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1
Tew, R. "Imaging theory of surface-breaking discontinuities." Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380008.
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Kearns, James A. "An investigation of the diffraction of an acoustic plane wave by a curved surface of finite impedance." Diss., Georgia Institute of Technology, 1989. http://hdl.handle.net/1853/16749.
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Millerat, Pierre. "Modélisation de la diffusion acoustique par un objet cylindrique : Application de la théorie géométrique de la diffraction." Valenciennes, 1994. https://ged.uphf.fr/nuxeo/site/esupversions/428e39ca-af7d-46a9-bb68-62b53d7f829e.
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Nous cherchons à appréhender le problème de la diffusion acoustique par un objet cylindrique. Cet objet étant soumis à une onde incidente plane normale au cylindre, nous établissons les relations théoriques indépendantes de la nature de l'objet et caractérisant le type de propagation. En utilisant la théorie géométrique de la diffraction, nous justifions la notion d'ondes de surface. Dans un second temps, nous étudions d'un point de vue théorique puis numérique les ondes diffusées par un cylindre fluide, un cylindre élastique et un tube élastique. Nous insistons plus particulièrement sur les caractéristiques numériques des ondes de surface (ondes de Wait, Franz, Rayleigh, Stoneley, Brekhovskikh,) et sur l'existence théorique d'ondes se propageant par de multiples réflexions à travers l'objet cylindrique. Nous étudions, enfin, les ondes se propageant axialement à la surface d'un objet cylindrique.
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Uhrig, Matthias Pascal. "Numerical simulation of nonlinear Rayleigh wave beams evaluating diffraction, attenuation and reflection effects in non-contact measurements." Thesis, Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/54368.
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Although several studies have proven the accuracy of using a non-contact, air-coupled receiver in nonlinear ultrasonic (NLU) Rayleigh wave measurements, inconsistent results have been observed when working with narrow specimens. The objectives of this research are first, to develop a 3D numerical finite element (FE) model which predicts nonlinear ultrasonic measurements and second, to apply the validated model on the narrow waveguide to determine causes of the previously observed experimental issues. The commercial FE-solver ABAQUS is used to perform these simulations. Constitutive law and excitation source properties are adjusted to match experiments conducted, considering inherent effects of the non-contact detection, such as frequency dependent pressure wave attenuation and signal averaging. Comparison of “infinite” and narrow width simulations outlines various influences which impair the nonlinear Rayleigh wave measurements. When the wave expansion is restricted, amplitudes of the fundamental and second harmonic components decrease more significantly and the Rayleigh wavefronts show an oscillating interaction with the boundary. Because of the air-coupled receiver’s finite width, it is sensitive to these edge effects which alter the observed signal. Thus, the narrow specimen adversely affects key factors needed for consistent measurement of material nonlinearity with an air-coupled, non-contact receiver.
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Du, Xinpeng. "Laser-Ultrasonic Measurement of Single-Crystal Elastic Constants from Polycrystalline Samples by Measuring and Modeling Surface Acoustic Wave Velocities." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1524177819455643.
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Sahed, Hocine. "Détection et étude de la perturbation dûe à un défaut dans la paroi d'un réservoir cylindrique sous pression." Valenciennes, 1995. https://ged.uphf.fr/nuxeo/site/esupversions/f640c3d5-db1c-4f28-b1d6-8a393ac30dd2.
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L’expérience a montré que la présence d'un défaut peut modifier considérablement les caractéristiques mécaniques d'une structure et dans certains défavorables, la rendre critique. Le défaut peut souvent être détecté par l'observation de ses effets sur un champ extérieur appliqué. Pour cela, plusieurs techniques non-destructives (rayons x, ultrasonores, etc. ) ont été développées. L’objectif de notre travail est l'analyse théorique de la détection d'un défaut et du problème de singularité posé dans un réservoir cylindrique, à pression. Ce mémoire présente les deux aspects du problème. En contrôle non destructif, la détection par émission acoustique et par diffraction d'ondes élastiques est analysée. Dans le premier cas, la position physique et mathématique est donnée. De la solution (existence et unicité) est déduite l'expression du champ émis (contraintes/déplacement), en un point d'observation dans la zone lointaine du milieu de propagation. Par contre, dans le second cas, on se ramène à une équation intégrale de diffraction d'une onde élastique par un défaut, dans un milieu élastique, isotrope infini. A partir de la formulation en fréquence, le problème variationnel est bien posé, dans un espace Sobolev, lié à l'énergie élastique. Ensuite, on aborde le problème de singularité posé par le défaut dans le réservoir cylindrique à pression. En utilisant le principe de superposition, la théorie des coques nous permet de traduire ce problème en problème mixte aux valeurs limites. Par le biais de la transformation intégrale de Fourrier, un système intégral de Fredholm est obtenu. L’analyse asymptotique et la résolution permettent la caractérisation de l'état perturbé du réservoir et de déterminer ainsi les différents paramètres du champ singulier. Enfin, une approche énergétique est utilisée pour construire un critère de stabilité du réservoir défectueuxThis dissertation deals firstly with a theorical analysis of flaw detection and secondly with the singularity problem in a cylindrical, pressurized vessel (tank). Flaw detection by acoustic emission and elastic waves scattering is discussed within the context of non-destructive inspection. Firstly, the position of the physical and mathematical problem is given. From the solution (Existence and Uniqueness), we express the radiated field (displacement/stress), in a zone far away from the propagation medium. Secondly, under some assumptions about the tank, we consider an infinite, isotopic , elastic medium. Hence, using Fourier transforms, an integral equation is obtained for an elastic wave scattering. We discuss some type of problems. In the framework of Sobolev space, an alternative formulation is well posed. Thirdly, our last interest is in the perturbation problem due to a defect in the cylindrical tank. Within the shell theory, we reduce the problem to a mixed boundary value problem. Using an inverse integral transform, a Fredhom system of integral equations is established. Thus, the asymptotic analysis and the resolution allow us to characterize the perturbed state of the vessel and to determine the parameters of singular fields. A stability criterion, with an energetic approach, is built in order to flaw propagation in the tank. Finally, some subjects for further research in this domain of investigation are suggested : The behavior of a vessel under propagation crack conditions; The oscillations during the process of crack propagation; State of the vibration mode in the vessel perturbed by a flaw; Theoretical study on the conditions of the arrest, initiation, bifurcation of the crack
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Malyarenko, Eugene V. "Lamb wave diffraction tomography." W&M ScholarWorks, 2000. https://scholarworks.wm.edu/etd/1539623991.
