17,641 research outputs found
Space station stabilization and control study Final engineering report
Simulation of stabilization and control for spinning, manned space station to provide artificial gravity station environmen
THE CHARACTERIZATION OF GROUND ICE DEPOSITS USING GROUND-PENETRATING RADAR TECHNIQUES
This study explores the capabilities of ground-penetrating radar (GPR) in the task of characterizing ground ice and the role this instrument can play in understanding the geomorphology of the cryosphere. The first article investigates the dielectric permittivity of ground ice using on-ice common-midpoint (CMP) GPR surveys conducted over massive stratified segregation ice, non-stratified segregation ice, and polygon ice wedges located on Ellesmere and Devon Islands, Nunavut. In comparison with ice cores, it was found that the dielectric permittivity of ground ice is most influenced by the volumetric ice content. This
relationship appears to follow a modified complex refractive index (CRIM) dielectric mixing model. The second study applies the Brewster angle of incidence method to determine the dielectric permittivity of ground ice using endfire CMP surveys conducted atop the active layer. This method was able to predict dielectric permittivities within one dielectric unit of those established in the first article
Particle Flow Calorimetry at the ILC
One of the most important requirements for a detector at the ILC is good jet
energy resolution. It is widely believed that the particle flow approach to
calorimetry is the key to achieving the goal of 0.3/sqrt(E[GeV]). This paper
describes the current performance of the PandoraPFA particle flow algorithm.
For 45 GeV jets in the Tesla TDR detector concept, the ILC jet energy
resolution goal is reached. At higher energies the jet energy resolution
becomes worse and can be described by the empirical expression: sigma_E/E ~
0.265/sqrt(E[GeV]) + 1.2x10^{-4}E[GeV].Comment: 5 pages, 2 .eps figures, to appear in Proc. LCWS06, Bangalore, March
200
Characterizing Human Mobility Patterns in a Large Street Network
Previous studies demonstrated empirically that human mobility exhibits Levy
flight behaviour. However, our knowledge of the mechanisms governing this Levy
flight behaviour remains limited. Here we analyze over 72 000 people's moving
trajectories, obtained from 50 taxicabs during a six-month period in a large
street network, and illustrate that the human mobility pattern, or the Levy
flight behaviour, is mainly attributed to the underlying street network. In
other words, the goal-directed nature of human movement has little effect on
the overall traffic distribution. We further simulate the mobility of a large
number of random walkers, and find that (1) the simulated random walkers can
reproduce the same human mobility pattern, and (2) the simulated mobility rate
of the random walkers correlates pretty well (an R square up to 0.87) with the
observed human mobility rate.Comment: 13 figures, 17 page
Feedback methods for inverse simulation of dynamic models for engineering systems applications
Inverse simulation is a form of inverse modelling in which computer simulation methods are used to find the time histories of input variables that, for a given model, match a set of required output responses. Conventional inverse simulation methods for dynamic models are computationally intensive and can present difficulties for high-speed
applications. This paper includes a review of established methods of inverse simulation,giving some emphasis to iterative techniques that were first developed for aeronautical applications. It goes on to discuss the application of a different approach which is based on feedback principles. This feedback method is suitable for a wide range of linear and nonlinear dynamic models and involves two distinct stages. The first stage involves
design of a feedback loop around the given simulation model and, in the second stage, that closed-loop system is used for inversion of the model. Issues of robustness within
closed-loop systems used in inverse simulation are not significant as there are no plant uncertainties or external disturbances. Thus the process is simpler than that required for the development of a control system of equivalent complexity. Engineering applications
of this feedback approach to inverse simulation are described through case studies that put particular emphasis on nonlinear and multi-input multi-output models
Energy Loss from Reconnection with a Vortex Mesh
Experiments in superfluid 4He show that at low temperatures, energy
dissipation from moving vortices is many orders of magnitude larger than
expected from mutual friction. Here we investigate other mechanisms for energy
loss by a computational study of a vortex that moves through and reconnects
with a mesh of small vortices pinned to the container wall. We find that such
reconnections enhance energy loss from the moving vortex by a factor of up to
100 beyond that with no mesh. The enhancement occurs through two different
mechanisms, both involving the Kelvin oscillations generated along the vortex
by the reconnections. At relatively high temperatures the Kelvin waves increase
the vortex motion, leading to more energy loss through mutual friction. As the
temperature decreases, the vortex oscillations generate additional reconnection
events between the moving vortex and the wall, which decrease the energy of the
moving vortex by transfering portions of its length to the pinned mesh on the
wall.Comment: 9 pages, 10 figure
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