45,113 research outputs found
MHD simulations of the formation and propagation of protostellar jets to observational length scales
We present 2.5-D global, ideal MHD simulations of magnetically and
rotationally driven protostellar jets from Keplerian accretion discs, wherein
only the initial magnetic field strength at the inner radius of the disc,
, is varied. Using the AMR-MHD code AZEUS, we self-consistently
follow the jet evolution into the observational regime ()
with a spatial dynamic range of . The simulations reveal a
three-component outflow: 1) A hot, dense, super-fast and highly magnetised 'jet
core'; 2) a cold, rarefied, trans-fast and highly magnetised 'sheath'
surrounding the jet core and extending to a tangential discontinuity; and 3) a
warm, dense, trans-slow and weakly magnetised shocked ambient medium entrained
by the advancing bow shock. The simulations reveal power-law relationships
between and the jet advance speed, , the average jet
rotation speed, , as well as fluxes of mass,
momentum, and kinetic energy. Quantities that do not depend on
include the plasma- of the transported material which, in all cases,
seems to asymptote to order unity. Jets are launched by a combination of the
'magnetic tower' and 'bead-on-a-wire' mechanisms, with the former accounting
for most of the jet acceleration---even for strong fields---and continuing well
beyond the fast magnetosonic point. At no time does the leading bow shock leave
the domain and, as such, these simulations generate large-scale jets that
reproduce many of the observed properties of protostellar jets including their
characteristic speeds and transported fluxes.Comment: 26 pages, 16 figures. Accepted for publication in MNRA
Dynamics and Structure of Three-Dimensional Poloidally Magnetized Supermagnetosonic Jets
A set of 3D MHD simulations of magnetized jets has been performed. The jets
contain an equipartition primarily poloidal magnetic field and the effect of
jet density on jet dynamics and structure is evaluated. The jet is precessed at
the origin to excite Kelvin-Helmholtz unstable helical modes. We extensively
compare the structure in these simulations with linear stability theory. The
jet that is dense with respect to the external medium develops a high speed
core surrounded by a less dense sheath consisting of slower moving jet fluid.
These simulations suggest that extended extragalactic jets propagate to such
large distances because they are surrounded by a lobe or cocoon whose density
is less than the jet density. (Abridged abstract.)Comment: 30 pages, AASTeX, to appear in ApJ, much better versions of Figures
2-5 are available at http://crux.astr.ua.edu/~rosen/hcr/hcr.htm
Filamentary fragmentation in a turbulent medium
We present the results of smoothed particle hydrodynamic simulations
investigating the evolution and fragmentation of filaments that are accreting
from a turbulent medium. We show that the presence of turbulence, and the
resulting inhomogeneities in the accretion flow, play a significant role in the
fragmentation process. Filaments which experience a weakly turbulent accretion
flow fragment in a two-tier hierarchical fashion, similar to the fragmentation
pattern seen in the Orion Integral Shaped Filament. Increasing the energy in
the turbulent velocity field results in more sub-structure within the
filaments, and one sees a shift from gravity-dominated fragmentation to
turbulence-dominated fragmentation. The sub-structure formed in the filaments
is elongated and roughly parallel to the longitudinal axis of the filament,
similar to the fibres seen in observations of Taurus, and suggests that the
fray and fragment scenario is a possible mechanism for the production of
fibres. We show that the formation of these fibre-like structures is linked to
the vorticity of the velocity field inside the filament and the filament's
accretion from an inhomogeneous medium. Moreover, we find that accretion is
able to drive and sustain roughly sonic levels of turbulence inside the
filaments, but is not able to prevent radial collapse once the filaments become
supercritical. However, the supercritical filaments which contain fibre-like
structures do not collapse radially, suggesting that fibrous filaments may not
necessarily become radially unstable once they reach the critical line-density.Comment: (Accepted for publication in MNRAS
The role of built environment energy efficiency in a sustainable UK energy economy
Energy efficiency in the built environment can make significant contributions to a sustainable energy economy. In order to achieve this, greater public awareness of the importance of energy efficiency is required. In the short term, new efficient domestic appliances, building technologies, legislation quantifying building plant performance, and improved building regulations to include installed plant will be required. Continuing these improvements in the longer term is likely to see the adoption of small-scale renewable technologies embedded in the building fabric. Internet-based energy services will see low-cost building energy management and control delivered to the mass market in order that plant can be operated and maintained at optimum performance levels and energy savings quantified. There are many technology options for improved energy performance of the building fabric and energy systems and it's not yet clear which will prove to be the most economic. Therefore, flexibility is needed in legislation and energy-efficiency initiatives
New tools in comparative political economy: The database of political institutions.
[Dataset available: http://hdl.handle.net/10411/15987]
Control Plane Compression
We develop an algorithm capable of compressing large networks into a smaller
ones with similar control plane behavior: For every stable routing solution in
the large, original network, there exists a corresponding solution in the
compressed network, and vice versa. Our compression algorithm preserves a wide
variety of network properties including reachability, loop freedom, and path
length. Consequently, operators may speed up network analysis, based on
simulation, emulation, or verification, by analyzing only the compressed
network. Our approach is based on a new theory of control plane equivalence. We
implement these ideas in a tool called Bonsai and apply it to real and
synthetic networks. Bonsai can shrink real networks by over a factor of 5 and
speed up analysis by several orders of magnitude.Comment: Extended version of the paper appearing in ACM SIGCOMM 201
AZEuS: An Adaptive Zone Eulerian Scheme for Computational MHD
A new adaptive mesh refinement (AMR) version of the ZEUS-3D astrophysical
magnetohydrodynamical (MHD) fluid code, AZEuS, is described. The AMR module in
AZEuS has been completely adapted to the staggered mesh that characterises the
ZEUS family of codes, on which scalar quantities are zone-centred and vector
components are face-centred. In addition, for applications using static grids,
it is necessary to use higher-order interpolations for prolongation to minimise
the errors caused by waves crossing from a grid of one resolution to another.
Finally, solutions to test problems in 1-, 2-, and 3-dimensions in both
Cartesian and spherical coordinates are presented.Comment: 52 pages, 17 figures; Accepted for publication in ApJ
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Dietary manipulation of broiler breeder growth through the feeding of conjugated linoleic acid
Analysing Magnetism Using Scanning SQUID Microscopy
Scanning superconducting quantum interference device microscopy (SSM) is a
scanning probe technique that images local magnetic flux, which allows for
mapping of magnetic fields with high field and spatial accuracy. Many studies
involving SSM have been published in the last decades, using SSM to make
qualitative statements about magnetism. However, quantitative analysis using
SSM has received less attention. In this work, we discuss several aspects of
interpreting SSM images and methods to improve quantitative analysis. First, we
analyse the spatial resolution and how it depends on several factors. Second,
we discuss the analysis of SSM scans and the information obtained from the SSM
data. Using simulations, we show how signals evolve as a function of changing
scan height, SQUID loop size, magnetization strength and orientation. We also
investigated 2-dimensional autocorrelation analysis to extract information
about the size, shape and symmetry of magnetic features. Finally, we provide an
outlook on possible future applications and improvements.Comment: 16 pages, 10 figure
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