Résumé : Tout comme l’aéronautique, l’industrie aérospatiale s’est tournée progressivement vers l’emploi des coques à base des matériaux composites. Cette transition a permis d’alléger considérablement les structures aéronautiques et aérospatiales, et par conséquent, a réduit la consommation de carburants ainsi que l’impact écologique des aéronefs et des lanceurs. Toutefois, la loi de masse stipule que cela ne peut être sans conséquence sur la perte par transmission acoustique des panneaux, surtout sur les basses fréquences. Que ce soit pour la conformité aux exigences des normes aéronautiques en terme de niveau de pression acoustique à l’intérieur des cabines ou la protection des charges utiles dans les coiffes des lanceurs, les traitements acoustiques ciblant les basses fréquences s’avèrent un défi d’envergure. En effet, avec des contraintes très strictes de minimum de masse et de volume ajoutés, il est difficile de traiter les problèmes d’absorption acoustique basses-fréquences, avec les traitements phoniques classiques. Afin de tirer avantage des effets résonants pour améliorer l’absorption sur les basses fréquences, ce projet se propose d’étudier l’intégration d’écrans résistifs à très faibles épaisseurs dans les revêtements acoustiques, tout en minimisant le poids. Il a été proposé dans le cadre de la chaire industrielle de recherche en aéroacoustique dont les principaux bailleurs de fonds sont Bombardier aerospace, Pratt & whitney et Bell helicopter. Aussi, des travaux de recherche connexes se sont déroulés en partenariat avec ULA (United Launch Alliance). Ce travail de maîtrise comporte une partie bibliographique qui présente une étude exhaustive des traitements basses-fréquences existants, tout en y portant un regard critique. La partie théorique met particulièrement l’emphase sur les différents modèles de propagation d’onde et les phénomènes de dissipations dans les milieux poreux. Dans cette partie, on dénombre également les différents types d’écrans ainsi que les modèles de propagation correspondants. Des critères permettant une étude comparative objective du point de vue masse/performance ont été proposés. En plus des paramètres non acoustiques, les conditions de montage et d’agencement des couches ont été étudiées avec des simulations numériques appuyées par des mesures expérimentales. Dans l’avant dernier chapitre, un modèle SEA (Statical Energy Analysis) d’une coiffe de lanceur a été construit avec tous ses détails à partir d’un exemple concret. Les simulations se sont terminées avec une étude comparative de la réduction de niveau de pression acoustique dans la coiffe. Le dernier chapitre résume les principaux résultats et conclusions de cette étude. / Abstract : Over the last few decades, the aerospace industry has witnessed a significant emergence of the use of composite shells. The latter are continuously replacing the metallic ones. This transition allowed a significant weight reduction of flying structures. Consequently, it substantially lessened the fuel consumption and mitigated the environmental footprint of aircrafts and space launch vehicles. However, evoking the mass law, this transition clearly cannot be without consequences on the acoustic transmission loss of fuselage panels. Neither can it be on payload fairings, especially over the low frequencies. Whether to meet with the standards and regulations in terms of acoustic pressure levels inside the pressurized cabins or to protect the payloads inside the launchers fairings, acoustic treatments design targeting the low frequencies seems to be a challenging issue. Indeed, with very stringent constraints in terms of added weight and volume, it is difficult to deal with the low frequencies noise and vibration, using passive monolayers. In order to take advantage of the resonant effects in enhancing low frequencies absorption, this work intends to investigate the integration of heavy treatments of very low thicknesses in acoustic coatings while minimizing weight. It was conducted within the frame work of the industrial research chair in aeroacoustics, whose main funders are : Bombardier Aerospace, Pratt & Whitney and Bell Helicopter. Also, other related researches were jointly carried out in partnership with ULA (United Launch Alliance). This master thesis includes a bibliographical section which consists of an overview of the existing low-frequencies solutions while keeping a critical eye on most of them. The following theoretical part focuses on the different models describing wave s’ propagation and dissipation phenomena in porous media. In the same section, we also list the different types of screens and the corresponding propagation models. Criteria for an objective comparative study in terms of weight versus performance were suggested. In addition to non-acoustic parameters, the layout and the mounting conditions of the acoustic packages were also investigated by means of numerical simulations, seconded by experimental measurements. In the penultimate chapter, an SEA detailed model of a launcher fairing was built based on data taken from a concrete example. The simulations ended with a comparative study of the sound pressure level reduction in the inner fairing cavity. The final chapter summarizes the main findings, conclusions and perspectives of this study.

