13,661 research outputs found
Feedback Control of Traveling Wave Solutions of the Complex Ginzburg Landau Equation
Through a linear stability analysis, we investigate the effectiveness of a
noninvasive feedback control scheme aimed at stabilizing traveling wave
solutions of the one-dimensional complex Ginzburg Landau equation (CGLE) in the
Benjamin-Feir unstable regime. The feedback control is a generalization of the
time-delay method of Pyragas, which was proposed by Lu, Yu and Harrison in the
setting of nonlinear optics. It involves both spatial shifts, by the wavelength
of the targeted traveling wave, and a time delay that coincides with the
temporal period of the traveling wave. We derive a single necessary and
sufficient stability criterion which determines whether a traveling wave is
stable to all perturbation wavenumbers. This criterion has the benefit that it
determines an optimal value for the time-delay feedback parameter. For various
coefficients in the CGLE we use this algebraic stability criterion to
numerically determine stable regions in the (K,rho) parameter plane, where rho
is the feedback parameter associated with the spatial translation and K is the
wavenumber of the traveling wave. We find that the combination of the two
feedbacks greatly enlarges the parameter regime where stabilization is
possible, and that the stability regions take the form of stability tongues in
the (K,rho)--plane. We discuss possible resonance mechanisms that could account
for the spacing with K of the stability tongues.Comment: 33 pages, 12 figure
Velocity field distributions due to ideal line vortices
We evaluate numerically the velocity field distributions produced by a
bounded, two-dimensional fluid model consisting of a collection of parallel
ideal line vortices. We sample at many spatial points inside a rigid circular
boundary. We focus on ``nearest neighbor'' contributions that result from
vortices that fall (randomly) very close to the spatial points where the
velocity is being sampled. We confirm that these events lead to a non-Gaussian
high-velocity ``tail'' on an otherwise Gaussian distribution function for the
Eulerian velocity field. We also investigate the behavior of distributions that
do not have equilibrium mean-field probability distributions that are uniform
inside the circle, but instead correspond to both higher and lower mean-field
energies than those associated with the uniform vorticity distribution. We find
substantial differences between these and the uniform case.Comment: 21 pages, 9 figures. To be published in Physical Review E
(http://pre.aps.org/) in May 200
How Pharmaceutical Industry Employees Manage Competing Moral Commitments
The pharmaceutical industry has been criticised for pervasive misconduct. These concerns have generally resulted in increasing regulation. While such regulation is no doubt necessary, it tends to assume that everyone working for pharmaceutical companies is equally motivated by commerce, without much understanding of the specific views and experiences of those who work in different parts of the industry. In order to gain a more nuanced picture of the work that goes on in the “medical affairs” departments of pharmaceutical companies, we conducted 15 semi-structured interviews with professionals working in medical departments of companies in Sydney, Australia. We show that this group of pharmaceutical professionals are committed to their responsibilities both to patients, research participants, and the public and to their companies. Despite the discrepancies between these commitments, our participants did not express much cognitive dissonance, and this appeared to stem from their use of two dialectically related strategies, one of which embraces commerce and the other of which resists the commercial imperative. We interpret these findings through the lens of institutional theory and consider their implications for pharmaceutical ethics and governance. Keywords: Qualitative research; Social values; Pharmaceutical industry; Pharmaceutical ethicsNHMR
Effective buoyancy and CAPE: Some implications for tropical cyclones
We review the widely used concepts of buoyancy and convective available potential energy (CAPE) in relation to deep convection in tropical cyclones and discuss their limitations. A fact easily forgotten in applying these concepts is that the buoyancy force of an air parcel, as often defined, is non-unique because it depends on the arbitrary definition of a reference density field. However, when calculating CAPE, the buoyancy of a lifted air parcel is related to the specific reference density field along a vertical column passing through that parcel. Both concepts can be generalized for a vortical flow and to slantwise ascent of a lifted air parcel in such a flow. In all cases, the air parcel is assumed to have infinitely small dimensions. In this article, we explore the consequences of generalizing buoyancy and CAPE for buoyant regions of finite size that perturb the pressure field in their immediate environment. Quantitative calculations of effective buoyancy, defined as the sum of the conventional buoyancy and the static vertical perturbation pressure gradient force induced by it, are shown for buoyant regions of finite width. For a judicious choice of reference density, the effective buoyancy per unit mass is essentially a unique force, independent of the reference density, but its distribution depends on the horizontal scale of the buoyant region. A corresponding concept of effective CAPE is introduced and its relevance to deep convection in tropical cyclones is discussed. The study is conceived as a first step to understanding the decreasing ability of inner-core deep convection in tropical cyclones to ventilate the mass of air converging in the frictional boundary layer as the vortex matures and decays
