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Aprendizagem ativa em ótica geométrica: experimentos e demonstrações investigativas / Active learning in geometrical optics: experiments and interactive demonstrations.Edson Valentim Roberto 08 June 2009 (has links)
Atualmente existe, praticamente, um consenso entre os pesquisadores de Ensino de Física, de que a metodologia tradicional não é a mais efetiva por não promover um real engajamento dos estudantes durante o processo de ensino/aprendizagem. Muitos pesquisadores denominam Aprendizagem Ativa (AA), os métodos que requerem tal engajamento e as pesquisas indicam que esta abordagem é mais efetiva. Ou seja, verifica-se um aumento da compreensão dos conceitos básicos de Física. Entretanto, ainda há pouca aplicação de novas metodologias em sala de aula, tanto no ensino médio quanto no superior. No nosso entender, as atividades experimentais (de laboratório ou demonstração) são as mais adequadas para a aplicação de metodologias de AA. Entretanto, particularmente no Brasil, existe pouco material didático experimental disponível que utilize essa metodologia. Este trabalho visa sanar tal necessidade no campo da Ótica Geométrica, oferecendo uma série de roteiros comentados para experimentos e demonstrações interativas. Estes roteiros usam a idéia de investigação orientada onde se pretende que os conhecimentos prévios dos alunos sejam expostos e confrontados com os conceitos científicos. Este material foi aplicado a estudantes do ensino médio (escolas públicas) e do curso de Licenciatura em Ciências Exatas do IFSC. Para a avaliação do mesmo foram realizados testes escritos e monitoramentos em sala de aula. / There is nowadays consensus among the physics education researchers that the traditional teaching methods are not the most effective in promoting real engagement of the students in the teaching/learning process. Several researchers named Active Learning (AL), methods that require this engagement and research evidences indicate that this approach is the most effective, a better comprehension of the basic physics concepts is verified. However, particularly in Brazil, there are few didactic materials available using AL approaches. This works aims to fulfill this need in the field of Geometrical Optics, offering a series of tutorials (guides) for experiments and interactive demonstrations. These tutorials are based in the idea of guided investigation where the students previous beliefs are exposed and confronted with the scientific concepts. This material was applied to high school students (in public schools) and college students (science teacher preparation course of IFSC). In order to evaluate this material, written tests were performed as well as observation of classrooms.
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Exploring bistatic scattering modeling for land surface applications using radio spectrum recycling in the Signal of Opportunity Coherent Bistatic SimulatorBoyd, Dylan R. 08 August 2023 (has links) (PDF)
The potential for high spatio-temporal resolution microwave measurements has urged the adoption of the signals of opportunity (SoOp) passive radar technique for use in remote sensing. Recent trends in particular target highly complex remote sensing problems such as root-zone soil moisture and snow water equivalent. This dissertation explores the continued open-sourcing of the SoOp coherent bistatic scattering model (SCoBi) and its use in soil moisture sensing applications. Starting from ground-based applications, the feasibility of root-zone soil moisture remote sensing is assessed using available SoOp resources below L-band. A modularized, spaceborne model is then developed to simulate land-surface scattering and delay-Doppler maps over the available spectrum of SoOp resources. The simulation tools are intended to provide insights for future spaceborne modeling pursuits.
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Predicting the effects of shadowing and scattering from planar surfaces on localizer course structure using geometrical optics and the uniform theory of diffraction as implemented in the near zone basic scattering codeRushton, Elliott C. January 1994 (has links)
No description available.
