An Extended Least Squares Method to Separate the Incident and Reflected Wave Fields for Long-Crested and Short-Crested Waves

An extended least square method for reflection analysis that separates long-crested or short-crested wave fields into the incident and reflected components from the measured wave surface elevations and from other wave parameters is presented. This method uses the least squares technique by minimizing the squared errors between the measured and the estimated wave heights. This method applies linear wave theory including the linear dispersion relationship and the transfer functions translating the surface wave elevations and other wave parameters. The wave parameters are measured simultaneously from several positions, and the wave probe measurements from three or an arbitrary number of the positions are selected for reflection analysis. A probe spacing algorithm is described that determines the total number of the wave probes and their positions between the wave maker and the reflecting structure and selects three probes from the pre-arranged probe array for reflection analysis. The algorithm automates the arrangement of wave probes for a wave basin test involving several wave conditions featuring the wave period and water depth, and the corresponding wavelength varies according to these conditions. New software, named REFANA (reflection analysis), has been written that conducts the reflection analysis using the extended least square method and determines the number of probes and their positions and selects three of them for the reflection analysis. The incident wave heights determined by REFANA approximate the input incident wave heights. The reflection coefficients computed by REFANA agree well with REFLS, a commercial software for reflection analysis. Moreover, probe positions can be arranged automatically using REFANA, which also minimizes the total number of required wave probes. Experimental measurements of wave reflection on two different breakwaters are conducted in the laboratory, and the reflection coefficients are evaluated using the software REFANA. The results from REFANA are compared to the REFLS commercial software. Also, an empirical function is developed to estimate reflection coefficient in front of breakwaters. The empirical function is a two sigmoid-curve (s-shaped) function, such as logistic function and error function, in terms of the surf similarity number. The empirical function with proper coefficients can approximate the reflection coefficient for a rough, sloped, and permeable breakwater

Second-harmonic generation of the lowest-order antisymmetric Lamb wave at a closed parallel crack

The second-harmonic generation of the fundamental antisymmetric Lamb wave at a closed parallel crack in an elastic plate is studied by numerical analysis. The closed crack is modeled as a spring-type interface with quadratic nonlinearity. Based on a perturbation method, the problem of nonlinear Lamb wave scattering is decomposed into two linearized problems, i.e., for the linear reflection/transmission of the incident Lamb wave at the crack and for the generation/radiation of the second-harmonic Lamb waves due to the contact nonlinearity. The reduced problems are solved by the finite element method in the frequency domain. Numerical results demonstrate significant effects of the crack resonance on the linear and nonlinear Lamb wave scattering responses, which appear as sharp peaks/dips in the reflection/transmission spectra and enhanced second-harmonic amplitudes at some frequencies. Two simple frequency selection rules are established which explain the enhanced generation of the second-harmonic Lamb waves. The time-domain analysis is also carried out to supplement the frequency-domain analysis, which confirms that the incident Lamb wave interacts with the crack at some specific frequencies in its bandwidth in a selective manner and enhances the generation of the second-harmonic components

Influence of the cross-section geometry of a cylindrical solid submerged in an acoustic medium on wave propagation

This paper studies wave propagation in the vicinity of a cylindrical solid formation submerged in an acoustic medium generated by point blast loads placed outside the inclusion. The full 3D solution is obtained first in the frequency domain as a discrete summation of responses for 2D problems defined by a spatial Fourier transform. Each 2D solution is computed using the Boundary Element Method, which makes use of two-and-a-half-dimensional Green's functions. This model is implemented to obtain Fourier spectra responses which make it possible to identify the behavior of both the axisymmetric and non-axisymmetric guided wave modes, when the cross-section of the elastic inclusion changes from circular to smooth oval.http://www.sciencedirect.com/science/article/B6TW5-44W4412-2/1/730c9aeee648dd67ed04d4a46003f0a

Quantitative subsurface defect evaluation by pulsed phase thermography: depth retrieval with the phase

