Approved for public release; distribution in unlimited. / With the introduction of wireless Local Area Networks (WLANs) in many organizations, it became much easier to intercept confidential files and personal health records. The present study focused on radio frequency propagation in a high-rise building, specifically, the attenuation between floors, and the possibility of intercepting signals through the floors. The current work is based on simulations using the Urbana software tool. It is used to predict the contour of the power levels of signals for a given physical model of the environment using high-frequency ray-tracing methods. The simulation results indicated that the signal levels for a 1 W transmitter could only be detected at the -70 dBm level within two floors (above or below). Even within the two floor range the signal distribution was very nonuniform due to the effects of multipath. The results indicated that closing doors reduced the signal levels, but only slightly for wood doors. Signals escaped the building through the window and were able to travel between floors via this path. The ray tracing accounted for only single diffraction, and therefore rays diffracted two or more times were not included. / Captain, Tunisian Air Force

This thesis aims at revealing the fundamental guiding and coupling properties of nonreciprocal electromagnetic surface waves on magneto-optical or gyromagnetic media and designing novel applications based on the properties. We introduce the background in the first chapter. We then describe the concept of nonreciprocity and the main calculation method in the second chapter. In the third chapter, we show that one-way waves can be sustained at the edge of a gyromagnetic photonic crystal slab under an external magnetic field. We also investigate the coupling between two parallel one-way waveguides. We reveal the condition for effective co-directional and contra-directional coupling. We also notice that the contra-directional coupling is related to the concept of a “trapped rainbow”. In the fourth chapter, we address the concept of a “trapped rainbow”. It aims at trapping different frequency components of the electromagnetic wave packet at different positions in space permanently. In previous structures, the entire incident wave is reflected due to the strong contra-directional coupling between forward and backward modes. To overcome this difficulty, we show that utilizing nonreciprocal waveguides under a tapered external magnetic field can achieve a truly “trapped rainbow” effect at microwave frequencies. We observe hot spots and relatively long duration times around critical positions through simulations and find that such a trapping effect is robust against disorders. Lastly, in the fifth chapter, we study the one-way waves in a surface magnetoplasmon cavity. We find that the external magnetic field can separate the clockwise and anti-clockwise cavity modes into two totally different frequency ranges. This offers us more choices, both in the frequency ranges and in the one-way directions, for realizing one-way components. We also show the waveguide-cavity coupling by designing a circulator, which establishes the foundation for potential applications. / <p>QC 20160816</p><p></p>

L’objectif de ce travail de thèse est l’étude des chocs pyrotechnique à la source par une approche fréquentielle et de leur simulation à l’aide de la TVRC (Théorie Variationnelle des Rayons Complexes). Cette étude s’appuie largement sur un essai réalisé par le CNES dans le cadre du pôle chocs pyrotechniques en juin 2006. Afin de traiter le problème posé par la simulation de la propagation des chocs dans le démonstrateur, la TVRC a été étendue aux coques orthotropes de courbure quelconque. Une formulation variationnelle adaptée a été exhibée et la relation de dispersion a été établie. Les travaux effectués montrent que la prise en compte des moyennes et hautes fréquences, traditionnellement négligées, peut s’avérer essentielle pour la compréhension des phénomènes de propagation des ondes de flexion dans les structures complexes. / This memory is about the study of pyroshocks near the source by a frequency approach and their simulation by the mean of the VCTR (variational Theory of Complex Rays). This study is based on a test made by the CNES (French Space Agency) in the frame of the Pyroshock Pole in june 2006. In order to deal with the problem of shock propagation in the structure on which the test has been proceeded, the VTCR has been extended to orthotropic shells with arbitrary curvature. An adapted variational formulation and the dispersion relation have been established. Taking mid- and high-frequencies into account can be a key point in order to understand propagation phenomena of flexural waves in complex structures.

Thesis (D.Sc.)--University of the Witwatersrand, Faculty of Science, 1959.

