9,856 research outputs found
Dynamic buckling estimates
Dynamic buckling estimates for sensitive structures subjected to finite-time loadin
Phase Transitions in Ultra-Cold Two-Dimensional Bose Gases
We briefly review the theory of Bose-Einstein condensation in the
two-dimensional trapped Bose gas and, in particular the relationship to the
theory of the homogeneous two-dimensional gas and the
Berezinskii-Kosterlitz-Thouless phase. We obtain a phase diagram for the
trapped two-dimensional gas, finding a critical temperature above which the
free energy of a state with a pair of vortices of opposite circulation is lower
than that for a vortex-free Bose-Einstein condensed ground state. We identify
three distinct phases which are, in order of increasing temperature, a phase
coherent Bose-Einstein condensate, a vortex pair plasma with fluctuating
condensate phase and a thermal Bose gas. The thermal activation of
vortex-antivortex pair formation is confirmed using finite-temperature
classical field simulations
An Exploratory Study of Forces and Frictions affecting Large-Scale Model-Driven Development
In this paper, we investigate model-driven engineering, reporting on an
exploratory case-study conducted at a large automotive company. The study
consisted of interviews with 20 engineers and managers working in different
roles. We found that, in the context of a large organization, contextual forces
dominate the cognitive issues of using model-driven technology. The four forces
we identified that are likely independent of the particular abstractions chosen
as the basis of software development are the need for diffing in software
product lines, the needs for problem-specific languages and types, the need for
live modeling in exploratory activities, and the need for point-to-point
traceability between artifacts. We also identified triggers of accidental
complexity, which we refer to as points of friction introduced by languages and
tools. Examples of the friction points identified are insufficient support for
model diffing, point-to-point traceability, and model changes at runtime.Comment: To appear in proceedings of MODELS 2012, LNCS Springe
Non-linear Plasma Wake Growth of Electron Holes
An object's wake in a plasma with small Debye length that drifts
\emph{across} the magnetic field is subject to electrostatic electron
instabilities. Such situations include, for example, the moon in the solar wind
wake and probes in magnetized laboratory plasmas. The instability drive
mechanism can equivalently be considered drift down the potential-energy
gradient or drift up the density-gradient. The gradients arise because the
plasma wake has a region of depressed density and electrostatic potential into
which ions are attracted along the field. The non-linear consequences of the
instability are analysed in this paper. At physical ratios of electron to ion
mass, neither linear nor quasilinear treatment can explain the observation of
large-amplitude perturbations that disrupt the ion streams well before they
become ion-ion unstable. We show here, however, that electron holes, once
formed, continue to grow, driven by the drift mechanism, and if they remain in
the wake may reach a maximum non-linearly stable size, beyond which their
uncontrolled growth disrupts the ions. The hole growth calculations provide a
quantitative prediction of hole profile and size evolution. Hole growth appears
to explain the observations of recent particle-in-cell simulations
Collisional Effects on Nonlinear Ion Drag Force for Small Grains
The ion drag force arising from plasma flow past an embedded spherical grain
is calculated self-consistently and non-linearly using particle in cell codes,
accounting for ion-neutral collisions. Using ion velocity distribution
appropriate for ion drift driven by a force field gives wake potential and
force greatly different from a shifted Maxwellian distribution, regardless of
collisionality. The low-collisionality forces are shown to be consistent with
estimates based upon cross-sections for scattering in a Yukawa (shielded) grain
field, but only if non-linear shielding length is used. Finite collisionality
initially enhances the drag force, but only by up to a factor of 2. Larger
collisionality eventually reduces the drag force. In the collisional regime,
the drift distribution gives larger drag than the shift distribution even at
velocities where their collisionless drags are equal. Comprehensive practical
analytic formulas for force that fit the calculations are provided.Comment: Fig 1. corrected in this versio
A Search for Lymphatic Tissue in the Mouse Pineal Gland
There is recent evidence that the pineal gland in some species may have lymphatic qualities during early post natal life. In this study the development and structure of the pineal complex in the CDF1 mouse is examined in fetal, neonatal, juvenile, and adult stages. The gland appears as an out growth of the dorsal wall of the third ventricle of the brain and at ten days gestation attains a mean size of 90 microns in the transverse and 35 microns in the median saggital plane. General and specific staining shows fetal and early post partum tissue to be compact and mitotic with scant vascularity and poorly developed connective tissue structure. Mitotic activity ceases around three weeks post partum marking the beginning of the juvenile period. Neuroglia become evident as cell volume increases throughout the juvenile stage. Parenchyma cells reach a size of 15 microns in the late juvenile and adult glands. In the adult the gland has grown to a mean diameter of 630 microns in the transverse by 390 microns in the median saggital plane. At this stage the gland is supported by an extremely fine connective tissue framework which is continuous with the capsule. There is no evidence of lymphatic aggregations or germinal centers as described in other species, although occasional lymphocytes are seen in the capsule of the gland and adjacent meningeal tissues of juvenile specimens
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