourier transform on the space variable and a
Laplace transform on the time variable) can be easier than performing them one
after the other when the variables are combined in invariant quantities. This
is naturally also true when performing two inverse transforms simultaneously,
when the conjugated variables are combined into a propagator. An immediate
application is found in the computation of the solutions of partial
differential equations. This article contains several general examples of such
"simultaneous double transforms" for arbitrary analytic functions of space and
time.Comment: 8 pages, 1 table of simultaneous double transformation pair
Local time in diffusive media and applications to imaging
Local time is the measure of how much time a random walk has visited a given
position. In multiple scattering media, where waves are diffuse, local time
measures the sensitivity of the waves to the local medium's properties. Local
variations of absorption, velocity and scattering between two measurements
yield variations in the wave field. These variations are proportionnal to the
local time of the volume where the change happened and the amplitude of
variation. The wave field variations are measured using correlations and can be
used as input in a inversion algorithm to produce variation maps. The present
article gives the expression of the local time in dimensions one, two and three
and an expression of its fluctuations, in order to perform such inversions and
estimate their accuracy.Comment: 10 pages, 2 figures and 3 table
A general framework for multiple scattering of polarized waves including anisotropies and Berry phase
We develop a framework for the multiple scattering of a polarized wave. We
consider particles with spin propagating in a medium filled with scatterers. We
write the amplitudes of each spin eigenstate in a local, mobile frame. One of
the axes is in the direction of propagation of the particle. We use this
representation to define a directional Green's operator of the homogeneous
medium and also to write the spin-dependent scattering amplitudes. We show that
this representation reveals a Berry phase. We establish a generalized
Green-Dyson equation for the multiple scattering problem in this framework. We
show that the generalized Green-Dyson equation can be solved by linear algebra
if one uses a representation of the rotations based on Wigner D-matrices. The
properties of light scattering are retrieved if we use spin 1 particles. Our
theory allows to take into account several kinds of anisotropies like circular
or linear dichroism and birefringence, Faraday effects andMie scattering within
the same formalism. Several anisotropies can be present at the same time.Comment: 32 pages, 4 figure
Spatiotemporal velocity-velocity correlation function in fully developed turbulence
Turbulence is an ubiquitous phenomenon in natural and industrial flows. Since
the celebrated work of Kolmogorov in 1941, understanding the statistical
properties of fully developed turbulence has remained a major quest. In
particular, deriving the properties of turbulent flows from a mesoscopic
description, that is from Navier-Stokes equation, has eluded most theoretical
attempts. Here, we provide a theoretical prediction for the {\it space and
time} dependent velocity-velocity correlation function of homogeneous and
isotropic turbulence from the field theory associated to Navier-Stokes equation
with stochastic forcing. This prediction is the analytical fixed-point solution
of Non-Perturbative Renormalisation Group flow equations, which are exact in a
certain large wave-number limit. This solution is compared to two-point
two-times correlation functions computed in direct numerical simulations. We
obtain a remarkable agreement both in the inertial and in the dissipative
ranges.Comment: 8 pages, 4 figures, improved versio
Observation of a non-adiabatic geometric phase for elastic waves
We report the experimental observation of a geometric phase for elastic waves
in a waveguide with helical shape. The setup reproduces the experiment by
Tomita and Chiao [A. Tomita, R.Y. Chiao, Phys. Rev. Lett. 57 (1986) 937-940,
2471] that showed first evidence of a Berry phase, a geometric phase for
adiabatic time evolution, in optics. Experimental evidence of a non-adiabatic
geometric phase has been reported in quantum mechanics. We have performed an
experiment to observe the polarization transport of classical elastic waves. In
a waveguide, these waves are polarized and dispersive. Whereas the wavelength
is of the same order of magnitude as the helix's radius, no frequency dependent
correction is necessary to account for the theoretical prediction. This shows
that in this regime, the geometric phase results directly from geometry and not
from a correction to an adiabatic phase.Comment: 13 figure
Locating a weak change using diffuse waves (LOCADIFF) : theoretical approach and inversion procedure
We describe a time-resolved monitoring technique for heterogeneous media. Our
approach is based on the spatial variations of the cross-coherence of coda
waveforms acquired at fixed positions but at different dates. To locate and
characterize a weak change that occurred between successive acquisitions, we
use a maximum likelihood approach combined with a diffusive propagation model.
