No description available.

Thesis (Ph.D.) - University of St Andrews, May 2010.

Reconnection of wingtip vortices behind aircrafts is thought to be a cause of wake turbulence, detrimental to air traffic control. We observe the reconnection process for three initial vortex tube set-ups; anti-parallel, orthogonal and anti-parallel with axial flow. From these we are able to identify each of the different reconnection processes observed and discussed for the magnetic reconnection case but not necessarily the vortex reconnection case; of both 2D and 3D reconnection. We use a finite different method to solve the Navier-Stokes equation for a large array of points. We analyse the results of the first two scenarios for a range of Reynolds numbers to observe how the viscous term of Navier-Stokes affects the reconnection process. We were able to show that for an increase in $Re$ we would see an increase in the reconnection rate due to the formation of thinner and stronger vortex sheets which are necessary for a faster reconnection. For higher values of $Re$ we observed a Kelvin-Helmholtz instability within the vortex sheets and the formation of additional vortex rings during the reconnection process. We simulate a range of axial flow values to observe how kinetic helicity and twist evolve with reconnection. We were able to identify the loss of twist in the vortex tubes due to 3D reconnection known as 'slipping'. In these and the orthogonal runs we observed the generation of null pairs and the formation of a separator between them. We utilised the plots of both vorticity isosurfaces and vorticity fieldlines to observe and analyse the reconnection process where isosurfaces have been the norm for vortex reconnection observations in previous work. The vorticity fieldlines allow us to observe the orientation of vorticity during reconnection and allow us to observe both the 'threads' and 'bridges' and their evolution together.

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Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Synchronous distributed groupware is a class of software applications allowing a geographically distributed group of people to collaborate in real time. There are different types of groupware, e.g., collaborative editing software, distributed meeting support tools, and multiplayer games. However, collaborators in groupware can become disconnected from the session. Disconnections can range in duration from a few seconds (e.g., due to a network outage) to hours or days (e.g., stowing a laptop while flying). Disconnection causes information loss and makes it difficult for users to understand both the state of the workspace and the current activities of other people upon reconnection. Thus, it is important to handle disconnection in groupware. However, handling disconnection is difficult for groupware developers. They need to determine varieties of strategies in order to address different disconnection scenarios. These strategies determine how stored information can be manipulated as the system waits for a disconnected user to rejoin, and how information should be replayed upon reconnection. If disconnection lasts for a long time, developers need to select and combine strategies in order to manage a trade-off between performance requirements (e.g., delivering stored information as quickly as possible upon reconnection) and understandability requirements (e.g., allowing users to watch missed information in an understandable manner). Developers might not know how to implement such strategies in a reusable manner. Because of this lack of knowledge in handling disconnection, developers might build disconnection-aware groupware applications that will not address the range of wide variety of user-level requirements that arise from different disconnection scenarios. Moreover, as there are a few disconnection-aware groupware applications, developers might not know the overhead (e.g., additional message transmission time, memory usage and programming complexity) of handling disconnection in synchronous groupware in practice. In order to mitigate developers’ problems to handle disconnection in synchronous groupware, this thesis provides codified solutions that capture and organize a wide range of strategies for handling disconnection, that manage the performance and understandability trade-off through selecting and combining suitable strategies and that show how to implement the strategies in a reusable manner. In order to determine the overhead of handling disconnection, a toolkit is developed following the designed solutions, and different applications are constructed using the toolkit. User studies and performance analyses are conducted that evaluate the toolkit and demonstrate its quality goals, such as offering simple application programming interface (API), high performance, and supporting different disconnection scenarios and timeframes. / Thesis (Ph.D, Computing) -- Queen's University, 2013-04-29 17:52:00.165

