Thesis (M. Sc.)--University of Hong Kong, 1998. / Includes bibliographical references (leaf 86-87).

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A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, July 14, 2015. / Hyperspectral images nd application in many areas of modern society, we use them for land surveying, core sample analysis, in the conservation and forestry industries and many more. A major problem in hyperspectral images is how to deal with noise. Many methods that analyse hyperspectral images either need clean images or accurate estimations of the noise statistics in the images. The goal of this dissertation is to present and compare methods for statistic estimation and noise removal. We use an arti cial hyperspectral image to study some existing methods and develop some new ones based on existing methods, speci cally the BM3D algorithm. We test methods that estimate the level of the noise present in an image, methods that estimate the structure of the noise and methods that remove noise. We analyse all the methods under an additive noise model and consider spectrally correlated and uncorrelated noise. Within our investigations we investigate di erent types of correlation. We will show the strengths that the various methods have and establish a way to approach treating a hyperspectral image with no information beyond the image itself. Using our observations and insights from the experiments on the arti cial data we analyse some radiance data from the AVIRIS instrument. We show that the additive signal independent part of the noise is small but not negligible. We also show some evidence for the structure of the noise in the AVIRIS instrument.

Graduation date: 2004

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A study of methods for reducing the acoustic noise in magnetic resonance imaging (MRI) scanners is presented in this thesis. The structural-acoustic coupling mechanism of MRI scanners was investigated using a method of structural-acoustic modal analysis. Mathematical expressions of generalized radiation impedances of gradient coil ducts with perforated panel inserts were developed and the effects of the perforated panel inserts on the acoustic noise in the duct were discussed. The possibility of using micro-perforated panel (MPP) absorbers in MRI scanners to reduce the acoustic noise was then investigated through analytical and computational modeling. A comprehensive experimental study was conducted after the analytical and computational investigation. Finally, design methods and procedures were developed specifically for the MPP absorbers in MRI scanners. Design considerations and recommendations were given as well. Several major conclusions can be made from this research. Firstly, the method of structural-acoustic modal analysis is effective for finding the structural-acoustic coupling modes which should be avoided in the design of MRI scanners. Secondly, a perforated panel insert produces significant effects on the radiation impedance of gradient coil ducts and MRI scanner bores. This attribute partly contributes to its capability of reducing the acoustic noise in a duct. Thirdly, the effectiveness of MPP absorbers in MRI scanners can be accurately predicted using a combination of theoretical analysis and computational modeling. Moreover, it has been proved that well designed MPP absorbers are effective in reducing the acoustic noise in MRI scanners. Lastly, the presented design methods and recommendations for the MPP absorbers can be relatively easily used by MRI designers or engineers to tackle the acoustic noise problem in MRI scanners. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2010-07-13 11:46:03.585

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The application of an ejector nozzle integrated with a reaction drive rotor configuration for a vertical takeoff and landing rotorcraft is considered in this research. The ejector nozzle is a device that imparts energy from a high speed airflow source to a lower speed secondary airflow inside a duct. The overall nozzle exhaust mass flow rate is increased through fluid entrainment, while the exhaust gas velocity is simultaneously decreased. The exhaust gas velocity is strongly correlated to the jet noise produced by the nozzle, making the ejector a good candidate for propulsion system noise reduction. Ejector nozzles are mechanically simple in that there are no moving parts. However, coupled fluid dynamic processes are involved, complicating analysis and design. Geometric definitions of the ejector nozzle are determined through a reduced fidelity, multi-disciplinary, representation of the rotary wing ejector. The resulting rotary wing ejector geometric sizing procedure relates standard vehicle and rotor design parameters to the ejector. Additionally, a rotary wing ejector performance procedure is developed to compare this rotor configuration to a conventional rotor. Performance characteristics and aerodynamic effects of the rotor and ejector nozzle are analytically studied. Ejector nozzle performance, in terms of exit velocities, is compared to the primary reaction drive nozzle; giving an indication of the potential for noise reduction. Computational fluid dynamics are paramount in predicting the aerodynamic effects of the ejector nozzle located at the rotor blade tip. Two-dimensional, steady-state, Reynolds-averaged Navier-Stokes (RANS) models are implemented for sectional lift and drag predictions required for the rotor aerodynamic model associated with both the rotary wing ejector sizing and performance procedures. A three-dimensional, unsteady, RANS simulation of the rotary wing ejector is performed to study the aerodynamic interactions between the ejector nozzle and rotor. Overall performance comparisons are made between the two- and three-dimensional models of the rotary wing ejector, and a similar conventional rotor.

Thesis (MReg&UrbPlan)--University of South Australia, 2002.