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[en] SYNTHESIS AND RIGOROUS ANALYSIS OF OMNIDIRECTIONAL DUAL-REFLECTOR ANTENNAS: THE CASE OF THE MAIN REFLECTOR WITH CIRCULAR GENERATRIX / [pt] SÍNTESE E ANÁLISE RIGOROSA DE ANTENAS OMNIDIRECIONAIS DE DUPLO-REFLETORES: O CASO DO REFLETOR PRINCIPAL COM GERATRIZ CIRCULARSANDRO ROGERIO ZANG 29 November 2012 (has links)
[pt] Este trabalho aborda o projeto de antenas omnidirecionais de duplorefletores,
onde o refletor principal é obtido a partir de uma geratriz circular. A
antena é composta de refletores circularmente simétricos e concêntricos que são
alimentados por uma corneta cônica coaxial excitada pelo modo TEM para a
produção de polarização vertical. Para realizar este estudo são utilizadas
técnicas de síntese e de análise eletromagnética rigorosa. A técnica de análise
eletromagnética rigorosa é baseada na associação do Método de Casamento de
Modos para representar os campos no interior do alimentador e do Método dos
Momentos para solucionar as equações integrais para o campo elétrico e para o
campo magnético e determinar a corrente elétrica induzidas sobre as paredes
metálicas externas e a amplitude dos modos sobre a abertura da corneta. A
técnica de síntese destas antenas é baseada na aplicação dos princípios da
Ótica Geométrica para modelar o subrefletor que irá produzir uma distribuição de
fase uniforme em uma abertura cônica colocada em frente ao refletor principal
com geratriz circular. O estudo exploratório sobre o desempenho destas antenas
é dividido em três partes. Na primeira, as soluções fornecidas pela síntese ótica
e aproximações para os campos na abertura são utilizadas para identificar
configurações de antenas compactas ou que maximizem o ganho ao longo do
plano horizontal. Entretanto, pode-se obter uma melhor iluminação da área de
cobertura através da inclinação do lobo principal (down tilt). Para reduzir o custo
de fabricação de um conjunto de antenas que atendam diferentes áreas de
cobertura, pode-se utilizar o mesmo refletor principal e modelar os subrefletores para deslocar a direção de máximo do diagrama. Assim, em uma segunda etapa
do estudo paramétrico, são considerados alguns refletores principais, e, para
cada um deles, é obtida uma família de subrefletores modelados para deslocar o
máximo do diagrama no plano vertical. Esta estratégia se torna efetiva na
medida em que o custo de fabricação do subrefletor é menor do que o do refletor
principal. Na terceira parte, utilizando o método híbrido composto pelo Método do
Casamento de Modos e pelo Método dos Momentos, é feito o modelamento da
corneta coaxial TEM com o objetivo de reduzir sua perda de retorno e estender
sua banda de operação. Por fim, ainda utilizando este método híbrido, é feita
uma análise rigorosa das antenas duplo-refletoras propostas neste trabalho,
procurando otimizar o desempenho destas antenas em termos do diagrama de
radiação e da perda de retorno. / [en] This work presents a design study of omnidirectional dual-reflector
antennas, where the main reflector is obtained from a circular generatrix. The
antenna is composed of two concentric circularly symmetric reflectors and it is
fed by a coaxial conical horn excited by TEM mode to produce vertical
polarization. To obtain the subreflector surface, the study employs a GO
synthesis technique to shape the subreflector in order to produce a uniform
phase distribution in a conical aperture placed in front of the main reflector. To
validate the results, it is employed a rigorous electromagnetic analysis technique
based on the association of Mode Matching Technique to represent the fields
inside the horn and Method of Moments to solve the integral equations of electric
and magnetic fields. The solution of Method of Moments yields the induced
electric current on the outer surface of the horn and the amplitude of the modes
on the aperture of the horn. The exploratory study is divided in three parts. First,
by using the GO approximations for the aperture fields, the geometry parameters
are explored to identify compact antenna configurations that maximize the gain
along the horizontal plane. However, a more uniform coverage of the ground can
be obtained by tilting the main lobe. Thus, in a second step of the parametric
study, it is considered a family of designs with the same main reflector and a set
of subreflectors that are designed for different tilt angle of main lobe. This
strategy is effective to reduce the manufacturing costs of a family of antennas
designed to provide different coverage. Third, by using the hybrid method, the TEM coaxial horn is shaped to reduce the return loss and extend its operating
band, and, finally, the overall antennas performance is optimized by controlling
radiation pattern and return loss.
