No description available.

Very Low Frequency (VLF) radio waves propagate within the Earth-ionosphere waveguide with very little attenuation. Modifications of the waveguide geometry affect the propagation conditions, and hence, the amplitude and phase of VLF signals. Changes in the ionosphere, such as the presence of the D-region during the day, or the precipitation of energetic particles, are the main causes of this modification. Using narrowband receivers monitoring remote VLF transmitters, the amplitude and phase of these signals are recorded. A multivariate data analysis technique, Principal Component Analysis (PCA), is applied to the data in order to determine parameters such as seasonal and diurnal changes which affect the variation of these signals. Data was then analysed for effects from extragalactic gamma ray bursts, terrestrial gamma ray flashes and solar flares. Only X-rays from solar flares were shown to have an appreciable affect on ionospheric propagation. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2009.

published_or_final_version / Physics / Master / Master of Science

The interaction between a bounded ultrasonic beam and a liquid wedge over a solid half-space is studied theoretically as well as experimentally. A semi-analytical technique called Distributed Point Source Method (DPSM) is adopted for modeling the ultrasonic field in a wedge-shaped fluid structure on a solid half-space. This study is important for analyzing and understanding the propagation of ultrasonic waves used for underwater communications and inspections. A better understanding of the elastic wave propagation in water and in submerged marine strata near the seashore requires extensive investigations of such problem geometries. The semi-analytical technique used in this dissertation considers a bounded acoustic beam striking a fluid-solid interface between a fluid wedge and a solid half-space. Solution of this problem is beyond the scope of the currently available analytical methods when the beam is bounded. However, it is important to model the bounded beams because, in all underwater communications and inspections, bounded beams are used. Currently, only numerical method [Boundary Element Method (BEM) or Finite Element Method (FEM)] based packages (e.g., PZFlex) are in principle capable of modeling ultrasonic fields in such structures. However, these packages are not very accurate and are very CPU-intensive for high-frequency ultrasonic problems. At high frequencies, FEM- and BEM-based packages require huge amount of computation memory and time for their executions that the DPSM technique can avoid. The effect of the angle variation between the fluid-solid interface and the fluid wedge on the wave propagation characteristics is studied and presented.

No description available.

No description available.

The thesis provides the required knowledge base for establishing Laser Induced Stress Wave Thermometry (LISWT) as a viable alternative to current infrared technologies for temperature measurement up to 1000°C with ±1°C resolution. The need for a non-contact, high resolution thermal measurement methodology applicable to Rapid Thermal Processing (RTP) motivated the work. A stress wave propagation model was developed and a complex, temperature-dependent elasto-viscoplastic constitutive law was identified. A stagger-grid finite difference scheme was followed to approximate the solution field subject to temperature and plate thickness variations. Extensive numerical experiments were conducted to identify the proper time and spatial steps. A Gabor wavelet transform scheme was also employed for the extraction of wafer thermal and geometric information from exploring wave attenuation and dispersion. Researched results concluded that wave group velocity is a nonlinear function of temperature. Nonlinearity became more prominent at high temperatures and low frequencies. As such, for LISWT to achieve better thermal resolution at high temperatures, low frequency components of the induced stress wave should be exploited. The results also showed that the influence of temperature on attenuation is relatively small. It is not recommended to use attenuation for resolving temperature variation as small as several degrees Celsius. In addition to temperature, geometry also was found to have an impact on wave dispersion and attenuation. The results showed that the influence of thickness on wave velocity is significant, thus suggesting that for LISWT to achieve high temperature resolution, wafer thickness must be accurately calibrated in order to eliminate all possible errors introduced by thickness variation. The study established the basic framework for LISWT to be applicable to silicon wafer RTP at elevated temperatures. The model and methods developed for the course of the research can be easily adapted to account for other nondestructive evaluation applications involving the use of surface, plate or bulk waves for material characterization and thermal profiling.

Three kinds of propagating waves physically admissible in a tubular section are derived to establish their dispersion characteristics in response to the presence of multi-layered viscoelastic coatings. One is the longitudinal wave that propagates in the axial direction. The other two are shear and longitudinal waves along the circumferential direction. To characterize the hollow cylinder with coating layers, wave dispersion and attenuation are studied using the “global matrix” technique. Since each layer is considered to be perfectly bonded to each other, displacement and strain continuity are imposed as the interfacial boundary conditions. Viscoelastic coating materials such as bitumen and epoxy serve to improve pipeline reliability, but they also dampen and dissipate wave energy. The viscoelastic materials are studied as well. By replacing the real material constants with complex material constants in the characteristic equation, the impact of the viscoelastic coatings on wave dispersion is established. Bisection method is followed to find the real and complex roots of the three characteristic equations derived. Roots thus obtained are manipulated to allow the phase velocity and attenuation dispersion to be plotted against frequency. The dispersion of phase velocity and wave attenuation for coated pipes are evaluated against a baseline model which is the bare, uncoated tubing to establish the propagation characteristics of the guided shear and longitudinal waves in the presence of multiple coating layers. The effects of increasing attenuation parameter and coating thickness are also investigated.

The thesis provides the required knowledge base for establishing Laser Induced Stress Wave Thermometry (LISWT) as a viable alternative to current infrared technologies for temperature measurement up to 1000°C with ±1°C resolution. The need for a non-contact, high resolution thermal measurement methodology applicable to Rapid Thermal Processing (RTP) motivated the work. A stress wave propagation model was developed and a complex, temperature-dependent elasto-viscoplastic constitutive law was identified. A stagger-grid finite difference scheme was followed to approximate the solution field subject to temperature and plate thickness variations. Extensive numerical experiments were conducted to identify the proper time and spatial steps. A Gabor wavelet transform scheme was also employed for the extraction of wafer thermal and geometric information from exploring wave attenuation and dispersion. Researched results concluded that wave group velocity is a nonlinear function of temperature. Nonlinearity became more prominent at high temperatures and low frequencies. As such, for LISWT to achieve better thermal resolution at high temperatures, low frequency components of the induced stress wave should be exploited. The results also showed that the influence of temperature on attenuation is relatively small. It is not recommended to use attenuation for resolving temperature variation as small as several degrees Celsius. In addition to temperature, geometry also was found to have an impact on wave dispersion and attenuation. The results showed that the influence of thickness on wave velocity is significant, thus suggesting that for LISWT to achieve high temperature resolution, wafer thickness must be accurately calibrated in order to eliminate all possible errors introduced by thickness variation. The study established the basic framework for LISWT to be applicable to silicon wafer RTP at elevated temperatures. The model and methods developed for the course of the research can be easily adapted to account for other nondestructive evaluation applications involving the use of surface, plate or bulk waves for material characterization and thermal profiling.

Thesis (Ph. D.)--California Institute of Technology, 1977. UM #77-24,050. / Advisor names found in the Acknowledgments pages of the thesis. Title from home page (viewed 03/09/2010). Includes bibliographical references.