Thèse de doctorat : Sciences de l'univers et de l'environnement : Paris 12 : 2007. / Titre provenant de l'écran-titre. Bibliogr. : 188 réf.

Thesis (PhD)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: The implementation and verification of a recently proposed Huy gens' principle in hollow lossless waveguide is described. The extended Huygens' principle is applicable to the scattering effect of arbitrary obstacles in waveguide and allows the coupling of a volume Finite Element discretization with a quasi Method of Moments surface element approach. A review of the technique is given and expressions for the two port scattering parameters are derived. The implementation is restricted to posts in rectangular waveguide, although the technique may also be applied to general scattering problems in waveguide. Finite Elements for the volume of the obstacle are selected and a basis set is proposed. The elemental finite element matrices are derived for a low and higher order basis set, while the validity of the derivations is established by considering a simple waveguide example. A coupled set of matrix equations is constructed that can be solved for the unknown surface electric and magnetic fields and the volume magnetic fields. A set of test examples is chosen to verify the implementation. The convergence behaviour is examined for two test cases. The dissertation concludes with an evaluation of the technique and recommendations for future work. / AFRIKAANSE OPSOMMING: Die implementasie en verifiëring van 'n onlangs voorgestelde Huygens beginsel in leë golfleier word beskryf. Die uitgebreide beginsel van Huygens is van toepassing op die strooiingseffek van arbitrêre voorwerpe in verlieslose golfleier en beskryf die koppeling tussen 'n volume eindige element diskretisasie met 'n oppervlakelement kwasi-moment metode. 'n Oorsig oor die tegniek word gegee en uitdrukkings vir die tweepoort strooiingsparameters word afgelei. Die implementering word beperk tot penne in reghoekige golfleier, alhoewel die tegniek op algemene golfleier strooiingsprobleme van toepassing is. Eindige elemente word gekies vir die volume van die voorwerp en 'n basisfunksie versameling afgelei van bestaande basisfunksies. Lae en hoë orde element matrikse word afgelei en die geldigheid van die afleidings word nagegaan deur 'n eenvoudige golfleier probleem op te los. 'n Gekoppelde stel matriksvergelykings word opgestel in terme van die onbekende oppervlak elektriese en magnetiese velde. 'n Aantal toetsvoorbeelde word gekies om die implementasie te verifieer. Konvergensiegedrag word ondersoek vir twee toetsgevalle. Die proefskrif sluit af met 'n evaluasie van die tegniek en aanbevelings vir verdere werk.