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As the worldwide aviation fleet continues to age, methods for accurately predicting the presence of structural flaws, such as hidden corrosion and disbonds, that compromise air worthiness become increasingly necessary. Ultrasonic guided waves, Lamb waves, allow large sections of aircraft structures to be rapidly inspected. However, extracting quantitative information from Lamb wave data has always involved highly trained personnel with a detailed knowledge of mechanical waveguide physics. In addition, human inspection process tends to be highly subjective, slow and prone to errors. The only practical alternative to traditional inspection routine is a software expert system capable of interpreting data with minimum error and maximum speed and reliability. Such a system would use the laws of guided wave propagation and material parameters to help signal processing algorithms automatically extract information from digitized waveforms. This work discusses several practical approaches to building such an expert system.;The next step in the inspection process is data interpretation, and imaging is the most natural way to represent two-dimensional structures. Unlike conventional ultrasonic C-scan imaging that requires access to the whole inspected area, tomographic algorithms work with data collected over the perimeter of the sample. Combined with the ability of Lamb waves to travel over large distances, tomography becomes the method of choice for solving NDE problems. This work explores different tomographic reconstruction techniques to graphically represent the Lamb wave data in quantitative maps that can be easily interpreted by technicians. Because the velocity of Lamb waves depends on the thickness, the traveltimes of the fundamental modes can be converted into a thickness map of the inspected region. Lamb waves cannot penetrate through holes and other strongly scattering defects and the assumption of straight wave paths, essential for many tomographic algorithms, fails. Diffraction tomography is a way to incorporate scattering effects into tomographic algorithms in order to improve image quality and resolution. This work describes the iterative reconstruction procedure developed for the Lamb Wave tomography and allowing for ray bending correction for imaging of moderately scattering objects.
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Hofmann, Matthias. "TEM-Untersuchungen zum Gefüge und zu mechanischen Spannungen in Metallisierungen für SAW-Bauelemente." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2007. http://nbn-resolving.de/urn:nbn:de:swb:14-1189001442992-14894.
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Higher frequencies in the MHz and GHz range and the increasing miniaturization lead to a higher load of the SAW (surface acoustic wave) metallizations. This higher SAW load and the intrinsic stresses result in a stress induced material transport, called acoustomigration. These microstructural changes can destroy the characteristic of the SAW device. Different Al based material combinations were investigated by different authors to improve the reliability of the metallizations and to delay the cost-intensive change to Cu based metallizations. The Cu based metallizations with TaSiN diffusion barriers were also investigated in this work. The barrier layers are necessary to impede the oxygen diffusion into the Cu layer and the Cu diffusion into the piezoelectric substrate. Also in this work the analytical TEM were used as a tool to investigate these microstructural changes in the SAW electrodes. Chemical changes in the metallizations were analysed by EDXS and EELS. The locally high resolved stress measurement in metallizations is a challenge for the future. The CBED (convergent beam electron diffraction) technique has shown the best resolution, however, it can only be applied to TEM lamellas. The aim of this work was to measure the stress within the SAW metallizations by using the CBED method. With it, we could correlate the microstructural changes with the causing stresses within the metallizations. To qualify the CBED method the thermal expansion of Al and Cu single crystals was measured by using a new model for thin TEM lamallas.
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Rogoff, Zigmund M. "Diffraction of acoustic waves at fluid-solid boundaries." Thesis, University of Nottingham, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319952.
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Hofmann, Matthias. "TEM-Untersuchungen zum Gefüge und zu mechanischen Spannungen in Metallisierungen für SAW-Bauelemente." Doctoral thesis, Technische Universität Dresden, 2006. https://tud.qucosa.de/id/qucosa%3A25031.
