2,806 research outputs found
Comparison of models of CO2-laser impedance fluctuations
There is a large opto-galvanic effect (OGE) in CO2-N2-He laser mixtures and this is exploited in laser frequency and power stabilisation systems. Two substantially different theories have been advanced to explain the effect. The two models are compared and it is concluded that the multi-step ionisation model is not adequate to describe the OGE in CO2 lasers, but the temperature perturbation or discharge cooling model describes the phenomenon with considerable precision
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
Ribbon Graph Minors and Low-Genus Partial Duals
We give an excluded minor characterisation of the class of ribbon graphs that
admit partial duals of Euler genus at most one
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]
Transport coefficients for the shear dynamo problem at small Reynolds numbers
We build on the formulation developed in Sridhar & Singh (JFM, 664, 265,
2010), and present a theory of the \emph{shear dynamo problem} for small
magnetic and fluid Reynolds numbers, but for arbitrary values of the shear
parameter. Specializing to the case of a mean magnetic field that is slowly
varying in time, explicit expressions for the transport coefficients,
and , are derived. We prove that, when the velocity
field is non helical, the transport coefficient vanishes. We then
consider forced, stochastic dynamics for the incompressible velocity field at
low Reynolds number. An exact, explicit solution for the velocity field is
derived, and the velocity spectrum tensor is calculated in terms of the
Galilean--invariant forcing statistics. We consider forcing statistics that is
non helical, isotropic and delta-correlated-in-time, and specialize to the case
when the mean-field is a function only of the spatial coordinate and time
; this reduction is necessary for comparison with the numerical
experiments of Brandenburg, R{\"a}dler, Rheinhardt & K\"apyl\"a (ApJ, 676, 740,
2008). Explicit expressions are derived for all four components of the magnetic
diffusivity tensor, . These are used to prove that the
shear-current effect cannot be responsible for dynamo action at small \re and
\rem, but for all values of the shear parameter.Comment: 27 pages, 5 figures, Published in Physical Review
The shear dynamo problem for small magnetic Reynolds numbers
We study large-scale dynamo action due to turbulence in the presence of a
linear shear flow, in the low conductivity limit. Our treatment is
nonperturbative in the shear strength and makes systematic use of both the
shearing coordinate transformation and the Galilean invariance of the linear
shear flow. The velocity fluctuations are assumed to have low magnetic Reynolds
number (Rm) but could have arbitrary fluid Reynolds number. The magnetic
fluctuations are determined to lowest order in Rm by explicit calculation of
the resistive Green's function for the linear shear flow. The mean
electromotive force is calculated and an integro-differential equation is
derived for the time evolution of the mean magnetic field. In this equation,
velocity fluctuations contribute to two different kinds of terms, the C and D
terms, in which first and second spatial derivatives of the mean magnetic
field, respectively, appear inside the spacetime integrals. The contribution of
the D terms is such that the time evolution of the cross-shear components of
the mean field do not depend on any other components excepting themselves.
Therefore, to lowest order in Rm but to all orders in the shear strength, the D
terms cannot give rise to a shear-current assisted dynamo effect. Casting the
integro-differential equation in Fourier space, we show that the normal modes
of the theory are a set of shearing waves, labelled by their sheared
wavevectors. The integral kernels are expressed in terms of the velocity
spectrum tensor, which is the fundamental quantity that needs to be specified
to complete the integro-differential equation description of the time evolution
of the mean magnetic field.Comment: Near-final version; Accepted for publication in the Journal of Fluid
Mechanics; References added; 22 pages, 2 figure
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
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
Supercriticality to subcriticality in dynamo transitions
Evidence from numerical simulations suggest that the nature of dynamo
transition changes from supercritical to subcritical as the magnetic Prandtl
number is decreased. To explore this interesting crossover we first use direct
numerical simulations to investigate the hysteresis zone of a subcritical
Taylor-Green dynamo. We establish that a well defined boundary exists in this
hysteresis region which separates dynamo states from the purely hydrodynamic
solution. We then propose simple dynamo models which show similar crossover
from supercritical to subcritical dynamo transition as a function of the
magnetic Prandtl number. Our models show that the change in the nature of
dynamo transition is connected to the stabilizing or de-stabilizing influence
of governing non-linearities.Comment: Version 3 note: Found a sign-error in an equation which propagated
further. Section 4 and Fig. 3,4,5 are updated in Version 3 (final form
Convective plan-form two-scale dynamos in a plane layer
We study generation of magnetic fields, involving large spatial scales, by
convective plan-forms in a horizontal layer. Magnetic modes and their growth
rates are expanded in power series in the scale ratio, and the magnetic eddy
diffusivity (MED) tensor is derived for flows, symmetric about the vertical
axis in a layer. For convective rolls magnetic eddy correction is demonstrated
to be always positive. For rectangular cell patterns, the region in the
parameter space of negative MED coincides with that of small-scale magnetic
field generation. No instances of negative MED in hexagonal cells are found. A
family of plan-forms with a smaller symmetry group than that of rectangular
cell patterns has been found numerically, where MED is negative for molecular
magnetic diffusivity over the threshold for the onset of small-scale magnetic
field generation.Comment: Latex. 24 pages with 3 Postscript figures, 19 references. Final
version (expanded Appendix 2, 4 references added, notation changed to a more
"user-friendly"), accepted in Geophysical and Astrophysical Fluid Dynamic
- …