Résumé : Les véhicules aériens et terrestres sont constitués de systèmes bâtis complexes. La structure principale est généralement composée de panneaux légers renforcés par des éléments rigides. Cette solution de conception est répandue parce qu’elle allie la force et un faible poids. Cependant, on sait qu’elle offre des résultats vibroacoustiques médiocres, c’est à dire que l’effet des perturbations externes qui touchent le système peut générer un niveau de bruit excessif à l’intérieur de la cabine des passagers. C’est une préoccupation majeure chez les fabricants, parce que ce niveau de bruit nuit sensiblement au confort ressenti par les clients et peut causer de la fatigue chez les conducteurs et les pilotes. Pour cette raison, les composants passifs constitués de matériaux dissipatifs assemblés en mode multicouche sont généralement intégrés à la structure. Ces assemblées bordées intègrent surtout des matériaux poroélastiques, qui sont plutôt répandus, grâce à l’agencement intéressant de bonnes propriétés d’isolation sonore et de faible poids. L’intégration en amont des traitements de contrôle du bruit au processus de conception est la clé de succès d’un produit. Pour ce faire, des outils pratiques numériques en mesure de capter le comportement dynamique des systèmes vibroacoustiques impliquant les structures, les cavités et les matériaux d’insonorisation sont requis. D’une part, la modélisation de ces systèmes couplés en utilisant des procédés à base d’éléments finis peut être, bien que précis, irréalisable pour des applications pratiques. D’autre part, les approches analytiques telles que la méthode de matrice de transfert sont souvent préférées grâce à leur facilité d’utilisation, même si elles manquent de précision en raison des hypothèses rigoureuses inhérentes au cadre analytique. Dans ce contexte, les procédures de structuration hybrides sont récemment devenues très populaires. En effet, les différentes techniques de modélisation sont généralement recherchées pour décrire les systèmes vibroacoustiques complexes arbitraires sur la plus large gamme de fréquences possible. L’objectif du projet proposé est de mettre au point un cadre hybride offrant une mé- thodologie simple pour tenir compte des traitements de contrôle du bruit dans l’analyse vibroacoustique par éléments finis. A savoir, le modèle de calcul qui en découle conserve la souplesse et la précision de la méthode des éléments finis en bénéficiant de la simplicité et de l’efficacité de la méthode de matrice de transfert pour obtenir une réduction de la charge de calcul pour la modélisation de composants acoustiques passifs. La performance de la méthode pour prédire la réponse vibroacoustique de structures planes homogènes avec des traitements acoustiques attachées est évaluée. Les résultats démontrent que la méthode hybride proposée est très prometteuse, parce qu’elle permet une réduction de l’effort de calcul tout en conservant suffisamment de précision par rapport à l’analyse complète par éléments finis. En outre, la méthode de matrice de transfert proposée de modélisation des traitements de contrôle des bruits est générale, comme on peut l’appliquer dans d’autres cadres outre l’application de l’élément fini considéré dans ce travail. // Abstract : Aerial and terrestrial vehicles consist of complex built-up systems. The main structure is typically made of light panels strengthened by stiffer components. Such design solution is widely used as it combines strength and low weight. However, it is known to give poor vibroacoustic performances, i.e. the effect of external disturbances acting on the system may generate an excessive noise level inside the passengers cabin. This is a main concern for the manufacturers, as it significantly affects the comfort experienced by the costumers and may fatigue drivers and pilots. For this reason, passive components consisting of dissipative materials assembled in a multilayer fashion are typically integrated within the structure. These lined assemblies mainly involve poroelastic materials, which are commonly used thanks to the appealing combination of good noise insulation properties and low weight. The early integration of noise control treatments in the design process is the key to a successful product. For this purpose, practical numerical tools able to capture the dynamic behavior of vibroacoustic systems involving structures, cavities and noise proofing materials are demanded. On the one hand, modeling such coupled systems using finite element based methods can be, albeit accurate, time consuming for practical applications. On the other hand, analytical approaches such as the transfer matrix method are often preferred thanks to their ease of use, although they suffer from a lack of accuracy due to the stringent assumptions inherent within the analytical framework. In this context, hybrid substructuring procedures have recently become quite popular. Indeed, different modeling techniques are typically sought to describe arbitrarily complex vibroacoustic systems over the widest possible frequency range. The aim of this thesis is to devise a hybrid framework providing a simple methodology to account for noise control treatments in vibroacoustic finite element analysis. Namely, the resulting computational model retains the flexibility and accuracy of the finite element method while taking advantage from the simplicity and efficiency of the transfer matrix method to obtain a reduction of the computational burden in the modeling of passive acoustic components. The performance of the method in predicting the vibroacoustic response of flat structures with attached homogeneous acoustic treatments is assessed. The results prove that the proposed hybrid methodology is very promising, as it allows for a reduction of the computational effort while preserving enough accuracy with respect to full finite element analysis. Furthermore, the proposed transfer matrix based methodology for noise control treatments modeling is general, as it can be used in alternative frameworks besides the finite element application considered in this work.