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Higher frequencies in the MHz and GHz range and the increasing miniaturization lead to a higher load of the SAW (surface acoustic wave) metallizations. This higher SAW load and the intrinsic stresses result in a stress induced material transport, called acoustomigration. These microstructural changes can destroy the characteristic of the SAW device. Different Al based material combinations were investigated by different authors to improve the reliability of the metallizations and to delay the cost-intensive change to Cu based metallizations. The Cu based metallizations with TaSiN diffusion barriers were also investigated in this work. The barrier layers are necessary to impede the oxygen diffusion into the Cu layer and the Cu diffusion into the piezoelectric substrate. Also in this work the analytical TEM were used as a tool to investigate these microstructural changes in the SAW electrodes. Chemical changes in the metallizations were analysed by EDXS and EELS. The locally high resolved stress measurement in metallizations is a challenge for the future. The CBED (convergent beam electron diffraction) technique has shown the best resolution, however, it can only be applied to TEM lamellas. The aim of this work was to measure the stress within the SAW metallizations by using the CBED method. With it, we could correlate the microstructural changes with the causing stresses within the metallizations. To qualify the CBED method the thermal expansion of Al and Cu single crystals was measured by using a new model for thin TEM lamallas.
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Zhang, Qin. "Acoustic pulse diffraction by curved and planar structures with edges." Thesis, University of British Columbia, 1990. http://hdl.handle.net/2429/32005.
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Efficient and accurate solutions of acoustic wave diffraction by a rigid step discontinuity and a curved half-plane are derived by the uniform geometrical theory of diffraction. These solutions can be used in seismic data processing to evaluate and, eventually, to improve the existing data processing procedures. They can also find applications in electromagnetics, microwave antenna design, acoustic design and sound engineering.
The rigid step discontinuity solution given in this thesis is more accurate than the existing
solutions which are based on Kirchhoff theory of diffraction. This solution removes the previous restriction on the source and the receiver arrangement. It also provides high efficiency by the use of ray theory. This solution is further generalized to two offset half-planes and an inclined wedge. Solutions for more complicated structures can be obtained
by superposition of these solutions with added interactions. The complex source position method is used to extend the omnidirectional point source solution to a beam source solution. The effect of changes of the directivity and orientation of the beam source is studied. Time-domain single and double diffraction coefficients are determined through direct Fourier transforming and convolution. An infinite impulse response filter is applied to the time-domain direct computation of single diffraction. This combination achieves a total saving of 75% of computing time over the frequency-domain approach.
Diffraction by a curved half-plane is analyzed with the inclusion of creeping wave diffraction and second order edge diffraction. An acoustic model of a curved half-plane is designed to verify the theory. The experimental results obtained by Mellema have verified the existence of the creeping wave diffraction and weak traces of the second order edge diffraction.Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate
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Assier, Raphaël Charles. "On the diffraction of acoustic waves by a quarter-plane." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610447.
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Chase, Yun S. "Sound diffraction by an underwater ridge with finite impedance." Thesis, Georgia Institute of Technology, 1995. http://hdl.handle.net/1853/17773.
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Lawrow, Alexander. "Domain-inverted transducers for acousto-optic devices." Thesis, University of Kent, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.267423.
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Shaw, Anurupa. "Ice thickness estimation using low frequencies, and an investigation of diffraction of sound in samples with micro structures using ultrasound." Thesis, Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/54242.
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In the first section, the thickness of ice on the lakes and canals is estimated by analyzing the sound spectrum generated by dispersion of Lamb type waves propagating in ice. In winters when the lakes and canals freeze, it is important to know the thickness of the ice layer before setting foot on it. When a stone is thrown on the ice layer, a fluting sound can be heard. This is recorded for different thicknesses of ice, and the sound spectrum is compared with the results simulated using a parameterized model. This model is created using a combination of plane waves for different incident angles and frequencies to generate dispersion curves for different thicknesses of ice. The frequencies of the reflected sound are then compared with the frequencies of musical instruments in order to assign different musical notes to different thicknesses of ice. The technique enables thickness estimation without the use of specialized equipment or time consuming drilling and may therefore be of practical value in the preservation of the lives of ice skaters and playing children. In the second half of the study, high frequencies (400 MHz and 1 GHz) are used to investigate samples with micro structures. Acoustic microscopy is a well established technique as far as smooth surfaces are concerned. V (z) curves are obtained from which, through surface wave generation, important features concerning elasticity and related properties can be extracted. Recently, high resolution imaging using high frequency focused transducers, based on acoustic microscopy has appeared. The surface profiles of the samples used in this study, have periodic structures but lack smoothness. The periodicity causes sound diffraction and the roughness influences the acoustic microscopic investigation. The small acoustic contrast between the substrate and the periodic corrugation on the material, gives us information about the additional stresses which develop and affect the bonding between the two materials. In this study, experiments are conducted using samples with corrugations of different periodicity, and a comparison is made between the results for smoother surfaces and results for the periodic structures of the same material. An attempt is made to analyse the effects described above.
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Willey, Carson Landis. "Ultrasonic Guided Wave Tomography for Wall Thickness Mapping in Pipes." University of Cincinnati / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1460729589.
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Horger, John. "Multiple plane wave analysis of acousto-optic diffraction of Gaussian shaped light beams." Thesis, Virginia Tech, 1987. http://hdl.handle.net/10919/44053.