The upper atmosphere of the sun, the solar corona, is approximately 1,000,000K hotter than the surface of the Sun, a property which cannot be explained by the normal processes of heat conduction and radiation. It is now commonly believed that the magnetic fields which fill the solar atmosphere, and propagate down into the interior of the Sun, are important for transferring and transforming energy from the strong plasma flows inside the Sun into the corona as heat. I have investigated an elementary flux interaction which forms a fundamental building block of the coronal heating process. This interaction involves two opposite polarity sources on the Sun's surface in the presence of an overlying magnetic field. To fully understand how this interaction transfers heat into the solar corona, the magnetic skeleton is required, which shows possible sites of heating that are due to magnetic reconnection. A magnetic field is best described by its magnetic skeleton. The most important parts of the magnetic skeleton to find are the null points, from which separatrix surfaces extend that divide magnetic flux of different topology. Part of this thesis proposes a new method of finding null points, for which the accuracy is shown and then compared with another commonly used method (which gave false results). Using these techniques for finding the magnetic skeleton in the magnetic interaction above, the evolution of the skeleton was found to head through seven distinct states, some of which were far more complicated than expected. This included a high number of separators (the intersection of two separatrix surfaces), which are a known location of magnetic reconnection. This separator reconnection was shown to be the main heating mechanism in this interaction, from which the total amount and rates of reconnection in the experiment was calculated. This led to the discovery of recursive reconnection, a process where magnetic flux is reconnected before reconnecting back to its original state, to allow for the process to repeat again. This recursive reconnection was shown to allow far more reconnection than would have been previously expected, all of which releases heat into the neighbouring areas of the atmosphere. Finally, the interaction was modelled with sources of different magnetic radii but of equal flux. This showed that when the antisymmetric nature of the previous interactions was removed, there was little change in the reconnection rates, but when the strength of the overlying magnetic field was increased, the reconnection rates were found to increase. This increase in the overlying magnetic field strength also produced a new magnetic feature called a bald-edge, which was found to replace some of the null points. These bald-edges were found to be associated with surfaces similar to separatrix surfaces that divide flux of different topology but do not extend from a null point. Also features similar to separators extend from these bald-edges.

No description available.

A recently proposed set of demagnetization diagnostics [Scudder et al., submitted to Physics of Plasmas, 2015] is related to the preconditions of Guiding Center Theory (GCT) and benchmarked in kinetic particle-in-cell simulations. Specifically, GCT requires that the time and length scales of the field are long compared to the Larmor motion of the particles. When this condition is violated, the particles become demagnetized and the assumptions of magnetohydrodynamics are no longer valid. In this thesis, these diagnostics are applied to different space plasma layers of different length scales. In the past, proxy diagnostics that are not based on fundamental GCT conditions have been used to search for, and provide evidence of, demagnetization in different space plasma layers. The problem with these proxy diagnostics is that they are not invertible to demagnetization. The diagnostics presented in this thesis are not only unique to demagnetization, but also have the additional advantages of being dimensionless, scalar, and independent of coordinate system. These diagnostics are applied to three space plasma layers of different length scales, resulting in new insights and methods for detecting particle demagnetization. First, the evidence for wave heating in the solar wind is reexamined through its fundamental assumptions of demagnetization. The proxy diagnostic commonly used for demagnetization is non-conservation of the Chew-Goldberger-Low conserved quantity. This diagnostic is a good proxy for the first adiabatic invariant in the supersonic regime. To test this and compare it to the assumptions of the Helios analysis [Marsch et al., Journal of Geophysical Research: Space Physics, 88(A4), 1983], the solar wind is modeled through a self-consistent Vlasov mapping. In addition, other experimental assumptions in that same Helios analysis are also examined. Second, a new method for estimating local length scales is demonstrated across a known bow shock crossing. This new method, based on one of the demagnetization diagnostics, is different from current methods in that it can be performed with single spacecraft data and does not require a special coordinate system. Third, a new set of invertible signatures of the electron diffusion region (EDR) is introduced and applied to five magnetopause events to search for layers of collisionless magnetic reconnection. Four of these magnetopause events have not been identified before in the literature. The five EDR diagnostics are large electron pressure anisotropy, non-perturbative GCT expansion parameters, order one electron pressure agyrotropy, and order one electron thermal mach number. These EDR diagnostics are compared to a wide range of degenerate diagnostics that are commonly used in reconnection studies. The results of this analysis show that, compared to these degenerate diagnostics, the EDR diagnostics are much more surgical in their identification of electron-scale current layers.

Thesis (Ph.D.)--University of Waikato, 2007. / Title from PDF cover (viewed May 12, 2008) Includes bibliographical references (p. [168]-175)