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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
511

Étude d'un matériau granulaire sec par ondes ultrasonores : effet non linéaire, atténuation et diffusion / Study of a dry granular material by ultrasound wave : nonlinear effect, attenuation and scattering

Laurent, Jérôme 07 July 2011 (has links)
Cette thèse expérimentale porte sur les ondes ultrasonores se propageant dans un milieu granulaire sec, sous contrainte. Dans une première partie, on s'intéresse à la propagation des ondes cohérentes de grande longueur d'onde. La mesure des vitesses de ces ondes en fonction de la contrainte et de la fréquence, montre que les propriétés élastiques du milieu résultent du contact de type Hertz-Mindlin, et que le milieu est non dispersif. La mesure d'atténuation indique un mécanisme dominant du type diffraction de Rayleigh. La propagation d'une onde de Biot à travers l'air dans un empilement sous gravité a été aussi mise en évidence. Dans une seconde partie, on s'est intéressé à la diffusion multiple d'onde acoustique. Nous avons déduit le coefficient de diffusion et le temps inélastique d'absorption en fonction de la contrainte et de la fréquence d'excitation. Le libre parcours moyen de transport a été déterminé en fonction du rapport de la longueur d'onde sur la taille de grains, montrant deux régimes distincts à basse et à haute fréquence. La dernière partie, porte sur le comportement non linéaire hystérétique d'un milieu granulaire, par méthode de résonance. Les modes fondamentaux d'ondes cohérentes ont été étudiés en fonction de l'amplitude d'excitation et de la contrainte de confinement. Un ramollissement élastique de 10%, pour l'onde longitudinale, et de 20%, pour l'onde transversale, sont observés, en bon accord avec un modèle de milieu effectif basé sur la théorie de Mindlin. Les effets d'interaction irréversible son-matière ont été examinés, tels que la compaction intermittente et le processus de cicatrisation. Enfin, on observe que l'atténuation de l'onde transversale dépend conjointement de l'amplitude d'excitation et de la fréquence / This experimental work deals with the ultrasound propagation in a dry granular medium under stress. The first chapter of the thesis concerns the coherent wave propagation in the long-wavelength limit. The measurement of sound velocity as a function of the applied stress and wave frequency confirm that the nonlinear elasticity of the medium originates from the Hertz-Mindlin contact and show that the medium is free of velocity-dispersion. On the other hand, the amplitude measurement reveals that the dominant mechanism of attenuation is due to the Rayleigh-like scattering. Additionally, the airborne Biot wave is observed and investigated. The second chapter is devoted to the diffusively scattered ultrasound. We particularly determine the diffusion coefficient and the inelastic absorption time as a function of stress and frequency. The mean transport free path was investigated versus the ratio of the wavelength to the grain size, which shows two distinct behaviours at low and high frequencies, respectively. The third and last chapter is focused on the hysteretic nonlinear behaviour studied by the resonance method. The fundamental modes of coherent waves are investigated as a function of the applied stress and excitation amplitude. The softening of elastic modulus is observed and can rises to 10% for the longitudinal wave and 20% for the transversal wave, which are in good agreement with the model prediction based on the Mindlin friction theory. Furthermore, the irreversible sound-matter interaction is examined; the intermittent compaction and the slow healing process are observed. Finally, the attenuation of transversal waves is shown to depend simultaneously on wave amplitude and frequency
512

Filtro Hanning modificado com tamanho de janela variável para estimação de amplitude do sinal da rede elétrica