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A short history of acousto-optics research is presented
along with a general description of how light and sound
interact.
The Multiple Scattering model is derived and used with a Gaussian light beam to observe the distortion in light beam profile within the sound field. Numerical results are presented for comparison to previous studies using thick holograms and two orders of light. The results from using two light orders are compared to four light order results.
A Hamming sound amplitude distribution is introduced as
a possible way to reduce the amount of light beam profile
distortion.
Master of Science
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Matsouli, Ioanna. "Study of magneto-acoustic effects in FeBOâ†3 by synchrotron radiation diffraction imaging." Thesis, University of Warwick, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.310013.
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Dalton, David Raymond. "Derivation and practical application of exact time domain solutions for diffraction of acoustic waves by a half plane." Thesis, University of British Columbia, 1987. http://hdl.handle.net/2429/27866.
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The history of diffraction theory, exact frequency domain solutions and selected past time domain solutions are briefly reviewed. Exact time domain solutions for scattering of plane, cylindrical and spherical acoustic waves by a half plane are derived by inverse Fourier transforming the frequency domain integral solutions. The solutions consist of two diffraction terms, a reflected term and a direct term. The diffracting edge induces step function discontinuities in the direct and reflected terms at two shadow boundaries. At each boundary, the associated diffraction term reaches a maximum amplitude of half the geometrical optics term and has a signum function discontinuity, so that the total field remains continuous. A physical interpretation is developed in terms of Huygen's principle near the diffracting edge.
Solutions for practical point source configurations are evaluated by numerically convolving the impulse diffraction responses with a wavelet. The numerical problems presented by convolution with a singular, truncated operator are solved by analytically derived correction techniques, which are favourably compared to those used by earlier authors. The diffraction solution collapses into a compact discretized formulation. The half plane is shown to be a limiting form of wedge solutions, which can thus be computed using similar algorithms.
Two zero offset sections are produced and compared to approximate Kirchhoff integral solutions. The exact diffraction hyperbola is noticeably non-symmetric, with higher amplitudes on the reflector side of the edge. Near the apex of the hyperbola the Kirchhoff solution is nearly equivalent to the exact diffraction term symmetric in amplitude about the reflection shadow boundary but fails to describe the other, low amplitude, term equivalent to half the response of a line scatterer. The differences are more noticeable on the flanks of the hyperbola, where the two terms are comparable in amplitude, and at shallow depths, due to an aperture effect.
Increasing either the depth of the edge or the angle of the seismic line to the normal to the edge results in a flatter diffraction hyperbola showing little amplitude variation with moveout. As the seismic line becomes parallel to the edge the diffraction curve becomes flat and is indistinguishable from a reflection event. At great depth diffraction events may be mistaken for reflection events as well.
Examples of CDP and CSP gathers, when compared to the Common Offset (CO) gathers, demonstrate that CO gathers are optimal for diffraction processing. Also, since the diffraction moveout and reflection moveout curves differ widely except for depth points near the edge, normal moveout stacking will distort the diffractions and diffraction stacking is essential to retain diffraction information.
Strips of varying width are modelled by superposition of half planes to demonstrate resolution effects and show that the limit of a strip is a line scatterer. A dipping strip and an offset half plane model are produced and added for later comparison with wedge solutions.Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
Graduate
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Viel, Alexis. "Diffraction d'ondes de cisaillement en acoustique picoseconde et mesure de leur coécient de réflexion à une interface : modélisation et expériences." Thesis, Bordeaux, 2017. http://www.theses.fr/2017BORD0610/document.
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L'optoacoustique picoseconde est une technique qui permet de sonder la matière à l'échelle submicronique. L'objectif de ce travail est la synthèse d'ondes de cisaillement de fréquences GHz dans des couches minces isotropes pour l'analyse des propriétés transverses de la matière. C'est grâce à la diffraction du champ acoustique à l'intérieur du transducteur que ces ondes de cisaillement sont synthétisées. Nous exposons dans une première partie le développement des outils théoriques nécessaires à l'analyse de la répartition spatiale du champ acoustique généré lors de l'interaction laser-matière. Les diagrammes de directivité ainsi établis permettent de prévoir les directions de rayonnement privilégiées. Par une méthode de post traitement adaptée,nous synthétisons ensuite des ondes transverses dans ces directions particulières. Lorsque le transducteur est chargé par un milieu dont on cherche à connaître les propriétés transverses, les ondes de cisaillement synthétisées sont réfléchies à l'interface entre le transducteur et ce milieu. L'analyse du coefficient de réflexion permet alors de déterminer les propriétés mécaniques du milieu à sonder. Nous illustrons cette méthode numériquement et expérimentalement dans le cas d'une interface titane/glycérolPicosecond optoacoustics is a technique that can probe properties of matter on a submicronscale. The aim of this work is to synthesize shear waves at GHz frequencies in a thin isotropiclayer in order to analyse its shear properties. Transverse waves are synthesized using diffraction.In the first part we develop theoretical tools in order to analyse the spatial repartition of theacoustic field generated by the laser-matter interaction. Directivity patterns predict the particulardirections for which amplitude of the shear waves is higher. Thus, we synthesized shearwaves in these particular directions using a dedicated post processing method. To investigateshear properties of a film lying on the sample, we study the reection of shear waves betweenthe transducer and the lying film. Starting with either simulated or experimental recordedwaveforms, we demonstrate the method in the specific case of the titanium/glycerol interface
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Ha, Duong Tuong. "Equations intégrales pour la résolution numérique de problèmes de diffraction d'ondes acoustiques dans R**(3)." Paris 6, 1987. http://www.theses.fr/1987PA066420.
