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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.
41

An investigation into the feasibility of a sea water and ice based acoustic UHE neutrino telescope

Bevan, S. January 2009 (has links)
No description available.
42

Near infrared studies of Jupiter's upper atmosphere

Lystrup, Makenzie Brook January 2008 (has links)
This thesis is an investigation of near infrared emissions from Jupiter’s upper atmosphere using observations from high-resolution spectrometers on two ground-based telescopes: the NASA Infrared Telescope Facility (IRTF) and the Keck II. Four studies are presented and discussed in terms of the energetics and dynamics of Jupiter’s upper atmosphere and its connection to the surrounding magnetosphere. The work based on the IRTF data investigates short time scale variability in Jupiter’s near infrared aurora and the effects of that variability on atmospheric heating by examining fluctuations in the velocity of the ion winds in the southern auroral region. These velocities are derived from the Doppler shifting of H3+ emission lines. This thesis presents the highest quality data of Jupiter's near infrared aurora available to date, from the 10-metre Keck II telescope. Many emission features are identified that have not previously been observed outside the laboratory and-emission from the highest reaches of the upper atmosphere are observed for the first time with a ground-based telescope. Three studies based on these data are presented. A mid- to low-latitude study of H3+ emissions is presented, with profiles of intensities and column densities, and a compared with results from a global circulation model. H3+ and CH4 emissions are employed in a study of Jupiter's northern and southern auroral regions; profiles of rotational and vibrational temperatures, column densities, and total emission are presented. Finally, a new method for deriving vertical ion density and temperature profiles in Jupiter's auroral regions is developed. Such profiles have never been recorded and it was previously thought that such vertical profiles in Jupiter’s upper atmosphere could not be made from ground-based observations, but only from spacecraft data. Profiles derived from this new method are presented in the study.
43

Novel processing of porous bioceramic structures

Muthutantri, A. I. January 2009 (has links)
Bone is one of the most commonly replaced tissues in the body. Bone tissue engineering has become one of the key areas of research as a successful treatment option for bone regeneration. Scaffolds are used in tissue engineering to direct tissue development and open pore scaffolds with a pore size range of 100 – 400 μm and porosity >90%, are preferred. Bioceramics have been used in numerous orthopaedic applications since the 1960s. While bioinert materials such as zirconia have been used in load bearing applications due to their impressive mechanical properties, bioactive materials such as hydroxyapatite (HA) have been used to promote bone growth. Porous bioceramic structures have found uses as scaffolds which act as frameworks to support and guide tissue growth in tissue engineering applications. There is constant demand for new processing methods for producing structures of both graded and uniform porosity. The current methods used for producing structures of graded porosity involve complex and multiple manufacturing steps. This thesis investigates the feasibility of using electrohydrodynamic (EHD) atomisation to produce foams with graded porosity as a ‘one-step’ processing method, using zirconia as the ceramic material due to its extensive use in industrial and biomedical applications. Modifications have been made to the electrospraying set–up configuration and a range of suspension concentrations have been utilised to determine the experimental conditions. Control of porosity, pore size and depth of penetration has been obtained by varying parameters such as spray time, sintering temperature and the sacrificial template. Secondly, a combination of the traditional slurry dipping method and the EHD method have been introduced and used to process scaffolds with better surface and mechanical properties than using each method individually. For this work, a nano–HA suspension has been used and the scaffolds produced have been characterised by X-ray diffraction, X-ray microtomography, scanning electron microscopy and compression testing. Due to limitations in the HA suspension, zirconia was used as the ceramic material to produce scaffolds using the EHD method, which was manipulated to enhance its efficiency. The effects of changing and modifying the polymeric template by subjecting the templates to various pre–treatment methods on the microstructure and mechanical properties were investigated. It has been possible to achieve porous scaffolds of mechanical strength within the recommended region for cancellous bone. Finally, these mechanically strong scaffolds have been dipped in a nano–HA suspension and sintered at a lower temperature with an attempt to make the scaffolds bioactive since zirconia is bioinert. These composite structures have been tested for their bioactivity using simulated body fluid (SBF). The SBF results have proved to be favourable and apatite growth has been observed on the composite scaffolds using SEM images, additional apatite peaks on the XRD spectra and the increase in the mechanical properties.
44

