Ces travaux de thèse ont porté sur l’étude de nouveaux matériaux pour l’absorption électromagnétique en chambre anéchoïque. Ce sujet a été imaginé à partir de l’étude d’une nouvelle matrice jusqu’alors jamais utilisée pour les absorbants électromagnétiques : la mousse époxy. Cette dernière présente en effet de nombreux avantages par rapport aux matrices habituellement utilisées dans le commerce : possibilité de réaliser des formes complexes, piégeage de la charge au cœur du matériau… Nous avons associé cette matrice avec différentes charges carbonées (noir de carbone, graphite et des fibres longues de carbone). L’association de la mousse époxy avec des fibres de carbone millimétriques a montré d’excellentes performances d’absorption pour un taux de charge très faibles : 0,5 %wt (S11 ≈ -40 dB entre 4 et 18 GHz en incidences normale et oblique). En variant la longueur des fibres de carbone, nous avons mis en évidence qu’il était possible d’améliorer les performances d’absorption en basses fréquences en utilisant des fibres plus longues. Enfin, nous avons dirigé nos recherches sur la réalisation d’un matériau absorbant ‘Vert’ à partir d’une matrice de liège. Ces nouveaux matériaux, réalisés à partir de matières biosourcées, ont montré de meilleures performances d’absorption qu’un absorbant du commerce de mêmes dimensions en incidence normale (S11 = -53 dB et S11 = - 27 dB respectivement à 4,2 GHz) et en incidence oblique (S11 = -50 dB et S11 = -30 dB respectivement à 4,2 GHz) et constituent donc des candidats potentiels pour le remplacement des matériaux absorbants du commerce actuels. / This thesis work focused on the study of new materials for electromagnetic absorption in anechoic chambers. This subject arose from the study of a new matrix which was never used for electromagnetic absorbers until then: the epoxy foam. This foam has many advantages compared to the matrices usually used in the trade like the possibility of cutting complex shapes out of them or trapping the charge in the core of the absorber...This matrix was associated to different carbonaceous load (carbon black, graphite and carbon fibers). The combination of epoxy foam with millimeter carbon fibers has shown better absorption performance at very low loading rates: 0.5 %wt (S11 ≈ -40 dB between 4 and 18 GHz under normal and oblique incidences). The use of different fiber lengths showed that it is possible to improve absorption performance at low frequencies using long carbon fibers. Finally, we directed our work on the creation of an absorbent material from a cork matrix. These new materials, made from bio-based materials, have shown better absorption performance than a commercial absorber, with the same dimensions in normal incidence (S11 = -54 dB and S11 = - 27 dB respectively at 4.26 GHz) and oblique incidence (S11 = -51 dB and S11 = -30 dB respectively at 4.26 GHz) and are therefore potential candidates for the replacement of existing commercial absorbent materials.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The purpose of this work is to study and characterize aerodynamically a free jet operating at subsonic regime and identify its acoustic signature. This study aims to analyze fundamentally the turbulent structures and the total noise produced at different Mach numbers. This kind of research is crucial to the understanding of these mechanisms of noise generation and propagation, and it is extremely important for aeronautical applications, for instance, the jet engine exhaust. The research is done by analysing the data obtained in experiments using pitot tube, hot-wire anemometer and acoustic measurements. This work also describes the experimental procedures for each step of analysis, as well as the characteristics of jet noise facilities. The data from measurements with a pitot tube was used to study the mean velocity profiles. The average properties are also analyzed with an anemometry system, likewise used to study the turbulent intensity at eleven axial lines, ranging from the centerline to the edge of the nozzle (lipline). These results are compared with literature and has verified the accuracy of hot-wire anemometers for turbulent intensities lower than 15%. The aerodynamic data are obtained for Mach numbers 0.25, 0.50 and 0.75, from the nozzle exit to thirteen diameters in the direction of the jet. The acoustic study is carried out by analyzing the sound pressure level obtained at six positions in the far field, with observer angles ranging from 40 to 110º. In this campaign more velocities are studied with Mach numbers from 0.18 to 1.00 with step of 0.05 are described. A database with the sound pressure level as a function of frequency is constructed from this information. / O objetivo deste trabalho é estudar e caracterizar aerodinamicamente um jato livre operando em regime subsônico e identificar a assinatura acústica do mesmo. Esse estudo busca analisar fundamentalmente as estruturas turbulentas e o ruído total produzido em diferentes números de Mach. Tal estudo é crucial para o entendimento desses mecanismos de geração e propagação, e encontra extrema importância para aplicações aeronáuticas, como, por exemplo, a exaustão de motores a reação (jato). A investigação é feita através da análise dos dados obtidos em experimentos utilizando tubo de pitot, anemômetro de fio-quente e ensaios acústicos. Neste trabalho também são descritos os procedimentos experimentais de cada etapa de análise, bem como as características dos laboratórios utilizados para o estudo do ruído de jato. Com os dados provenientes das medições com tubo de pitot são estudados os perfis de velocidade média. As propriedades médias também são analisadas com o sistema de anemometria, que ainda é utilizado para estudo da intensidade turbulenta em onze linhas axiais, variando da linha de centro até a borda do bocal (lipline). Estes resultados são comparados com a literatura e é constatada a acurácia dos anemômetros de fio-quente para intensidades turbulentas menores que 15%. Os dados aerodinâmicos mencionados são obtidos para números de Mach 0,25, 0,50 e 0,75, a partir da saída do bocal até treze diâmetros na direção do jato. O estudo acústico é feito através da análise do nível de pressão sonora obtido em seis posições no campo distante, com ângulos de observação variando de 40 a 110º. Diferentes velocidades também foram analisadas, desta vez, com números de Mach de 0.18 a 1.00 com passo de 0.05. Um banco de dados com o nível de pressão sonora em função da frequência é construído a partir destas informações. / Mestre em Engenharia Mecânica