Les Essais Non Destructifs (END) par ultrasons permettent de caractériser le béton, sans le dégrader en raison de leurs liens avec ses propriétés mécaniques et sa composition. Cependant, les signaux mesurés résultant de diffusions successives et multiples des ondes sont complexes à analyser. Afin d’optimiser les techniques ultrasonores, il est nécessaire de mieux comprendre les interactions onde-matière dans ce type de milieu et de modéliser au mieux les phénomènes associés. Afin d’aller au-delà des limites des modèles analytiques d’homogénéisation, dans ce travail de thèse un modèle numérique bidimensionnel décrivant la propagation d’ondes ultrasonores dans un milieu hétérogène, adapté au béton, est construit dans le logiciel SPECFEM2D. Ce modèle est comparé à des modèles analytiques, et validé expérimentalement à l’aide d’un milieu synthétique à forte hétérogénéité en comparant les deux paramètres effectifs cohérents : vitesse de phase et atténuation. Il permet également de prendre en compte la viscoélasticité du mortier par l’intermédiaire d’un facteur de qualité. Celui-ci est déterminé à partir des mesures effectuées pour une série de mortiers étudiés.L’outil numérique complet peut être utilisé à plusieurs fins: d’une part, la réalisation d’études afin d’évaluer l’influence de certains paramètres sur la propagation d’onde (la forme et la distribution des granulats), et d’autre part, la simulation des configurations de mesure mises en œuvre sur structure afin de les optimiser en fonction des paramètres qui interviennent, en particulier la fréquence des ondes. Cette meilleure maîtrise des mesures permettra de conduire à terme à l’amélioration du diagnostic. / Ultrasonic non-destructive testing (NDT) is used to characterize concrete, without degrading it, because of its relationship to its mechanical properties and composition. However, the measured signals resulting from successive diffusions and thus from multiple scattering are therefore complex to analyze. In order to optimize ultrasonic techniques, it is thus necessary to better understand the wave-material interactions in this type of medium and to better model the associated phenomena. In order to go beyond the limits of analytical homogenization models, in this thesis a two-dimensional numerical model describing the propagation of ultrasonic waves in a heterogeneous medium, adapted to concrete, is built in the SPECFEM2D software package. This model is compared to analytical models, and validated experimentally using a synthetic medium with high heterogeneity by comparing the two effective parameters of coherent waves: phase velocity and attenuation. This numerical model also makes it possible to take into account the viscoelasticity of the mortar by means of a quality factor. This quality factor is determined from measurements made for a series of mortars that we study. The complete set of numerical tools developed in this work can be used for several purposes: firstly, to carry out studies to evaluate the influence of certain parameters on wave propagation (the shape and distribution of aggregates), and secondly, the simulation of the measurement configurations implemented for a structure in order to optimize them in terms of the parameters involved, in particular the wave frequency. This better control of the measures will ultimately lead to better diagnosis.

