Thesis (Ph.D. (Space Physics)--North-West University, Potchefstroom Campus, 2008.

Thesis (Ph.D. (Space Physics)--North-West University, Potchefstroom Campus, 2008.

Thesis (Ph.D. (Space Physics)--North-West University, Potchefstroom Campus, 2008.

Le centre de la Galaxie abrite un trou noir supermassif, Sagittarius A*. Sa proximité en fait un laboratoire privilégié pour étudier les phénomènes d’accrétion à l’œuvre autour des trous noirs et contraindre le cycle d’activité de ces astres. Sgr A* est actuellement extrêmement peu lumineux et malgré des sursauts d’activité quotidiens sa luminosité demeure au moins huit ordres de grandeur en dessous de sa luminosité d’Eddington. Cet objet est ainsi l’un des trous noirs supermassifs connus les moins lumineux. Les mécanismes radiatifs à l’origine des variations quotidiennes observées ne sont pas clairement identifiés. Nous présentons les résultats d’une campagne d’observation multi-longueurs d’onde visant à mesurer le spectre de ces événements simultanément en rayons X et en infrarouge proche, à l’aide de l’observatoire XMM-Newton et de l’instrument VLT/NACO. Les données infrarouges obtenues grâce à la technique de spectro-imagerie en bande large ont permis d’étudier la variabilité du spectre de Sgr A* en infrarouge. Les incertitudes liées aux erreurs systématiques sont encore importantes mais les premiers tests réalisés semblent indiquer que l’indice spectral pourrait dépendre de la luminosité du trou noir. Sur des échelles de temps plus grandes, nous montrons également que Sgr A* n’a pas toujours été aussi peu actif. Des traces de son activité passée sont en effet visibles dans la matière moléculaire directement autour du trou noir, notamment sous la forme d’un rayonnement réfléchi visible dans la raie de fluorescence du fer à 6.4 keV. Nous avons réalisé une étude complète et systématique des variations de cette émission détectée dans la zone moléculaire centrale en utilisant les observatoires Chandra et XMM-Newton. Nos résultats confirment que Sgr A* a connu des sursauts intenses au cours des derniers siècles, au moins six ordre de grandeur en dessus de la luminosité actuelle. En particulier, nous avons mis en évidence, pour la première fois, la présence de deux événements transitoires distincts de relativement courte durée, probablement liés à des événements catastrophiques. Ces résultats constituent une première étape pour relier l’activité de ce trou noir spécifique aux autres noyaux de galaxie présents dans l’Univers. / Sagittarius A⋆ is the supermassive black hole at the Galactic center. Due to its proximity, this specimen is an excellent laboratory to study the accretion processes occurring around black holes and to constrain the duty cycle of these objects. Sgr A* is currently extremely faint and despite the detection of daily flares, its luminosity remains at least eight orders of magnitude below its Eddington luminosity, making this specimen one of the least luminous known supermassive black holes. The radiative processes responsible for the daily variations of its luminosity have not been clearly identified yet. We present the results of a multi-wavelength campaign observing Sgr A* simultaneously in X-rays and in the near-infrared, using the XMM-Newton observatory and the VLT/NACO instrument. We studied the spectral variability of Sgr A* using the infrared data we obtained through a spectro-imaging technique. Uncertainties linked to the systematic errors are still large but the first tests applied seem to show that the spectral index of Sgr A* could depend on the black hole luminosity. On longer timescales, we demonstrate that Sgr A* experienced a higher level of activity in the recent past. Indeed, echoes of its past activity can be detected in the molecular material surrounding the black hole. They are traced by a strong signal in the iron fluorescence line at 6.4 keV. We achieved a complete and systematic study of this variable emission detected from the central molecular zone, using Chandra and XMM-Newton observatories. Our results confirm that Sgr A* experienced intense flares in the past few centuries, with a luminosity at least six orders of magnitude higher than its current one. In particular, we highlight for the first time the existence of two distinct transient events of relatively short duration, which are probably due to catastrophic events. These results are the first step needed to include Sgr A*’s activity into a broader understanding of the galactic nuclei.

