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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.
21

An isentropic model and the effect of stratospheric planetary waves

Kinnersley, Jonathan Stewart January 1991 (has links)
This thesis describes the formulation and operation of a two-dimensional model of the atmosphere from 0 to about 100 km, using an isentropic vertical coordinate above the tropopause. The model is used in this thesis to study the dynamical effects of stratospheric planetary waves, arising from their transport of Ertel's potential vorticity (PV). The use of isentropic coordinates leads to conceptual simplifications, and to practical advantages in parameterising planetary-wave eddy fluxes in the stratosphere. The model has an interactive troposphere and reproduces the observed annual cycle of the equatorial tropopause temperature when an estimate of the planetary wave PV flux is included in the stratosphere. A feature of the radiation scheme used in this model, and apparently not shared by other 2D models, is the inclusion of the effect of the variation of daylight hours with height. This has a significant influence on the dynamical fields, especially during the equinoxes. The model simulates the observed stratosphere well during autumn and in the southern hemisphere (SH) winter. However, to simulate spring, summer and the NH winter, an accurate estimate of the real PV flux is needed. Since no such estimate was available, three methods were used in this thesis to derive values of the flux for the year from July 1980 to June 1981. One method calculated the flux directly from approximate winds, another estimated the flux due to thermal dissipation of zonal asymmetries in PV, and the third was a variation of an existing method, which finds the PV flux needed to produce the evolution of the observed zonal wind in conjunction with the diabatic circulation. Bearing in mind the significant uncertainties present in all three estimates, the following description of the actual PV flux was put forward. The flux, when large, takes the form of a single negative peak, moving from middle to high latitudes over its life-time.
22

Models of three body scattering and escape

Roy, Alan January 2001 (has links)
Three-body encounters in a stellar system can result in the formation of a binary system if the third star carries away sufficient kinetic energy to leave the other two bound. Subsequent encounters with such a binary are one of the main mechanisms for dynamical evolution of the system. In chapter two the energy change in a hard binary is calculated when the third star moves on a distant nearly parabolic orbit. Previous results fail in certain important cases (e.g. a circular binary), and the new results give a complete treatment. Chapters three and four are motivated by the problem of determining the rate at which stars escape from a globular cluster. This process has important consequences for the evolution of systems of globular star clusters. Hénon(1988) developed a <i>model</i> problem which mimics chaotic aspects of a star's motion within a globular cluster. This model (chapter three) describe a particle bouncing on an inclined surface and is used to understand the mechanism underpinning the escape process in globular clusters. Orbital trajectories of escaping stars are investigated in chapter four by applying parts of the theory presented in chapter three. The motion of a star within a globular cluster is modelled using Hill's Equations. By applying numerical integration the trajectory of a star, with given initial conditions, may be followed in phase space. Different cases are considered depending on whether the cluster moves in a circular or noncircular orbit around the centre of the galaxy. In the circular case the orbital motion is viewed on an appropriate surface of section. Escaping stars are all found to lie inside a tubular surface which connects the cluster with the outside world. In the noncircular case the initial conditions leading to stable/unstable orbits are investigated. The escape process for highly eccentric orbits is modelled by supposing that the star is subjected to an impulsive force each time the cluster moves through the galactic pericentre.
23

Measurement of the ¹⁸Ne(α,p)²¹Na reaction rate, and its implications for Nuclear Astrophysics