Darwin Tames an Andromeda Dwarf: Unraveling the Orbit of NGC 205 Using a Genetic Algorithm
NGC 205, a close satellite of the M31 galaxy, is our nearest example of a
dwarf elliptical galaxy. Photometric and kinematic observations suggest that
NGC 205 is undergoing tidal distortion from its interaction with M31. Despite
earlier attempts, the orbit and progenitor properties of NGC 205 are not well
known. We perform an optimized search for these unknowns by combining a genetic
algorithm with restricted N-body simulations of the interaction. This approach,
coupled with photometric and kinematic observations as constraints, allows for
an effective exploration of the parameter space. We represent NGC 205 as a
static Hernquist potential with embedded massless test particles that serve as
tracers of surface brightness. We explore 3 distinct, initially stable
configurations of test particles: cold rotating disk, warm rotating disk, and
hot, pressure-supported spheroid. Each model reproduces some, but not all, of
the observed features of NGC 205, leading us to speculate that a rotating
progenitor with substantial pressure support could match all of the
observables. Furthermore, plausible combinations of mass and scale length for
the pressure-supported spheroid progenitor model reproduce the observed
velocity dispersion profile. For all 3 models, orbits that best match the
observables place the satellite 11+/-9 kpc behind M31 moving at very large
velocities: 300-500 km/s on primarily radial orbits. Given that the observed
radial component is only 54 km/s, this implies a large tangential motion for
NGC 205, moving from the NW to the SE. These results suggest NGC 205 is not
associated with the stellar arc observed to the NE of NGC 205. Furthermore, NGC
205's velocity appears to be near or greater than its escape velocity,
signifying that the satellite is likely on its first M31 passage.Comment: 34 pages, 20 figures, accepted for publication in the Astrophysical
Journal, A pdf version with high-resolution figures may be obtained from
http://www.ucolick.org/~kirsten/ms.pd
Consanguineous marriage, prepregnancy maternal characteristics and stillbirth risk: A population-based case-control study
Introduction. Consanguineous marriage is associated with increased risks for congenital anomalies, low birthweight, and other adverse perinatal outcomes. In this population-based, case-control study we investigated the association between consanguineous marriage (first-cousin marriage) and stillbirth risk, using prospectively collected information from prepregnancy visits. Material and methods. From 2007 to 2009, we identified 283 stillbirths (cases) and 2088 randomly selected live control births through prepregnancy visits in rural Golestan, Iran. The associations between consanguinity and prepregnancy maternal characteristics and stillbirth risk were examined using multivariate logistic regression. Results. The rate of consanguineous marriage was 19.4% among cases and 13.6% among controls. Consanguinity was associated with increased stillbirth risk [odds ratio (OR) 1.53; 95% CI 1.10-2.14]. The association was significantly increased for preterm stillbirth (< 37 gestational weeks) (OR 2.43; 95% CI 1.46-4.04) but not for term stillbirth (≥ 37 weeks) (OR 1.14; 95% CI 0.75-1.74). Low and high maternal age, underweight, obesity, nulliparity, a history of infertility or miscarriage, previous obstetric complications (preeclampsia, preterm delivery, and stillbirth in previous pregnancies) were also associated with increased stillbirth risks. Conclusions. Consanguineous marriage is associated with increased risk of stillbirth, particularly preterm stillbirth. Findings for other maternal risk factors for stillbirth in rural Iran are consistent with previously reported findings from high-income countries. © 2015 Nordic Federation of Societies of Obstetrics and Gynecology
Small scale structures in three-dimensional magnetohydrodynamic turbulence
We investigate using direct numerical simulations with grids up to 1536^3
points, the rate at which small scales develop in a decaying three-dimensional
MHD flow both for deterministic and random initial conditions. Parallel current
and vorticity sheets form at the same spatial locations, and further
destabilize and fold or roll-up after an initial exponential phase. At high
Reynolds numbers, a self-similar evolution of the current and vorticity maxima
is found, in which they grow as a cubic power of time; the flow then reaches a
finite dissipation rate independent of Reynolds number.Comment: 4 pages, 3 figure
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