No description available.

No description available.

Thesis (PhD)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: Multi-mode coupled cavity filters have been the mainstay of commercial satellite communications systems since their introduction in 1970. Multi-mode filters use more than one resonant mode in a single cavity resonator, thereby reducing the size and weight of the filter. This is especially advantageous for satellite applications where the size and weight of the communications payload must be minimised. The use of more modes also increases the complexity of the filter design. Iris design plays an integral part in the design of multi-mode coupled cavity filters. Currently, irises are mostly designed using either the small aperture theory derived by Bethe, or a standard numerical technique. A recent study comparing these two techniques shows that approximations made in standard correction factors applied to small aperture theory, can lead to unacceptable errors. While numerical techniques are clearly required for the accurate design of complex irises, the standard numerical technique is only suited to the design of relatively simple iris structures. This dissertation presents three new CAD procedures for the design of irises in multi-mode coupled cavity devices. The new procedures are verified by numerical examples and measurements. Two new multi-mode coupled cavity devices were designed and tested using the CAD procedures developed in this work. The first is a new diplexer structure that realises two fourth order Chebyschev filter channels in three quadruple-mode cavities. Through correct placement and accurate design of irises, channel isolation was increased beyond that of existing multi-mode diplexer structures. The second application is the development of a new coupling structure for the replacement of coupling and tuning screws. A third order triple-mode cavity filter, including this new fixed coupling element, was designed using the CAD procedures developed in this dissertation. / AFRIKAANSE OPSOMMING: Multi-modale gekoppelde resoneerder filters word reeds vanaf die eerste verskyning van kommersiele sateliet-kommunikasiestelsels in 1970 gebruik. Vir hierdie toepassing is die fisiese grootte en massa van die filters in die kommunikasiestelsels van groot belang. Multimodale filters gebruik meer as een resonante modus in ‘n golfgeleier resoneerder om die fisiese grootte en massa van die filter te verminder. Hoe meer resonante modusse in een golfgeleier resoneerder, hoe kleiner en ligter die filter. Die gebruik van meer modusse kompliseer egter die ontwerp van die filter. ‘n Baie belangrike aspek van multi-modale filterontwerp is die ontwerp van die irisse wat die golfgeleier resoneerder skei. Die irisse word tans ontwerp, of deur middel van Bethe se klein iris teorie, of met die gebruik van ‘n standaard numeriese metode. ‘n Onlangse studie toon aan dat sekere standaard toevoegings tot Bethe se teorie, benaderings bevat wat tot foutiewe iris ontwerp kan lei. Numeriese metodes is dus noodsaaklik vir akkurate iris ontwerp. Hierdie studie wys egter dat die bestaande numeriese tegnieke beperk is tot die ontwerp van relatief eenvoudige irisse. Hierdie proefskrif bied drie nuwe rekenaargesteunde ontwerpsprosedures vir die doeltreffende ontwerp van verskillende klasse irisse in multi-modale golfgeleier resoneerder filters. Die nuwe metodes word getoets aan die hand van numeriese voorbeelde, en/of gemete resultate. Twee nuwe komplekse multi-modale golfgeleier resoneerder filter toepassings is ontwerp deur gebruik te maak van die nuwe ontwerpsprosedures. Die eerste is ‘n nuwe diplekser struktuur met twee vierde-orde Chebyschev filterkanale in drie golfgeleier resoneerders wat elk vier resonante modusse ondersteun. Deur korrekte en akkurate plasing van die irisse word ‘n groot verbetering in kanaalisolasie bo ‘n bestaande multi-modale diplekser verkry. Die tweede toepassing is die ontwikkeling van ‘n nuwe koppelstruktuur vir die vervanging van die gebruiklike koppel- en aanpassingskroewe. ‘n Derde-orde trippel-modus filter, gebaseer op hierdie nuwe struktuur, is ook ontwerp deur van die nuwe ontwerpsalgoritmes gebruik te maak.

Thesis (Ph.D.)--Auburn University, 2005. / Abstract. Vita. Includes bibliographic references (ℓ. 131-144)

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2004. / Includes bibliographical references (leaves 98-103). Also available in electronic version. Access restricted to campus users.

When an intense femtosecond laser pulse propagates in a gas, it undergoes filamentation, a spectacular process where the pulse spatial, spectral and temporal characteristics change considerably. A thin short-lived plasma column is formed in the wake of the propagating pulse. My PhD work has been dedicated to the further understanding of the filamentation process. In a first part, I compare the properties of a usual filament with those of a filament formed by a femtosecond laser pulse with a Bessel beam profile. Using a laser pulse of same intensity and duration, I show that a Bessel beam can form a longer and more uniform plasma column in air, but that the plasma density is significantly lower. In a second part, I show that it is possible to increase considerably the lifetime of the plasma column, using a dual femtosecond/nanosecond laser pulse technique. To obtain an increased lifetime over a significant segment of a plasma column, I rely on the properties of Bessel beams in the nonlinear regime developed in the first chapter. In a third part, I study the dynamics of free electrons that are produced in the filamentation process. To do this, I have developed a specially designed current probe. Experiments reveal a very rich behaviour. The longitudinal displacements of electrons in the plasma column depend sensitively on the nature of the gas and its pressure as well as on the laser polarization of the laser. I propose a model to explain this behaviour. The direction of electron flow results from the competition between pure laser forces and a Coulomb wake field force. In the last chapter, I study filamentation in a Helium gas. This required improving the laser characteristics in order to reach the necessary power for filamentation. Improved characteristics have been achieved by implementing a planar compression stage which shortened the laser pulse from 50 fs to 10 fs without appreciable energy loss. The first experimental evidence for filamentation in He is presented at the end of the thesis. Agreement is found with a numerical simulation.