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Résolution de problème extérieur pour l'équation des ondes. Etude du cas des ondes harmoniques. Méthode de Schenk, Méthode variationnelle de Nedelec, proposition d'un nouveau système d'équations intégrales pour la résolution du problème du mur absorbant. Analyse de diverses fonctions intégrales pouvant être utilisées pour des calculs d'ondes transitoires (emploi de formules de potentiels retardés). Proposition d'un cadre fonctionnel lié aux formules d'énergie des ondes. Formulations variationnelles espace temps et shémas de type Galerkin basés sur ces mêmes formules d'énergie. Analyse des schémas de type collocation utiles pour discrétiser l'équation intégrale de Kirchoff.
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Marmorat, Simon. "Modèles asymptotiques et simulation numérique pour la diffraction d'ondes par des petites hétérogénéités." Thesis, Université Paris-Saclay (ComUE), 2015. http://www.theses.fr/2015SACLY001/document.
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Cette thèse est consacrée à l'étude du problème de la diffraction d'une onde acoustique par un ensemble de petites hétérogénéités pénétrables ainsi qu'au développement de méthodes de simulation numérique dédiées à la résolution efficace de ce type de problèmes. La principale nouveauté de ces travaux provient du fait que nous traitons ce problème dans le domaine temporel.La première partie de ce manuscrit est consacrée à l'analyse asymptotique du problème de diffraction, menée à bien grâce à la méthode des développements asymptotiques raccordés, le petit paramètre étant la taille caractéristique des défauts $varepsilon$. Ceci nous permet d'obtenir un développement du champ acoustique comme perturbation du problème sans défauts. Nous prouvons un résultat de consistance entre le champ exact et son développement asymptotique en $varepsilon$.Dans la seconde partie, en s'appuyant sur les résultats de l'analyse asymptotique, nous proposons deux modèles approchés pour le problème de diffraction. Ces deux modèles sont bien-posés et leur solution sont chacune des approximations précises du champ total. La principale caractéristique de ces modèles approchés est qu'ils s'appuient tous deux sur une équation d'onde dans le milieu ambiant (sans défauts), couplée à des termes sources auxiliaires permettant de rendre compte de la présence des défauts. Il est ainsi envisageable, pour traiter ces problèmes approchés, d'utiliser une méthode de discrétisation par éléments finis présentant des performances de temps de calcul similaires au cas de la propagation d'une onde dans l'espace libre, puisque l'opérateur des ondes sous-jacent s'appuie sur une géomètrie indépendante des petits défauts. Nous présentons un certain nombre de résultats numériques permettant de valider les deux modèles proposés ainsi qu'une analyse d'erreur numériqueThis work is dedicated to the study of the diffraction of acoustic waves by a set of small inclusions, as well as to the development of numerical methods for the simulation of such phenomenons. The main novelty of this work is that we deal with time-domain waves.The first part of this manuscript deals with the asymptotic analysis of the diffraction problem, which is carried out by matched asymptotics, the small parameter being the characteristic size of the defects $varepsilon$. This furnishes an asymptotic expansion of the acoustic field as a perturbation of the defect-free problem. We prove a consistency result between the total field and its $varepsilon$-asymptotic expansion.In the second part, using the results of the asymptotic analysis, we introduce two approximate models for the diffraction problem. These models are well-posed and their solution are precise approximations of the total acoustic field. One of the main features of these approximate models is that they both lie on a wave equation in the surrounding medium (without defects), coupled to auxiliary source terms which account for the presence of the inclusions. It is then possible to discretize these approximate models using a finite element method, leading to a numerical method which performs as fast as in the defect-free case, since the underlying wave operator is independent of the defects. We present several numerical results which validate both approximate models as well as some insights about numerical error analysis
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Trannoy, Guillaume. "Dispositif tactile acoustique : Etude de la température et de l'intéraction doigt-coque." Thesis, Paris 6, 2014. http://www.theses.fr/2014PA066585/document.