Bradfield-Smith, William January 1999 (has links)
Experimental work has been carried out at Louvain-la-Neuve to study the reactions <SUP>13</SUP>N(α,p)<SUP>16</SUP>O and <SUP>18</SUP>Ne(α,p)<SUP>21</SUP>Na in inverse kinematics with a gaseous helium target. The experimental method devised was tested using the reaction <SUP>13</SUP>N(α,p)<SUP>16</SUP>O, as the cross section was calculable from data on <SUP>16</SUP>O(p,α)<SUP>13</SUP>N[1,2,3,4], the inverse reaction. This test experiment showed that the experimental error obtainable in the deduction of the cross section resonance strength's was 30%, making the technique of practical use in the investigation of (α,p) reactions of interest to Nuclear Astrophysics. The reaction <SUP>18</SUP>Ne(α,p)<SUP>21</SUP>Na, which is important as a break-out mechanism from the hot CNO cycle into the rp-process during explosive hydrogen burning, has been investigated, and values for the cross sections resonance strengths have been extracted from the experimental data. A stellar reaction rate, based only upon the observed resonances, has been calculated and compared with theoretical predictions[5]. A good agreement was obtained at and above a temperature of 2.5 10<SUP>9</SUP><I>K</I>, whilst at lower temperatures the experimentally reaction rate obtained fell rapidly below the calculated value. This discrepancy was due to the fact that the theoretical calculation of the stellar reaction rate used resonances at energies below 2.5 MeV, not observed experimentally. At low temperature the reaction flux through these resonances dominates the stellar reaction rate. The experimental stellar reaction rate, though only a lower limit, has been applied to a one mass zone X-ray burst model[6]. This network calculation has shown that break-out via <SUP>18</SUP>Ne(α,p)<SUP>21</SUP>Na is sufficient to trigger the burst for a type I X-ray burster, and allow mass to flow from the CNO region to the mass 100 region via the rp-process.
24

N-body algorithms and applications to galactic dynamics

Purchase, Steve January 2000 (has links)
Typically galaxies contain 10<sup>11</sup> stars and globular clusters contain 10<sup>6</sup> stars. In order to investigate the dynamical evolution of these systems one may perform numerical simulations (<i>N</i>-body simulations); which in turn are restricted to 10<sup>3</sup> to 10<sup>7</sup> particles depending on the required accuracy and adopted evolutionary model. For the <i>N</i>-body direct method the computational effort scales at least as O(<i>N</i><sup>3</sup>), and thus the need for efficient and accurate evolutionary models and advances in hardware in apparent. A review of the <i>N</i>-body evolutionary models is presented and, with the advent of parallel super computers, the parallelisation of these evolutionary models is investigated. In particular, we discuss the parallelisation of a direct method <i>N</i>-body code; focusing upon a new parallelisation strategy, implementing the evolutionary model with a portable parallel language (High Performance Fortran), and reducing the communication costs between processors by a suitable implementation of the Hypersystolic algorithm. A dynamical system, such as a galaxy, may oscillate about a stable equilibrium if it is excited above that state. These modes of oscillation may persist long enough to have observable consequences, despite being weakly damped. The moods of oscillation are investigated with a range of evolutionary models. The simplest mode to detect and simulate is the "fundamental" mode or "<i>l = </i>0" mode, which manifests itself as a radical oscillation of the entire system. To investigate this mode a King W<sub>0</sub> = 1 theoretical model is adopted. The effect of adopting a softened gravitational potential to generate the forces between stars on the fundamental mode is investigated with a direct method <i>N</i>-body evolutionary model, and these results are compared to those which use an SCF (self-consistent field) type code. The presence of small and amplitude fundamental mode oscillations is detectable when the perturbation particle method is used, which would otherwise be undetectable for another <i>N</i>-body evolutionary model, due to particle noise. Furthermore, the source of heavy damping in direct method <i>N</i>-body simulations is found to be phase mixing. The experimental fundamental mode oscillations are found to match well with the theoretically predicted frequencies. A more complicated mode, the "sloshing" mode or "<i>l = </i>1" is also studied. This mode manifests itself in the density centre shifting or sloshing about. To investigate this mode a King <i>W</i><sub>0</sub> = 5 model is adopted, and the results compared to an analytical predicted frequency.
25

Data analysis and background studies for the Cobra Neutrinoless Double-Beta decay experiment