Mendonça, Thiago Ribeiro Furtado de 13 July 2015 (has links)
Submitted by Renata Lopes (renatasil82@gmail.com) on 2015-12-16T11:40:36Z No. of bitstreams: 1 thiagoribeirofurtadodemendonca.pdf: 2671419 bytes, checksum: 9751de9437139252fdb339ca32b4f21d (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2015-12-16T11:57:40Z (GMT) No. of bitstreams: 1 thiagoribeirofurtadodemendonca.pdf: 2671419 bytes, checksum: 9751de9437139252fdb339ca32b4f21d (MD5) / Made available in DSpace on 2015-12-16T11:57:41Z (GMT). No. of bitstreams: 1 thiagoribeirofurtadodemendonca.pdf: 2671419 bytes, checksum: 9751de9437139252fdb339ca32b4f21d (MD5) Previous issue date: 2015-07-13 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Em face ao aumento da demanda por energia elétrica e maior confiabilidade em seu fornecimento, diversas metodologias de controle e proteção da rede elétrica estão sendo desenvolvidas. Para isso é de fundamental importância a utilização de técnicas de processamento de sinais que permitam a extração de forma precisa e rápida dos parâmetros da rede elétrica. O advento de novas tecnologias permitiu a modernização dos sistemas de geração, transmissão e distribuição de energia elétrica, melhorando a eficiência energética e confiabilidade. Por outro lado, observou-se a proliferação na indústria de novas cargas, em sua maioria de natureza não linear, responsáveis pela geração de componentes harmônicos e inter-harmônicos que distorcem o sinal do sistema elétrico (tensão e corrente), dificultando a estimação de parâmetros importantes. Com o intuito de continuar provendo energia de forma segura e confiável, alguns pré-requisitos devem ser atendidos, tais como a baixa distorção harmônica, amplitude estável, baixa oscilação da frequência fundamental, entre outros. Hoje em dia, devido ao maior nível de cargas e equipamentos não lineares conectados a rede, rastrear estes parâmetros de qualidade de energia não é uma tarefa fácil. Para isso, novas técnicas de processamento de sinais devem ser combinadas às teorias de sistemas de potência como forma de estimar e acompanhar as mudanças destes parâmetros. Nesse contexto, diversos algoritmos vêm sendo implementados, cada um com suas especificidades e desempenho. O presente trabalho propõe o desenvolvimento de um novo filtro baseado no filtro Hanning, que é capaz de eliminar harmônicos e atenuar inter-harmônicos com maior eficiência além de ser mais estável quando se considera a implementação em ponto fixo. Este filtro foi combinado com um algoritmo capaz de ajustar seus coeficientes de acordo com a estimação da frequência fundamental de forma a garantir amostragem síncrona, ou seja, a frequência de amostragem será sempre múltipla inteira da frequência atual. Dessa forma, a estimação de amplitude e reconstrução de componentes são feitas com maior precisão mesmo quando se considera a natureza variante da frequência fundamental do sinal bem como a maior quantidade de harmônicos e inter-harmônicos presentes, retratando com maior fidelidade o sinal real do sistema de potência. Os resultados mostram-se superiores às técnicas já existentes, considerando sinais com frequências fora da nominal bem como com grande quantidade de componentes harmônicos e inter-harmônicos, atingindo uma melhoria de 74% em relação ao MAF e de quase 90% em relação ao Hanning convencional. / Due to the growing demand for energy provided in a more reliable way, several control, protection and monitoring methodologies are been developed. For this reason it is of great importance the utilization of appropriate signal processing techniques that allow a precise and fast extraction of parameters from the electrical grid. The advent of new technologies has enabled the modernization of the electric energy generation, transmission and distribution systems, improving its efficiency and service reliability. On the other hand, it was noted a widespread in the application of non-linear loads, responsible for generating harmonics and inter-harmonics components that distorts the electrical signal (voltage and current), hindering the estimation process of useful parameters. With the aim to keep providing energy in a safe and reliable way, some prerequisites must be taken into account, such as the low harmonic distortion, stable amplitude, low oscillation of the fundamental frequency among others. Nowadays, due to the widespread use of nonlinear loads and equipment coupled to the grid, tracking these power quality parameters is not an easy task. That is why novel approaches and techniques of signal processing must be combined with knowledge of power systems theory as a form to estimate and keep track of these measurements. Within this context several algorithms are been developed, each one with its specificity and performance. The present work proposes the development of a new filter, based on the Hanning filter, capable of eliminating harmonics and attenuate with more efficiency the inter-harmonic components besides being more stable for fixed point implementation. This filter is combined with an algorithm that enable the adjustment of its coefficients according to the frequency estimation, assuring a synchronous sampling process, that is, the sampling rate will always be an integer multiple of the actual system frequency. In this way, the amplitude estimation and component reconstruction are performed with better accuracy even when considering the time-varying nature of the fundamental frequency of the signal and the high amount of distortion due to harmonics and inter-harmonics, depicting with greater fidelity the true signal of power system. Results have shown better performance than the already known techniques, considering signal with off-nominal frequency as well as high level of harmonic and inter-harmonic distortion, reaching 74% of improvement in relation with MAF and almost 90% in relation of conventional Hanning.
513

Évaluation non destructive de la contamination du béton par les chlorures avec la technique radar / Nondestructive evaluation of the chlorides contamination in concrete with ground penetrating radar

