1,051 research outputs found
Direct observation of twist mode in electroconvection in I52
I report on the direct observation of a uniform twist mode of the director
field in electroconvection in I52. Recent theoretical work suggests that such a
uniform twist mode of the director field is responsible for a number of
secondary bifurcations in both electroconvection and thermal convection in
nematics. I show here evidence that the proposed mechanisms are consistent with
being the source of the previously reported SO2 state of electroconvection in
I52. The same mechanisms also contribute to a tertiary Hopf bifurcation that I
observe in electroconvection in I52. There are quantitative differences between
the experiment and calculations that only include the twist mode. These
differences suggest that a complete description must include effects described
by the weak-electrolyte model of electroconvection
Curve veering and mode localization in a buckling problem
It is shown on a simple example that small disorder, or mistuning, may alter drastically the eigensolution of buckling problems in nearly periodic structures with weak internal coupling. Specifically, the phenomena of eigenvalue loci veering and mode shape localization, which are known to occur for free vibration problems, are evidenced in the case of structural buckling as well.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43439/1/33_2004_Article_BF00945875.pd
Experimental snap loading of synthetic ropes
Abstract. Large tensile forces, known as snap loads, can occur when a slack rope becomes taut. Such forces may damage the rope or masses connected to it. Experiments are described in which one end of a rope is attached to the top of a drop tower and the bottom end is attached to a weight. The weight is raised to a certain height and then released. The force at the top of the rope and the acceleration of the weight are recorded during the first snap load that occurs. Repeated drop tests are performed on each rope. The effects of the type of rope, drop height, drop weight, whether the rope has been subjected to static precycling, and the number of previous dynamic tests are examined. A mathematical model is proposed for the rope force as a function of the displacement and velocity of the weight
Three-dimensional pattern formation, multiple homogeneous soft modes, and nonlinear dielectric electroconvection
Patterns forming spontaneously in extended, three-dimensional, dissipative
systems are likely to excite several homogeneous soft modes (
hydrodynamic modes) of the underlying physical system, much more than quasi
one- and two-dimensional patterns are. The reason is the lack of damping
boundaries. This paper compares two analytic techniques to derive the patten
dynamics from hydrodynamics, which are usually equivalent but lead to different
results when applied to multiple homogeneous soft modes. Dielectric
electroconvection in nematic liquid crystals is introduced as a model for
three-dimensional pattern formation. The 3D pattern dynamics including soft
modes are derived. For slabs of large but finite thickness the description is
reduced further to a two-dimensional one. It is argued that the range of
validity of 2D descriptions is limited to a very small region above threshold.
The transition from 2D to 3D pattern dynamics is discussed. Experimentally
testable predictions for the stable range of ideal patterns and the electric
Nusselt numbers are made. For most results analytic approximations in terms of
material parameters are given.Comment: 29 pages, 2 figure
Proper Motions in the Galactic Bulge: Plaut's Window
A proper motion study of a field of 20' x 20' inside Plaut's low extinction
window (l,b)=(0 deg,-8 deg), has been completed. Relative proper motions and
photographic BV photometry have been derived for ~21,000 stars reaching to
V~20.5 mag, based on the astrometric reduction of 43 photographic plates,
spanning over 21 years of epoch difference. Proper motion errors are typically
1 mas/yr and field dependent systematics are below 0.2 mas/yr.
Cross-referencing with the 2MASS catalog yielded a sample of ~8,700 stars, from
which predominantly disk and bulge subsamples were selected photometrically
from the JH color-magnitude diagram. The two samples exhibited different
proper-motion distributions, with the disk displaying the expected reflex solar
motion as a function of magnitude. Galactic rotation was also detected for
stars between ~2 and ~3 kpc from us. The bulge sample, represented by red
giants, has an intrinsic proper motion dispersion of (sigma_l,sigma_b)=(3.39,
2.91)+/-(0.11,0.09) mas/yr, which is in good agreement with previous results,
and indicates a velocity anisotropy consistent with either rotational
broadening or tri-axiality. A mean distance of 6.37^{+0.87}_{-0.77} kpc has
been estimated for the bulge sample, based on the observed K magnitude of the
horizontal branch red clump. The metallicity [M/H] distribution was also
obtained for a subsample of 60 bulge giants stars, based on calibrated
photometric indices. The observed [M/H] shows a peak value at [M/H]~-0.1 with
an extended metal poor tail and around 30% of the stars with supersolar
metallicity. No change in proper motion dispersion was observed as a function
of [M/H]. We are currently in the process of obtaining CCD UBVRI photometry for
the entire proper-motion sample of ~21,000 stars.Comment: Submitted to AJ April 17th 2007. Accepted June 8th 2007. 45 pages, 14
figure
Universal critical temperature for Kosterlitz-Thouless transitions in bilayer quantum magnets
Recent experiments show that double layer quantum Hall systems may have a
ground state with canted antiferromagnetic order. In the experimentally
accessible vicinity of a quantum critical point, the order vanishes at a
temperature T_{KT} = \kappa H, where H is the magnetic field and \kappa is a
universal number determined by the interactions and Berry phases of the thermal
excitations. We present quantum Monte Carlo simulations on a model spin system
which support the universality of \kappa and determine its numerical value.
This allows experimental tests of an intrinsically quantum-mechanical universal
quantity, which is not also a property of a higher dimensional classical
critical point.Comment: 5 pages, 4 figure
Population health and the economy: Mortality and the Great Recession in Europe
We analyze the evolution of mortality‐based health indicators in 27 European countries before and after the start of the Great Recession. We find that in the countries where the crisis has been particularly severe, mortality reductions in 2007–2010 were considerably bigger than in 2004–2007. Panel models adjusted for space‐invariant and time‐invariant factors show that an increase of 1 percentage point in the national unemployment rate is associated with a reduction of 0.5% (p < .001) in the rate of age‐adjusted mortality. The pattern of mortality oscillating procyclically is found for total and sex‐specific mortality, cause‐specific mortality due to major causes of death, and mortality for ages 30–44 and 75 and over, but not for ages 0–14. Suicides appear increasing when the economy decelerates—countercyclically—but the evidence is weak. Results are robust to using different weights in the regression, applying nonlinear methods for detrending, expanding the sample, and using as business cycle indicator gross domestic product per capita or employment‐to‐population ratios rather than the unemployment rate. We conclude that in the European experience of the past 20 years, recessions, on average, have beneficial short‐term effects on mortality of the adult population.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/142224/1/hec3495_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/142224/2/hec3495.pd
- …