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2013. / Made available in DSpace on 2014-08-06T17:50:58Z (GMT). No. of bitstreams: 1 323502.pdf: 3970340 bytes, checksum: 265fd164c272384c43c6a3899db0789f (MD5) Previous issue date: 2013 / O objetivo geral deste trabalho é demonstrar a viabilidade de aplicação numérica do modelo físico de Huygens na descrição de ondas acústicas. Uma ampla revisão bibliográfica sobre o assunto foi realizada, incluindo trabalhos na área de Eletromagnetismo, contexto em que a primeira versão discretizada do modelo de Huygens foi proposta, utilizando a consolidada teoria de linhas de transmissão. Entretanto, ainda que o método tenha se desenvolvido consideravelmente em seu contexto original, percebe-se que as aplicações na solução de problemas em Acústica ainda são escassas. Inicialmente, dedica-se um capítulo para discorrer sobre os fundamentos da modelagem numérica, expondo as razões principais que levaram a escolha do tema do trabalho. Posteriormente, alguns tópicos fundamentais relacionados à teoria de processamento de sinal e auralização foram incluídos. A seguir é apresentada a abordagem teórica pioneira do método numérico, concebida a partir do modelo de Huygens, denominada de Método de Matrizes de Linha de Transmissão (TLM). Neste ponto, buscou-se revisar as analogias tradicionalmente estabelecidas com a teoria Acústica. Posteriormente, demonstram-se os aspectos teóricos de uma nova abordagem energética mecanicista para o modelo, denominada de Modelagem Discreta de Huygens (DHM). Em um passo seguinte, foram realizadas comparações conceituais entre a Modelagem Discreta de Huygens e outros métodos numéricos tradicionais, como o método de Volumes Finitos e também aqueles que utilizam como base a equação da onda, tais como Diferenças Finitas (FDM) e Elementos Finitos (FEM). Por último, apresenta-se uma série de resultados utilizando o conjunto de algoritmos desenvolvidos ao longo deste trabalho. Destacam-se as análises comparativas entre os tempos de simulação obtidos pelos métodos FEM e DHM para três cavidades distintas. Resultados relevantes também foram obtidos utilizando uma metodologia diferenciada para representação do coeficiente de reflexão de materiais de absorção no domínio do tempo, a partir de dados de impedância no domínio da frequência. Buscou-se estabelecer a comparação dos resultados obtidos, com modelos elaborados em FEM, assim como através de modelos analíticos consolidados, visando a confirmação das evidências iniciais quanto à potencialidade do método. <br> / Abstract : The main idea of this work is to show the feasibility of applying the Huygens' physical model for the numerical description of acoustic wave propagation. A extensive literature review on the subject was performed, including some work in the field of electromagnetism, using the consolidated theory of transmission lines. Although the method has been considerably developed in their original area, applications in solving acoustic problems are still quite scarce. A chapter was dedicated to discuss the fundamentals of numerical modeling, explaining the reasons that led to the choice made for this work. Fundamental topics related to signal processing and auralization were also included. In the following, the pioneering numerical approach based on the Huygens' physical model is presented, called Transmission Line Matrix, seeking to make analogies with the acoustics theory. Subsequently, it was shown the theoretical aspects of the energetical approach to the Huygens' model, called Discrete Huygens' Modeling. In a next step, comparisons were made among the concepts of this approach and other numerical methods that apply the wave equation as the fundamental model, such as finite difference and finite element. Following, results were presented using an algorithm developed as consequence of this work, in order to solve acoustic cavities of different sizes, and then compare with results obtained from numerical models simulated using finite element and analytical models. In the Annex, some data already published by different authors for acoustical problems were considered in order to emphasize the range of applications and the quality of results.

Ce travail de thèse est une étude originale de l'équilibre et de la circulation des populations d'électrons dans la magnétosphère de Saturne à l'aide des données multi-instrumentales de la sonde Cassini-Huygens. A partir de spectres inter-étalonnés des instruments particules de basse énergie (de 0.6 eV à 26 keV) et de haute énergie (de 12keV à 1 MeV), j'ai identifié les populations électroniques présentes dans la magnétosphère de Saturne : une population thermique (quelques eV), une population suprathermique (100-1000 eV), une population d'électrons énergétiques (MeV), et une population de photoélectrons issus de la photo-ionisation du tore de gaz neutre, observée pour la première fois dans la magnétosphère interne (< 5 Rayons saturniens). A partir des profils radiaux des moments fluides des populations électroniques dominantes, j'ai identifié la présence de 3 grandes régions magnétosphériques caractérisées par des régimes plasma différents, séparées par deux frontières localisées à 9 et à 15 Rayons saturniens. L'analyse statistique des profils de moments a révélé une dynamique importante de la couche de plasma et une asymétrie de la distribution des électrons thermiques et suprathermiques en longitude. L'analyse de l'évolution des moments fluides à l'intérieur de chacune des régions magnétosphériques et entre ces régions m'a permis d'identifier d'une part les régions de source, de perte et de transport des populations électroniques, et d'autre part les processus physiques dominants dans ces régions. Sur base de cette analyse, un schéma de circulation des populations d'électrons dans la magnétosphère de Saturne est enfin proposé.