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Ce travail de thèse adresse les phénomènes en jeux dans les interfaces tactiles par ondes de Lamb avec, d’une part, l’influence des perturbations engendrées par les touchers d’utilisateurs et, d’autre part, l’influence des conditions environnementales sur les matériaux et l’algorithme de localisation.Dans une première partie, des mesures expérimentales du comportement viscoélastique de l’index ont été réalisées et relient l’effort et l’angle d’appui à la superficie de contact elliptique par une loi en puissance. Les impédances acoustiques caractéristiques du doigt et de divers matériaux ont été mesurées pour les ondes longitudinales et transversales. Ces paramètres sont représentatifs de la capacité à perturber la propagation d’ondes dans une cavité acoustique. En s’appuyant sur ces résultats, un doigt artificiel a été conçu, permettant des mesures automatisées ou des calibrations répétables de surfaces tactiles.Dans une seconde partie, l’influence des conditions de température et d’humidité relative sur les propriétés mécaniques a été étudiée par l’analyse des vitesses et des atténuations des ondes de volumes dans deux thermoplastiques différents. Alors que l’ABS injecté présente des vitesses de propagation linéairement dépendantes avec la température, elles ont une dépendance quadratique dans le cas d’un polyamide fritté. La sensibilité des ondes de Lamb aux conditions climatiques est estimée à partir des propriétés des matériaux et permet de déduire les modifications des spectres de réponse d’un toucher sur une coque, autorisant une compensation de l’algorithme de localisation. Enfin, différentes pistes d’amélioration du contrôle des ondes guidées et de l’algorithme sont proposées en s’appuyant sur l’utilisation de matériaux viscoélastiques et de représentations temps-fréquenceThis work addresses the phenomena taking place in Lamb wave tactile interfaces by studying the perturbation due to a user touch, and the influence of environmental conditions on materials and localization algorithm.In the first part, viscoelastic measurements of the index finger show a power law relation between force, angle and elliptic area of the contact. The acoustic impedances of index finger and different materials have been measured for longitudinal and transversal waves. These parameters are representatives of the ability to perturb the propagation of waves in the acoustic cavity. From these results, an artificial finger has been designed and allows automated measurements and repeatable calibration of tactile surfaces to be carried out.In the second part, the influence of temperature and humidity on mechanical properties is investigated by analyzing velocities and attenuations of bulk waves in two different thermoplastics. While velocities are linearly dependent on temperature for injected ABS, they are quadratically dependent for the sintered polyamide. Sensibility of Lamb waves to the environmental conditions is estimated with material properties and allows the modifications of the spectral response of a touch to be calculated. That enables compensations in the localization algorithm.Finally, various improvements in the control of guided waves and localization algorithm are proposed. They rely on viscoelastic materials and time-frequency representations
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Holtz, Marcel. "Verschiebestrom induzierte Deformationswellen in LiNbO3 gemessen mittels Schrotrausch-limitierter Femtosekunden-Röntgenbeugung." Doctoral thesis, Humboldt-Universität zu Berlin, 2018. http://dx.doi.org/10.18452/19400.
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Im Rahmen dieser Dissertation wird die Kopplung von einer ultraschnellen optischen Interband-Anregung und der Erzeugung von akustischen Phononen im ferroelektrischen Prototyp-Material Lithiumniobat (LiNbO3) studiert. Unter Verwendung der Anrege-Abfrage-Technik wird LiNbO3 mit ultrakurzen optischen Lichtimpulsen angeregt und die Antwort des LiNbO3-Kristallgitters mit harten Femtosekunden-Röntgenimpulsen abgefragt. Zur Erzeugung der ultrakurzen Röntgenimpulse (100 fs) wird eine kompakte, lasergetriebene Femtosekunden-Röntgenquelle mit einer 1 kHz Repetitionsrate genutzt. Die Stärke und die zeitliche Evolution der beobachteten integralen Röntgen-Reflektivität ausgewählter Bragg-Reflexionen ist charakteristisch für die Propagation von starken akustischen Deformationswellen in einem homogen angeregten Einkristall. Überraschenderweise weisen sowohl die Polarisation als auch die Propagationsgeschwindigkeit der aktivierten Phononmoden eine hohe Anisotropie in Abhängigkeit der Ausrichtung der ferroelektrischen Achse relativ zur Polarisation der optischen Anrege-Impulse auf. Die bekannten Kopplungsmechanismen, wie das elektronische Deformationspotential oder die anharmonische Phonon-Phonon-Wechselwirkung, sind zur Erklärung der akustischen Deformationswellen mit der beobachteten Amplitude und der hohen Anisotropie ungeeignet. Stattdessen konnte gezeigt werden, dass die impulsive mechanische Spannung über den piezoelektrischen Effekt erzeugt wird. Die dafür notwendige makroskopische elektrische Polarisation resultiert aus einem photoinduzierten Verschiebestrom, welcher die Folge der optischen Interband-Anregung mittels Zwei-Photonen-Absorption ist. Diese optisch-elektrisch-mechanische Kopplung zwischen dem nichtlinearen Leitfähigkeitstensor fünfter Ordnung, dem piezoelektrischen Tensor und den relevanten Phononmoden ermöglicht eine vollkommene Beschreibung der experimentellen Beobachtungen.The coupling between an ultrafast optical interband excitation and the generation of acoustic phonons in the ferroelectric prototype material lithium niobate (LiNbO3) is studied. Employing the pump-probe technique, LiNbO3 is excited optically with ultrashort light pulses while the impact on the crystal lattice is probed by femtosecond hard x-ray pulses. A compact, laser-driven femtosecond x-ray source with a 1-kHz repetition rate provides 100-fs pulses for time-resolved experiments. With this experimental arrangement, the response of the LiNbO3 crystal lattice to a nonlinear optical excitation can be investigated. The strength and temporal evolution of the observed integrated x-ray reflectivity of selected Bragg reflections are consistent with the propagation of a strong acoustic strain wave in a homogeneously excited single crystal. Surprisingly, both the polarization of the activated phonon modes and the propagation velocity show a high anisotropy as a function of the ferroelectric axis orientation relative to the polarization of the optical excitation pulses. The previously known coupling mechanisms, such as the electronic deformation potential or the anharmonic phonon-phonon interaction, are insufficient to explain the observed deformation amplitude and high anisotropy of the acoustic strain waves. Instead, it is shown that the impulsive mechanical stress is generated via the piezoelectric effect. The required macroscopic electrical polarization is a result of a photoinduced shift current, which in turn is generated by the optical interband excitation via two-photon absorption. This optical-electrical-mechanical coupling between the fifth-order nonlinear conductivity tensor, the piezoelectric tensor, and the relevant phonon modes fully describe the experimental observations.