Ces travaux de thèse sont consacrés à l’examen du rôle du rayonnement thermique dans le processus de propagation des flammes issues de la combustion des particules d’aluminium dans l’air. Le sujet étant complexe et d’un intérêt industriel, il nécessite de prendre en compte le couplage de nombreux phénomènes physico-chimiques afin de prédire finement les conséquences des explosions de poussières. Une analyse bibliographique approfondie est proposée, concernant les mécanismes d’inflammation et de combustion des particules d’aluminium et aussi concernant les connaissances relatives à la propagation des flammes de poussières. La question spécifique de la nature des échanges thermiques et de l’influence du rayonnement thermique est étudiée. La revue bibliographique souligne les approximations et les hypothèses simplificatrices utilisées dans la littérature permettant donc de définir les pistes d’améliorations. Compte tenu des limitations importantes concernant la physique de ces flammes, un outil de simulation de physique numérique nommé « RADIAN », proche de la simulation numérique directe, a été développé proposant un couplage fin entre les différents modes d’échanges thermiques et la combustion pour modéliser la propagation de la flamme dans un nuage de poussières. La méthode des éléments discrets (MED) est utilisée pour modéliser numériquement les échanges radiatifs entre les particules et les échanges conductifs entre gaz et particules. La méthode des différences finies est utilisée pour modéliser numériquement la conduction thermique dans la phase gazeuse et la combustion. Un modèle radiatif est proposé se basant sur la théorie de Mie sur les interactions rayonnement-particules. Les résultats des simulations sont comparés avec des solutions analytiques et des données expérimentales de la littérature. Mais en plus, une étude expérimentale est aussi conduite afin de mesurer la distribution du flux radiatif devant la flamme et la vitesse de combustion laminaire pour des flammes Méthane-Sic, Méthane-Alumine et Al-air. Un bon accord entre les simulations et les expériences est démontré. La loi de Beer-Lambert relative au transfert radiatif devant le front de flamme s’avère inapplicable et une nouvelle solution analytique est proposée. La présence de particules absorbantes du rayonnement promeut la propagation de la flamme. En particulier, il a été montré expérimentalement et confirmé numériquement que les mélanges riches d’AL-air sont susceptibles d’accélérer rapidement. / In this thesis, the role of thermal radiation in aluminum-air flames propagation is studied. The subject being complex and of industrial interest, it requires the coupling of many physiochemical phenomena to accurately predict the consequences of dust explosions. A thorough literature review is proposed about the ignition and the combustion of aluminum particles and about the available theoretical models of dust flames propagation. The specific question of the nature of thermal exchanges and the influence of thermal radiation is studied. The bibliographic review underlines the simplifying assumptions and hypotheses used in the literature making possible the definition of improvement areas. Because of the limited amount of knowledge available to address these questions, a numerical tool “RADIAN” is developed enabling an accurate coupling between the different modes of heat exchange and combustion. The Discrete Element Method (DEM) is used to numerically model the radiative exchanges between particles and the gas-particle thermal conduction. The Finite Difference method is used to numerically model the thermal conduction through the gas phase and combustion. A radiative model based on Mie theory for radiation-particles interactions is incorporated. The results of the simulations are compared with available analytical solutions and experimental data. An original experimental study is also conducted to measure the distribution of irradiance ahead of the flame front and the laminar burning velocity for methane-air-Sic, methane-air-alumina and Al-air flames. A good agreement between numerical simulations and experiments is demonstrated. The Beer-Lambert’s law for radiative transfer in front of the flame front is found to be inapplicable and a new analytical solution is proposed. The presence of absorbing particles may promote the flame propagation. In particular, it is shown experimentally and confirmed theoretically/numerically that Al-air rich mixtures are likely to rapidly accelerate.

