2,926 research outputs found
Global MRI with Braginskii viscosity in a galactic profile
We present a global-in-radius linear analysis of the axisymmetric
magnetorotational instability (MRI) in a collisional magnetized plasma with
Braginskii viscosity. For a galactic angular velocity profile we
obtain analytic solutions for three magnetic field orientations: purely
azimuthal, purely vertical and slightly pitched (almost azimuthal). In the
first two cases the Braginskii viscosity damps otherwise neutrally stable
modes, and reduces the growth rate of the MRI respectively. In the final case
the Braginskii viscosity makes the MRI up to times faster than its
inviscid counterpart, even for \emph{asymptotically small} pitch angles. We
investigate the transition between the Lorentz-force-dominated and the
Braginskii viscosity-dominated regimes in terms of a parameter \sim \Omega
\nub/B^2 where \nub is the viscous coefficient and the Alfv\'en speed.
In the limit where the parameter is small and large respectively we recover the
inviscid MRI and the magnetoviscous instability (MVI). We obtain asymptotic
expressions for the approach to these limits, and find the Braginskii viscosity
can magnify the effects of azimuthal hoop tension (the growth rate becomes
complex) by over an order of magnitude. We discuss the relevance of our results
to the local approximation, galaxies and other magnetized astrophysical
plasmas. Our results should prove useful for benchmarking codes in global
geometries.Comment: 14 pages, 5 figure
Synchronization of Coupled Boolean Phase Oscillators
We design, characterize, and couple Boolean phase oscillators that include
state-dependent feedback delay. The state-dependent delay allows us to realize
an adjustable coupling strength, even though only Boolean signals are
exchanged. Specifically, increasing the coupling strength via the range of
state-dependent delay leads to larger locking ranges in uni- and bi-directional
coupling of oscillators in both experiment and numerical simulation with a
piecewise switching model. In the unidirectional coupling scheme, we unveil
asymmetric triangular-shaped locking regions (Arnold tongues) that appear at
multiples of the natural frequency of the oscillators. This extends
observations of a single locking region reported in previous studies. In the
bidirectional coupling scheme, we map out a symmetric locking region in the
parameter space of frequency detuning and coupling strength. Because of large
scalability of our setup, our observations constitute a first step towards
realizing large-scale networks of coupled oscillators to address fundamental
questions on the dynamical properties of networks in a new experimental
setting.Comment: 8 pages, 8 figure
Multirhythmicity in an optoelectronic oscillator with large delay
An optoelectronic oscillator exhibiting a large delay in its feedback loop is
studied both experimentally and theoretically. We show that multiple
square-wave oscillations may coexist for the same values of the parameters
(multirhythmicity). Depending on the sign of the phase shift, these regimes
admit either periods close to an integer fraction of the delay or periods close
to an odd integer fraction of twice the delay. These periodic solutions emerge
from successive Hopf bifurcation points and stabilize at a finite amplitude
following a scenario similar to Eckhaus instability in spatially extended
systems. We find quantitative agreements between experiments and numerical
simulations. The linear stability of the square-waves is substantiated
analytically by determining stable fixed points of a map.Comment: 14 pages, 7 figure
Novelty Search in Competitive Coevolution
One of the main motivations for the use of competitive coevolution systems is
their ability to capitalise on arms races between competing species to evolve
increasingly sophisticated solutions. Such arms races can, however, be hard to
sustain, and it has been shown that the competing species often converge
prematurely to certain classes of behaviours. In this paper, we investigate if
and how novelty search, an evolutionary technique driven by behavioural
novelty, can overcome convergence in coevolution. We propose three methods for
applying novelty search to coevolutionary systems with two species: (i) score
both populations according to behavioural novelty; (ii) score one population
according to novelty, and the other according to fitness; and (iii) score both
populations with a combination of novelty and fitness. We evaluate the methods
in a predator-prey pursuit task. Our results show that novelty-based approaches
can evolve a significantly more diverse set of solutions, when compared to
traditional fitness-based coevolution.Comment: To appear in 13th International Conference on Parallel Problem
Solving from Nature (PPSN 2014
Improving accuracy and efficiency of mutual information for multi-modal retinal image registration using adaptive probability density estimation
Mutual information (MI) is a popular similarity measure for performing image registration between different modalities. MI makes a statistical comparison between two images by computing the entropy from the probability distribution of the data. Therefore, to obtain an accurate registration it is important to have an accurate estimation of the true underlying probability distribution. Within the statistics literature, many methods have been proposed for finding the 'optimal' probability density, with the aim of improving the estimation by means of optimal histogram bin size selection. This provokes the common question of how many bins should actually be used when constructing a histogram. There is no definitive answer to this. This question itself has received little attention in the MI literature, and yet this issue is critical to the effectiveness of the algorithm. The purpose of this paper is to highlight this fundamental element of the MI algorithm. We present a comprehensive study that introduces methods from statistics literature and incorporates these for image registration. We demonstrate this work for registration of multi-modal retinal images: colour fundus photographs and scanning laser ophthalmoscope images. The registration of these modalities offers significant enhancement to early glaucoma detection, however traditional registration techniques fail to perform sufficiently well. We find that adaptive probability density estimation heavily impacts on registration accuracy and runtime, improving over traditional binning techniques. © 2013 Elsevier Ltd
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