We illustrate this technique, called LOCADIFF, with numerical simulations. In
several illustrative examples, we show that the change can be located with a
precision of a few wavelengths and its effective scattering cross-section can
be retrieved. The precision of the method depending on the number of source
receiver pairs, time window in the coda, and errors in the propagation model is
investigated. Limits of applications of the technique to real-world experiments
are discussed.Comment: 11 pages, 14 figures, 1 tabl
Imagerie de chargements locaux en régime de diffusion multiple
Dans les milieux fortement hétérogènes, les ondes ne se propagent pas de façon balistique. Elles peuvent interagir de nombreuses fois avec les hétérogénéités du milieu et entrer ainsi dans le régime de diffusion multiple. Dans ce régime, les méthodes classiques d'imagerie basées sur les trajets des ondes directes ou simplement diffusées sont inefficaces. Les formes d'ondes multiplement diffusées (coda) sont trop complexes pour être modélisées exactement mais elles sont parfaitement reproductibles et très sensibles aux variations du milieu de propagation. Des travaux récents ont démontré la possibilité de mesurer de faibles changements de vitesse d'un milieu grâce aux ondes diffuses, à l'échelle de la croûte terrestre (coda sismique) comme à l'échelle des matériaux (coda ultrasonore). Ces travaux s'intéressent majoritairement à des changements globaux ou régionaux des différents milieux. La problématique de cette thèse concerne la possibilité d'utiliser la coda pour étudier des changements locaux du milieu. Deux études complémentaires sont développées : Le problème direct consiste à modéliser les variations de la coda engendrées par un changement local. Nous distinguons le cas d'un changement local de structure (fort contraste d'impédance) du cas d'un changement local de vitesse (faible contraste d'impédance). Le problème inverse consiste à utiliser les mesures de variations de la coda pour tenter de localiser et de caractériser les changements survenus. Les applications potentielles de ces travaux concernent entre autres le suivi temporel de structures géologiques ainsi que le contrôle non destructif de matériaux hétérogènes. Avec cet objectif, les différentes méthodes développées sont illustrées par des simulations numériques d'ondes acoustiques et sismiques ainsi que par des expériences en ultrasons dans des éléments en béton.In highly heterogeneous media, waves don't propagate ballistically. They can interact several times with the heterogeneities of the medium and enter the multiple scattering regime. In this regime, classical imaging techniques, based on direct or singly scattered waves fail. Multiply scattered waveforms (coda) are too complex for being exactly modeled but are perfectly reproducible and very sensitive to small variations of the medium. Recent works demonstrated the possibility of measuring small velocity variations with diffuse waves, either at the geophysical scale (seismic coda) or at the material scale (ultrasonic coda). These works are mainly focused in monitoring global or regional changes of the medium. The present thesis deals with the possibility of using coda waves to study local changes of the medium. Two complementary studies are developed: The forward problem addresses the modeling of the coda variations generated by a local change. We distinguish the case of a structural change (strong impedance contrast) from the case of a velocity change (small impedance contrast). The inverse problem consists in using the coda variations measurements to locate and characterize the changes that occurred. Potential applications may concern, among others, monitoring of geological structures and non-destructive testing of heterogeneous materials. This in mind, we illustrate the different studies with numerical simulations of acoustic and seismic waves and with ultrasound experiments in concrete blocks.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF
Permanent education in health: a review
OBJECTIVE : To undertake a meta-synthesis of the literature on the main concepts and practices related to permanent education in health. METHODS : A bibliographical search was conducted for original articles in the PubMed, Web of Science, LILACS, IBECS and SciELO databases, using the following search terms: “public health professional education”, “permanent education”, “continuing education”, “permanent education health”. Of the 590 articles identified, after applying inclusion and exclusion criteria, 48 were selected for further analysis, grouped according to the criteria of key elements, and then underwent meta-synthesis. RESULTS : The 48 original publications were classified according to four thematic units of key elements: 1) concepts, 2) strategies and difficulties, 3) public policies and 4) educational institutions. Three main conceptions of permanent education in health were found: problem-focused and team work, directly related to continuing education and education that takes place throughout life. The main strategies for executing permanent education in health are discussion, maintaining an open space for permanent education, and permanent education clusters. The most limiting factor is mainly related to directly or indirect management. Another highlight is the requirement for implementation and maintenance of public policies, and the availability of financial and human resources. The educational institutions need to combine education and service aiming to form critical-reflexive graduates. CONCLUSIONS : The coordination between health and education is based as much on the actions