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Mohamed, Fathi Husain Alhadi. "Mitigation of Amplitude and Phase Distortion of Signals Under Modified Von Karman Turbulence Using Encrypted Chaos Waves." University of Dayton / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1467943201.
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Tsingos, Nicolas. "Simulation de champs sonores de haute qualité pour des applications graphiques interactives." Phd thesis, Université Joseph Fourier (Grenoble), 1998. http://tel.archives-ouvertes.fr/tel-00528829.
Full textAbstract:
Ce travail porte sur la simulation de champs sonores de haute qualité pour des applications graphiques interactives. Dans ce cadre, nous nous sommes intéressé à trois problèmes : le calcul interactif des effets de l'occultation des ondes sonores par des obstacles, l'intégration du son dans un système d'animation et de réalité virtuelle et la simulation adaptative des réflections du son dans des environnements réverbérants. Nous présentons une méthode originale permettant d'approcher les effets des obstacles sur la propagation des ondes sonores. Cette méthode qualitative est fondée sur le calcul de l'obstruction des premiers ellipsoïdes de Fresnel. Pour cela nous utilisons le rendu câblé des cartes graphiques spécialisées pour effectuer un calcul interactif entre une source et un récepteur ponctuels dans des environnements généraux. Une extension plus quantitative, basée sur la théorie de Fresnel-Kirchhoff est également décrite. Nous décrivons également un système interactif de simulation acoustique. Il permet le rendu synchronisé du son et de l'image dans le cadre d'applications d'animation de synthèse et de réalité virtuelle. Nous y avons intégré notre approche de traitement des occultations sonores. Nous présentons comment d'autres effets, comme les réflections spéculaires du son ou l'effet Doppler sont également pris en compte. Enfin, nous introduisons une technique originale de simulation adaptative fondée sur un formalisme proche de la radiosité hiérarchique utilisée en synthèse d'images. Elle permet de prendre en compte efficacement des réflections globales spéculaires et diffuses dans le cadre d'échanges énergétiques dépendants du temps. La solution obtenue est indépendante du point d'écoute et ouvre la porte à des applications de parcours interactifs de l'environnement virtuel. En outre, la complexité du processus peut être contrôlée, permettant des applications plus quantitatives, comme la prévision des qualités acoustiques de lieux d'écoute. Ces trois contributions peuvent permettre de réaliser un système de simulation complet d'une scène sonore virtuelle pouvant être utilisé dans une variété d'applications. Celles-ci ne se limitent toutefois pas à l'acoustique, mais peuvent être étendues à la simulation de propagation d'ondes radioélectriques pour la téléphonie mobile ou les réseaux sans fils.
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Ferrand, Adrien. "Développement de modèles asymptotiques en Contrôle Non Destructif (CND) par ultrasons : Interaction des ondes élastiques avec des irrégularités géométriques et prise en compte des ondes de tête." Phd thesis, Université Sciences et Technologies - Bordeaux I, 2014. http://tel.archives-ouvertes.fr/tel-01072253.
Full textAbstract:
L'onde de tête est l'onde de première arrivée observée au cours d'une inspection TOFD (Time Of Flight Diffraction). La technique TOFD est une méthode d'inspection très répandue en CND (Contrôle Non Destructif) par ultrasons, faisant appel à deux capteurs piézoélectriques positionnés symétriquement et en vis-à-vis, avec un écartement constant, au-dessus de la surface d'entrée de la pièce à inspecter. Une étude numérique montre que la propagation de l'onde de tête près d'une surface d'entrée irrégulière n'est plus un phénomène de propagation uniquement surfacique comme dans le cas d'une surface plane, mais fait aussi intervenir un phénomène de propagation volumique induit par des diffractions du champ ultrasonore sur les irrégularités de surface. Pour modéliser ces phénomènes, une méthode générique de tracé de rayons fondée sur le principe de Fermat généralisé est développée et détermine le parcours effectif dans une pièce de surface irrégulière de toutes les ondes ultrasonores se propageant dont l'onde de tête. L'évaluation des phénomènes de diffraction par des modèles d'amplitude suivant une approche rayons permet ensuite d'obtenir une simulation complète (temps de vol, front d'onde et amplitude) de l'onde de tête pour plusieurs types d'irrégularités surfaciques. Des validations théoriques et expérimentales de l'outil de simulation développé ont été effectuées et se sont avérées concluantes.