Cluster multi-point observations of the magnetotail plasma sheet

Henderson, Paul David January 2008 (has links)
This thesis presents observations of the terrestrial magnetotail plasma sheet made by the European Space Agency Cluster mission. The Cluster mission is composed of four identical spacecraft, the first such multi-spacecraft mission, and enables, for the first time, the disambiguation of time versus space phenomena. Using the data from 2003, when the spacecraft were at their smallest average separation to date, many small-scale processes, both microphysical and macrophysical, are investigated. In the first study presented, two small flux ropes, a possible signature of multiple X-line reconnection, are investigated. By the development and utilisation of various multi-spacecraft methods, the currents and magnetic forces internal and external to the flux ropes, as well as the internal structure of the flux ropes, are investigated. In addition, a theory of their early evolution is suggested. In the second study presented, various terms of the generalised Ohm's law for a plasma are determined, including, for the first time, the divergence of the full electron pressure tensor, during the passage past the spacecraft of an active reconnection X-line. It is found that the electric field contribution from the divergence of the electron pressure tensor is anti-correlated with the contribution from the Hall term in the direction normal to the neutral sheet. In addition, further signatures of reconnection are quantified, such as parallel electric field generation and Hall quadrupolar magnetic field and current systems. In the final study presented, the anti-correlation between the divergence of the electron pressure tensor and Hall terms is investigated further. It is found that the anti-correlation is general, appearing in the direction normal to the neutral sheet because of a cross tail current. In a simple magnetohydrostatic treatment, a force balance argument leads to the conclusion that the gradient of the anti-correlation is a function of the ratio of the electron to ion temperatures, as well as providing information regarding the spatial scales of the pressure tensors.
45

Low coronal signatures of coronal mass ejections : coronal 'waves' and dimmings

Attrill, G. D. R. January 2009 (has links)
Coronal mass ejections (CMEs) are vast eruptions of magnetised plasma that explode from the solar atmosphere. This thesis focuses on understanding the nascent stages of CMEs, and their magnetic development as they expand into the interplanetary space of our solar system. This is an important part of our effort to understand the space weather environment that we live in, and increasingly interact with through satellite communications technologies. Predominantly through combining extreme ultra-violet imaging and magnetogram data, two low coronal signatures of CMEs, namely coronal waves and dimmings, are studied. A comprehensive list of observational properties of EIT coronal waves is compiled and potential counterparts in radio, H!, soft X-rays and He II wavelengths are also discussed. New observational constraints on EIT coronal waves are presented, most notably diffuse coronal waves are shown to have a magnetic nature. Finding that many observational constraints are not satisfactorily explained by current theories, a new model for understanding the physical nature of diffuse coronal waves is developed. The new model interprets diffuse coronal “wave” bright fronts to be the low coronal magnetic footprint of CMEs. Implications for developing our understanding of how CMEs become large-scale in the low corona are discussed. Application of the model demonstrates how an understanding of the formation of complex global-scale coronal dimmings can be derived. For the first time it is shown that study of the evolution and magnetic nature of coronal dimming regions can be used to probe the post-eruptive evolution of the CME. Finally, a study is presented regarding why and how CME-related dimmings recover, despite the “open” magnetic connectivity of the ejecta to the Sun being maintained as indicated by electron heat flux measurements at 1 AU.
46

Investigation of impact crater processes using experimental and numerical techniques