of health services as on management and educational institutions. Thus, it becomes a challenge to implement the teaching-learning processes that are supported by critical-reflexive actions. It is necessary to carry out proposals for permanent education in health involving the participation of health professionals, teachers and educational institutions.OBJETIVO : Realizar metasíntesis de la literatura sobre los principales conceptos y prácticas relacionados con la educación permanente en salud. MÉTODOS : Se realizó búsqueda bibliográfica de artículos originales en las bases de datos PubMed, Web of Science, Lilacs, IBECS y SciELO, utilizando los siguientes descriptores: “ public health professional education”, “permanent education”, “continuing education”, “permanent education health ”. De un total de 590 artículos identificados, posterior a los criterios de inclusión y exclusión, fueron seleccionados 48 para análisis, los cuales fueron sometidos al análisis individual, análisis comparativo, análisis con criterios de agrupamiento de elementos-clave y sometidos a metasíntesis. RESULTADOS : Los 48 artículos originales fueron clasificados como elementos-clave en cuatro unidades temáticas: 1) Concepciones; 2) Estrategias y dificultades; 3) Políticas públicas e 4) Instituciones formadoras. Se encontraron tres concepciones principales de educación permanente en salud: ubicación del problema y enfocarlo en el trabajo en equipo, directamente relacionado con la educación continua y educación que se da a lo largo de la vida. Las principales estrategias para efectivar la educación permanente fueron la ubicación del problema, mantenimiento de espacios para la educación permanente y polos de educación permanente. El mayor factor limitante estuvo relacionado con la gerencia directa o indirecta. Fueron mencionadas la necesidad de implementación y mantenimiento de políticas públicas, así como la disponibilidad de recursos financieros y humanos. Las instituciones formadoras tendrían la necesidad de articular educación y servicio para la formación de egresados críticos-reflexivos. CONCLUSIONES : La articulación educación y salud se encuentra pautada tanto en las acciones de los servicios de salud, cuanto en la gestión y de instituciones formadoras. Así, se torna un desafío implementar procesos de educación-aprendizaje que sean respaldados por acciones crítico-reflexivas. Es necesario realizar propuestas de educación permanente en salud con la participación de profesionales de los servicios, profesores y profesionales de las instituciones de educación.OBJETIVO : Realizar metassíntese da literatura sobre os principais conceitos e práticas relacionados à educação permanente em saúde. MÉTODOS : Foi realizada busca bibliográfica de artigos originais nas bases de dados PubMed, Web of Science, Lilacs, IBECS e SciELO, utilizando os seguintes descritores: “ public health professional education ”, “ permanent education”, “continuing education ”, “ permanent education health ”. De um total de 590 artigos identificados, após os critérios de inclusão e exclusão, foram selecionados 48 para análise, os quais foram submetidos à análise individual, análise comparativa, análise com critérios de agrupamentos de elementos-chave e submetidos à metassíntese. RESULTADOS : Os 48 artigos originais foram classificados como elementos-chave em quatro unidades temáticas: 1) Concepções; 2) Estratégias e dificuldades; 3) Políticas públicas; e 4) Instituições formadoras. Foram encontradas três concepções principais de educação permanente em saúde: problematizadora e focada no trabalho em equipe, diretamente relacionada à educação continuada e educação que se dá ao longo da vida. As principais estratégias para efetivação da educação permanente foram a problematização, manutenção de espaços para a educação permanente e polos de educação permanente. O maior fator limitante foi relacionado à gerência direta ou indireta. Foram indicadas a necessidade de implementação e manutenção de políticas públicas, além de disponibilidade de recursos financeiros e de recursos humanos. As instituições formadoras teriam necessidade de articular ensino e serviço para a formação de egressos críticos-reflexivos. CONCLUSÕES : A articulação educação e saúde encontra-se pautada tanto nas ações dos serviços de saúde, quanto de gestão e de instituições formadoras. Assim, torna-se um desafio implementar processos de ensino-aprendizagem que sejam respaldados por ações crítico-reflexivas. É necessário realizar propostas de educação permanente em saúde com a participação de profissionais dos serviços, professores e profissionais das instituições de ensino
On the Achievable Rates for the Return-Link of Multi-Beam Satellite Systems Using Successive Interference Cancellation
This paper investigates the theoretically reachable rates in the return-link of multi-user multi-beam satellite systems. The return-link can actually be analyzed by means of multiple-input multiple-output signal processing techniques. While MIMO inspired multi-user detection for the return-link of satellite systems has been partly investigated in the past, a fundamental look at the achievable gains by these advanced signal processing techniques for satellite communications is missing. Depending on the number of colors used in the system, the available bandwidth per beam can be traded off against the level of co-channel interference. This paper explores the fundamental limits dictated by information theory to the communication rates on satellite multi-beam scenarios, and the impact of a realistic beam patterns on these data rates
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