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Opening New Radio Windows and Bending Twisted BeamsNordblad, Erik January 2011 (has links)
In ground based high frequency (HF) radio pumping experiments, absorption of ordinary (O) mode pump waves energises the ionospheric plasma, producing optical emissions and other effects. Pump-induced or natural kilometre-scale field-aligned density depletions are believed to play a role in self-focussing phenomena such as the magnetic zenith (MZ) effect, i.e., the increased plasma response observed in the direction of Earth's magnetic field. Using ray tracing, we study the propagation of ordinary (O) mode HF radio waves in an ionosphere modified by density depletions, with special attention to transmission through the radio window (RW), where O mode waves convert into the extraordinary (X, or Z) mode. The depletions are shown to shift the position of the RW, or to introduce RWs at new locations. In a simplified model neglecting absorption, we estimate the wave electric field strength perpendicular to the magnetic field at altitudes normally inaccessible. This field could excite upper hybrid waves on small scale density perturbations. We also show how transmission and focussing combine to give stronger fields in some directions, notably at angles close to the MZ, with possible implications for the MZ effect. In a separate study, we consider electromagnetic (e-m) beams with helical wavefronts (i.e., twisted beams), which are associated with orbital angular momentum (OAM). By applying geometrical optics to each plane wave component of a twisted nonparaxial e-m Bessel beam, we calculate analytically the shift of the beam's centre of gravity during propagation perpendicularly and obliquely to a weak refractive index gradient in an isotropic medium. In addition to the so-called Hall shifts expected from paraxial theory, the nonparaxial treatment reveals new shifts in both the transverse and lateral directions. In some situations, the new shifts should be significant also for nearly paraxial beams.
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[pt] APLICAÇÃO DE TÉCNICA DE SÍNTESE DE LENTES CIRCULARMENTE SIMÉTRICAS / [en] APPLICATION OF A TECHNIQUE FOR THE SYNTHESIS OF CIRCULARLY SYMMETRIC LENSESRODRIGO SAMICO BALTER 24 January 2024 (has links)
[pt] Este trabalho tem como objetivo aplicar uma técnica de síntese ótica
de lentes dielétricas circularmente simétricas, que são iluminadas por uma
fonte pontual com diagrama circularmente simétrico. A interface da lente
(interface do dielétrico-ar) será modelada utilizando as aproximações da Ótica
Geométrica e da Física Ótica. Da Ótica Geométrica, o controle da densidade
de potência em campo distante é obtido pela aplicação de conservação de
energia no interior dos tubos de raios que emergem do centro de fase da
fonte e a determinação da direção dos raios que emergem da superfície do
dielétrico é obtida através da aplicação da Lei de Snell para os raios incidentes
na interface. Da Física Ótica, é possível obter as correntes elétrica e magnética
na superfície externa da lente para determinar o campo radiado na integração
dessas correntes pela superfície a partir da suposição de que as dimensões da
lente e da curvatura da interface sejam muito maiores que um comprimento de
onda no espaço livre. Devido a simetria circular, simplificações na formulação
e nos esquemas numéricos para a solução serão apresentadas, resultando em
técnica de projeto mais eficientes. A descrição da superfície da interface da
lente é obtida através de técnica numérica. / [en] This work aims to apply a technique of optical synthesis of symmetrically
circular dielectric lenses, which are illuminated by a point source with a
symmetrically circular diagram. The lens interface (dielectric-air interface) will
be modeled using the approximations of Geometric Optics and Physical Optics.
From Geometric Optics, control of the power density in the far field is obtained
by applying energy conservation inside the ray tubes that emerge from the
phase center of the source and the determination of the direction of the rays
emerging from the dielectric surface is obtained through the application of
Snell’s law for the incident rays at the interface. From Physical Optics, it is
possible to obtain the electric and magnetic currents on the external surface
of the lens to determine the radiated field in the integration of these currents
along the surface assuming that the lens dimensions and interface curvature
are much larger than a wavelength in free space. Due to circular symmetry,
simplifications in formulation and numerical schemes for the solution will be
presented, resulting in a more efficient design technique. The description of the
lens interface surface is obtained through numerical technique.