Este trabalho visa o projeto, modelagem e fabricação de estruturas multicamadas baseadas em polímeros para aplicações como sensores ópticos integrados. A grande motivação para este trabalho está no fato de que estas estruturas, diferente da geometria Mach-Zehnder, dispensam o uso de litografia pois são completamente planares. Isto permite uma diminuição no custo de fabricação dos dispositivos além de permitir que estruturas mais curtas sejam utilizadas. Em se tratando de óptica integrada, as dimensões reduzidas da estrutura impõem severas penalidades no processo de lançamento de potência óptica na mesma. Por isso, será considerada neste estudo a utilização de prismas para o acoplamento de entrada do acoplador. Esta técnica, além de reduzir drasticamente os problemas de alinhamento decorrentes de acoplamento convencional do tipo \"End Fire\", permite uma transferência de potência óptica superior a 80% entre a fibra e o guia retangular. As variações na transferência de potência entre os guias de ondas da estrutura multicamada serão medidas através de um fotodetetor MSM integrado ao sensor. Este detetor, além de sua extrema facilidade de fabricação e baixos custos, torna o conjunto sensor mais robusto. Em se tratando de uma proposta de plataforma para sensores, serão também investigadas possíveis aplicações para esta estrutura, como por exemplo: refratômetro integrado, sensor de glicose, sensor de adulteração de combustível, etc. A análise das estruturas será efetuada por meio de técnicas de modelagem analíticas (Técnica da Matriz de Transferência - TMT e Teoria de Modos Acoplados - TMA), e numéricas (Método da Propagação de Feixe de Ângulo Largo formulado em Diferenças Finitas - WA-FD-BPM). Esta última permite que a estrutura do fotodetetor seja levada em consideração simultaneamente nas simulações. / This work concerns with design, modeling and fabrication of polymer based planar multilayer structures for integrated optic sensor applications. The motivation for this work is that planar multilayer structures, differently from the Mach-Zehnder geometry, do not require a lithographic process. As a consequence, significantly cheaper and shorter structures can be realized. The reduced dimensions of the structure, by its turn, pose a severe penalty in terms of optical power coupling. Therefore, this investigation will focus primarily on input (and output) prism coupling configuration. This technique, besides reducing the alignment requirements observed for conventional end-fire coupling, allows optical power coupling efficiency as high as 80% from fiber to rectangular waveguide. Any optical power transfer between the waveguides of the multilayer structure will be detected by an MSM photodetector integrated with the sensor. This low cost photodetector, besides improving the structure robustness, is quite ease to fabricate. Since the idea of this work is to develop a platform for integrated optic sensors, it will also be investigated possible applications for this structure, such as: integrated optic refractometer, glucose sensor and fuel adulteration sensor. The analysis of such structures will be carried out by means of analythical (Transfer Matrix Technique-TMT and Coupled Mode Theory-CMT) and numerical (Wide-Angle Finite Difference Beam Propagation Method-WA-FD-BPM) modeling techniques. The WA-FD-BPM technique allows one to simulate the multilayer waveguide and the MSM photodetector simultaneously.

Utilizando a técnica da matriz de transferência e as ideias de finite-size scaling, de renormalização fenomenológica e de invariância conforme estudamos dois modelos de polímeros interagentes na rede quadrada. Em ambos, a atividade de um monômero pertencente ao polímero vale x = e. Quando as interações são entre as ligações primeiras-vizinhas que pertencem ao polímero definimos um fator de Boltzmann associado à interação como sendo = e-l, onde l é a energia de interação entre ligações. Se as interações são entre os monômeros que pertencem a sítios primeiros-vizinhos mas não-consecutivos o fator de Boltzmann associado à interação é z = e-m , sendo m a energia de interação entre os monômeros. Através do estudo de pares de tiras de larguras L-L obtivemos estimativas para os diagramas de fases dos dois modelos. Ambos apresentam três fases: uma fase não-polimerizada, uma fase polimerizada usual e uma fase polimerizada densa, na qual o polímero se encontra colapsado. Nessa fase, o polímero assume uma configuração que maximiza o número de interações, de modo que a densidade de sítios ocupados pelo polímero tende a 1 e a densidade de interações assume valores muito próximos de 1. Os diagramas de fases dos dois modelos são qualitativamente semelhantes, havendo diferenças quantitativas, já esperadas. As transições entre a fase não polimerizada e a fase polimerizada densa são de primeira ordem. A transição entre a fase não polimerizada e a fase polimerizada usual é de segunda ordem, e um ponto dessa fronteira, que corresponde ao modelo sem interações ( = 1 ou z = 1, dependendo do modelo), é bem conhecido [1-3] e vale xc = 0,37905227 ± 0,00000012. A transição entre as fases polimerizadas é de primeira ordem para valores pequenos de x, mas muda para uma transição de segunda ordem