Processos de fragmentação são ferramentas fundamentais no estudo de íons por espectrometria de massas, pois permitem a obtenção de informação sobre a estrutura e termoquímica destes íons . Desenvolvemos uma técnica de dissociação de íons baseada na ativação destes através da absorção multifotônica sequencial de radiação infravermelha de um filamento incandescente. Nesta técnica, íons aprisionados na cela de um espectrômetro de massas por transformada de Fourier são submetidos a radiação do filamento por intervalos de tempo relativamente longos (da ordem de segundos), e passam por um processo de multíplos eventos de absorção e emissão de radiação, até a energia interna dos íons atingir um nível suficiente para a fragmentação. A partir deste modelo para o processo foram implementados métodos computacionais que permitem a simulação da dissociação. A realização destas simulações exige o conhecimento do espectro vibracional do íon, que normalmente tem que ser obtido por algum método teórico de cálculo, como foi feito neste trabalho paro o caso de alguns dos íons estudados. O método de dissociação unimolecular induzida por absorção de radiação infravermelha foi aplicado na determinação das energias de ativação e de dissociação dos íons moleculares de acetofenona, p-cimeno e cumeno. Este método também mostrou-se capaz de diferenciar diferentes estruturas isomericas, e foi com esta finalidade aplicado no estudo da isomerização ceto-enol do íon molecular de acetofenona. Demonstrou-se também a capacidade do método dissociar os íons moleculares de n-butilbenzeno e brometo de alila. / Fragmentation processes are fundamental tools for the mass spectrometric study of ions because they provide information on structure and termochemistry of these ions. We have developed a dissociation technique that is based on the multiphoton activation of trapped ions by infrared radiation emitted by an incandescent filament. In this technique, ions trapped in the cell of a Fourier Transform mass spectrometer are exposed to the filament radiation for long time periods (seconds), and a process of multiple absorption and emition events occurs, until the internal energy of the ions is high enough for fragmentation. Computational methods have been implemented from this model allowing for simulation of the dissociation process. For this simulation the ion vibrational spectra, that normally must be obtained from theoretical calculations, is also required, as has been done in this work for some of the studied ions. The method of unimolecular dissociation induced by infrared radiation was applied to the determination of activation and dissociation energies of the molecular ions of acetophenone, p-cymene and cumene. This method also proved to be capable in the differentiation of isomeric structures, and was so applied in the study for the keto-enol isomerization of the acetophenone molecular ion. It was also shown that the method can dissociate the molecular ions of n-butylbenzene an allyl bromide.

One of the key issues of our society is to decrease energy demand. About 40 % of energy consumption in Europe is related to building operation, mostly for heating. The required thermal comfort is related mainly to temperature of air and temperature of surrounding surfaces. Especially in older buildings with low level of thermal insulation the inner surface of perimeter wall can be several degrees colder than air. In such case the perception of thermal comfort will be lowered also depending on the layout of the room. Other issue can be related to thermal radiation asymmetry. The amount of energy that is eradiated from a surface is driven by material property called emissivity. A surface with sufficiently low emissivity will became “thermal mirror”. It is possible to elaborate about intelligent thermal management of the internal space with usage of low-emissive surfaces; where thermal radiation can be suppressed or can be enhanced. This depends on the surface emissivity and its real temperature. Recently, growing interest about the low-emissive surfaces can be seen. They can represent reasonable complement to conventional insulation, but they have certain limits. Potentially, even higher energy savings can be reached with so called retro reflection. If the thermal radiation of an object is reflected back, its thermal loss will be lower. In such case it can be expected that equivalent thermal comfort can be reached even with lower overall temperature of the room. The aim of the work is study of material combinations with potential for utilization as low-emissive interior surfaces. Research has been conducted on low-emissive paints and low-emissive foils. Another part of the work is related to retro reflection of thermal radiation as a potential way to influence thermal comfort. The work shows, that energy saving up to 20% can be obtained.

No description available.

<p>Among computational electromagnetic methods, Integral Equation (IE) solvers have a great capability in solving open-region problems such as scattering and radiation, due to no truncation boundary condition required. Volume Integral Equation (VIE) solvers are especially capable of handling arbitrarily shaped geometries and inhomogeneous materials. However, the numerical system resulting from a VIE analysis is a dense system, having $N^2$ nonzero elements for a problem of $ N $ unknowns. The dense numerical system in conjunction with the large number of unknowns resulting from a volume discretization prevents a practical use of the VIE for solving large-scale problems.</p> <p>In this work, two fast algorithms of $ O(N \log N) $ complexity to generate an rank-minimized $ H^2 $-representation for electrically large VIEs are developed. The algorithms systematically compress the off-diagonal admissible blocks of full VIE matrix into low-rank forms of total storage of $O(N)$. Both algorithms are based on nested cross approximation, which are purely algebraic. The first one is a two-stage algorithm. The second one is optimized to only use one-stage, and has a significant speedup. Numerical experiments on electrically large examples with over 33 million unknowns demonstrate the efficiency and accuracy of the proposed algorithms. </p> <p>Important applications of VIEs in thermal and quantum engineering have also been explored in this work. Creating spin(circularly)-polarized infrared thermal radiation source without an external magnetic field is important in science and engineering. Here we study two materials, magnetic Weyl semimetals and manganese-bismuth(MnBi), which both have permittivity tensors of large gyrotropy, and can emit circularly-polarized thermal radiations without an external magnetic field. We also design symmetry-broken metasurfaces, which show strong circularly-polarized radiations in simulations and experiments. In spin qubit quantum computing systems, metallic gates and antennas are necessary for quantum gate operations. But their existence greatly enhances evanescent wave Johnson noise (EWJN), which induces the decay of spin qubits and limits the quantum gate operation fidelity. Here we first use VIE to accurately simulate realistic quantum gate designs and quantify the influence on gate fidelity due to this noise.</p>