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Continuous parallel plate waveguide lenses for future low-cost and high-performances multiple beams antennas / Lentilles continues en guide d’ondes à plans parallèles pour des applications multi-faisceaux à bas coût et à haute performanceDoucet, François 25 February 2019 (has links)
Les travaux présentés dans cette thèse portent sur l’étude et la conception de lentilles continues en guide d’ondes à plans parallèles (PPW) pour des applications multi-faisceaux. La conversion du front d'onde est assurée par une lentille formée d’une lame et d'une cavité transversale. Ce concept, proposant une approche mécanique simplifiée et possiblement purement métallique, est particulièrement attractif pour les futurs systèmes de communications par satellites (GEO/LEO) requérants des solutions à moindre coût tout en maintenant de hautes performances. La première partie de cette thèse présente la réalisation d’un outil d’analyse numérique basée sur l’optique géométrique (GO). Une prédiction rapide et précise des performances en rayonnement est obtenue. En combinant cet outil avec des processus d’optimisation, des performances en dépointage sur un large secteur angulaire sont ensuite démontrées, incluant une stabilité des diagrammes en rayonnement (ouverture à mi-puissance, niveaux de lobes secondaires et pertes en dépointage). Un prototype fonctionnant dans la bande Ka est réalisé, validant les performances précédemment obtenues. De hautes efficacités de rayonnement sont également mises en avant sur l’ensemble de la plage de fréquence. Enfin un second prototype plus compact basé sur de multiples lentilles est proposé et étudié, démontrant des performances similaires à la première solution fabriquée et mesurée. / The activities presented in this PhD focus on the study and development of continuous parallel plate waveguide (PPW) lenses for multiple beam applications. The wave front conversion is ensured by a PPW transversal ridge and cavity. The proposed concept, based on a simplified mechanical approach and possibly full-metal, is particularly suitable for future satellite communication systems (GO/LEO) requiring low-cost antennas while maintaining high performances. The first part of the thesis deals with the development of a numerical analysis tool based on geometrical optics (GO). A fast and accurate prediction of the radiation performance is provided.Combining the tool with optimization processes, large scanning performances have been demonstrated, including a stability of the radiation pattern performance (HPBW, SLL, scan loss). A prototype working at Ka band has been manufactured, validating the performances demonstrated previously. High radiation efficiencies are also proved over the entire frequency range. Finally, a second prototype targeting performances in compactness and based on multiple lenses is proposed and studied, showing similar performances as the first solution fabricated and measured.
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Análisis de sistemas radiantes sobre geometrías arbitrarias definidas por superficies paramétricasSaiz Ipiña, Juan Antonio 01 December 1995 (has links)
En esta tesis se presenta un método para analizar antenas montadas sobre estructuras arbitrarias. La Optica Geométrica (GO) y la Teoría Uniforme de la difraccion (UTD), han sidoempleadas para analizar los efectos que la estructura produce sobre el diagrama de radiación de la antena emisora. Para la descripción geométrica de la estructura, han sido utilizados parches NURBS (Non Uniform Rational B-Spline), por lo que el método presentado, es compatible con la mayoría de los programas gráficos disponibles en el mercado.EL tratamiento de geometrías arbitrarias requiere un código eficiente en el análisis de tres dimensiones.Por otro lado, para obtener resultados satisfactorios, la descripción de la superficie de la estructura, debe ser muy próxima al modelo real, sin embargo, esto complica el tratamiento computacional. Aquí la estructura es modelada mediante un conjunto de parches NURBS, que unidos entre sí, describen el modelo completo. Esta descripción permite manipular superficies arbitrarias con un bajo numero de parches, lo que significa un volumen de información reducido.La descripción inicial por NURBS del modelo, es acompañada con información complemetaria como por ejemplo: la tipología de las superficies, las curvas frontera, el tipo de material, etc. Esto es imprescindible para la aplicación de criterios de selección dedicados a la aceleración del proceso.El método tras leer la descripción del modelo, descompone los parches NURBS en superficies racionales de Bezier. Un parche de Bezier es también una superficie paramétrica definida en términos de una combinación lineal de los polinomios de Bernstein.Las antenas son modeladas usando modelos numéricos simples basados en agrupaciones de dipolos infinitesimales eléctricos y magnéticos. Esta caracterización de la antena es muyventajosa ya que con un numero reducido de datos de entrada, la fuente queda definida para cualquier dirección del espacio y el valor del campo radiado puede ser calculado