quando x aumenta. Nessa fronteira existe um ponto tri crítico, que foi estimado em ( xPTC = 1,5 ± 0,1, yPTC = 1,1 ± 0,1 ), para o modelo de ligações interagentes. No caso do modelo de monômeros interagentes, não foi possível obter uma estimativa conclusiva a respeito da localização do ponto tri crítico. No encontro das três fronteiras existe um ponto crítico terminal, no qual terminam a linha de transições de segunda ordem entre a fase não polimerizada e a fase polimerizada usual, a linha de transições descontínuas entre as duas fases polimerizadas e a linha de transições de primeira ordem entre as fases não polimerizadas e a polimerizada densa. Os valores estimados por nós são (xPCT = 0,244 ± 0,002, yPCT = 3,86 ± 0,03) para o modelo de ligações interagentes e (xPCT = 0,345 ± 0,001, zPCT = 1,52 ± 0,001) para monômeros interagentes. O ponto , no qual termina a fronteira de segunda ordem entre a fase não-polimeriza e a fase polimerizada usual e onde ocorre pela primeira vez a transição de colapso é um ponto crítico terminal em ambos os modelos. Os expoentes críticos e associados à fronteira entre a fase não polimerizada e polimerizada usual também foram calculados, e encontramos os valores = 0,7507 ± 0,0008 e = 0,2082 ± 0,0004, para y = 1, e = 0,7498 ± 0,0004 e = 0,205 ± 0,003, para y = 1,2, para o modelo de ligações interagentes. Para o modelo de monômero interagentes, os dados foram = 0,7507 ± 0,0007 e = 0,2089 ± 0,0009, para z = 1, e = 0,7500 ± 0,0004 e = 0,205 ± 0,008, para z = 1,2. Observando os valores dos expoentes, vemos que eles ficam constantes dentro das barras de erros, de modo que a transição é uma transição de segunda ordem usual. Os valores concordam muito bem com os valores esperados, que são (exatamente) = 3/4 e = 5/24 [4]. / Using the transfer matrix technique, finite-size scaling, phenomenological renormalization group, and conformal invariance ideas, we studied the thermodynamic behavior of two interacting models of polymers on the square lattice. In both models one monomer that belongs to the polymer has an activity x = e. When the interactions are between first neighbor bonds that belong to the polymer, we define a Boltzmann factor y = e-l, where l is the interaction energy between two bonds. If the interactions are between monomers located at first neighbor but nonconsecutive sites, the associated Boltzmann factor is z = e-m, where m is the interaction energy between two monomers. We consider pairs of strips of widths L-L\' and found estimates for the phase diagrams of both models. They have three phases: a non-polymerized phase, an usual polymerized phase and a dense phase, in which the polymer is colapsed. In this phase, the configuration of the polymer is that maximizes the number of interactions, and the density of sites occupied by the polymer goes to 1, while the density of interactions is very close to 1. The phase diagrams of two models are qualitatively similar, but. there are quantitative differences between them, as we already expected. The transition between non polymerized phase and dense phase is of first order. The transition between non-polymerized phase and usual polymerized phase is of second order, and one point of this frontier, which corresponds to the non-interacting model (y = 1 ou z = 1, depending on the specific model), is well known [1-3J and has the value xc = 0,37905227 ± 0,00000012. The transition between the two polymerized phases is of first order for small values of x, and it changes to a second order transition when x increases. At this frontier there is a tri critical point, and we found ( xTCP = 1,5 ± 0,1, yTCP = 1,1 ± 0,1 ) for the interacting bond model. It was not possible to obtain a conclusive estimation of the location of the tri critical point for the model of interacting monomers. At the point that all transition lines meet there is a critical endpoint, in which the second order transition line between non polymerized phase and usual polymerized phase, the first order transition line between polymerized phases and the first order transition line between non polymerized phase and dense polymerized phases finish. We found (xCEP = 0,244 ± 0,002, yCEP = 3,86 ± 0,03) for the interacting bond model and (xCEP = 0,345 ± 0,001, zCEP = 1,52 ± 0,001) for the interacting monomer model. The point is where ends the second order transition between non polymerized phase and usual polymerized phase and at this point the collapse transition happens at the first time. Then, in our models, the point is a critical endpoint. We also found the critical exponents and of the second order transition line between non polymerized phase and usual 1\'olymerizcd phase. The values we obtained are = 0,7507 ± 0,0008 and = 0,2082 ± 0,0004, for y = 1, and = 0,7498 ± 0,0004 and = 0,205 ± 0,003, for y = 1,2, to the model of interacting bonds. The interacting monomers model has = 0,7507 ± 0,0007 and = 0,2089 ± 0,0009, for z = 1, and = 0,7500 ± 0,0004 and = 0,205 ± 0,008, for z = 1,2. Looking at these results we can see that the exponents remain constant within error bars, thus the transition is a usual second order transition. Furthermore, these values are in a very good agreement with the expected values, which are = 3/4 and = 5/24 [4].