Ultra wideband (UWB) technology plays a significant role in wireless communication. The complex antenna transfer function (CATF) of an UWB antenna provides important information required for better channel designs and communication systems. In this dissertation the CATF of a Double ridge guide horn (DRGH) antenna is determined and used as a standard antenna for UWB measurements. Two methods were used: the two antenna method in an anechoic chamber and a modified gain-transfer method in a compact antenna test range (CATR). Measurements were performed with a vector network analyser (VNA) in the frequency domain, in the anechoic chamber and the CATR. The distance measurements required to calculate the CATF from the S-parameter measurements were performed in the time domain. The CATF of the standard antenna was determined using two identical antennas and then it was shown that a modified gain-transfer method can be used to determine the CATF of any unknown antenna in a CATR, using the standard antenna as a reference. Some of the challenges were to obtain the correct equations and measurement method to obtain the CATF in a CATR. The standard antenna was used to investigate uncertainty contributions for the measurements in the CATR. / Dissertation (MEng)--University of Pretoria, 2013. / gm2014 / Electrical, Electronic and Computer Engineering / unrestricted

The increasing use of turbochargers is leading to an outstanding research to understand the internal flow in turbomachines. In this frame, computational fluid dynamics (CFD) is one of the tools that can be applied to contribute to the analysis of the fluid-dynamic processes occurring in a turbine. The objective of this thesis is the development of a methodology for performing simulations of radial turbomachinery optimizing the available computational resources. This methodology is used for the characterization of a vaned-nozzle turbine under steady and pulsating flow conditions. An important effort has been devoted in adjusting the case configuration to maximize the accuracy achievable with a certain computational cost. Concerning the cell size, a local mesh independence analysis is proposed as a procedure to optimize the distribution of cells in the domain, thus allowing to use a finer mesh in the most suitable places. Particularly important in turbomachinery simulations is the influence of the approach for simulating rotor motion. In this thesis two models have been compared: multiple reference frame and sliding mesh. The differences obtained using both methods were found to be significant in off-design regions. Steady flow CFD results have been validated against global measurements taken on a gas-stand. The modeling of a turbine, installed either on a turbocharger test rig or an engine, requires the calculation of the flow in the ducts composing the system. Those ducts could be simulated assuming a one-dimensional (1D) approximation, and thus reducing the computational cost. In this frame of ideas, two CFD boundary conditions have been developed. The first one allows performing coupled 1D-3D simulations, communicating the flow variables from each domain through the boundary. The second boundary condition is based in a new formulation for a stand-alone anechoic end, which intends to represent the flow behavior of an infinite duct. Finally, the turbine was simulat / Fajardo Peña, P. (2012). Methodology for the Numerical Characterization of a Radial Turbine under Steady and Pulsating Flow [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/16878 / Palancia

A high spatial resolution acoustic directivity acquisition system (ADAS) has been developed to acquire anechoic measurements of the far field radiation of musical instruments that are either remote controlled or played by musicians. Building upon work performed by the BYU Acoustic Research Group in the characterization of loudspeaker directivity, one can rotate a musical instrument with sequential azimuthal angle increments under a fixed semicircular array of microphones while recording repeated notes or sequences of notes. This results in highly detailed and instructive directivity data presented in the form of high-resolution balloon plots. The directivity data and corresponding balloon plots may be shown to vary as functions of time or frequency. This thesis outlines the development of a prototype ADAS and its application to different sources including loudspeakers, a concert grand piano, trombone, flute, and violin. The development of a method of compensating for variations in the played amplitude at subsequent measurement positions using a near-field reference microphone and Frequency Response Functions (FRF) is presented along with the results of its experimental validation. This validation involves a loudspeaker, with known directivity, to simulate a live musician. It radiates both idealized signals and anechoic recordings of musical instruments with random variations in amplitude. The concept of coherence balloon maps and surface averaged coherence are introduced as tools to establish directivity confidence. The method of creating composite directivities for musical instruments is also introduced. A composite directivity comes from combining the directivities of all played partials to approximate what the equivalent directivity from a musical instrument would be if full spectral excitation could be used. The composite directivities are derived from an iterative averaging process that uses coherence as an inclusion criterion. Sample directivity results and discussions of experimental considerations of the piano, trombone, flute, and violin are presented. The research conducted is preliminary and will be further developed by future students to expand and refine the methods presented here.