fácilmente.El análisis electromagnético de los efectos que contribuyen al campo dispersado por la geometría comienza con una selección rigurosa de la geometría iluminada desde la fuente.Unicamente los parches de Bezier iluminados serán almacenados por el ordenador durante el análisis. La filosofía de este proceso es descartar aquella parte de la geometría que no contribuye a los efectos de dispersión.El campo total calculado es la superposición de los siguientes efectos pertenecientes a la GO y a la UTD: campo directo procedente de la fuente, campo reflejado por los parches de Bezier, campo difractado por las aristas del modelo definidas como curvas de Bezier, ondas de superficie, dobles reflexiones, reflexione-difraccion y difraccion-reflexión. El método ha sido diseñado para analizar campo cercano y lejano. El mayor gasto computacional se debe a la búsqueda de los puntos de dispersión, por lo que antes de emplear los algoritmos de intersección es necesario aplicar un conjunto de criterios rápidos dependientes de la dirección de observación.El principio de Fermat en combinación con el Gradiente Conjugado (CGM) es usado para obtener de manera eficiente los puntos de dispersión sobre la estructura. Para cada efecto, laposible ocultación de la trayectoria completa del rayo es examinada, por ello, si el rayo corta alguno de los parches de Bezier su contribución será descartada. Los dobles efectos son tratados como una generalización de los simples efectos.El método desarrollado es eficiente ya que precisa de un numero reducido de superficies para modelar objetos complejos lo que se traduce en bajos requerimientos de memoria y reducidos tiempos de calculo. / In this thesis a method to analyze antennas on board of complex bodies is presented. The Geometrical Optics (GO) and Uniform Theory of Diffraction (UTD) have been used to analyze the effect of the structure in the radiation pattern of the antennas. The bodies are geometrically modelled by using NURBS (Non Uniform Rational B-Spline) surfaces. In addition to be accurate and efficient, the method is compatible with most of the modern CAGD (Computer Aided Geometric Design) available programs.The treatment of arbitrary geometries requires a code which can carry out an efficient 3D analysis. To obtain accurate results the description of the surface must be close to the real model, however this complicates the computational procedure. Here the structure is modeled by a collection of individual N.U.R.B.S. surface patches joined to form a complete description of the surface model. The NURBS description is able to manipulate free form surfaces with a low number of patches, and therefore, with a low amount of information. The initial description of the model by NURBS surfaces is accompanied with other complementary data for example : the topology of the surfaces, the boundary curves, the types of material and other inputs. It is very interesting to apply criteria to make the complete analysis faster.The method reads the NURBS description of the model and transforms the NURBS into the rational BEZIER surfaces. A rational BEZIER patch is also a parametric surface defined in terms of a linear combination of Bernstein polynomials.The antennas are modelled using simple numerical models based on arrays of electric and magnetic infinitesimal dipoles. This antenna modelization is very advantageous because with a little input data, the source is defined in any direction and the field value is readily accessible.The electromagnetic analysis of the contributive effects to the scattering field by the geometry, starts with the rigorous selection of the geometry illuminated from the source. Only the Bezier patches illuminated will be in memory of the computer during the analysis. The philosophy of this previous process is to discard in the process the part of the geometry which does not contribute to the scattering effects.The total field is the superposition of the following GO and UTD field components: direct field from the source, reflected fields from the Bezier patches of the model, diffracted fields from the arbitrary edges defined as a Bezier curves, creeping waves, double reflected field and diffracted-reflected and reflected-diffracted fields. The search of specular and diffraction points are the most CPU time consuming, thus before using the intersection algorithms it is necessary to apply a set of fast selection criteria which depend on the observation direction.The Fermat principle in conjunction with the Conjugate Gradient Method (CGM) is used for obtaining efficiently the reflection points and diffraction points on the structure. For each effect the complete ray path is examined to see whether or not it is interrupted by any Bezier patch of the model, in this case the field component is not computed. The double effects are treated using a generalization of the single effects algorithms. The method has been developed to analyze the near and far field cases for different frequencies.The developed method is quite efficient because it makes use of a small number of surfaces to model complex bodies, so it requires few memory and low computing time.