As redes ópticas de comunicação têm possibilitado, cada vez mais, o oferecimento de serviços do tipo faixa larga. A rede de comunicação totalmente óptica está se tornando a meta tecnológica mais ambiciosa. Grandes esforços têm sido concentrados no desenvolvimento e aperfeiçoamento de materiais e dispositivos não-recíprocos, como isoladores e circuladores constituem uma importante classe de dispositivos ópticos. Os isoladores são utilizados em sistemas ópticos para prevenir a reflexão de luz em lasers e amplificadores. Os circuladores são empregados em esquemas de derivação de sinais que utilizam multiplexação em comprimento de onda (WDM). O funcionamento destes dispositivos é baseado nas propriedades de materiais magnetoópticos. As metas desta dissertação são apresentar as principais características dos materiais magnetoópticos, explorando as características não-recíprocas dos modos TM. Guias planares e tridimensionais são analisados neste trabalho. Para tanto, são obtidas as expressões das componentes dos campos e as equações características dos modos de interesse em estruturas planares por meio da técnica de matriz de transferência (TMT). A análise de propagação de onda em guias planares com materiais magnetoópticos é feita por meio de simulação numérica empregando o método de propagação de feixe (BPM) baseado em diferenças finitas (FD) e o esquema de Crank-Nicholson (CN) na discretização da solução da equação de onda. A condição de fronteira transparente (TBC) é incorporada ao algoritmo FD-BPM com a finalidade de se evitar reflexões de ondas eletromagnéticas para dentro da janela computacional. O método do índice efetivo é empregado na análise de guias de onda tridimensionais do tipo rib. / Optical communication networks have allowed a continuous increase of broadband services offer. The all-optical communication networks are becoming the most ambitious technological goal. Great efforts have been concentrated on the materiaIs and devices development and improvement to make it possible. Nonreciprocal devices, such as isolators and circulators constitute an important class of optical devices. Isolators are used in optical systems to avoid reflection of light in lasers and amplifiers. Circulators are used in signal derivation schemes that use wavelength division multiplexing (WDM). The operation of these devices is based on the properties of magnetooptic materiaIs. The purposes of this dissertation are to present the main features of the magnetooptic materiaIs as well as to analyze the eletromagnetic wave propagation in magnetooptic waveguides, exploring nonreciprocal features of TM modes. Planar and three-dimensional waveguides are analysed in this present study. Therefore expressions of electromagnetic field components and characteristic equations of the modes of interest in planar structures are obtained by using transfer matrix technique (TMT). The wave propagation analysis in planar magnetooptic waveguides is realized by using the finite-difference beam propagation method (FD-BPM) and Crank-Nicholson scheme (CN) applied to wave equation solution discretization. In order to avoid electromagnetic wave reflection into computational window, the transparent boundary condition (TBC) is incorporated to the FD-BPM formalism. The effective index method (EIM) is used in the analysis of three-dimensional rib magnetooptic waveguides.

<p>Intake and Exhaust system noise makes a huge contribution to the interior and exterior noise of automobiles. There are a number of linear acoustic tools developed by institutions and industries to predict the acoustic properties of intake and exhaust systems. The present project discusses and validates, through measurements, the proper modelling of these systems using BOOST-SID and discusses the ideas to properly convert a geometrical model of an exhaust muffler to an acoustic model. The various elements and their properties are also discussed.</p><p>When it comes to Acoustic properties there are several parameters that describe the performance of a muffler, the Transmission Loss (TL) can be useful to check the validity of a mathematical model but when we want to predict the actual acoustic behavior of a component after it is installed in a system and subjected to operating conditions then we have to determine other properties like Attenuation, Insertion loss etc,.</p><p>Zero flow and Mean flow (M=0.12) measurements of these properties were carried out for mufflers ranging from simple expansion chambers to complex geometry using two approaches 1) Two Load technique 2) Two Source location technique. For both these cases, the measured transmission losses were compared to those obtained from BOOST-SID models.</p><p>The measured acoustic properties compared well with the simulated model for almost all the cases.</p>

Untersucht wird der Auswirkung von Anisotropie auf den unordnungsinduzierten Metall-Isolator-Übergang (MIÜ) im Rahmen des dreidimensionalen Anderson-Modells der Lokalisierung für (schwach) gekoppelte Ebenen bzw. Ketten. Mittels numerischer Verfahren (Lanczos- und Transfer-Matrix-Methode) werden Eigenwerte und -vektoren bzw. die Lokalisierungslänge berechnet. Zur Bestimmung des kritischen Exponenten dieses Phasenüberganges 2. Ordnung wird ein allgemeiner Skalenansatz verwendet, der auch den Einfluss einer irrelevanten Skalenvariablen und Nichtlinearitäten berücksichtigt. Ein Kapitel untersucht die verwendeten numerischen Verfahren, verschiedene Methoden werden verglichen und die Portierbarkeit zu Parallelrechnern diskutiert. Der MIÜ wird mit zwei unabhängigen Methoden charakterisiert: Eigenwertstatistik und Transfer-Matrix-Methode. Die Systemgrößenunabhängigkeit der betrachteten Größen am Phasenübergang wird benutzt um den MIÜ zu identifizieren. Sie resultiert aus der Multifraktalität der kritischen Eigenzustände, die für den isotropen Fall bis zu einer Systemgröße von 111^3 Gitterplätzen gezeigt wird. Es stellt sich heraus, daß der MIÜ auch bei sehr starker Anisotropie existiert und bereits bei geringerer Potentialunordnung als im isotropen Fall auftritt. Für den Fall sehr schwach gekoppelter Ebenen wird gezeigt, daß der kritische Exponent mit dem des isotropen Falles übereinstimmt und damit die übliche Einteilung in Universalitätsklassen bestätigt.