by Yue Ping Zhang. / Publication date from spine. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1995. / Includes bibliographical references (leaves 194-200). / DEDICATION / ACKNOWLEDGMENTS / Chapter / Chapter 1. --- Introduction --- p.1 / Chapter 1.1 --- Brief Description of Tunnels --- p.1 / Chapter 1.2 --- Review of Tunnel Imperfect Waveguide Models --- p.2 / Chapter 1.3 --- Review of Tunnel Geometrical Optical Model --- p.4 / Chapter 1.4 --- Review of Tunnel Propagation Experimental Results --- p.6 / Chapter 1.5 --- Review of Existing Tunnel UHF Radio Communication Systems --- p.13 / Chapter 1.6 --- Statement of Problems to be Studied --- p.15 / Chapter 1.7 --- Organization --- p.15 / Chapter 2 --- Propagation in Empty Tunnels --- p.18 / Chapter 2.1 --- Introduction --- p.18 / Chapter 2.2 --- Propagation in Empty Tunnels --- p.18 / Chapter 2.2.1 --- The Imperfect Empty Straight Rectangular Waveguide Model --- p.19 / Chapter 2.2.2 --- The Hertz Vectors for Empty Straight Tunnels --- p.20 / Chapter 2.2.3 --- The Propagation Modal Equations for Empty Straight Tunnels --- p.23 / Chapter 2.2.4 --- The Propagation Characteristics of Empty Straight Tunnels --- p.26 / Chapter 2.2.5 --- Propagation Numerical Results in Empty Straight Tunnels --- p.30 / Chapter 2.3 --- Propagation in Empty Curved Tunnels --- p.36 / Chapter 2.3.1 --- The Imperfect Empty Curved Rectangular Waveguide Model --- p.37 / Chapter 2.3.2 --- The Hertz Vectors for Empty Curved Tunnels --- p.39 / Chapter 2.3.3 --- The Propagation Modal Equations for Empty Curved Tunnels --- p.41 / Chapter 2.3.4 --- The Propagation Characteristics of Empty Curved Tunnels --- p.43 / Chapter 2.2.5 --- Propagation Numerical Results in Empty Curved Tunnels --- p.47 / Chapter 2.4 --- Summary --- p.50 / Chapter 3 --- Propagation in Occupied Tunnels --- p.53 / Chapter 3.1 --- Introduction --- p.53 / Chapter 3.2 --- Propagation in Road Tunnels --- p.53 / Chapter 3.2.1 --- The Imperfect Partially Filled Rectangular Waveguide Model --- p.54 / Chapter 3.2.2 --- The Scalar Potentials for Road tunnels --- p.56 / Chapter 3.2.3 --- The Propagation Modal Equations for Road Tunnels --- p.59 / Chapter 3.2.4 --- Propagation Numerical Results in Road Tunnels --- p.61 / Chapter 3.3 --- Propagation in Railway Tunnels --- p.64 / Chapter 3.3.1 --- The Imperfect Periodically Loaded Rectangular Waveguide Model --- p.65 / Chapter 3.3.2 --- The Surface Impedance Approximation --- p.66 / Chapter 3.3.2.1 --- The Surface Impedance of a Semi-infinite Lossy Dielectric Medium --- p.66 / Chapter 3.3.2.2 --- The Surface Impedance of a Thin Lossy Dielectric Slab --- p.67 / Chapter 3.3.2.3 --- The Surface Impedance of a Three-layered Half Space --- p.69 / Chapter 3.3.2.4 --- The Surface Impedance of the Sidewall of a Train in a Tunnel --- p.70 / Chapter 3.3.3 --- The Hertz Vectors for Railway Tunnels --- p.71 / Chapter 3.3.4 --- The Propagation Modal Equations for Railway Tunnels --- p.73 / Chapter 3.3.5 --- The Propagation Characteristics of Railway Tunnels --- p.76 / Chapter 3.3.6 --- Propagation Numerical Results in Railway Tunnels --- p.78 / Chapter 3.4 --- Propagation in Mine Tunnels --- p.84 / Chapter 3.4.1 --- The Imperfect periodically Loaded Rectangular Waveguide Model --- p.85 / Chapter 3.4.2 --- The Hertz Vectors for Mine Tunnels --- p.86 / Chapter 3.4.3 --- The Propagation modal Equations for Mine Tunnels --- p.88 / Chapter 3.4.4 --- The Propagation Characteristics of Mine Tunnels --- p.95 / Chapter 3.4.5 --- Propagation Numerical Results in Mine Tunnels --- p.96 / Chapter 3.5 --- Summary --- p.97 / Chapter 4 --- Statistical and Deterministic Models of Tunnel UHF Propagation --- p.100 / Chapter 4.1 --- Introduction --- p.100 / Chapter 4.2 --- Statistical Model of Tunnel UHF Propagation --- p.100 / Chapter 4.2.1 --- Experiments --- p.101 / Chapter 4.2.1.1 --- Experimental Set-ups --- p.102 / Chapter 4.2.1.2 --- Experimental Tunnels --- p.104 / Chapter 4.2.1.3 --- Experimental Techniques --- p.106 / Chapter 4.2.2 --- Statistical Parameters --- p.109 / Chapter 4.2.2.1 --- Parameters to Characterize Narrow Band Radio Propagation Channels --- p.109 / Chapter 4.2.2.2 --- Parameters to Characterize Wide Band Radio Propagation Channels --- p.111 / Chapter 4.2.3 --- Propagation Statistical Results and Discussion --- p.112 / Chapter 4.2.3.1 --- Tunnel Narrow Band Radio Propagation Characteristics --- p.112 / Chapter 4.2.3.1.1 --- Power Distance Law --- p.114 / Chapter 4.2.3.1.2 --- The Slow Fading Statistics --- p.120 / Chapter 4.2.3.1.3 --- The Fast Fading Statistics --- p.122 / Chapter 4.2.3.2 --- Tunnel Wide Band Radio Propagation Characteristics --- p.125 / Chapter 4.2.3.2.1 --- RMS Delay Spread --- p.126 / Chapter 4.2.3.2.2 --- RMS Delay Spread Statistics --- p.130 / Chapter 4.3 --- Deterministic Model of Tunnel UHF Propagation --- p.132 / Chapter 4.3.1 --- The Tunnel Geometrical Optical Propagation Model --- p.134 / Chapter 4.3.2 --- The Tunnel Impedance Uniform Diffracted Propagation Model --- p.141 / Chapter 4.3.2.1 --- Determination of Diffraction Points --- p.146 / Chapter 4.3.2.2 --- Diffraction Coefficients for Impedance Wedges --- p.147 / Chapter 4.3.3 --- Comparison with Measurements --- p.151 / Chapter 4.3.3.1 --- Narrow Band Comparison of Simulated and Measured Results --- p.151 / Chapter 4.3.3.1.1 --- Narrow Band Propagation in Empty Straight Tunnels --- p.151 / Chapter 4.3.3.1.2 --- Narrow Band Propagation in Curved or Obstructed Tunnels --- p.154 / Chapter 4.3.3.2 --- Wide Band Comparison of Simulated and Measured Results --- p.158 / Chapter 4.3.3.2.1 --- Wide Band Propagation in Empty Straight Tunnels --- p.159 / Chapter 4.3.3.2.2 --- Wide Band Propagation in an Obstructed Tunnel --- p.163 / Chapter 4.4 --- Summary --- p.165 / Chapter 5 --- Propagation in Tunnel and Open Air Transition Region --- p.170 / Chapter 5.1 --- Introduction --- p.170 / Chapter 5.2 --- Radiation of Radio Waves from a Rectangular Tunnel into Open Air --- p.171 / Chapter 5.2.1 --- Radiation Formulation Using Equivalent Current Source Concept --- p.171 / Chapter 5.2.2 --- Radiation Numerical Results --- p.175 / Chapter 5.3 --- Propagation Characteristics of UHF Radio Waves in Cuttings --- p.177 / Chapter 5.3.1 --- The Attenuation Constant due to the Absorption --- p.178 / Chapter 5.3.2 --- The Attenuation Constant due to the Roughness of the Sidewalls --- p.182 / Chapter 5.3.3 --- The Attenuation Constant due to the tilts of the Sidewalls --- p.183 / Chapter 5.3.4 --- Propagation Numerical Results in Cuttings --- p.184 / Chapter 5.4 --- Summary --- p.187 / Chapter 6 --- Conclusion and Recommendation for Future Work --- p.189 / APPENDIX --- p.193 / The Approximate Solution of a Transcendental Equation --- p.193 / REFERENCES --- p.194

by Lee Wai Hung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (leaves 118-121). / Abstracts in English and Chinese. / Chapter 1 --- INTRODUCTION --- p.1 / Chapter 1.1 --- Background --- p.1 / Chapter 1.2 --- Structure of Thesis --- p.8 / Chapter 1.3 --- Methodology --- p.8 / Chapter 2 --- BACKGROUND THEORY --- p.10 / Chapter 2.1 --- Radio Wave Propagation Modeling --- p.10 / Chapter 2.1.1 --- Basic Propagation Phenomena --- p.10 / Chapter 2.1.1.1 --- Propagation in Free Space --- p.10 / Chapter 2.1.1.2 --- Reflection and Transmission --- p.11 / Chapter 2.1.2 --- Practical Propagation Models --- p.12 / Chapter 2.1.2.1 --- Longley-Rice Model --- p.13 / Chapter 2.1.2.2 --- The Okumura Model --- p.13 / Chapter 2.1.3 --- Indoor Propagation Models --- p.14 / Chapter 2.1.3.1 --- Alexander Distance/Power Laws --- p.14 / Chapter 2.1.3.2 --- Saleh Model --- p.15 / Chapter 2.1.3.3 --- Hashemi Experiments --- p.16 / Chapter 2.1.3.4 --- Path Loss Models --- p.17 / Chapter 2.1.3.5 --- Ray Optical Models --- p.18 / Chapter 2.2 --- Ray Tracing： Brute Force approach --- p.20 / Chapter 2.2.1 --- Physical Layout --- p.20 / Chapter 2.2.2 --- Antenna Information --- p.20 / Chapter 2.2.3 --- Source Ray Directions --- p.21 / Chapter 2.2.4 --- Formulation --- p.22 / Chapter 2.2.4.1 --- Formula of Amplitude --- p.22 / Chapter 2.2.4.2 --- Power Reference E o --- p.23 / Chapter 2.2.4.3 --- Power spreading