Ali M'zé, Wahabi 21 March 2018 (has links)
Le géoradar, ou Ground Penetrating Radar (GPR) en anglais, est une méthode non destructive couramment utilisée pour l'auscultation des ouvrages en béton. L'intérêt de cette méthode réside sur sa capacité à ausculter rapidement des très grandes surfaces, elle est de plus en plus employée en Génie Civil. Habituellement, cette méthode est utilisée en Génie Civil pour la localisation les aciers de renforcements, ou bien pour l'estimation de l'épaisseur d'enrobage du béton. Toutefois, la méthode GPR peut aussi être utilisée pour l'auscultation du béton. En effet, le béton est un matériau diélectrique poreux qui peut modifier la propagation des ondes électromagnétiques (EM). Les résultats les plus récents présentent la capacité du GPR à évaluer la teneur en eau. Cependant, le GPR pourrait très bien aussi être utilisé pour la détection des ions chlorure présents dans la solution interstitielle du béton, car comme les chlorures modifient la conductivité du béton ils sont susceptibles d'atténuer les ondes électromagnétiques. Néanmoins, seulement quelques études ont été menées dans ce domaine. Par conséquent, dans cette étude, nous proposons d'utiliser les ondes EM du géoradar pour estimer conjointement la teneur en eau et la teneur en chlorure du béton pour différents corps d'épreuves. Pour cela, plusieurs séries de corps d'épreuves sont utilisées avec des modes de contaminations par les chlorures différents. Une procédure de mesure de la vitesse à partir de l'analyse des signaux réfléchis est proposée. On démontre que la vitesse des ondes EM est essentiellement affectée par la teneur en eau alors que l'atténuation est sensible à la fois à la teneur en eau et à la teneur en chlorures. Ensuite, dans un second temps, nous testons différents modèles de permittivité pour prédire les mesures de constante diélectrique et du facteur de pertes évalués à partir des mesures par GPR ou de résistivité électrique. / Ground Penetrating Radar (GPR) is an usual nondestructive testing method for the assessment of concrete structures. The benefit of this method lies within its ability to assess quickly a large scale of concrete surface. Generally, GPR is used for the localization of reinforcements or for the thickness measurements. However, GPR can be also used for the diagnosis of concrete because concrete is a porous dielectric material which can modify the propagation of the electromagnetic (EM) waves. Most common results present the ability of GPR to assess moisture. But, GPR could be also used to detect the presence of chlorides into the interstitial concrete solution as chlorides can modify the concrete conductivity and altered the electromagnetic signal waves. However, only few studies have been carry-out on that field. Therefore, in this study, we propose to use GPR electromagnetic waves to evaluate both the water content and the chloride content inside the interstitial concrete solution of several tests concrete samples. So, several groups of concrete samples with the same formulation will be conditioned for different chloride contamination modes. Thereafter, a velocity measurement process will be proposed from the reflected signal wave analysis. In that process, we will show that the velocity is only affected by the water content while the attenuation strongly affected by both the water content and the chloride content. Furthermore, we will test several permittivity models to predict the dielectric permittivity and the loss factor estimated from the concrete samples measurements with the GPR device and the electrical resistivity device.
514

Ultrafast, broadband and multi-pulse transmissions for ultrasonic imaging / Émission d'ondes multi-impulsions, planes et larges bandes pour l'acquisition d'images ultrasonores