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Modélisation de la propagation électromagnétique en milieux inhomogènes basée sur les faisceaux gaussiens : application à la propagation en atmosphère réaliste et à la radio-occultation entre satellites / Electromagnetic propagation modeling in inhomogeneous media with refractive index gradients based on Gaussian beams : application to realistic atmospheric propagation and radio occultation between satellitesL'hour, Charles-Antoine 19 April 2017 (has links)
La thèse, dont le sujet est "Modélisation de la propagation électromagnétique en milieux à gradient d'indice basée sur les faisceaux gaussiens - Application à la propagation en atmosphère réaliste et à la radio-occultation entre satellites" a été commencée le 2 décembre 2013, au Département ÉlectroMagnétisme et Radar (DEMR) de l'Onera de Toulouse et avec le laboratoire LAPLACE de l'Université Paul Sabatier. Elle est co-financée par l'ONERA et par la Région Midi-Pyrénées. L'encadrement a été assuré par Jérôme Sokoloff (Laplace/UPS, directeur de thèse), Alexandre Chabory (ENAC, co-directeur) et Vincent Fabbro (ONERA). L'École Doctorale est l' "École Doctorale Génie Électrique, Électronique, Télécommunications : du système au nanosystème". Le faisceau gaussien a été principalement utilisé dans la recherche scientifique afin d'étudier les systèmes optiques tels que les lasers. Des études plus rares et plus récentes ont proposé de l'utiliser pour modéliser la propagation des ondes sismiques. Ses propriétés spatiales et spectrales ont amené certains auteurs à étudier son utilisation dans des modèles de propagation atmosphériques. Cette thèse a consisté à développer un modèle, appelé GBAR (Gaussian Beam for Atmospheric Refraction), de propagation troposphérique réaliste et déterministe en utilisant le formalisme des faisceaux gaussiens. La démarche adoptée a consisté à reprendre les équations fondamentales introduites par Cerveny et Popov décrivant de façon itérative la propagation d'un faisceau gaussien en milieu inhomogène, sous hypothèse de haute fréquence (modèle asymptotique). De nouvelles équations ont été développées à partir d'elles pour obtenir une description analytique de la propagation d'un faisceau gaussien dans un milieu troposphérique décrit par les variations spatiales de l'indice de réfraction. L'hypothèse de base pour l'obtention de la formulation analytique est que le gradient de l'indice de réfraction peut être considéré vertical et constant au voisinage du faisceau. Les équations analytiques pour la description de la propagation d'un seul faisceau ont ensuite été étendues à la modélisation d'un champ quelconque dans un milieu troposphérique pouvant contenir de fortes variations du gradient d'indice, y compris des inversions de gradient. Ceci a été réalisé en couplant les équations analytiques avec la procédure de décomposition multi-faisceaux développée dans sa thèse pas Alexandre Chabory. Le modèle GBAR a été validé dans des milieux troposphériques réalistes issus de simulations du modèle météo méso-échelle WRF (Weather Research and Forecasting). Dans un troisième temps, le modèle a été utilisé pour simuler des inversions de données de radio-occultation. Des outils existent pour fournir un modèle d'interprétation de ces données pour estimer les propriétés physiques de l'atmosphère à partir des mesures en phase, amplitude, Doppler et délai des signaux GNSS transmis entre satellites en orbite autour de la Terre / The subject of this PhD thesis is " Electromagnetic propagation modeling in inhomogeneous media with refractive index gradients based on Gaussian beams - Application to realistic atmospheric propagation and radio occultation between satellites ". The study started on december 2nd, 2013 at the DEMR (Département Électromagnétisme et Radar) department of the ONERA research laboratory, in Toulouse, France. It was funded both by the ONERA and Région Midi-Pyrénées. It was supervised by Jérôme Sokoloff (LAPLACE/UPS, thesis director), Alexandre Chabory (ENAC, thesis co-director) and Vincent Fabbro (ONERA). The doctoral school was "École Doctorale Génie Électrique, Électronique, Télécommunications : du système au nanosystème ". The Gaussian beam was mostly used in scientific investigations to study optical systems such as lasers. Rarer and more recent works suggested the use of the Gaussian beam formalism in order to model the propagation of seismic waves. The properties of the Gaussian beam also led some authors to develop models for atmospheric propagation. In this thesis a model based on Gaussian beams called GBAR (Gaussian Beam for Atmospheric Refraction) was developped for tropospheric propagation in realistic and deterministic conditions. The scientific approach consisted in rewritting the fundamental equations introduced by Cerveny and Popov describing iteratively the propagation of a Gaussian beam in inhomogeneous media, under the high-frequency assumption (asymptotic model). New equations were derived from them in order to get analytical equations of the propagation of a Gaussian beam in inhomogeneous media described by the variations