Ce travail est motivé par la spectroscopie de photoélectrons et l'étude de la cinématique des galaxies où les données correspondent respectivement à une séquence temporelle de spectres et à une image multispectrale. L'objectif est d'estimer les caractéristiques (amplitude, position spectrale et paramètre de forme) des raies présentes dans les spectres, ainsi que leur évolution au sein des données. Dans les applications considérées, cette évolution est lente puisque deux spectres voisins sont souvent très similaires : c'est une connaissance a priori qui sera prise en compte dans les méthodes développées. Ce problème inverse est abordé sous l'angle de la séparation de sources retardées, où les spectres et les raies sont attribués respectivement aux mélanges et aux sources. Les méthodes de l'état de l'art sont inadéquates car elles supposent la décorrélation ou l'indépendance des sources, ce qui n'est pas le cas. Nous tirons parti de la connaissance des sources pour les modéliser par une fonction paramétrique. Nous proposons une première méthode de moindres carrés alternés : les paramètres de formes sont estimés avec l'algorithme de Levenberg-Marquardt, tandis que les amplitudes et les positions sont estimées avec un algorithme inspiré d'Orthogonal Matching Pursuit. Une deuxième méthode introduit un terme de régularisation pour prendre en compte l'évolution lente des positions; un nouvel algorithme d'approximation parcimonieuse conjointe est alors proposée. Enfin, une troisième méthode contraint l'évolution des amplitudes, positions et paramètres de forme par des fonctions B-splines afin de garantir une évolution lente conforme au physique des phénomènes observés. Les points de contrôle des B-splines sont estimés par un algorithme de moindre carrés non-linéaires. Les résultats sur des données synthétiques et réelles montrent que les méthodes proposées sont plus efficaces que les méthodes de l'état de l'art et aussi efficaces qu'une méthode bayésienne adaptée au problème mais avec un temps de calcul sensiblement réduit. / This work is motivated by photoelectron spectroscopy and the study of galaxy kinematics where data respectively correspond to a temporal sequence of spectra and a multispectral image. The objective is to estimate the characteristics (amplitude, spectral position and shape) of peaks embedded in the spectra, but also their evolution within the data. In the considered applications, this evolution is slow since two neighbor spectra are often very similar: this a priori knowledge that will be taken into account in the developed methods. This inverse problem is approached as a delayed source separation problem where spectra and peaks are respectively associated with mixtures and sources. The state-of-the-art methods are inadequate because they suppose the source decorrelation and independence, which is not the case. We take advantage of the source knowledge in order to model them by a parameterized function. We first propose an alternating least squares method: the shape parameters are estimated with the Levenberg-Marquardt algorithm, whilst the amplitudes and positions are estimated with an algorithm inspired from Orthogonal Matching Pursuit. A second method introduces a regularization term to consider the delay slow evolution; a new joint sparse approximation algorithm is thus proposed. Finally, a third method constrains the evolution of the amplitudes, positions and shape parameters by B-spline functions to guarantee their slow evolution. The B-spline control points are estimated with a non-linear least squares algorithm. The results on synthetic and real data show that the proposed methods are more effective than state-of-the-art methods and as effective as a Bayesian method which is adapted to the problem. Moreover, the proposed methods are significantly faster.

Electromagnetic compatibility (EMC) is an essential part in today’s society and there are more products around us that emit electromagnetic waves than ever before. To make sure that all these products function properly under all circumstances EMC testing is needed. One test that is conducted is radiated immunity testing. A susceptibility antenna is needed to perform immunity testing. This thesis aims to show that a tuneable antenna could be used for immunity testing in the frequency band 26-150 MHz and in the future replace the current antenna, which is not tuneable, used at SAAB Support and Services EMC in Östersund. A simulation program called EZNEC+ was used to simulate different antennas that were tested in the semi-anechoic chamber at SAAB. Two antenna types showed better efficiency and reached lower in frequency than SAAB’s current antenna. These antennas were a bowtie antenna and an x-shaped antenna, both extending in only two spacial directions instead of the normal three. Their drawback was a less uniform E-field at the lowest frequencies, where the E-field was much stronger to the sides compared to straight in front of the antenna. This results in a narrow lobe width, but if this drawback could be accepted or mitigated both antennas are possible replacements for the current antenna at SAAB and should be further investigated.