with path length 1/d --- p.23 / Chapter 2.2.4.4 --- Antenna Patterns --- p.23 / Chapter 2.2.4.5 --- Reflection and Transmission Coefficients --- p.24 / Chapter 2.2.4.6 --- Polarization --- p.26 / Chapter 2.2.5 --- Mean Received Power --- p.26 / Chapter 2.2.6 --- Effect of Thickness --- p.27 / Chapter 2.3 --- Neural Network --- p.27 / Chapter 2.3.1 --- Architecture --- p.28 / Chapter 2.3.1.1 --- Multilayer feedforward network --- p.28 / Chapter 2.3.1.2 --- Recurrent Network --- p.29 / Chapter 2.3.1.3 --- Fuzzy ARTMAP --- p.29 / Chapter 2.3.1.4 --- Self organization map --- p.30 / Chapter 2.3.1.5 --- Modular Neural network --- p.30 / Chapter 2.3.2 --- Training Method --- p.32 / Chapter 2.3.3 --- Advantages --- p.33 / Chapter 2.3.4 --- Definition --- p.34 / Chapter 2.3.5 --- Software --- p.34 / Chapter 3 --- HYBRID MODULAR NEURAL NETWORK --- p.35 / Chapter 3.1 --- Input and Output Parameters --- p.35 / Chapter 3.2 --- Architecture --- p.36 / Chapter 3.3 --- Data Preparation --- p.42 / Chapter 3.4 --- Advantages --- p.42 / Chapter 3.5 --- Limitation --- p.43 / Chapter 3.6 --- Applicable Environment --- p.43 / Chapter 4 --- INDIVIDUAL MODULES IN HYBRID MODULAR NEURAL NETWORK --- p.45 / Chapter 4.1 --- Conversion between spherical coordinate and Cartesian coordinate --- p.46 / Chapter 4.1.1 --- Architecture --- p.46 / Chapter 4.1.2 --- Input and Output Parameters --- p.47 / Chapter 4.1.3 --- Testing result --- p.48 / Chapter 4.2 --- Performing Rotation and translation transformation --- p.53 / Chapter 4.3 --- Calculating a hit point --- p.54 / Chapter 4.3.1 --- Architecture --- p.55 / Chapter 4.3.2 --- Input and Output Parameters --- p.55 / Chapter 4.3.3 --- Testing result --- p.56 / Chapter 4.4 --- Checking if an incident ray hits a Scattering Surface --- p.59 / Chapter 4.5 --- Calculating separation distance between source point and hitting point --- p.59 / Chapter 4.5.1 --- Input and Output Parameters --- p.60 / Chapter 4.5.2 --- Data Preparation --- p.60 / Chapter 4.5.3 --- Testing result --- p.61 / Chapter 4.6 --- Calculating propagation vector of secondary ray --- p.63 / Chapter 4.7 --- Calculating polarization vector of secondary ray --- p.63 / Chapter 4.7.1 --- Architecture --- p.64 / Chapter 4.1.2 --- Input and Output Parameters --- p.65 / Chapter 4.7.3 --- Testing result --- p.68 / Chapter 4.8 --- Rejecting ray from simulation --- p.72 / Chapter 4.9 --- Calculating receiver signal --- p.73 / Chapter 4.10 --- Further comment on preparing neural network --- p.74 / Chapter 4.10.1 --- Data preparation --- p.74 / Chapter 4.10.2 --- Batch training --- p.75 / Chapter 4.10.3 --- Batch size --- p.78 / Chapter 5 --- CANONICAL EVALUATION OF MODULAR NEURAL NETWORK --- p.80 / Chapter 5.1 --- Typical environment simulation compared with ray launching --- p.80 / Chapter 5.1.1 --- Free space --- p.80 / Chapter 5.1.2 --- Metal ground reflection --- p.81 / Chapter 5.1.3 --- Dielectric ground reflection --- p.84 / Chapter 5.1.4 --- Empty Hall --- p.86 / Chapter 6 --- INDOOR PROPAGATION ENVIRONMENT APPLICATION --- p.90 / Chapter 6.1 --- Introduction --- p.90 / Chapter 6.2 --- Indoor measurement on the Third Floor of Engineering Building --- p.90 / Chapter 6.3 --- Comparison between simulation and measurement result --- p.92 / Chapter 6.3.1 --- Path 1 --- p.93 / Chapter 6.3.2 --- Path 2 --- p.95 / Chapter 6.3.3 --- Path 3 --- p.97 / Chapter 6.3.4 --- Path 4 --- p.99 / Chapter 6.3.5 --- Overall Performance --- p.100 / Chapter 6.4 --- Delay Spread Analysis --- p.101 / Chapter 6.4.1 --- Location 1 --- p.103 / Chapter 6.4.2 --- Location 2 --- p.105 / Chapter 6.4.3 --- Location 3 --- p.107 / Chapter 6.4.4 --- Location 4 --- p.109 / Chapter 6.4.5 --- Location 5 --- p.111 / Chapter 6.5 --- Summary --- p.112 / Chapter 7 --- CONCLUSION --- p.I / Chapter 7.1 --- Summary --- p.113 / Chapter 7.2 --- Recommendations for Future Work --- p.115 / PUBLICATION LIST --- p.117 / BIBLIOGRAHY --- p.118

