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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
101

The structural building blocks of turbulent wall-bounded flow

Dennis, D. J. C. January 2009 (has links)
The investigation has been conducted in the form of experimental measurements, using high-speed stereoscopic particle image velocimetry, of a turbulent boundary layer in the Cambridge University Engineering Department’s water tunnel facility. All the results produced in this project showed indications that long structures are a ubiquitous feature of turbulent boundary layer flow. The instantaneous velocity fields and a variety of flow-field statistics (averages and correlations) all demonstrated the presence of both high and low speed long structures that had reasonably consistent characteristics. The results also indicated that hairpin-like vortices were frequently present in the turbulent boundary layer. Although the instantaneous fields showed that these vortices were generally asymmetric and distorted from the archetypal hairpin shape, conditionally averaged fields showed that the average shape was very much like an ideal hairpin. As such the hairpin vortex seems an ideal candidate as a representative eddy in any statistical model of a turbulent boundary layer. The view of the turbulent boundary layer as seen in the results of this project is of hairpin-like vortices creating Q2 and Q4 events, that are aligned such that the streamwise component of successive events can merge and lead to the formation of long structures.
102

Cryogenic developments and signal amplification in environmental scanning electron microscopy

Fletcher, A. L. January 1998 (has links)
This thesis describes the development of a cryogenic imaging system for an environmental scanning electron microscope (ESEM). The ESEM is an important new development in electron microscopy since it enables specimens to be viewed in a small pressure of gas - generally this gas is water vapour, although an alternative must be used for cryogenic applications. The presence of the gas also contributes to the imaging mechanism, a process whereby the signal electrons are amplified by the gaseous molecules prior to detection. The purpose of the cryogenic system was to image the complicated, four phase microstructure of ice cream. Although viewed routinely by conventional electron microscopy techniques, the harsh temperature and pressure regimes involved (around -120°C at 10<SUP>.6</SUP> torr) increase the likelihood of introducing artefacts. Therefore, a methodology was developed for imaging ice cream with ESEM in a small pressure of an alternative imaging gas at a much warmer temperature of -80°C. In order to stabilise the ice phase in the samples at a higher temperatures, a system was designed for mixing gases, so that a small amount of water vapour could be mixed into the imaging gas. This system lifted the temperature restrictions of ice cream imaging so it can, in principle, now be imaged at its storage temperature of -20°C. In the search for alternative imaging gases to water vapour, questions were raised about the fundamental way in which the signal electrons interact with the gas. In order to understand the electron amplification properties of the different gases, a Faraday cage was designed and the electron amplification was investigated. We suggest that the ratio of the peak amplification to the plateau amplification gives a semi-quantitative method of determining the imaging quality of the gas. Furthermore, by isolating experimentally the effects of different components of the signal, it was found that the low energy secondary electrons dominate the signal at low pressure, whereas the effect of backscattered electrons becomes more important as the pressure is raised. In addition, the performance of two ESEM detector designs were compared. The new gaseous secondary electron detector (GSED), which was designed to reduce the contribution of some sources of signal, was found to achieve its aim, but some of its overall contrast was sacrificed when compared to the original environmental secondary detector (ESD).
103

Electron transport in n-type SiGe double quantum dots

Ferguson, A. January 2003 (has links)
This thesis presents an experimental investigation into the properties of highly doped (<i>n<sub>phosphorous </sub></i> ~ 1.4 x 10<sup>19</sup><i>cm<sup>-</sup></i><sup>3</sup>) n-type silicon-germanium double quantum dots. The structures are fabricated with a diameter of between 40nm and 70nm by means of electron beam lithography. Their electronic properties are then measured using a variety of cryogenic techniques, including a dilution refrigerator with a base temperature of 20mK. The main results from this investigation are described below. The ‘Coulomb Oscillations’ form a hexagonal lattice as the electrochemical potentials on the two dots are shifted by means of electrostatic gates. This indicates that the double dot is indeed a double well potential, and shows that the electron number on the dots can be independently altered. The elastrostatic gates are shown to have a wide degree of control over not just the electrochemical potential on the dots, but also the shape of the potential well. In this way, the two wells can be induced to coalesce; one of the wells can be merged into its nearby lead; or the coupling parameters between the dots and dots and leads can be altered. A supplementary piece of work, undertaken in collaboration with Paul Cain, is presented in appendix A. This describes an original scheme for quantum computation in which the ammonia molecule is used as the qubit. It is confined within a fullerene and placed on a crystal surface where gates and global microwave pulses manipulate, and interact it with other such encapsulated ammonia molecules. Finally, measurements is performed with an electrometer which has the ability to distinguish the polarisability of the ammonia molecule’s eigenstates.
104