Cette thèse élargie le champ des applications de l’analogie microonde en adaptant les techniques de fabrication permettant de contrôler les analogues et en développant un outil versatile et précis pour réaliser les mesures. L’analogie microonde consiste à transposer à l’échelle microonde les objets dont nous souhaitons étudier la diffraction en créant un analogue respectant le même rapport dimension sur longueur d’onde, et les mêmes propriétés morphologiques électromagnétiques. Une attention particulière a été portée aux objets faiblement diffractant ayant des niveaux SER aussi faibles que -60 dBm2. A la suite de la caractérisation des réflexions parasites et de celle du bruit aléatoire perturbant les mesures, une nouvelle technique d’optimisation du paramétrage des appareils de mesure a été proposée. Elle comporte notamment un réglage des puissances de source en fonction des angles de bistatisme et un filtrage temporel, par switch, qui a été mis en place et paramétré pour filtrer les signaux parasites mesurés. Les bénéfices de ces diverses optimisations des paramètres de mesures ont été démontrés et ils ont permis de mesurer précisément des niveaux de SER très faibles. De plus, la mise en œuvre de la fabrication additive a permis de réaliser des analogues de géométrie maîtrisée avec des permittivités relatives locales à la carte ayant des parties réelles dans la gamme de 1 à 3 ; la permittivité étant ajustée par contrôle de la porosité. Les trois principales études présentées concernent : des sphéroïdes de faible permittivité, analogues de micro-algues, des agrégats de suies de forme complexes, et des scènes forestières composées d’analogues d’arbres et de véhicules. / This thesis widens the application of the microwave analogy by adopting the appropriate techniques to create objects of controlled shapes and electromagnetic properties using novel manufacturing technologies, as well as by developing a versatile setup providing accurate measurements. Microwave analogy is a useful approach to investigate a scattering problem when the targets have nanometric or metric sizes. The experiment is scaled to the microwave range and the target is mimicked by a centimeter-sized analog, while maintaining the same initial wavelength over target’s dimension ratio and conserving the same geometric and electromagnetic properties. A special attention is given to low scattering targets with RCS levels down to -60 dBm2. The random noise of the measurement setup was characterized and a novel optimization technique was proposed which consists of an angular decomposition of the bistatic region with different power profiles. The undesired reflections within the anechoic chamber were identified and a Hardgating system was installed, allowing to filter out the stray signals. In addition, additive manufacturing technologies were involved in the fabrication of analogs and a novel technique was proposed to obtain targets with “on-demand” shapes and local values of their complex permittivity. We can manufacture low scattering analogs of the real part of permittivity for any value between 1 and 3 by creating well controlled porous structures. The three main targets are studied: low permittivity spheroids, analogs of microalgae, soot aggregates analogs with complex shape, and scaled forest scene composed of tree analogs with some metal vehicle analogs.

Les matériaux absorbants des ondes électromagnétiques sont des éléments importants pour l'évaluation de nombreux systèmes électroniques militaires mais également civils. Ces matériaux sont utilisés, par exemple, pour la réduction des interférences électromagnétiques (EMI) dans divers composants sans fils, la réduction de la surface équivalente radar (SER) ou comme absorbants à l'intérieur des chambres de mesures. C’est cette dernière application qui est visée par les travaux de cette thèse. L’objectif de mes travaux de thèse est d’optimiser des matériaux absorbants utilisés dans les chambres anéchoïques. La géométrie et la composition du matériau absorbant sont les deux paramètres qui influencent la capacité d’absorption de l’onde électromagnétique par un matériau. Ce seront donc les deux pistes d’optimisation explorés durant cette thèse. Notre but est d’obtenir les absorbants présentant les plus faibles coefficients de réflexion et de transmission, soit une absorption élevée, ceci dans une large bande de fréquence. / The electromagnetic absorber materials are important elements for evaluating various electronic and civil systems. These materials are used, for example, for minimizing electromagnetic interferences (EMI) in different wireless components, for minimizing the radar cross section (RCS) or for usage in anechoic chambers. The latter application is the targeted work in this thesis. The objective of this work is to optimize the absorber materials used in anechoic chambers. The geometry and the material composition are the two parameters that influence the absorption of the electromagnetic wave inside the material itself. This are the two topics of optimization explored in this thesis. Our objective is to obtain material absorbers that have low reflection and transmission coefficients and high absorption in a large frequency band.

The master thesis summaries the findings results of a study focused on the spread of noise vibrating flat plates. Part of this thesis is measurements of flat plate noise in the anechoic chamber and finally their processing and evaluation. The next part of the thesis is focused on FEM acoustic simulation and comparing results with measurements.