Benane, Mehdi Yanis 10 December 2018 (has links)
L'échographie est un outil de diagnostic largement utilisé grâce à des vertus telles que l'acquisition / traitement de données en temps réel, la facilité d'utilisation et la sécurité pour le patient / praticien pendant l'examen. Cependant, comparée à d'autres méthodes d'imagerie telles que la tomographie à rayons X et l'imagerie par résonance magnétique, l'échographie présente l'inconvénient de fournir une qualité d'image relativement basse. Dans cette thèse, nous étudions une méthode capable d'augmenter la qualité d'image, permettant ainsi de meilleurs diagnostics échographiques. Afin d'augmenter le rapport signal / bruit des signaux reçus, nous proposons d'utiliser des signaux modulés en fréquence (chirps). Pour éviter l'effet négatif de la bande passante limitée de la sonde, nous modulons en amplitude les signaux d'excitations afin d'augmenter l'énergie du signal dans les bandes de fréquences où la sonde est moins efficace. Pour compresser l'énergie des échos, nous utilisons des filtres de Wiener afin d'obtenir un bon compromis résolution spatiale / stabilité du bruit. Nous combinons cette méthode appelée REC (Resolution Enhancement Technique) avec l’imagerie ultrarapide. Nous montrons des résultats simulés et expérimentaux (in-vitro, ex-vivo et in-vivo) prometteurs. De plus, nous adaptons REC afin de compenser l'effet d'atténuation tissulaire. Cette amélioration est validée expérimentalement sur des phantoms. Nous adaptons également REC à la propagation non linéaire des ondes ultrasonores, en proposant une technique d'inversion d'impulsions qui utilise REC pour fournir une meilleure résolution et un meilleur rapport contraste / bruit. Ensuite, nous appliquons REC à différents schémas d’acquisition tels que les ondes divergentes et la transmission multi-lignes (MLT). Nous montrons également que la qualité d’image peut être augmentée davantage en tenant compte de la réponse impulsionnelle spatiale de la sonde lorsque REC et MLT sont combinés / Ultrasound imaging is a diagnostic tool widely used thanks to such virtues as real-time data acquisition / processing, ease of use and safety for the patient / practitioner during examination. However, when compared to other imaging methods such as X-ray tomography and Magnetic Resonance Imaging, the echography has the disadvantage to provide relatively low image quality. In this thesis, we study a method that is able to increase the ultrasound image quality, thus paving the way towards improved diagnostics based on echography and novel ultrasound applications. In order to increase the echo signal to noise ratio of the received signals, we propose to use linear frequency modulated signals, also called chirps. To avoid the negative effect of the bandlimited acquisition probe, we apply a pre-enhancement step on the probe excitation signals in order to boost the signal energy in the frequency bands where the probe is less efficient. To compress the echo energy in reception, we use Wiener filters that allow obtaining a good trade-off between the spatial resolution and noise stability. We apply the previously detailed pipeline, also called REC (Resolution Enhancement Technique) on ultrafast imaging schemes. We show promising results in simulation and in-vitro, ex-vivo, in-vivo acquisitions. Furthermore, we adapt REC in such way that the frequency dependent tissue attenuation effect is compensated for. This improvement is validated in simulation and phantom experiments. We also adapt REC to the nonlinear propagation of ultrasound waves, by proposing a pulse inversion technique that uses REC to provide a better image resolution and contrast to noise ratio. Then, we demonstrate the generality of the REC method by applying it to different acquisition schemes such as diverging wave compounding and Multi Line Transmit (MLT). We also show that the image quality can be increased more by taking into account the spatial impulse response of the ultrasound probe when REC and MLT are combined
515

Methodes acoustiques pour caractériser les propriétés mécaniques des muscles : approche fondamentale sur le tissu musculaire de souris. Vers une application clinique pour la dystrophie musculaire de Duchenne / Acoustics methods for characterizing mechanical properties of muscle : fundamental approach to muscular mouse tissue. Toward a clinical application for Duchenne muscular dystrophy

Blasco, Hugues 09 December 2010 (has links)
La dystrophie musculaire de Duchenne (DMD) entraine une perte progressive de la force musculaire. L'objectif de ce présent travail est d'étudier la sensibilité d'une méthode acoustique échographique (50 MHz) et d'une méthode résonante (25 kHz) aux changements de propriétés des tissus musculaires de souris mdx modèles. Les paramètres mesurés sont l'atténuation ultrasonore pour la méthode échographique et le module complexe de cisaillement pour la méthode résonante. Dans ce manuscrit nous exposons le développement de ces deux méthodes adaptées à l'étude de deux tissus musculaires de souris : le diaphragme et la peau. La méthode échographique a permis de quantifier les changements de propriétés biologiques du diaphragme en fonction du pourcentage de zone non musculaire sur des souris âgées de 3 mois à 24 mois. La méthode résonante, génère un champ de pression dans le tissu induisant un cisaillement dans le tissu. Cette méthode a permis d'estimer des différences de propriétés mécaniques sur le diaphragme et sur la peau entre les tissus sains et tissu pathologiques. Les résultats obtenus autorisent à penser que le développement de la méthode résonante pour des applications in vivo chez l'Homme atteint de DMD est possible. / The muscular dystrophy of Duchenne (DMD) lead a progressive loss of the muscular strength. The objective of this present work is to study the sensibility of an ultrasound acoustic method (50 MHz) and a résonant method (25 kHz) to the changes of properties of muscle tissues of mdx mice models. The mesured parameters are the ultrasound attenuation for the echographic method and the complexe shear modulus for the résonant method.In this manuscript we explain the development of these two methods adapted to the study of two muscular tissues of mouse: the diaphragm and the skin. The ultrasound method allowed to quantify the changes of biological properties of the diaphragm according to the percentage of non muscular area on 3-month-old mice in 24 months. The résonant acoustic method, generates a field pressure in the tissue leading a shearing tissue. This method allowed to estimate différences of mechanical properties on the diaphragm and on the skin between healthy tissues and pathological tissues. We think the obtained results authorize the development of the resonant method for in vivo applications to human touched by DMD.
516