New modelling tools intended to contribute to the development of components for quieter vehicles are developed. The tools are based on continuum models for wave propagation in poro– and visco–elastic media. By using geometric attributes of the studied components, the computational cost may be radically decreased compared to traditional methods. By assigning known analytical functions for one or two of the spatial directions, the spatial dimension of the remaining numerical problem is reduced. This reduction of spatial dimensions is performed in two di↵erent ways. The first one treats wave propagation in infinitely extended homogeneous and hollowed cylindrical rods, or wave guides, consisting of visco–elastic media. The wave solutions obtained are then used to model rubber vibration isolators of finite length by mode–matching the fields to the radial boundary conditions of interest. The second one is a method for modelling rotationally symmetric multilayered structures consisting of poro–elastic, elastic and fluid domains. By using a harmonic expansion for the azimuthal spatial dependence, the original three–dimensional problem is split up into several, much smaller, two– dimensional ones, radically decreasing the computational load. Moreover, using a mixed measurement/modelling approach, the audible frequency range characteristics of a viscous damper from a truck is studied, illustrating the influence of the rubber bushings by which it is attached to surrounding structures. The modelling approaches presented in this thesis are intended as tools aiding the design process of new vehicles, enabling new technology striving for more sustainable vehicle concepts. More specifically, the tools aim to improve the modelling of sound and vibration properties which are often penalised when seeking new, more sustainable vehicle designs. / <p>QC 20120522</p> / Centre for Eco2 Vehicle Design

No description available.

Elastic Wave propagation in structures with irregular boundaries is studied by transforming the plates with irregular surfaces to sinusoidal wave-guides. Guided elastic wave in a two-dimensional periodically corrugated plate is studied analytically. The plate material is considered as homogeneous, isotropic and linearly elastic. In a periodically corrugated wave-guide, all possible spectral orders of wave numbers are considered. The dispersion equation is obtained by applying the traction free boundary conditions at the two surfaces. The analysis is carried out in the wave-number domain for both symmetric and anti-symmetric modes. Non-propagating 'stop bands' and propagating 'pass bands' are investigated. Experimental analyses with two different pairs of transducers are also performed and compared with the results from the mathematical analysis. Newly developed semi-analytical DPSM technique has been also adopted in this dissertation to model the ultrasonic field in sinusoidally corrugated plate. Distributed Point Source Method (DPSM) is gradually gaining popularity in the field of Non-Destructive Evaluation (NDE). DPSM can be used to calculate the ultrasonic field (pressure, velocity and displacement in a fluid or stress and displacement in a solid) generated by ultrasonic transducers. So far the technique has been used to model ultrasonic field in homogeneous or multilayered fluid structures. In this dissertation the method is extended to model the ultrasonic field generated in both fluid and solid media. The Prime objective of using DPSM technique in this dissertation is to model the ultrasonic field generated in the corrugated wave guide. This method has never been used to model ultrasonic field in solids. Development of stress and displacement Green's functions in solids are presented. In addition to the wave propagation problem in the sinusoidal wave guide, a few unsolved problems such as ultrasonic field generated by bounded acoustic beams in multilayered fluid structures, near a fluid-solid interface and in flat solid isotropic plates are also presented in this dissertation.

New modelling tools intended to contribute to the development of components for quieter vehicles are developed. The tools are based on continuum models for wave propagation in poro- and visco-elastic media. By using geometric attributes of the studied components, the computational cost may be radically decreased compared to traditional methods. By assigning known analytical functions for one or two of the spatial directions, the spatial dimension of the remaining numerical problem is reduced. This reduction of spatial dimensions is performed in two di↵erent ways. The first one treats wave propagation in infinitely extended homogeneous and hollowed cylindrical rods, or wave guides, consisting of visco-elastic media. The wave solutions obtained are then used to model rubber vibration isolators of finite length by mode-matching the fields to the radial boundary conditions of interest. The second one is a method for modelling rotationally symmetric multilayered structures consisting of poro-elastic, elastic and fluid domains. By using a harmonic expansion for the azimuthal spatial dependence, the original three-dimensional problem is split up into several, much smaller, two- dimensional ones, radically decreasing the computational load.Moreover, using a mixed measurement/modelling approach, the audible frequency range characteristics of a viscous damper from a truck is studied, illustrating the influence of the rubber bushings by which it is attached to surrounding structures.The modelling approaches presented in this thesis are intended as tools aiding the design process of new vehicles, enabling new technology striving for more sustainable vehicle concepts. More specifically, the tools aim to improve the modelling of sound and vibration properties which are often penalised when seeking new, more sustainable vehicle designs.