Les sondes de permittivité sont des instruments de prospection géophysique non destructifs qui donnent accès aux propriétés électriques, aux basses fréquences (10 Hz-10 kHz), de la proche subsurface. Ce faisant, elles renseignent sur la composition, porosité, température et éventuelle hétérogénéité des premiers mètres sous la surface.Utilisant généralement 4 électrodes, le principe des sondes de permittivité est simple : il consiste à injecter un courant sinusoïdal de phase et d’amplitude connues entre deux électrodes (dipôle émetteur) et à mesurer l'impédance mutuelle (le rapport complexe entre la tension et le courant injecté) entre ce dipôle émetteur et un dipôle récepteur. La permittivité complexe du matériau de surface, à savoir sa constante diélectrique et sa conductivité électrique, sont alors déduites de la mesure de l’amplitude et de la phase de cette impédance mutuelle. Les fréquences d’opération des sondes de permittivités sont basses là où l’approximation quasi-statique s’applique. A ce jour, les propriétés électriques de seulement deux surfaces planétaires extraterrestres ont été étudiées par des sondes de permittivité : celle de Titan par l’instrument PWA-MIP/HASI/Huygens/Cassini-Huygens et celle du noyau de la comète 67P/Churyumov–Gerasimenko par SESAME-PP/Philae/Rosetta.Nous présentons la première analyse des données obtenues par SESAME-PP à la surface de la comète 67P/Churyumov–Gerasimenko. Grâce à un travail précis (1) de modélisation numérique de l’instrument et de son fonctionnement, (2) de campagne de mesures (en laboratoire et dans des grottes de glace) afin de valider la méthode d’analyse et (3) d’hypothèses réalistes sur l’environnement proche de la sonde, nous avons pu contraindre la composition et surtout la porosité des premiers mètres du noyau cométaire montrant qu’ils étaient plus compacts que son intérieur. Nous avons également travaillé à une nouvelle analyse des données obtenues en 2005 par PWA-MIP proposant notamment de nouveaux scénarios pour le changement brutal de propriétés électriques observé 11 min après l’atterrissage de Huygens. Ces nouveaux scénarios s’appuient, entre autres, sur les mesures de caractérisation électrique menées au LATMOS sur des échantillons de composés organiques (tholins), analogues possibles des matériaux recouvrant la surface de Titan. / Permittivity probes are non-destructive geophysical prospecting instruments that give access to the low frequency (10 Hz – 10 kHz) electrical properties of the close subsurface. This provides us with information on the composition, porosity, temperature, and heterogeneity of the first meters of the subsurface.Using 4 electrodes, the technique consists in injecting a sinusoidal current of known phase and amplitude between two electrodes (transmitting dipole) and measuring the mutual impedance (complex ratio of measured potential over injected current) between this dipole and a receiving dipole. The complex permittivity (i.e. dielectric constant and conductivity) of the subsurface material is derived from the measured phase and amplitude of the mutual impedance. The frequency range of operation of permittivity probes is low, therefore the quasi static approximation applies. To this day the electrical properties of only two extra-terrestrial surfaces have been studied by permittivity probes, the surface of Titan by the instrument PWA-MIP/HASI/Huygens/Cassini-Huygens and the surface of the nucleus of comet 67P/Churyumov–Gerasimenko by SESAME-PP/Philae/Rosetta.We present a first analysis of the data collected by SESAME-PP at the surface of the comet 67P/Churyumov–Gerasimenko. With the help of (1) precise numerical models of the instrument, (2) field measurements (in a controlled and natural environment) in order to validate the analysis method, and (3) realistic hypothesis on the close environment we were able to constrain the composition and porosity of the first meters of the comet’s nucleus, showing that the subsurface is more compact than its interior. We also reanalysed of the data collected in 2005 by PWA-MIP, offering new explanations for the abrupt change in the electrical properties observed 11 minutes after the landing of Huygens. These new scenarios were built in the light of lab measurements performed at LATMOS on samples of organic matter (tholins), possible analogue of Titan’s surface material.

[pt] Este trabalho trata do problema da difração das ondas eletromagnéticas por obstáculos do tipo gume de faca com perfil transversal qualquer. A partir da integral de Huygens-Fresnel (formulação escalar) e da integral de Stratton-Chu (formulação vetorial) é estabelecida uma solução em forma fechada para o problema, cujas interpretação física e aplicações práticas são bastante simples. Os resultados teóricos são verificados por comparação com medidas experimentais para três tipos de perfil transversal: semi-circular, triangular e rampa. Através da mesma formulação é investigado o comportamento do campo atrás do obstáculo, onde existe uma região de interferência formada pelas diversas contribuições independentes do campo difratado. Procurou-se ainda, nos casos dos obstáculos triangular e rampa, interpretar os resultados finais em termos do raio da 1(primeira) zona de Fresnel. Embora o modelo utilizado seja limitado por não considerar a dimensão do obstáculo no sentido de propagação (longitudinal), as conclusões obtidas são de grande importância para os enlaces radioelétricos que operam por difração. / [en] The present work deals with the problem of the diffraction of electromagnetic waves by knife-edge obstacles with arbitrary transverse profile. From the Huygens-Fresnel (acalar formulation) and the Stratton-Chu ( vectorial formulation) integrals, a closed-form solution for the problem is establications. The theoretical results are verified by comparison with experimental measurements fot three kinds od profiles: semi-circular, triangular and ramp. Using this same formulation, the behavior of the field behind the obstacle is investigated, where a region of interference exists caused by the several independet contributions of the diffracted field. An attempt was made to interpret the final results in terms of the first Fresnel zone radius in cases of triangular and ramp obstacles. Although the model used here is limited for not taking into account the longitudinal dimension of the obstacles, the obtained conclusions have major importance to radio links operating in diffraction.