Attenuation, Stasis, or Amplification: Change in the Causal Effect of Coercive Policies

Smith, Gregory Lyman January 2020 (has links)
No description available.
517

Vliv kvality optických konektorů na přenos / Effect of optical connectors quality on transmission

Bernkopf, Miroslav January 2008 (has links)
This diploma thesis deals with optic fiber connectors and their influence on transmissions realized via fiber optics. Optical fibers and lightguides play the key role in telecommunications because of constant demands of higher bandwidth and data rates. Original metalic wires do not meet market requirements and so the fiber optics are more and more being used. There are various types of fibers and there are also many ways of connections. This work deals with demountable way of connection – optical connectors, which have the biggest influence on a total attenuation and route length. Connector has to meet various requirements such as cleannes, quality, mechanical construction and ferrule polishing.
518

Mikroohybové optovláknové snímače / Microbending fiber optic sensors

Švestka, Jiří January 2013 (has links)
This thesis deals with the influence microbending the transmitted signal at the bare optical fiber. Microbending influence on the transmitted signal is investigated in this work by changing the weight, power and water level in the tank by using this test fixture. The results of the experiments are proposed to enter the laboratory tasks to measure the impact microbending the transmitted signal in the subject line fiber-optic sensors (MOES).
519

Měření parametrů optických spojů / Measurement of optical connections

Kučera, Zdeněk January 2014 (has links)
Communication via fiber optical cable (links) is the latest advancement of the transmission media currently used for data transmission. Despite initial problems, using of this medium in recent years has greatly expanded due to their indisputable advantages. Optical cable is a great combination of reliability and security of data transfer. Its constantly improving transmission parameters contribute to the high speeds in the long-distance transmission of data. In the theoretical part of this work some kinds of optical cables are mentioned, also the most important parameters and the most widely used methods of measurement of these cables. Apart from that other methods of joining optical cables and the most used types of optical cables are also mentioned. The values of the decline and reflection, which are led into the device after connecting these parts into the optical lines. The device using the measuring method of the reverse reflection is also described in this thesis altogether with its most important parts and the way of the display of the measured track on the graphic image of the device. The problematics of identification and decline dead zone and the factors influencing the lenght of dead zone – the ability to identify two independent events on the link patchcord of the variable lenght, is also described in details in this thesis. In the practical part a series of measurements and evaluation (numerical and graphical) of the measured values of decline lenghts and identificating dead zones of the OTDR device will be carried out.
520

KALIBRACE ULTRAZVUKOVÉHO PRŮZVUČNÉHO SYSTÉMU VÝPOČETNÍ TOMOGRAFIE / CALIBRATION OF AN ULTRASONIC TRANSMISSIVE COMPUTED TOMOGRAPHY SYSTEM

Filipík, Adam January 2009 (has links)
Tato dizertace je zaměřena na medicínskou zobrazovací modalitu – ultrazvukovou počítačovou tomografii – a algoritmy zlepšující kvalitu zobrazení, zejména kalibraci USCT přístroje. USCT je novou modalitou kombinující ultrazvukový přenos signálů a principy tomografické rekonstrukce obrazů vyvíjených pro jiné tomografické systémy. V principu lze vytvořit kvantitativní 3D obrazové objemy s vysokým rozlišením a kontrastem. USCT je primárně určeno pro diagnózu rakoviny prsu. Autor spolupracoval na projektu Institutu Zpracování dat a Elektroniky, Forschungszentrum Karlsruhe, kde je USCT systém vyvíjen. Jeden ze zásadních problémů prototypu USCT v Karlsruhe byla absence kalibrace. Tisíce ultrazvukových měničů se liší v citlivosti, směrovosti a frekvenční odezvě. Tyto parametry jsou navíc proměnné v čase. Další a mnohem závažnější problém byl v pozičních odchylkách jednotlivých měničů. Všechny tyto aspekty mají vliv na konečnou kvalitu rekonstruovaných obrazů. Problém kalibrace si autor zvolil jako hlavní téma dizertace. Tato dizertace popisuje nové metody v oblastech rekonstrukce útlumových obrazů, kalibrace citlivosti měničů a zejména geometrická kalibrace pozic měničů. Tyto metody byly implementovány a otestovány na reálných datech pocházejících z prototypu USCT z Karlsruhe.

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