1,643 research outputs found
Bipartite partial duals and circuits in medial graphs
It is well known that a plane graph is Eulerian if and only if its geometric
dual is bipartite. We extend this result to partial duals of plane graphs. We
then characterize all bipartite partial duals of a plane graph in terms of
oriented circuits in its medial graph.Comment: v2: minor changes. To appear in Combinatoric
Barriers to the up-take of telemedicine in Australia - A view from providers
Introduction: The continued poorer health status of rural and remote Australians when compared with their urban counterparts is cause for concern. The use of advanced technology to improve access to health care has the potential to assist in addressing this problem. Telemedicine is one example of such technology which has advanced rapidly in its capacity to increase access to healthcare services or provide previously unavailable services. The important anticipated benefits of greater access to healthcare services are improved health outcomes and more cost-effective delivery
Creation and evolution of magnetic helicity
Projecting a non-Abelian SU(2) vacuum gauge field - a pure gauge constructed
from the group element U - onto a fixed (electromagnetic) direction in isospace
gives rise to a nontrivial magnetic field, with nonvanishing magnetic helicity,
which coincides with the winding number of U. Although the helicity is not
conserved under Maxwell (vacuum) evolution, it retains one-half its initial
value at infinite time.Comment: Clarifying remarks and references added; 12 pages, 1 figure using
BoxedEPSF, REVTeX macros; submitted to Phys Rev D; email to
[email protected]
Low magnetic Prandtl number dynamos with helical forcing
We present direct numerical simulations of dynamo action in a forced Roberts
flow. The behavior of the dynamo is followed as the mechanical Reynolds number
is increased, starting from the laminar case until a turbulent regime is
reached. The critical magnetic Reynolds for dynamo action is found, and in the
turbulent flow it is observed to be nearly independent on the magnetic Prandtl
number in the range from 0.3 to 0.1. Also the dependence of this threshold with
the amount of mechanical helicity in the flow is studied. For the different
regimes found, the configuration of the magnetic and velocity fields in the
saturated steady state are discussed.Comment: 9 pages, 14 figure
Large Scale Structures a Gradient Lines: the case of the Trkal Flow
A specific asymptotic expansion at large Reynolds numbers (R)for the long
wavelength perturbation of a non stationary anisotropic helical solution of the
force less Navier-Stokes equations (Trkal solutions) is effectively constructed
of the Beltrami type terms through multi scaling analysis. The asymptotic
procedure is proved to be valid for one specific value of the scaling
parameter,namely for the square root of the Reynolds number (R).As a result
large scale structures arise as gradient lines of the energy determined by the
initial conditions for two anisotropic Beltrami flows of the same helicity.The
same intitial conditions determine the boundaries of the vortex-velocity tubes,
containing both streamlines and vortex linesComment: 27 pages, 2 figure
Shell to shell energy transfer in MHD, Part I: steady state turbulence
We investigate the transfer of energy from large scales to small scales in
fully developed forced three-dimensional MHD-turbulence by analyzing the
results of direct numerical simulations in the absence of an externally imposed
uniform magnetic field. Our results show that the transfer of kinetic energy
from the large scales to kinetic energy at smaller scales, and the transfer of
magnetic energy from the large scales to magnetic energy at smaller scales, are
local, as is also found in the case of neutral fluids, and in a way that is
compatible with Kolmogorov (1941) theory of turbulence. However, the transfer
of energy from the velocity field to the magnetic field is a highly non-local
process in Fourier space. Energy from the velocity field at large scales can be
transfered directly into small scale magnetic fields without the participation
of intermediate scales. Some implications of our results to MHD turbulence
modeling are also discussed.Comment: Submitted to PR
Helicity cascades in rotating turbulence
The effect of helicity (velocity-vorticity correlations) is studied in direct
numerical simulations of rotating turbulence down to Rossby numbers of 0.02.
The results suggest that the presence of net helicity plays an important role
in the dynamics of the flow. In particular, at small Rossby number, the energy
cascades to large scales, as expected, but helicity then can dominate the
cascade to small scales. A phenomenological interpretation in terms of a direct
cascade of helicity slowed down by wave-eddy interactions leads to the
prediction of new inertial indices for the small-scale energy and helicity
spectra.Comment: 7 pages, 8 figure
Saturation of Magnetorotational Instability through Magnetic Field Generation
The saturation mechanism of Magneto-Rotational Instability (MRI) is examined
through analytical quasilinear theory and through nonlinear computation of a
single mode in a rotating disk. We find that large-scale magnetic field is
generated through the alpha effect (the correlated product of velocity and
magnetic field fluctuations) and causes the MRI mode to saturate. If the
large-scale plasma flow is allowed to evolve, the mode can also saturate
through its flow relaxation. In astrophysical plasmas, for which the flow
cannot relax because of gravitational constraints, the mode saturates through
field generation only.Comment: 9 pages, 10 figures to appear in ApJ, Jun 2009, 10 v69
Nonlinearity in a dynamo
Using a rotating flat layer heated from below as an example, we consider
effects which lead to stabilizing an exponentially growing magnetic field in
magnetostrophic convection in transition from the kinematic dynamo to the full
non-linear dynamo. We present estimates of the energy redistribution over the
spectrum and helicity quenching by the magnetic field. We also study the
alignment of the velocity and magnetic fields. These regimes are similar to
those in planetary dynamo simulations.Comment: Accepted to Geophys. Astrophys. Fluid Dyna
Primordial Magnetic Field Limits from Cosmic Microwave Background Bispectrum of Magnetic Passive Scalar Modes
Primordial magnetic fields lead to non-Gaussian signals in the cosmic
microwave background (CMB) even at the lowest order, as magnetic stresses and
the temperature anisotropy they induce depend quadratically on the magnetic
field. In contrast, CMB non-Gaussianity due to inflationary scalar
perturbations arises only as a higher order effect. Apart from a compensated
scalar mode, stochastic primordial magnetic fields also produce scalar
anisotropic stress that remains uncompensated till neutrino decoupling. This
gives rise to an adiabatic-like scalar perturbation mode that evolves passively
thereafter (called the passive mode). We compute the CMB reduced bispectrum
() induced by this passive mode, sourced via the
Sachs-Wolfe effect, on large angular scales. For any configuration of
bispectrum, taking a partial sum over mode-coupling terms, we find a typical
value of , for a magnetic field of nG, assuming a nearly
scale-invariant magnetic spectrum . We also evaluate, in full, the bispectrum
for the squeezed collinear configuration over all angular mode-coupling terms
and find . These values are more than times larger than the
previously calculated magnetic compensated scalar mode CMB bispectrum.
Observational limits on the bispectrum from WMAP7 data allow us to set upper
limits of nG on the present value of the cosmic magnetic field of
primordial origin. This is over 10 times more stringent than earlier limits on
based on the compensated mode bispectrum.Comment: 9 page
- …