International Telemetering Conference Proceedings / October 28-31, 1996 / Town and Country Hotel and Convention Center, San Diego, California / Exploration of Titan is one of the primary objectives of the Cassini/Huygens mission Saturn due to launch in 1997. Limited data will be provided by Huygens as it descends to the surface via parachute and by Cassini as it orbits Saturn and occasionally passes near Titan. Interest in Titan is high because of its planet-class size, dense atmosphere, and the possibility of continents and seas. Already, there are discussions for a follow-on mission to Titan. There are several proposed designs such as balloons and boats to explore Titan's ethane seas. In all cases, reliable data links back to Earth are absolutely essential. However, simply increasing the power has its limits due to constraints on launch weights. There are a number of possible options for getting data back from Titan. These alternatives, and their effect on the mission profile are discussed.

This thesis makes contributions to the solution of Hadamard's problem through an examination of the question of the validity of Huygens'principle for the non-self-adjoint scalar wave equation on a Petrov type D spacetime. The problem is split into five further sub-cases based on the alignment of the Maxwell and Weyl principal spinors of the underlying spacetime. Two of these sub-cases are considered, one of which is proved to be incompatible with Huygens' principle, while for the other, it is shown that Huygens' principle implies that the two principal null congruences of the Weyl tensor are geodesic and shear-free. Furthermore, an unpublished result of McLenaghan regarding symmetric spacetimes of Petrov type D is independently verified. This result suggests the possible existence of counter-examples of the Carminati-McLenaghan conjecture.

No description available.

In any research program we begin with a hypothesis and when our expected results do not concur with the observed results we must try and understand the dynamics behind the changed process. In this study we were trying to understand the flux between regional groundwater systems, surface waters and sedimentation processes in order to predict the fate of contaminants entering one of the larger bodies of water in the world- Lake Ontario. This lake has increased levels of tritium due to anthropogenic inputs. Our first approach to the problem was to look at tritium fluxes within the system . Hydrological balances were constructed and a series of sediment cores were taken longitudinally and laterally across the lake. The second approach was to quantify the sediment accumulation rate (SAR) within the depositional basins and zones of erosion in order to improve the linkage between erosion control (sedimentation) and the water quality program. In the last chapter the movement of tritium, by molecular diffusion, through the clayey-silts of Lake Ontario is quantified in terms of an effective diffusion coefficient. In these sediments effective diffusion equals molecular diffusion. In a laboratory experiment four cores of lake sediment were spiked with tritium . The resulting concentration gradient changes in the sediment porewaters after six weeks could be modeled by an analytical one- dimensional diffusive transport equation. Results calculated the average lab diffusion coefficient to be 2. 7 x 10 - 5cm 2. sec -1 which is twice that determined by Wang et al, 1952 but still reasonable. Short cores (50 cm) from lake Ontario had observed tritium concentrations with depth that reflected a variable diffusive profile. The increases and decreases in tritium with depth could be correlated between cores. Monthly tritium emission data was obtained and correlations between peaks in the tritium profile and emissions were observed. Monthly variations in release emissions corresponded to approximately a one centimeter slice of core. An average calculated diffusion coefficient of theses cores was 1. 0 x 10 -5 cm 2. sec -1 which compares to Wang's coefficient of 1. 39 x 10 -5 cm 2. sec -1. This implies that tritium is moving through the sediment column at a rate equal to diffusion. The results were obtained for smoothed values. It was not possible to model the perturbations of the data with a one dimensional model. The dynamics of the system imply that tritium could be used as a biomonitor for reactor emissions, mixing time and current direction scenarios and that a better understanding of this process could be gained by future coring studies and a new hypothesis.