7,304 research outputs found
Feasibility of using teleradiology to improve tuberculosis screening and case management in a district hospital in Malawi.
Malawi has one of the world's highest rates of human immunodeficiency virus (HIV) infection (10.6%), and southern Malawi, where Thyolo district is located, bears the highest burden in the country (14.5%). Tuberculosis, common among HIV-infected people, requires radiologic diagnosis, yet Malawi has no radiologists in public service. This hinders rapid and accurate diagnosis and increases morbidity and mortality
Force network ensemble: a new approach to static granular matter
An ensemble approach for force distributions in static granular packings is
developed. This framework is based on the separation of packing and force
scales, together with an a-priori flat measure in the force phase space under
the constraints that the contact forces are repulsive and balance on every
particle. We show how the formalism yields realistic results, both for
disordered and regular ``snooker ball'' configurations, and obtain a
shear-induced unjamming transition of the type proposed recently for athermal
media.Comment: 4 pages, 4 figures, changed conten
Shear band dynamics from a mesoscopic modeling of plasticity
The ubiquitous appearance of regions of localized deformation (shear bands)
in different kinds of disordered materials under shear is studied in the
context of a mesoscopic model of plasticity. The model may or may not include
relaxational (aging) effects. In the absence of relaxational effects the model
displays a monotonously increasing dependence of stress on strain-rate, and
stationary shear bands do not occur. However, in start up experiments transient
(although long lived) shear bands occur, that widen without bound in time. I
investigate this transient effect in detail, reproducing and explaining a t^1/2
law for the thickness increase of the shear band that has been obtained in
atomistic numerical simulations. Relaxation produces a negative sloped region
in the stress vs. strain-rate curve that stabilizes the formation of shear
bands of a well defined width, which is a function of strain-rate. Simulations
at very low strain-rates reveal a non-trivial stick-slip dynamics of very thin
shear bands that has relevance in the study of seismic phenomena. In addition,
other non-stationary processes, such as stop-and-go, or strain-rate inversion
situations display a phenomenology that matches very well the results of recent
experimental studies.Comment: 10 pages, 10 figure
Universal and wide shear zones in granular bulk flow
We present experiments on slow granular flows in a modified (split-bottomed)
Couette geometry in which wide and tunable shear zones are created away from
the sidewalls. For increasing layer heights, the zones grow wider (apparently
without bound) and evolve towards the inner cylinder according to a simple,
particle-independent scaling law. After rescaling, the velocity profiles across
the zones fall onto a universal master curve given by an error function. We
study the shear zones also inside the material as function of both their local
height and the total layer height.Comment: Minor corrections, accepted for PRL (4 pages, 6 figures
The Effect of Air on Granular Size Separation in a Vibrated Granular Bed
Using high-speed video and magnetic resonance imaging (MRI) we study the
motion of a large sphere in a vertically vibrated bed of smaller grains. As
previously reported we find a non-monotonic density dependence of the rise and
sink time of the large sphere. We find that this density dependence is solely
due to air drag. We investigate in detail how the motion of the intruder sphere
is influenced by size of the background particles, initial vertical position in
the bed, ambient pressure and convection. We explain our results in the
framework of a simple model and find quantitative agreement in key aspects with
numerical simulations to the model equations.Comment: 14 pages, 16 figures, submitted to PRE, corrected typos, slight
change
Millikelvin magnetic relaxation measurements of alpha-Fe2O3 antiferromagnetic particles
In this paper we report magnetic relaxation data for antiferromagnetic
alpha-Fe2O3 particles of 5 nm mean diameter in the temperature range 0.1 K to
25 K. The average spin value of these particles S=124 and the uniaxial
anisotropy constant D=1.6x10^-2 K have been estimated from the experimental
values of the blocking temperature and anisotropy field. The observed plateau
in the magnetic viscosity from 3 K down to 100 mK agrees with the occurrence of
spin tunneling from the ground state Sz = S. However, the scaling M vs Tln(nu
t) is broken below 5 K, suggesting the occurrence of tunneling from excited
states below this temperature.Comment: 4 pages (two columns), 4 figure
Focusing and Diffraction of Light by Periodic Si Micropyramidal Arrays
This research was devoted to modeling of the optical properties of Si
micropyramids aimed at designing optimal structures for applications as light
concentrators in mid-wave infrared (MWIR) focal place arrays (FPAs). It is
shown that completely different optical properties of such structures can be
realized using two types of boundary conditions (BCs): i) periodical and ii)
perfectly matched layer. The first type (periodical BC) allowed us to describe
the Talbot effect under plane wave coherent illumination conditions. This
effect was experimentally demonstrated in the proposed structures. The second
type (perfectly matched layer BC) allows describing the optical properties of
individual micropyramids concentrating or focusing light on the photodetector.
The optimal geometries of micropyramids required for maximizing the intensity
of photonic nanojets emerging from their truncated tips are determined.Comment: 4 pages, 5 figures, GOMACTech 202
The break-up of Ekman theory in a flow subjected to background rotation and driven by a non-conservative body force
We present an experimental/numerical study of a dipolar flow structure in a shallow layer of electrolyte driven by electromagnetic forcing and subjected to background rotation. The aim of this study is to determine the influence of a non-conservative body force on the range of applicability of the classical Ekman boundary layer theory in rapidly rotating systems. To address this question, we study the response of the flow to the three control parameters: the magnitude of the forcing, the rotation rate of the system, and the shallowness of the layer. This response is quantified taking into account the magnitude of the flow velocity (represented by the Reynolds number), the symmetry between both vortex cores, and the vertical profile of the horizontal velocity. As in the case without background rotation, the response of the flow exhibits two scaling regimes (a linear and a nonlinear regime) in which the flow exhibits different vertical profiles of velocity. The transition between the two regimes occurs when the convective acceleration becomes of the same order as the viscous damping. This suggests that the applicability of the Ekman theory depends on the existence of a balance between the forcing and the damping due to the Ekman layers and does not depend solely on the value of the Rossby number as for decaying flows. On the other hand, the cyclone/anticyclone asymmetry is governed exclusively by the Rossby number. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4766818
A stochastic flow rule for granular materials
There have been many attempts to derive continuum models for dense granular
flow, but a general theory is still lacking. Here, we start with Mohr-Coulomb
plasticity for quasi-2D granular materials to calculate (average) stresses and
slip planes, but we propose a "stochastic flow rule" (SFR) to replace the
principle of coaxiality in classical plasticity. The SFR takes into account two
crucial features of granular materials - discreteness and randomness - via
diffusing "spots" of local fluidization, which act as carriers of plasticity.
We postulate that spots perform random walks biased along slip-lines with a
drift direction determined by the stress imbalance upon a local switch from
static to dynamic friction. In the continuum limit (based on a Fokker-Planck
equation for the spot concentration), this simple model is able to predict a
variety of granular flow profiles in flat-bottom silos, annular Couette cells,
flowing heaps, and plate-dragging experiments -- with essentially no fitting
parameters -- although it is only expected to function where material is at
incipient failure and slip-lines are inadmissible. For special cases of
admissible slip-lines, such as plate dragging under a heavy load or flow down
an inclined plane, we postulate a transition to rate-dependent Bagnold
rheology, where flow occurs by sliding shear planes. With different yield
criteria, the SFR provides a general framework for multiscale modeling of
plasticity in amorphous materials, cycling between continuum limit-state stress
calculations, meso-scale spot random walks, and microscopic particle
relaxation
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