Baldwin, Emily Clare January 2008 (has links)
Impact events throughout the history of the Solar System have occurred at all scales, from craters produced by the hypervelocity impact of cosmic dust observed on lunar return samples, to the giant planet-sculpting impacts that have shaped the solid bodies of the Solar System. Investigating the impact process in the laboratory allows us to understand crater formation at a small scale where strength effects dominate; however, it is difficult to scale directly to planetary sized impacts because gravity governs the cratering process at this large scale. Through computer modeling, it is possible to bridge the gap from small to large scale impact events. The influence of target porosity, saturation and an overlying water layer on crater morphology is investigated in the laboratory using a two-stage light gas gun to fire 1 mm diameter stainless steel projectiles at ~5 km s^{-1} into sandstone targets. Larger craters were formed in the higher porosity targets and saturated targets. A critical water depth of 11.6\pm 0.5 times the projectile diameter was required to prevent cratering in an unsaturated target, compared with 12.7\pm 0.6 for saturated targets. The sensitivity of this critical water depth to impact velocity, projectile diameter and density is examined through use of the AUTODYN numerical code, for both laboratory and planetary scale impact events. Projectile survivability into water and sand targets is investigated in the lab for stainless steel and shale projectiles impacting at 2-5 km s^{-1}; up to 30% of the projectile is found to survive. AUTODYN simulations shows that basalt or sandstone meteorites impacting a simulated lunar surface survive the impact at velocities <5 km s^{-1} and at a range of angles, which has positive implications for detecting terrestrial meteorites on the Moon. Groundwork has also been laid for the modelling of the deliberate collision of the SMART-1 spacecraft into the Moon. Finally, lunar and terrestrial impact events are simulated in order to quantify the depth of excavation as a function of transient crater diameter for a range of crater and basin sizes. The output is found to lie in the range 0.08-0.15, with the South Pole Aitken basin excavating material to a depth comparable to the thickness of the farside crust.
47

Galaxy clustering and galaxy clusters from the UKIDSS DXS

Kim, Jae Woo January 2011 (has links)
Recent wide and deep surveys allow us to investigate the large scale structure of the Universe at high redshift. We present studies of the clustering of high redshift galaxies and galaxy clusters, using reprocessed UKIDSS DXS catalogues. The UKIDSS DXS is one of the deepest near-IR surveys to date and provides sufficient samples of the distant Universe. Firstly we measure the angular correlation function of high redshift red galaxies which are Extremely Red Objects (EROs) and Distant Red Galaxies (DRGs) in Chapters 3 and 4 from DXS SA22 and Elais-N1 fields. We found that their angular correlation functions can be described by a broken power-law. Thus we estimated clustering properties on small and large scales separately. Then we found that red or bright samples are more strongly clustered than those having the opposite characteristics. In addition old, passive EROs are found to be more clustered than dusty, star-forming EROs. The effect of cosmic variance on angular clustering was also investigated. Chapter 5 describes the halo modelling for the angular clustering of EROs. EROs reside in in dark matter haloes having > 1012.9h−1M, and have a bias of 1.93 at z = 1.12 and 3.17 at z = 1.55. From a direct comparison between the observed clustering and the cosmological model, they show good agreement. However the cosmological simulation may predict too many red satellites, especially at high redshfit. In Chapter 6, we present the details of our cluster detection algorithm based on the red sequence technique. This algorithm successfully found published galaxy clusters in the DXS Elais-N1 field. We also found many overdensities in the DXS SA22 field. Two prominent galaxy clusters were confirmed by spectroscopic observations, and we identified a supercluster candidate. Finally the clustering strength of candidate galaxy clusters showed good agreement with previous results and was consistent with the ΛCDM prediction. In the near future the full DXS catalogue will provide an opportunity to detect various high redshift populations. With other advanced surveys, it will be possible to investigate more details of the large scale structure of the Universe.
48

Turbulence characterisation for astronomical observatories

Shepherd, Harry William January 2012 (has links)
Atmospheric turbulence has two effects in astronomy; (i) the broadening of the point spread function due to phase fluctuations limiting the resolution of imaging and (ii) producing intensity fluctuations known as scintillation. Adaptive Optics (AO) can be installed on telescopes to correct for the effect of phase, and with the push to large telescopes more complex AO systems such as Multi Conjugate AO (MCAO) and Multi Object AO (MOAO) are desired. Operation of these systems requires a detailed profile of the turbulent atmosphere in real time. In this thesis we consider two turbulence profilers, SLOpe Detection And Ranging (SLODAR) and SCIntillation Detection and Ranging (SCIDAR), two cross beam profilers that retrieve data using covariance of phase variations (SLODAR) and intensity variations (SCIDAR). We present a modification of SLODAR to allow an estimate for non resolved turbulence to be made by considering scintillation in the subapertures of a Shack Hartmann wavefront sensor. A new SCIDAR (Stereo--SCIDAR) is described, allowing dynamic re--conjugation to improve altitude resolution. Practical considerations for the implementation of a SLODAR instrument are considered, including a discussion of potential false measurements of non Kolmogorov power spectra in the ground and surface layers of turbulence. Data is presented from SLODAR observing campaigns on La Palma, and at Paranal. Evidence is presented for orographic effects on measured turbulence, including those due to man made structures.
49