The work presented in this thesis studies acoustic perturbations in slowly varying pipes. The slow variation is introduced in the form of a small parameter ${\epsilon}$ and through this in turn gives rise to a slow axial scale $X$ such that $X = {\epsilon}x$ where $x$ is the normal axial coordinate. This allows an asymptotic approach and the WKB method is used to solve the subsequent mathematical problems. The first deals with the existence of a trapped mode in a hard-walled pipe of varying radius conveying fluid. For the derived leading order propagating mode solution, its amplitude becomes singular at transition points $X_{t}$ and $X_{t'}$ where $X_{t} > 0$ and $X_{t'} < 0$ and thus is unable to propagate past these points. Because of the break down in the solution, this leads to the theory that in the neighbourhood of these points there exists a boundary layer in which the original assumption about having slow variation does not hold. By first seeking the thickness of the layer, valid solutions can then be derived and then matched to the outer solutions in order to produce a uniform solution which holds for the entire axial domain. Once this is achieved, it is then used to derive trapped mode solutions. In this case, the theory used is that of two single turning points which are then combined to obtain the full solution. It is illustrated through consideration of examples and the dependence on ${\epsilon}$ is also shown through various plots. This problem will be considered for a symmetric and asymmetric duct and for differing duct parameters. Problems may arise when the two turning points lie close together and so we seek to improve on the method used by deriving a solution to trapped modes encompassing both turning points, which will be proposed together with some illustrations in order to justify its use and reliability. Next, the case of mode propagations on a thin elastic shell of varying radius conveying fluid is studied. The acoustic solutions of a straight shell in vacuo are first briefly reviewed and then built up by the addition of radius variation and the presence of a stationary fluid. The work presented first outlines the analysis for wave propagation in a slowly-varying thin elastic shell in vacuo. It is found that the shell and the fluid terms are coupled through the fluid pressure term, which is added to the equation governing the radial shell displacements since the pressure is assumed to affect radial motion only. Once the newly corrected equation for the radial shell displacements has been obtained, together with the axial and azimuthal displacements equations, this new system of governing equations is then separated into leading order ${\epsilon}^{0}$ and first order ${\epsilon}^{1}$ systems. In order to simplify the calculations, only the zeroth azimuthal order $m = 0$ will be studied here. With this simplification, a notable result is that the solutions of the torsional motion is decoupled from the axial and radial solutions. Once the dispersion equation is extracted from the leading order system, it can be seen that the axial and radial solutions are in fact coupled. The solution to the in vacuo with varying radius problem is first briefly presented and it is then followed by the solution to the fluid inclusion problem with varying radius, which makes up the main part of this section. The solution is studied for various frequencies and at various points along the shell. In addition, the axial and radial components of the first three modes are examined along with their amplitudes and energy distributions. Finally, mean flow is added and the same analysis is carried out, paying particular attention to the differences which arise in comparison to the stationary flow case.