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Guedj, Michel. "Application de l'analyse d'images a la physique des roches." Paris, ENMP, 1987. http://www.theses.fr/1987ENMP0080.
Full textAbstract:
On simule des diagraphies acoustiques par la propagation d'ondes ultrasonores a travers un echantillon de roche dans des conditons (pression temperature, saturation) parfaitement determinees. On injecte de la resine dans la roche, ce qui entraine des phenomenes de diffraction. Pour une analyse d'images automatique, un algorithme original de segmentation par extraction de contours est propose et les problemes methodologiques lies a l'analyse d'images de milieu poreux sont explicites
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Chen, Yamin, and 陳雅旻. "Diffraction Analysis and Compensation of Surface Acoustic Wave Transversal Filters." Thesis, 2002. http://ndltd.ncl.edu.tw/handle/65437901119414857089.
Full textAbstract:
碩士國立交通大學
電信工程系
90
This thesis describes the compensation of the distortion in the transversal SAW filter due to diffraction effect.The origin of the diffraction phenomenon is presented and the mathematical model of the diffraction response is established. Several compensation methods which are classified into in time domain and frequency domain are evaluated. Accordingly, a computer program with effective algorithm is developed.Its capability is demonstrated in the design of an IF bandpass filter applied in Digital Audio Broadcast(DAB).
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Chen, Chao-Yi, and 陳潮億. "Diffraction of Acoustic Waves by Circular and Ring Apertures." Thesis, 1997. http://ndltd.ncl.edu.tw/handle/40805868573505610020.
Full textAbstract:
碩士國立成功大學
水利及海洋工程學系
85
The integral-equation method is applied to determine the diffracted acousticfield about circular and ring apertures in soft and hard baffles. To verifythe accuracy of the numerical algorithm , the diffraction of acoustic wavesabout soft and hard circular disks have been computed and compared withanalytical results . The same numerical method is then applied to computethe diffracted acoustic waves about circular and ring apertures in soft andhard baffles . The Babinet principle has also been used to check whether thecomputed diffracted acoustic wave about a soft (hard) disk is identical toabout a circular aperture in a hard (soft) baffle. Finally , the effectof different parameters on the diffraction is systematically studied andcompared. These parameters include the location of acoustic source, thewavenumber , as well as the size of the aperture relative to the wave length.A general discussion based on a comparison of diffracted acoustic fields abouthard and soft baffles is given.
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Chung, Chen Jian, and 陳建仲. "Diffraction of Acoustic Waves by a Circular Disk of Arbitary Impedance." Thesis, 1995. http://ndltd.ncl.edu.tw/handle/86953304817538065640.
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Huston, Robert Delmar. "Acoustic phase measurements from volume scatter in the ocean." Thesis, 1987. http://hdl.handle.net/1828/5724.
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A primary goal of this thesis has been to demonstrate that stable, useful
measurements of the orientation of the acoustic signal vector as a function of
range and time can be obtained from ocean backscatter, and that this orientation,
or acoustic phase, can be related to the local sound speed distribution.
Such a measurement is quite distinct from the related problem of detecting the
rate of phase change, which forms the basis of Doppler technology. Doppler
measurements can be made using echoes from a single point, or a sparsely
distributed set of targets. Consistent and useful measurement of absolute phase,
is inherently more difficult, since it depends upon the positions of individual scatterers, which are normally random and sparse relative to the acoustic
wavelength.Graduate
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De, Lautour N. J. (Nathaniel J. ). "Radiative transfer in multiply layered media." 2006. http://hdl.handle.net/2292/1904.
Full textAbstract:
The theory of radiative transfer is applied to the problem of multiple wave scattering in a one-dimensional multilayer. A new mathematical model of a multilayer is presented in which both the refractive index and width of each layer are randomized. The layer widths are generated by a new probability distribution which allows for strong layer width disorder. An expression for the transport mean free path of the multilayer is derived based on its single-scattering properties. It will be shown that interference between the field reflected from adjacent layer interfaces remains significant even in the presence of strong layer width disorder. It will be proven that even when the scattering is weak, the field in a random multilayer localizes at certain frequencies. The effect of increasing layer width randomization on this form of localization is quantified. The radiative transfer model of time-harmonic scattering in multilayers is extended to narrow-band pulse propagation in weakly scattering media. The tendency of pulses to broaden in this medium is discussed. A radiative transport model of the system is developed and compared to numerical solutions of the wave equation. It is observed that pulse broadening is not described by simple transfer theory. The radiative transfer model is extended by the addition of a Laplacian term in an attempt to model the effect of ensemble average pulse broadening. Numerical simulation results in support of this proposal are given, and applications for the theory suggested. Finally, the problem of acoustic wave scattering by planar screens is considered. The study was motivated by the idea that multiple scattering experiments may prove possible in a medium composed of such scatterers. Successful multiple scattering in a medium of planar scatterers will depend on the scattering cross-section at angles away from normal incidence. The scattering cross-section is calculated for a circular disc using a new technique for solving the acoustic wave equation on planar surfaces. The method is validated by comparison with available analytic solutions and the geometric theory of diffraction.