La Thermographie de Phase Pulsée (TPP) est une technique d’Évaluation Non-Destructive basée sur la Transformée de Fourier pouvant être considérée comme étant le lien entre la Thermographie Pulsée, pour laquelle l’acquisition de données est rapide, et la Thermographie Modulée, pour laquelle l’extraction de la profondeur est directe. Une nouvelle technique d’inversion de la profondeur reposant sur l’équation de la longueur de diffusion thermique : μ=(α /πf)½, est proposée. Le problème se résume alors à la détermination de la fréquence de borne fb, c à d, la fréquence à laquelle un défaut à une profondeur particulière présente un contraste de phase suffisant pour être détecté dans le spectre des fréquences. Cependant, les profils de température servant d’entrée en TPP, sont des signaux non-périodiques et non-limités en fréquence pour lesquels, des paramètres d’échantillonnage Δt, et de troncature w(t), doivent être soigneusement choisis lors du processus de discrétisation du signal. Une méthodologie à quatre étapes, basée sur la Dualité Temps-Fréquence de la Transformée de Fourier discrète, est proposée pour la détermination interactive de Δt et w(t), en fonction de la profondeur du défaut. Ainsi, pourvu que l’information thermique utilisée pour alimenter l’algorithme de TPP soit correctement échantillonnée et tronquée, une solution de la forme : z=C1μ, peut être envisagée, où les valeurs expérimentales de C1 se situent typiquement entre 1.5 et 2. Bien que la détermination de fb ne soit pas possible dans le cas de données thermiques incorrectement échantillonnées, les profils de phase exhibent quoi qu’il en soit un comportement caractéristique qui peut être utilisé pour l’extraction de la profondeur. La fréquence de borne apparente f’b, peut être définie comme la fréquence de borne évaluée à un seuil de phase donné φd et peut être utilisée en combinaison avec la définition de la phase pour une onde thermique : φ=z /μ, et le diamètre normalisé Dn=D/z, pour arriver à une expression alternative. L'extraction de la profondeur dans ce cas nécessite d'une étape additionnelle pour récupérer la taille du défaut.Pulsed Phase Thermography (PPT) is a NonDestructive Testing and Evaluation (NDT& E) technique based on the Fourier Transform that can be thought as being the link between Pulsed Thermography, for which data acquisition is fast and simple; and Lock-In thermography, for which depth retrieval is straightforward. A new depth inversion technique using the phase obtained by PPT is proposed. The technique relies on the thermal diffusion length equation, i.e. μ=(α /π·f)½, in a similar manner as in Lock-In Thermography. The inversion problem reduces to the estimation of the blind frequency, i.e. the limiting frequency at which a defect at a particular depth presents enough phase contrast to be detected on the frequency spectra. However, an additional problem arises in PPT when trying to adequately establish the temporal parameters that will produce the desired frequency response. The decaying thermal profiles such as the ones serving as input in PPT, are non-periodic, non-band-limited functions for which, adequate sampling Δt, and truncation w(t), parameters should be selected during the signal discretization process. These parameters are both function of the depth of the defect and of the thermal properties of the specimen/defect system. A four-step methodology based on the Time-Frequency Duality of the discrete Fourier Transform is proposed to interactively determine Δt and w(t). Hence, provided that thermal data used to feed the PPT algorithm is correctly sampled and truncated, the inversion solution using the phase takes the form: z=C 1 μ, for which typical experimental C 1 values are between 1.5 and 2. Although determination of fb is not possible when working with badly sampled data, phase profiles still present a distinctive behavior that can be used for depth retrieval purposes. An apparent blind frequency f’b , can be defined as the blind frequency at a given phase threshold φd , and be used in combination with the phase delay definition for a thermal wave: φ=z /μ, and the normalized diameter, Dn=D/z, to derive an alternative expression. Depth extraction in this case requires an additional step to recover the size of the defect.La Termografía de Fase Pulsada (TFP) es una técnica de Evaluación No-Destructiva basada en la Transformada de Fourier y que puede ser vista como el vínculo entre la Termografía Pulsada, en la cual la adquisición de datos se efectúa de manera rápida y sencilla, y la Termografía Modulada, en la que la extracción de la profundidad es directa. Un nuevo método de inversión de la profundidad por TFP es propuesto a partir de la ecuación de la longitud de difusión térmica: μ=(α /π·f)½. El problema de inversion se reduce entonces a la determinación de la frecuencia límite fb (frecuencia a la cual un defecto de profundidad determinada presenta un contraste de fase suficiente para ser detectado en el espectro de frecuencias). Sin embargo, las curvas de temperatura utilizadas como entrada en TFP, son señales no-periódicas y no limitadas en frecuencia para las cuales, los parámetros de muestreo Δt, y de truncamiento w(t), deben ser cuidadosamente seleccionados durante el proceso de discretización de la señal. Una metodología de cuatro etapas, basada en la Dualidad Tiempo-Frecuencia de la Transformada de Fourier discreta, ha sido desarrollada para la determinación interactiva de Δt y w(t), en función de la profundidad del defecto. Así, a condición que la información de temperatura sea correctamente muestreada y truncada, el problema de inversión de la profundidad por la fase toma la forma : z=C 1 μ, donde los valores experimentales de C 1 se sitúan típicamente entre 1.5 y 2. Si bien la determinación de fb no es posible en el caso de datos térmicos incorrectamente muestreados, los perfiles de fase exhiben de cualquier manera un comportamiento característico que puede ser utilizado para la extracción de la profundidad. La frecuencia límite aparente f’b , puede ser definida como la frecuencia límite evaluada en un umbral de fase dado φd , y puede utilizarse en combinación con la definición de la fase para una onda térmica: φ=z /μ, y el diámetro normalizado Dn , para derivar una expresión alternativa. La determinación de la profundidad en este caso, requiere de una etapa adicional para recuperar el tamaño del defecto