Die vorliegende Arbeit beschreibt einen semi-analytischen Ansatz zur Modellierung der Dichteverteilung von DM im Galaktischen Halo. Aus den verschiedenen Substrukturmodellen wird die γ-Strahlungsintensität, welche die Erde erreicht, berechnet. Eine Spannbreite plausibler γ-Strahlungsintensitäten aufgrund der Paarvernichtung Galaktischer DM wird vorgeschlagen, welche die Vorhersagen verschiedener früherer Studien umfasst, und es werden die durchschnittlichen Massen, Abstände und ausgedehnten Strahlungsprofile der γ-strahlungsintensivsten DM-Verdichtungen berechnet. Schließlich werden die DM-Modelle für eine umfassende Berechnung der Nachweismöglichkeit Galaktischer Substrukturen mit CTA verwendet. Die instrumentelle Sensitivität zum Nachweis der γ-strahlungsintensivsten DM-Substruktur wird für eine mit CTA geplanten großflächigen Himmelsdurchmusterung außerhalb der Galaktischen Ebene berechnet. Die Berechnung wird mit CTA Analyse- Software und einer Methode durchgeführt, welche auf einer Likelihood beruht. Eine alternative, ebenfalls Likelihood-basierte Analysemethode wird entwickelt, mit welcher DM-Substrukturen als äumliche Anisotropien im Multipolspektrum des Datensatzes einer Himmelsdurchmusterung nachgewiesen werden können. Die Analysen ergeben, dass eine Himmelsdurchmusterung mit CTA und eine anschließende Suche nach γ-Strahlung von DM-Substrukturen Wirkungsquerschnitte für eine Paarvernichtung in der Größenordnung von (σv) > 1 × 10−24 cm3 s−1 für eine DM-Teilchenmasse von mχ ∼ 500 GeV auf einem Vertrauensniveau von 95% ausschließen kann. Diese Sensitivität ist vergleichbar mit Langzeitbeobachtungen einzelner Zwerggalaxien mit CTA. Eine modellunabhängige Analyse ergibt, dass eine Himmelsdurchmusterung mit CTA Anisotropien im diffusen γ-Strahlungshintergrund oberhalb von 100 GeV für relative Schwankungen von CPF > 10−2 nachweisen kann. / In the current understanding of structure formation in the Universe, the Milky Way is embedded in a clumpy halo of dark matter (DM). Regions of high DM density are expected to emit enhanced γ-radiation from the DM relic annihilation. This γ-radiation can possibly be detected by γ-ray observatories on Earth, like the forthcoming Cherenkov Telescope Array (CTA). This dissertation presents a semi-analytical density modeling of the subclustered Milky Way DM halo, and the γ-ray intensity at Earth from DM annihilation in Galactic subclumps is calculated for various substructure models. It is shown that the modeling approach is able to reproduce the γ-ray intensities obtained from extensive dynamical DM simulations, and that it is consistent with the DM properties derived from optical observations of dwarf spheroidal galaxies. A systematic confidence margin of plausible γ-ray intensities from Galactic DM annihilation is estimated, encompassing a variety of previous findings. The average distances, masses, and extended emission profiles of the γ-ray-brightest DM clumps are calculated. The DM substructure models are then used to draw reliable predictions for detecting Galactic DM density clumps with CTA, using the most recent benchmark calculations for the performance of the instrument. A Likelihood-based calculation with CTA analysis software is applied to find the instrumental sensitivity to detect the γ-ray-brightest DM clump in the projected CTA extragalactic survey. An alternative Likelihood-based analysis method is developed, to detect DM substructures as anisotropies in the angular power spectrum of the extragalactic survey data. The analyses predict that the CTA extragalactic survey will be able to probe annihilation cross sections of ⟨σv⟩ > 1 × 10−24 cm3 s−1 at the 95% confidence level for a DM particle mass of mχ ∼ 500 GeV from DM annihilation in substructures. This sensitivity is compatible with long-term observations of single dwarf spheroidal galaxies with CTA. Independent of a particular source model, it is found that the CTA extragalactic survey will be able to detect anisotropies in the diffuse γ-ray background above 100 GeV at a relative amplitude of CP_F > 10−2.