12,683 research outputs found
Breaking Kelvin: Circulation conservation and vortex breakup in MHD at low Magnetic Prandtl Number
In this paper we examine the role of weak magnetic fields in breaking
Kelvin's circulation theorem and in vortex breakup in two-dimensional
magnetohydrodynamics for the physically important case of a low magnetic
Prandtl number (low ) fluid. We consider three canonical inviscid solutions
for the purely hydrodynamical problem, namely a Gaussian vortex, a circular
vortex patch and an elliptical vortex patch. We examine how magnetic fields
lead to an initial loss of circulation and attempt to derive scaling
laws for the loss of circulation as a function of field strength and diffusion
as measured by two non-dimensional parameters. We show that for all cases the
loss of circulation depends on the integrated effects of the Lorentz force,
with the patch cases leading to significantly greater circulation loss. For the
case of the elliptical vortex the loss of circulation depends on the total area
swept out by the rotating vortex and so this leads to more efficient
circulation loss than for a circular vortex.Comment: 21 pages, 12 figure
Tracing the Pathway from Drift-Wave Turbulence with Broken Symmetry to the Generation of Sheared Axial Mean Flow
This study traces the emergence of sheared axial flow from collisional drift
wave turbulence with broken symmetry in a linear plasma device---CSDX. As the
density profile steepens, the axial Reynolds stress develops and drives a
radially sheared axial flow that is parallel to the magnetic field. Results
show that the non-diffusive piece of the Reynolds stress is driven by the
density gradient and results from the spectral asymmetry of the turbulence and
thus is dynamical in origin. Taken together, these findings constitute the
first simultaneous demonstration of the causal link between the density
gradient, turbulence and stress with broken spectral symmetry, and the mean
axial flow
Influence of zonal flows on unstable drift modes in ETG turbulence
The linear instability of the electron temperature gradient (ETG) driven
modes in the presence of zonal flows is investigated. Random and deterministic
- like profiles of the zonal flow are considered. It is shown that the
presence of shearing by zonal flows can stabilize the linear instability of ETG
drift modes
Nonlinear shock acceleration beyond the Bohm limit
We suggest a physical mechanism whereby the acceleration time of cosmic rays
by shock waves can be significantly reduced. This creates the possibility of
particle acceleration beyond the knee energy at ~10^15eV. The acceleration
results from a nonlinear modification of the flow ahead of the shock supported
by particles already accelerated to the knee momentum at p ~ p_*. The particles
gain energy by bouncing off converging magnetic irregularities frozen into the
flow in the shock precursor and not so much by re-crossing the shock itself.
The acceleration rate is thus determined by the gradient of the flow velocity
and turns out to be formally independent of the particle mean free path
(m.f.p.). The velocity gradient is, in turn, set by the knee-particles at p ~
p_* as having the dominant contribution to the CR pressure. Since it is
independent of the m.f.p., the acceleration rate of particles above the knee
does not decrease with energy, unlike in the linear acceleration regime. The
reason for the knee formation at p ~ p_* is that particles with are
effectively confined to the shock precursor only while they are within limited
domains in the momentum space, while other particles fall into
``loss-islands'', similar to the ``loss-cone'' of magnetic traps. This
structure of the momentum space is due to the character of the scattering
magnetic irregularities. They are formed by a train of shock waves that
naturally emerge from unstably growing and steepening magnetosonic waves or as
a result of acoustic instability of the CR precursor. These losses steepen the
spectrum above the knee, which also prevents the shock width from increasing
with the maximum particle energy.Comment: aastex, 13 eps figure
On radial geodesic forcing of zonal modes
The elementary local and global influence of geodesic field line curvature on
radial dispersion of zonal modes in magnetised plasmas is analysed with a
primitive drift wave turbulence model. A net radial geodesic forcing of zonal
flows and geodesic acoustic modes can not be expected in any closed toroidal
magnetic confinement configuration, since the flux surface average of geodesic
curvature identically vanishes. Radial motion of poloidally elongated zonal
jets may occur in the presence of geodesic acoustic mode activity.
Phenomenologically a radial propagation of zonal modes shows some
characteristics of a classical analogon to second sound in quantum condensates.Comment: Submitted to Phys. Plasmas (2011
Continuous Correction of Differential Path Length Factor in Near-Infrared Spectroscopy
In continuous-wave near-infrared spectroscopy (CW-NIRS), changes in the concentration of oxyhemoglobin and deoxyhemoglobin can be calculated by solving a set of linear equations from the modified Beer-Lambert Law. Cross-talk error in the calculated hemodynamics can arise from inaccurate knowledge of the wavelength-dependent differential path length factor (DPF). We apply the extended Kalman filter (EKF) with a dynamical systems model to calculate relative concentration changes in oxy- and deoxyhemoglobin while simultaneously estimating relative changes in DPF. Results from simulated and experimental CW-NIRS data are compared with results from a weighted least squares (WLSQ) method. The EKF method was found to effectively correct for artificially introduced errors in DPF and to reduce the cross-talk error in simulation. With experimental CW-NIRS data, the hemodynamic estimates from EKF differ significantly from the WLSQ (p\u3c0.001 ). The cross-correlations among residuals at different wavelengths were found to be significantly reduced by the EKF method compared to WLSQ in three physiologically relevant spectral bands 0.04 to 0.15 Hz, 0.15 to 0.4 Hz and 0.4 to 2.0 Hz (p\u3c0.001 ). This observed reduction in residual cross-correlation is consistent with reduced cross-talk error in the hemodynamic estimates from the proposed EKF method
Consumer credit information systems: A critical review of the literature. Too little attention paid by lawyers?
This paper reviews the existing literature on consumer credit reporting, the most extensively used instrument to overcome information asymmetry and adverse selection problems in credit markets. Despite the copious literature in economics and some research in regulatory policy, the legal community has paid almost no attention to the legal framework of consumer credit information systems, especially within the context of the European Union. Studies on the topic, however, seem particularly relevant in view of the establishment of a single market for consumer credit. This article ultimately calls for further legal research to address consumer protection concerns and inform future legislation
Method for Measuring the Dielectric Constant of Ferroelectric Ceramics at S-Band Frequencies
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66190/1/j.1151-2916.1960.tb13658.x.pd
A magnetically collimated jet from an evolved star
Planetary nebulae often have asymmetric shapes, which could arise due to
collimated jets from evolved stars before evolution to the planetary nebula
phase. The source of jet collimation in these stars is unknown. Magnetic fields
are thought to collimate outflows that are observed in many other astrophysical
sources, such as active galactic nuclei and proto-stars, although hitherto
there are no direct observations of both the magnetic field direction and
strength in any collimated jet. Theoretical models have shown that magnetic
fields could also be the dominant source of collimation of jet in evolved
stars. Here we report measurements of the polarization of water vapour masers
that trace the precessing jet emanating from the asymptotic giant branch star
W43A at 2.6 kpc from the Sun, which is undergoing rapid evolution into a
planetary nebula. The masers occur in two clusters at opposing tips of the
jets, ~1,000 AU from the star. We find direct evidence that the magnetic field
is collimating the jet.Comment: Published in Nature 440 (March 2nd 2006). High-res figures can be
found at http://www.jb.man.ac.uk/~wouter/papers/w43a/w43a.htm
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