of the refractive index. The basic assumption under to get the analytical equations is to consider that the refractive index gradient is vertical and constant around the beam axis. The analytical equations that describe the propagation of a Gaussian beam were extended to model the propagation of an arbitrary field in a tropospheric medium with strong variations and inversions of the refractive index. This was done by coupling the analytical equations with the multibeam expansion procedure developped by Alexandre Chabory in his PhD thesis. The GBAR model was validated in tropospheric conditions, using refractive index grids from the WRF (Weather Research and Forecasting) mesoscale meteorological model. In the third and final phase, the GBAR model was used to simulate Radio Occultation data inversions. Tools exist to allow for interpretations of Radio Occultation data in order to estimate the physical properties of the atmosphere from measured phased, amplitude, Doppler shift and delay of GNSS signals transmitted between satellites orbiting around the Earth
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Física dos cristais líquidos e gravitação : pontos de encontroPereira, Erms Rodrigues 14 April 2011 (has links)
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Previous issue date: 2011-04-14 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Aspects of the physics of nematic liquid crystals are studied in this thesis from
the viewpoint of riemannian geometry through analogue models of gravitation.
The topics chosen for study were: geometric and wave optics, elastic waves, hydrodynamics
and heat conduction. The main analogue model used is based on the
interpretation of Fermat’s principle as a process to obtain null geodesics, where the
liquid crystalline material is seen as a riemannian manifold. This approach predicts
that the metric effectively felt by the light ray depends on the configuration
of molecules in the liquid crystal and on the parallel and perpendicular refractive
indexes to the axis of symmetry of the liquid-cristal molecule. It is known that,
for the particular case of the existence of topological defects within the material,
effective metric similar to cosmological defects (like global monopoles and cosmic
strings) are obtained. This thesis develops itself on the situation where there are
topological defects of hedgehog type and (k = 1, c = 0) disclination type in the
nematic phase of the liquid crystalline material. The first problem studied, as
a review, deals with the wave optics, with respect to the light diffracted by the
cited defects. Since plane waves of small wavelength have identical trajectories
to light rays, the use of analog model is therefore justified. Thus, we show that
light scattered by these defects generates a characteristic diffraction pattern, being
the location given by an algebraic expression dependent on the parallel and perpendicular
refractive indexes to the axis of symmetry of the molecule. We also
show how theses patterns depend on the temperature of the material. The second
studied problem deals with the geometrical optics and hydrodynamics of the nematic
liquid crystals. From a molecular configuration similar to a (k = 1, c = 0)
disclination, we let the material flow radially towards the axis of the defect. Then,
using the hydrodynamic fact that velocity gradients in the material locally change
the refractive index of the molecule, we find the velocity profile that must exist
around the defect so that the metric actually experienced by light traveling in the
plane perpendicular to the axis the defect is the Schwarzschild one in the equatorial
plane, with the Schwarzschild radius interior to the object. We found that
the absolute values of the velocity of liquid crystalline fluid can be order of a few
meters per second, differing greatly from the values obtained by Gordon metric for
an isotropic fluid under identical conditions. The third studied problem deals with
the elastic oscillations in the presence of topological defects. Similarly to the first
problem, the trajectory of the sound is obtained by an elastic version of Fermat’s
principle and, then, compared with a null geodesic. We show how topological defects
influence the sound trajectories and the sound diffracted by them. The fourth
problem deals with the heat conduction in the vicinity of defects. Considering that
the defects come from an addition or removal of portion of the material, letting the
medium relaxes elastically, effective metric of the space disturbed by the defect are
found, with expressions similar to those obtained by the analogous model based on
Fermat’s principle. These metrics generate a modified thermal conductivity tensor,
allowing the study of the temperature field in this situation. We show that,
depending on the values of parallel and perpendicular thermal conductivity to the
axis of symmetry of the molecule and on the defect in question, the temperature
gradient can be accentuated or attenuated on the defect, allowing control of the
response thermal temperature of the material, according to the presence of defects.