Radar signature characterization from wind turbine scattering

textThe growth in the number of wind farms has raised significant concerns in the radar community due to their potential interference on radar systems. The motion of the turbine blades creates unwanted Doppler clutter that can interfere in the tracking of moving targets. Large turbine structures can also produce electromagnetic shadows that may make observing objects behind a wind farm difficult. Detailed characterization of the clutter is the first step towards effective mitigation techniques. The goal of this dissertation research is to gain a better understanding of the dynamic radar signatures resulting from scattering by wind turbines. First, the scattering characteristics of turbines in the presence of ground surface are studied. Image theory in conjunction with a shooting-and-bouncing ray code, Ahilo, is used to carry out the dynamic signature simulation. The observed features in the simulation are corroborated with laboratory model measurements. Second, the effects of higher order motions of a turbine undergoing rotation on the radar signatures are investigated and characterized. Mathematical models for the motions are proposed and used to simulate the joint time-frequency and inverse synthetic aperture radar characteristics of the turbine undergoing these motions. The motions are studied for an isolated turbine as well as for a turbine rotating above a ground. Selected motions are corroborated by laboratory model measurements. Next, a method to remove the dynamic clutter produced by wind turbines is presented. A physics-based basis is constructed to model the radar backscattering from a wind turbine. This basis is used in conjunction with the matching pursuit algorithm to iteratively remove the Doppler clutter due to wind turbines. The algorithm is tested using radar return generated using Ahilo. Finally, radar features of wind turbines are simulated and studied in the HF (high frequency) band. The features are presented in the range-Doppler plane for single as well as arrays of turbines. Doppler aliasing due to the limited pulse repetition frequency of HF radars is examined. Shadowing characteristics of arrays of turbines are simulated and analyzed. Electromagnetic modeling details including effects of thin-wire modeling, non-conducting turbine components, and the presence of a conducting ground surface are discussed.Electrical and Computer Engineerin

Numerical modeling of infrared thermography techniques via ANSYS

Several inspection techniques have been developed over years. Recently, infrared thermography (IRT) technology has become a widely accepted as a nondestructive inspection (NDI) technique for different fields and various applications as well. Infrared thermography stands as one of the most an attractive and a successful NDI technique that has ability to detect the object\u27s surface/subsurface defects remotely based on observing and measuring the surface\u27s emitted infrared heat radiation by using an infrared camera. The finite element modeling FEM ANSYS was successfully used for the modelling of several IRT techniques; such as Pulsed Thermography (PT) and Lock-in Thermography (LT) that can be used to detect the in-plane defects which are parallel to its surface; besides a Laser Spot Thermography (LST) technique that can be used to detect the cracks which are perpendicular to its surface. Furthermore; this thesis describes how LST method can be extended to a new technique, Laser Digital Micromirror Thermography (LDMT), based on using a digital micromirror device (DMD) that has ability to generate multi-hot spots onto the specimen\u27s surface being examined by using single laser source. In one hand, this thesis aims to show investigations about infrared thermography technology as a non-destructive inspection (IRT-NDI) by using numerical modeling methods via ANSYS. On the other hand, this thesis presents FEM ANSYS as a powerful tool allows doing several inspections, analyses, and evaluations of thermography techniques tests based on numerical modeling simulations and comparing their results to the corresponding experiments in literature experiment tests to validate these simulations and show a reasonable agreement to use ANSYS as a thermography inspection tool for future study and researches --Abstract, page iii

Numerical modeling of infrared thermography techniques via ANSYS

Several inspection techniques have been developed over years. Recently, infrared thermography (IRT) technology has become a widely accepted as a nondestructive inspection (NDI) technique for different fields and various applications as well. Infrared thermography stands as one of the most an attractive and a successful NDI technique that has ability to detect the object\u27s surface/subsurface defects remotely based on observing and measuring the surface\u27s emitted infrared heat radiation by using an infrared camera. The finite element modeling FEM ANSYS was successfully used for the modelling of several IRT techniques; such as Pulsed Thermography (PT) and Lock-in Thermography (LT) that can be used to detect the in-plane defects which are parallel to its surface; besides a Laser Spot Thermography (LST) technique that can be used to detect the cracks which are perpendicular to its surface. Furthermore; this thesis describes how LST method can be extended to a new technique, Laser Digital Micromirror Thermography (LDMT), based on using a digital micromirror device (DMD) that has ability to generate multi-hot spots onto the specimen\u27s surface being examined by using single laser source. In one hand, this thesis aims to show investigations about infrared thermography technology as a non-destructive inspection (IRT-NDI) by using numerical modeling methods via ANSYS. On the other hand, this thesis presents FEM ANSYS as a powerful tool allows doing several inspections, analyses, and evaluations of thermography techniques tests based on numerical modeling simulations and comparing their results to the corresponding experiments in literature experiment tests to validate these simulations and show a reasonable agreement to use ANSYS as a thermography inspection tool for future study and researches --Abstract, page iii