Laminar flows at high Reynolds numbers

Harper, J. F. January 1964 (has links)
No description available.
105

Millimetre and submillimetre bolometric interferometry

Campbell, E. S. January 2005 (has links)
I start by presenting simulations of the noise performance of Superconductor-Insulator-Superconductor (SIS) direct detectors, combined with op-amp based readout electronics. This involves the combination of non-linear device modelling, with a detailed noise model of both the SIS junction and the readout-electronics. Using this model, I show that an SIS direct detector can be competitive in terms of noise performance with a transition edge sensor, provided the junction is biased well below the gap, and that the gap is tailored to the observing frequency. I then describe a set of experiments aimed at measuring the noise performance of a tantalum aluminium SIS device combined with op-amp based readout. I compare the results of these experiments with the predictions of my model, and show that they are in excellent agreement. Using the modelling software I have developed, I predict the lowest noise equivalent power achievable with this device as being 1.3 x 10<sup>-17</sup>W/√Hz, over a video bandwidth of 5 kHz. The expected dynamic range is 91 dB. I also describe the development and testing of the first submillimetre, single-chip, beam combining interferometer. This uses a superconducting chip, on which both the beam combination and detection occur. Two designs of chip were tested, one using a directional coupler for the beam combination, the other using a 90° hybrid. Both designs resulted in the measurement of fringe patterns that agree remarkably well with theory. The final part of this work concerns the development of a new modal theory of interferometry. The aim of this work was to accurately describe the partially coherent behaviour of multi-moded bolometric interferometers. My first attempt uses an approach based on the eigenmodes of the optical system. This was successful, but it became apparent that a more elegant solution existed. This alternative approach introduces the concept of eigenfields. These are the fundamental field distributions associated with an optical system. The can be found using the singular value decomposition, and, as I demonstrate, they allow the accurate simulation of multi-modal bolometric interferometers.
106

Design of gradient coils for magnetic resonance imaging by genetic algorithm

Fisher, B. J. January 1997 (has links)
Magnetic field gradients are a fundamental requirement for magnetic resonance imaging (MRI). Consequently, as MRI techniques have become more advanced in recent years, the demands on the hardware producing these fields have increased. This thesis aims to demonstrate a novel method for designing gradient coils using a genetic algorithm (GA). The technique is shown to be both powerful and flexible, allowing incorporation of many practical considerations directly into the optimisation procedure and even facilitating designs based on unconventional geometries, with both flat and dual-diameter coils studied. The optimisation itself is written to favour robust coil designs and constraints are specified to ensure that all designs are relatively simple and thus cheap to construct. The effect on the achieved outcome of individual intrinsic and implementation-specific variable design parameters is examined. The manufacture, testing, evaluation and clinical application of two gradient coils designed using the GA are discussed in the contexts of human brain and knee imaging. The former is accepted as a particularly stringent test of gradient performance, and artefacts due to the inevitable imperfections of the magnetic field produced by such a gradient coil are largely removed using an algorithm compatible with the design software. An attempt is made to compare the results obtained with data available from the literature, and this leads to discussion of the difficulties in assessing and presenting measures of the quality of gradient hardware, but also shows that the designs are competitive with those produced by other strategies. Subsequent to acquiring high quality images of the knee at full flexion, the advantages of flat gradient coils are further made apparent by a number of computer simulations which indicate their potential for the future of MRI. The advances made during this thesis have made the GA technique the method of choice locally, and have enabled several major imaging studies.
107

Region segmentation of images using a scale-space approach

Iyer, B. K. January 2002 (has links)
Region segmentation is the process by which an image is segmented into its constituent regions, where a region is a group of connected pixels with similar properties. Closely related to region segmentation is edge detection where the edges form the boundaries between regions. Region segmentation is important in many areas of computer vision and image processing. It is often the first step towards interpreting an image, where the regions correspond to objects in the image. The first part of this dissertation introduces the topic of region segmentation of images and also discusses the common data structures by which regions are represented. Region segmentation by scale-space methods, namely the datasieve (a recursive non-linear morphological filter), is proposed and the next part of the dissertation introduces the datasieve. Datasieves, as the name suggests, are methods by which data can be "sieved" based on the size of elements of the data. Datasieves have been found to be robust and are computationally efficient. They have been used in a number of applications such as position estimation of objects in an image, object recognition, motion estimation, and region segmentation. A scale-tree of the image can be generated based on the scale-space analysis using the datasieve. The dissertation demonstrates some problems in using the datasieve-based scale-tree, in its direct form, for region segmentation of images. The concept of the "seed node" from the datasieve scale-tree is introduced, based on which the datasieve is used as the primary tool for developing methods for segmenting grey scale and color images. Emphasis is also given to demonstrate segmentation results of a multi-scale nature using the datasieve. The possibilities of using the properties of the seed node for applications such as object tracking are shown. The theme of the dissertation then shifts to incorporate non-linear image pyramids in developing faster methods for region segmentation and edge detection.
108

Coherence in correlated matter

Cheung, A. C. H. January 2008 (has links)
In Part I, we propose a new method for dynamic nuclear polarisation in a quasi one-dimensional quantum wire utilising the spin-orbit interaction, the hyperfine interaction, and a finite source-drain potential difference. In contrast with previous methods, our scheme does not rely on external magnetic or optical sources which makes local control much more feasible. Using this method, a significant polarisation of a few per cent is possible in currently available InAs wires which may be detected by conductance measurements. This may prove useful for nuclear magnetic resonance studies in nanoscale systems as well as in spin-based devices where external magnetic and optical sources will not be suitable. In Part II, we study an electron-hole gas within a microcavity, which is a system that exhibits spontaneous quantum coherence. We consider a model of electrons and holes interacting with each other via Coulombic forces and coupled to light in the cavity. We propose a variational mean-field ansatz for the ground state of zero temperature that consists of a coherent photonic part and bound electron-hole pairs. By minimising the free energy, variational equations are derived and their solutions presented in the low and high excitonic density regimes, corresponding to analytical results. In the dilute limit, atomic excitons (bound electron-hole pairs) are Bose condensed; and at high densities, there is pairing in momentum space to give an excitonic insulator. The intermediate regime is calculated numerically. Finally, we discuss the phase diagram and make correspondence with finite temperatures.
109

Ion energy, sheath potential and secondary electron emission in the tokamak edge

Pitts, R. A. January 1991 (has links)
No description available.
110

Experimental investigation of 'He Films Adsorbed on Graphite'

Neumann, Michael January 2008 (has links)
No description available.

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