Η παρούσα εργασία εστιάζεται στη μελέτη και την αλγοριθμική υλοποίηση των κλασσικών θεωριών Ενεργού Μέσου που αναπτύχθηκαν από τους Maxwell-Garnett και Bruggeman, μεταγενέστερες επεκτάσεις τους καθώς επίσης και ενός μοντέλου βασισμένο στη «θεωρία Mie», με σκοπό να διερευνηθεί εάν και σε ποιόν βαθμό, μπορούν να συμβάλλουν στην προσομοίωση του φαινομένου της τραχύτητας των διεπιφανειών μιας πολυστρωματικής δομής. Στόχος ακόμα της εργασίας είναι η ανάπτυξη φιλικού προς το χρήστη λογισμικού σε περιβάλλον Matlab, το οποίο να ενσωματώνει όλες τις υλοποιήσεις των ανωτέρω θεωριών Ενεργού Μέσου αλλά και εν μέρει να αυτοματοποιεί μέσω αυτών, τη αντιμετώπιση του φαινομένου της τραχύτητας. Το λογισμικό που αναπτύχθηκε συνδυάστηκε με υφιστάμενο λογισμικό της θεωρίας Χαρακτηριστικού Πίνακα για την συνολική αποτίμηση των οπτικών ιδιοτήτων σύνθετων επιστρώσεων. Στο πειραματικό σκέλος της εργασίας, αναπτύχθηκαν με τις μεθόδους Doctor Blade και εξάχνωσης με πυροβόλο δέσμης ηλεκτρονίων, δείγματα μονής επίστρωσης (διηλεκτρικών 𝑇𝑇𝑇2,𝑍𝑛𝑇), διπλής επίστρωσης τύπου «διηλεκτρικό-μέταλλο» (𝑊𝑇3/𝐴𝑔) και τριπλής επίστρωσης τύπου «διηλεκτρικό–μέταλλο-διηλεκτρικό» (𝑊𝑇3/𝐴𝑔/𝑊𝑇3). Για την ασφαλή αξιολόγηση των διαφόρων θεωρητικών μοντέλων που αναπτύχθηκαν, διενεργήθηκαν πολλαπλές μετρήσεις μορφολογίας, πάχους, τραχύτητας και οπτικής συμπεριφοράς (AFM, SEM, XRR, Προφιλομετρία ακίδας, UV-Vis φασματομετρία) Έτσι, τα πεδία σύγκρισης των μοντέλων με τα πειραματικά δεδομένα ήταν τα διαγράμματα φασματικής διαπερατότητας/ανακλαστικότητας και τα επιμέρους πάχη των συμπαγών στρωμάτων και των διεπιφανειών. Από τις απόπειρες προσομοίωσης των παραπάνω επιστρώσεων προκύπτουν σημαντικά συμπεράσματα αναφορικά με την εγκυρότητα των θεωρητικών μοντέλων ως προς την ικανότητα περιγραφής της οπτικής συμπεριφοράς. Το μοντέλο “Mie” περιγράφει εξαιρετικά την οπτική συμπεριφορά απλών υμενίων με έντονη παρουσία αέρα στο εσωτερικό τους, ανεξάρτητα από τα χαρακτηριστικά τραχύτητας που παρουσιάζουν. Αντίθετα, η εγκυρότητα των θεωριών Ενεργού Μέσου σε αυτές τις περιπτώσεις εξαρτάται αυστηρά από τα χαρακτηριστικά τραχύτητας. Παρόλα αυτά οι θεωρίες Ενεργού Μέσου και ιδιαίτερα η περίπτωση Bruggeman, επιτυγχάνουν μια ιδιαίτερα σημαντική βελτίωση στην πρόβλεψη της οπτικής συμπεριφοράς πολυστρωματικών επιστρώσεων – όπου η θεωρία Mie αποτυγχάνει - με την ενσωμάτωσης της τραχύτητας επιφάνειας και διεπαφών. Τέλος δίνονται κατευθύνσεις για την περεταίρω βελτίωση των μοντέλων εξομοίωσης της τραχύτητας αλλά και για την ανάπτυξη ενός ολοκληρωμένου προγράμματος που θα αντιμετωπίζει συνολικά το θέμα της θεωρητικής σχεδίασης επιστρώσεων λεπτών υμενίων. Αποτελέσματα της παρούσας, χρησιμοποιήθηκαν στην δημοσιευθείσα εργασία: • E. Koubli, S. Tsakanikas, G. Leftheriotis*, G. Syrrokostas and P. Yianoulis, Optical properties and stability of near-optimum WO3/Ag/WO3, Solid State Ionics. / This work focuses on the study and algorithmic implementation of the classical Effective Medium theories developed by Maxwell-Garnett and