The present study utilizes the ultrasonic travel time technique to diagnose grid generated turbulence. Ultrasonic flow metering technology relies on the measurement and computation of small perturbations in the travel time of acoustic ultrasonic waves through the dynamic medium. The statistics of the travel time variations of ultrasonic waves that are caused by turbulence probably affect the performance of ultrasonic flow meters. Motivation for the study stems from the large travel time variations observed in typical ultrasonic flow and circulation meters. Turbulent flow data was collected downstream of a grid introduced in a uniform flow in the wind tunnel using ultrasonic techniques. Grid turbulence is well defined in literature, and is nearly homogeneous and isotropic. The experimental investigation was performed under well-controlled laboratory conditions. The grid mesh sizes varied from 0.25-0.5in, and flow velocities from 0-20m/s. The ultrasonic transducers were of 100 kHz working frequency; and all of the data was collected with them oriented perpendicular to the mean flow. Path lengths were increased from 2-10in; and the data acquisition and control system featured a very high speed data acquisition card with an analog to digital converter that enabled excellent resolution of ultrasonic signals. Experimental data was validated by comparison to other studies. The work aims to investigate the influence of the grid-generated turbulent flow on acoustic wave propagation, in terms of the variance of the travel time. The effect of turbulence on acoustic wave propagation was observed. The experimental data was used to compute average travel times, acoustic travel time variances, and standard deviation amplitude fluctuations. The data was collected in the region estimated to be homogeneous and isotropic. Average travel time data support the assumption that only the large (as compared to the wavelength ) turbulent inhomogeneities influence acoustic wave propagation. Variance data confirm the presence of a non-linear trend in the acoustic travel times with increasing path length. Amplitude fluctuations data confirm a correlation between areas of caustic formation and large amplitude fluctuations.

Sistemas de ensaios não destrutivos por ondas guiadas despertam cada vez mais a atenção tanto da indústria, como da academia. Isso deve-se, principalmente, às possibilidades que as ondas guiadas permitem, como maior área de triagem que o ultrassom convencional. Porém a técnica tem suas limitações, já que esta apenas gera uma estimativa da localização de um defeito e não a sua dimensão. Nessa limitação, algoritmos tomográficos apresentam uma possibilidade de avanço da técnica, pois permitem determinar não só a localização de corrosões e defeitos, mas também seu dimensionamento. Este trabalho apresenta tomografia de difração como uma alternativa para avaliação de integridade estrutural. Primeiramente, utilizou-se análise por métodos numéricos para mostrar a validade dos algoritmos e posteriormente foi realizado um experimento em uma chapa real com o objetivo de reconstruir a imagem do defeito. / Guided waves nondestructive testing systems are increasingly attracting industrial and academic attention. The mainly reason for this attention is the possibility of screening a large area than conventional ultrasound technique. However, Guided Waves Testing has limitations, since it gives only an estimation of the location of a defect, but not the dimensions. Tomographic algorithms come up with an improvement of the technique, because it allows discovery not only the location of corrosions and defects, but the dimensions too. This work brings Diffraction Tomography as an alternative to structural health monitoring. First, a numerical analysis was implemented to demonstrate the validity of the algorithms, after that an experiment in a real plate was made with the objective to recover the defect image.