Mock galaxy catalogues and their application to future galaxy surveys

Merson, Alexander Ian January 2013 (has links)
We present a method for constructing end-to-end mock galaxy catalogues using a semi-analytical model of galaxy formation, applied to the halo merger trees extracted from a cosmological N-body simulation. These mocks are lightcone catalogues, which incorporate the evolution of galaxy properties with cosmic time. Interpolation is used to determine the epoch at which a galaxy will appear in the past lightcone of the observer. We discuss several applications of mock catalogues. Firstly, we consider the effectiveness of the BzK colour selection technique. The mock catalogue predictions are in reasonable agreement with the observed number counts of BzK galaxies. We predict that over 75 per cent of the model galaxies with K≤23, and redshift 1.4<z<2.5, are selected by the BzK technique. Interloper galaxies, outside the target redshift range, are predicted to dominate bright samples of BzK galaxies (i.e. with K≤21). Fainter K-band cuts are necessary to reduce the predicted interloper fraction. Secondly, we use a mock catalogue to calibrate a galaxy group-finding algorithm, via an objective method based upon the recovery of the distributions of several, easily measurable group properties. We find that it is extremely difficult to determine unique values for the linking lengths by minimising the χ^2 statistic for individual properties, and that it is necessary to combine χ^2 for more than one group property to reduce the parameter space. However, based upon our calibration, we conclude that the optimal linking lengths depend upon the multiplicity of the groups and the group property that one wishes to recover. For our final application, we use a lightcone catalogue to estimate the cosmology-independent angular correlation function, ω(θ), for samples of galaxies, selected in bins of apparent magnitude, in a thin redshift slice comparable to the size of photometric redshift errors. We compare our estimates of ω(θ) with the GALFORM predictions of the 3-dimensional real-space and redshift-space correlation functions. The amplitude of the real-space and redshift-space correlation functions display a trend with increasing luminosity. However, this trend is less clear in ω(θ) due to noisy estimates for the brightest two apparent magnitudes bins.
50

The measurement of the absolute viscosity of fluids exhibiting anomalous flow properties and a study of the phenomenon of thixotropy

Rae, Donald January 1952 (has links)
The theory of the measurement of the absolute viscosity (the' ratio of shearing stress to rate of shear) of instantaneously thixotropic fluids from flow along a tube and from flow between concentric cylinders makes assumptions requiring critical examination. An instrument using flow along a tube is suitable only for instantaneously thixotropic fluids: an instrument using flow between concentric cylinders is also suitable for normally thixotropic fluids. The design of these instruments is particularly related, "both to the conditions necessary for the validity of the given theory and to the characteristics of the fluids used. The usually accepted treatment of the tube flow is in error if the fluid has a yield value. The fluids used, ball-mill dispersions of solid particles in liquid media, have static and dynamic yield values that are different, and this necessitates a special measuring procedure in both instruments. The relation between the rate of shear and the shearing stress determined by either instrument is the same within the limits of experimental error: this error is small so that the validity of the methods of measurement is demonstrated. The dynamic yield values of these fluids are independent of temperature, but the absolute viscosity at any given shearing stress (including the limiting viscosity for high rates of shear measured in another instrument of the concentric cylinder type) varies as the viscosity of the medium. At low shearing stresses the absolute viscosity can be represented as a function of shearing stress by a simple empirical equation. As the concentration of particles is reduced, the limiting viscosity approaches the viscosity of the medium more rapidly than predicted by Einstein's equation.

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