Encouraging a greater understanding of the physics of liquid crystals and its use
as a background in analogue models of gravity is the main theme of each analyzed
problem. / Aspectos da física dos cristais líquidos nemáticos são estudados nesta tese do
ponto de vista da geometria riemannina, por meio de modelos análogos de gravitação.
Os tópicos escolhidos para estudo foram: óptica geométrica e ondulatória,
ondas elásticas, hidrodinâmica e condução de calor. O principal modelo análogo
empregado baseia-se na interpretação do princípio de Fermat como um processo de
obtenção de geodésicas nulas, onde o material líquido-cristalino é visto como sendo
uma variedade riemanniana. Esta abordagem prevê que a métrica efetivamente
sentida pelo raio luminoso dependa da configuração das moléculas dentro do cristal
líquido e dos índices de refração paralelo e perpendicular ao eixo de simetria da
molécula líquido-cristalina. É sabido que, para o caso especial da existência de defeitos
topológicos dentro do material, métricas efetivas semelhantes às de defeitos
cosmológicos (como monopolos globais e cordas cósmicas) são obtidas. Esta tese
desenrola-se sobre a situação onde existem defeitos topológicos do tipo ouriço e do
tipo desclinação (k = 1, c = 0) na fase nemática do material líquido-cristalino. O
primeiro problema estudado, em caráter de revisão, trata da óptica ondulatória, no
que concerne a difração de luz pelos defeitos citados. Uma vez que ondas planas de
comprimento de onda pequeno possuem trajetórias idênticas aos raios luminosos,
o emprego do modelo análogo é justificado. Assim, mostramos que a luz espalhada
por esses defeitos gera padrões de difração bem característicos, sendo a localização
dada por expressão algébrica dependente dos índices de refração paralelo e perpendicular
ao eixo de simetria da molécula líquido-cristalina. Também mostramos
de que forma esses padrões dependem da temperatura do material. O segundo
problema estudado trata da óptica geométrica e da hidrodinâmica dos cristais líquidos
nemáticos. A partir de uma configuração de moléculas semelhantes à de
uma desclinação (k = 1, c = 0), permitimos que o material flua radialmente na
direção do eixo do defeito. Em seguida, fazendo uso do fato hidrodinâmico de
que gradientes de velocidade no material modificam localmente os índices de refração
da molécula, encontramos o perfil de velocidade que deve existir em torno
do defeito para que a métrica efetivamente sentida pela luz, que viaja no plano
perpendicular ao eixo do defeito, seja a de Schwarzschild no plano equatorial, com
raio de Schwarzschild interior ao objeto. Encontramos que os valores absolutos
da velocidade de fluido líquido-cristalino podem ser da ordem de alguns metros
por segundo, diferindo enormemente dos valores obtidos pela métrica de Gordon
para um fluido isotrópico em condições idênticas. O terceiro problema estudado
aborda as oscilações elásticas na presença de defeitos. Semelhantemente ao primeiro
problema, a trajetória do som é obtida por uma versão elástica do princípio
de Fermat e, então, comparada com uma geodésica nula. Mostramos como defeitos
topológicos influenciam nas trajetórias sonoras, assim como no som difratado
por eles. O quarto problema trata da condução de calor na vizinhança de defeitos.
Considerando que os defeitos são resultantes de uma adição ou remoção de
porção de material, dando-se seguimento a uma relaxação elástica do meio, métricas
efetivas do espaço perturbado pelo defeito são encontradas, com expressões
semelhantes às obtidas pelo modelo análogo baseado no princípio de Fermat. Essas
métricas geram um tensor condutividade térmica modificado, dando cabo ao
estudo do campo de temperatura nessa situação. Mostramos que, dependendo dos
valores da condutividade térmica perpendicular e paralela ao eixo de simetria da
molécula líquido-cristalina e do defeito em questão, o gradiente de temperatura
pode ser acentuado ou atenuado sobre o defeito, permitindo o controle da resposta
térmica do material à temperatura, de acordo com a presença de defeitos. Suscitar
um entendimento maior da física dos cristais líquidos e de seu emprego como
background em modelos análogos de gravitação é o tema principal de cada um dos
problemas analisados.
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