Reeve, Christopher J. January 2009 (has links)
No description available.
26

Breakout from the hot-CNO cycle via the ¹⁸Ne(α,p)²¹Na reaction

Groombridge, Darren January 2001 (has links)
The <sup>18</sup>Ne(a,p)<sup>21</sup>Na reaction is of great importance to Nuclear Astrophysics as it provides a route to breakout from the hot-CNO cycle, possibly leading to the formation of the elements up to <i>A</i>~100. This particular reaction has been studied using a <sup>18</sup>Ne beam, available at Louvain-la-Neuve, together with a helium gas target system previously developed for the investigation of (a,p) reactions with a radioactive beam. This study covered an energy region from ~1.7-2.9 MeV in the centre of mass frame o the <sup>18</sup>Ne+a system. A change in the detector geometry resulted in an increase in the detection efficiency and significantly reduced the proton background that hindered the previous measurement. A direct measurement of the energy loss of the <sup>18</sup>Ne beam, as it passed through He gas, was undertaken to reduce a major source of uncertainty in the determination of the stellar reaction rate. This showed a linear relationship, between beam energy and distance traversed within the gas, over an energy scan of ~8-16 MeV and gave an energy loss of (1.55 ±0.01) MeV/cm and (15.96 ± 0.02) MeV for the energy of the <sup>18</sup>Ne beam upon entry into the gas. This information was used, together with kinematic information from the protons, to provide information on the level structure in the compound nucleus, <sup>22</sup>Mg. Ten states have been identified within an energy region of ~10-11 MeV and are in good agreement with information that is currently known. These resonances were used to calculate an enhanced stellar reaction rate which shows reasonable agreement with theoretical predictions [44] at and above a temperature of 1.5 GK.
27

Fourier analysis of the light curves of eclipsing variables and its application to YZ(21) Cassiopeiae

Kurutac, Mehmet January 1976 (has links)
The interpretation of the light curves of eclipsing variables has attracted the attention of many astronomers and many methods have been developed. These methods have mainly dealt with the problem in the time-domain. Recently, a new method started to appear in a series of papers published by Kopal, who has shown that a transition of the problem from the time-domain into the frequency-domain is indeed the right approach in the interpretation of the light curves of eclipsing binaries. Kopal's Fourier approach to the problem deals not with the single measurements of the light curve, but with areas, referred to by Kopal as "the theoretical moments of the light curve, " which are represented by the light curve. The main aim of this work is to evaluate the theoretical moments of the light curve for annular eclipses and apply it to the case of YZ Cassiopeiae.
28

Superconducting filter designs for Jodrell Bank astronomy observatory

Li, Yuanzhi January 2004 (has links)
With the increasing radio communications activities of recent years, the radio spectrum is becoming intensely crowded and this trend is set to increase at an extraordinary rate. Radio astronomy is particularly sensitive to interference of this type. A high temperature superconducting (HTS) filter, at the front-end of the receiver, has the potential to effectively eliminate the interference from adjacent bands. Such a filter will have negligible loss, have extremely sharp filter skirts and be small enough to fit into the current low temperature systems. HTS microwave filters started to emerge during the early 1990s, and since then much effort has been put into filter designs for narrow band applications. However, there are still not many wide band HTS filters reported. Here we are interested in a wide band filter for radio astronomy applications. Four HTS microstrip filters have been designed according to the specification from Jodrell Bank Radio Observatory. Among them, a nine-pole Chebyshev HTS wideband filter has been fabricated and tested. Good agreement between simulated and experimental results has been obtained. The filter has also been tested in Jodrell Bank radio astronomy. The telescope receiver system test showed that the HTS filter contributed negligible noise to one of the receiving channels when compared to the other channel without an HTS filter. The prototype filter is to be used on the Lovell Telescope in Jodrell Bank Observatory.
29

The production of metal mirrors for use in astronomy

Brooks, David January 2001 (has links)
This thesis demonstrates the possibility of manufacturing larger mirrors from nickel coated aluminium with a considerable cost and risk benefits compared to zero expansion glass ceramic or borosilicate. Constructing large mirrors from aluminium could cut the cost of production by one third. A new generation of very large telescopes is being designed, on the order of 100 meters diameter. The proposed designs are of mosaic type mirrors similar to the Keck Telescope primary. The enormous mass of glass required inhibits the construction, simply by its cost and production time. Very little research has been done on the processes involved in the production of large metal mirrors. However the thermal efficiency and potential improved mirror seeing benefits are documented. Space telescopes and optical telecommunications could also benefit with the application of metal mirrors. Presented here are the processes and results that culminated in the rebirth of the Birr Telescope. The main section concerns the material selection and processes in the construction of a 1.83 meter diameter 1.4 tonne aluminium primary mirror. The aluminium mirror technology developed was also applied to the construction of an aspheric thin meniscus deformable mirror. Methods employed in its production are described. Documented are the advanced computer controlled polishing methods employed in producing a one third scale model of the hyperbolic secondary mirror for the Gemini Telescopes. These were developed using an active polishing lap.
30

Radio-loud and radio-quiet quasars

Miller, Philip Geoffrey January 1992 (has links)
No description available.

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