Bruggeman, subsequently extended versions of them as well as a “Mie Theory” based model, in order to investigate whether - and to which extend - they can be exploited for the simulation of surface and interface roughness of a multilayer structure. It also aims in the development of user-friendly software (Matlab graphical user interface), which will integrate the implementations of all the above theories, in order to easily attain different approaches to the composite medium and to deal with the phenomenon of roughness. The overall assessment of the optical properties of composite, multilayer coatings was achieved by the combination of the software described above with an existing Transfer Matrix program. The experimental part of this work included the development, through Doctor Blade and E-gun Vapor Deposition techniques, of single-layer coatings (𝑇𝑇𝑇2, ZnO dielectrics), double-layer coatings of the "dielectric-metal" type (𝑊𝑇3/𝐴𝑔) and triple-layer coatings of the "dielectric-metal-dielectric" type (𝑊𝑇3/𝐴𝑔/𝑊𝑇3). For the safe evaluation of the developed theoretical models, multiple experimental techniques (AFM, SEM, XRR, Step Profilometry, and UV-Vis Spectrometry) were used for the measurement of the morphology, thickness, roughness and optical performance of the coatings. Thus, the evaluation of the theoretical models was based upon the comparison to the experimental data, using criteria such as the spectral transmittance/reflectance response and the thickness of the individual compact layers and interfaces of the coatings. The simulations of the above coatings yielded important conclusions regarding the validity of the theoretical models in the description of optical behavior. "Mie" model is found to describe extremely well the optical properties of simple thin-films containing significant amounts of air, regardless of the roughness characteristics that they present. On the contrary, the validity of Effective Medium theories in such cases is strongly dependent on the characteristics of roughness. Nevertheless Effective Medium theories and particularly the case of Bruggeman are proved to significantly contribute in the forecasting of multilayer coatings’ optical behavior - where Mie theory fails – by the incorporation of surface and interfaces roughness. Finally, directions are given for further improvement for the roughness simulation models as well as for the development of a complete program that would holistically account for the theoretical design/simulation of thin-film coatings. Results of the present work were used in the following published paper: • E. Koubli, S. Tsakanikas, G. Leftheriotis*, G. Syrrokostas and P. Yianoulis, Optical properties and stability of near-optimum WO3/Ag/WO3, Solid State Ionics.

Thesis (Ph.D.); Submitted to Iowa State Univ., Ames, IA (US); 12 Dec 2003. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2048" Stavroula Foteinopoulou. 12/12/2003. Report is also available in paper and microfiche from NTIS.

Les moteurs thermoacoustiques sont des oscillateurs autonomes constitués d'un résonateur acoustique partiellement occupé par un matériau poreux (stack) soumis à un important gradient de température grâce à un apport de chaleur externe. Lorsque le gradient de température imposé le long du stack devient supérieur à un certain gradient critique, appelé seuil de déclenchement, l'interaction fluide-parois se traduit par l'amplification d'une onde acoustique auto-entretenue de fort niveau sur le mode le plus instable du résonateur. L'objet des travaux présentés dans ce mémoire est double. D'une part, il est de proposer un formalisme pour la description du fonctionnement de moteurs thermoacoustiques facilement généralisable à l'ensemble de ces systèmes, qu'ils soient à ondes stationnaires ou à ondes progressives. D'autre part, il est de proposer une approche expérimentale pour la caractérisation du noyau thermoacoustique (incluant le stack et la portion de guide inhomogène en température), qui permette de décrire le comportement de systèmes thermoacoustiques sans formuler d'hypothèses sur la forme du champ de température ou la géométrie du stack.Une modélisation analytique des conditions marginales de stabilité et du taux d'amplification de l'onde est tout d'abord proposée, basée sur l'écriture des matrices de transfert des différents éléments qui constituent le moteur. Ces matrices de transfert associées aux conditions aux limites du système étudié conduisent à une équation caractéristique dont la forme dépend de la géométrie de moteur considérée. La solution de cette équation est une pulsation acoustique complexe dont la partie imaginaire correspond au coefficient d'amplification thermoacoustique.La mesure de la matrice de transfert du noyau thermoacoustique constitue la partie expérimentale des travaux exposés. Elle est réalisée pour différentes conditions de chauffage au moyen d'une méthode à quatre microphones. Dans un premier temps, les résultats expérimentaux sont introduits dans le modèle développé précedemment pour prédire le seuil de déclenchement de divers moteurs thermoacoustiques équipés de ce noyau. Les résultats obtenus grâce à cette méthode sont très proches des observations expérimentales, validant ainsi le banc de mesure et le modèle décrivant les conditions de stabilité. Dans un second temps, les données expérimentales sont utilisées pour affiner un modèle analytique décrivant les mécanismes couplés de propagation acoustique et de transport de la chaleur dans le noyau thermoacoustique : ceci permet notamment d'ajuster les valeurs de paramètres acoustiques et thermiques au moyen d'une méthode inverse.Au-delà du seuil de déclenchement, l'amplification et la saturation de l'onde résultent pour une bonne part du transport de chaleur thermoacoustique et de la convection forcée liée à la génération d'un écoulement redressé (vent acoustique), ces deux mécanismes étant généralement responsables d'une dynamique d'évolution complexe de l'amplitude de pression acoustique au cours du régime transitoire. La dernière partie de ces travaux est consacrée à l'introduction de ces deux effets dans le modèle décrit ci-avant, donnant ainsi accès à la description du régime transitoire de l'onde. Une modélisation simplifiée des transports de chaleur associés au vent acoustique de Rayleigh est notamment proposée, qui permet de montrer que cet effet joue vraisemblablement un rôle important dans la dynamique des régimes transitoires observés expérimentalement dans un générateur thermoacoustique quart d'onde. / Thermoacoustic engines are autonomous oscillators generally made of an acoustic resonator, partially filled with a porous material (stack) submitted to a strong, externally supplied, temperature gradient. When the imposed temperature gradient along the stack becomes larger than a critical value, called the onset threshold, the interaction between the fluid and the solid plates leads to a high-level self-sustained acoustic wave at the frequency of the most unstable mode of the resonator. The purpose of the work presented in this report can be separated into two parts. On the one hand, a theoretical modelling is proposed for the description of thermoacoustic engines, which is derived for standing-wave thermoacoustic systems as well as for travelling-wave systems. On the other hand, an experimental approach is proposed for the characterisation of the thermoacoustic core (corresponding to the stack and the thermal buffer tube), allowing to describe the behaviour of thermoacoustic systems without making any assumptions concerning the shape of the temperature profile or the stack geometry. An analytical network modelling of thermoacoustic engines is proposed for the calculation of the threshold conditions and the amplification rate of the acoustic wave. The transfer matrices, combined with appropriate boundary conditions, lead to the derivation of the characteristic equation of the system. The solution of this equation is a complex angular frequency whose imaginary part represents the thermoacoustic amplification coefficient.The experimental part of this work consists in measuring the transfer matrix of the thermoacoustic core. This is realized for several heat supply conditions, by means of a four-microphones method. First, these experimental results are used for the prediction of onset conditions of standard thermoacoustic engines equipped with the thermoacoustic core. The results obtained with this method are very close to the experimental observations, which allow to judge of the consistency of the experimental apparatus and of the model describing the stability conditions. Second, the experimental datas are used in order to fit an analytical model describing the coupled mechanisms of acoustic propagation and heat transfers in the thermoacoustic core : this allows to estimate the values of acoustic and thermal parameters using an inverse method.Beyond the onset threshold, the amplification and saturation of the wave essentially result from the thermoacoustic heat flux and the convection coming from the generation of a mass flow (acoustic streaming). Both these mechanisms are generally responsible of complicated dynamics for the transient acoustic pressure amplitude. The last part of this work is dedicated to the introduction of both these effects in the previously described model, which leads to the calculation of the transient regime. A simplified modelling of Rayleigh streaming enhanced heat transfers is proposed, which allows to show that this effect may play an important role in the transient dynamics observed in an experimental quarter-wavelength thermoacoustic engine.