8,839 research outputs found
Holographic dark energy described at the Hubble length
We consider holographic cosmological models of dark energy in which the
infrared cutoff is set by the Hubble's radius. We show that any interacting
dark energy model with a matter like term able to alleviate the coincidence
problem (i.e., with a positive interaction term, regardless of its detailed
form) can be recast as a noninteracting model in which the holographic
parameter evolves slowly with time. Two specific cases are analyzed. First, the
interacting model presented in [1] is considered, and its corresponding
noninteracting version found. Then, a new noninteracting model, with a specific
expression of the time-dependent holographic parameter, is proposed and
analyzed along with its corresponding interacting version. We constrain the
parameters of both models using observational data, and show that they can be
told apart at the perturbative level.Comment: 15 pages, 6 figure
Dynamics of shallow impact cratering
We present data for the time-dependence of wooden spheres penetrating into a
loose non-cohesive packing of glass beads. The stopping time is a factor of
three longer than the time needed to travel the total penetration
distance at the impact speed . The acceleration decreases
monotonically throughout the impact. These kinematics are modelled by a
position- and velocity-dependent stopping force that is constrained to
reproduce prior observations for the scaling of the penetration depth with the
total drop distance.Comment: 4 pages, experimen
Packing Fractions and Maximum Angles of Stability of Granular Materials
In two-dimensional rotating drum experiments, we find two separate influences
of the packing fraction of a granular heap on its stability. For a fixed grain
shape, the stability increases with packing fraction. However, in determining
the relative stability of different grain shapes, those with the lowest average
packing fractions tend to form the most stable heaps. We also show that only
the configuration close to the surface of the pile figures prominently.Comment: 4 pages, 4 figure
Jamming during the discharge of granular matter from a silo
In this work we present an experimental study of the jamming that stops the
free flow of grains from a silo discharging by gravity. When the outlet size is
not much bigger than the beads, granular material jams the outlet of the
container due to the formation of an arch. Statistical data from the number of
grains fallen between consecutive jams are presented. The information that they
provide can help to understand the jamming phenomenon. As the ratio between the
size of the orifice and the size of the beads is increased, the probability
that an arch blocks the outlet decreases. We show here that there is a power
law divergence of the mean avalanche size for a finite critical radius. Beyond
this critical radius no jamming can occur and the flow is never stopped. The
dependence of the arch formation on the shape and the material of the grains
has been explored. It has been found that the material properties of the grains
do not affect the arch formation probability. On the contrary, the shape of the
grains deeply influences it. A simple model to interpret the results is also
discussed.Comment: Submitted to Phys. Rev.
Fluctuation-Induced Casimir Forces in Granular Fluids
We have numerically investigated the behavior of driven non-cohesive granular
media and found that two fixed large intruder particles, immersed in a sea of
small particles, experience, in addition to a short range depletion force, a
long range repulsive force. The observed long range interaction is
fluctuation-induced and we propose a mechanism similar to the Casimir effect
that generates it: the hydrodynamic fluctuations are geometrically confined
between the intruders, producing an unbalanced renormalized pressure. An
estimation based on computing the possible Fourier modes explains the repulsive
force and is in qualitative agreement with the simulations.Comment: 4 pages, 3 figures. Accepted in Phys. Rev. Letter
Penetration depth for shallow impact cratering
We present data for the penetration of a variety of spheres, dropped from
rest, into a level non-cohesive granular medium. We improve upon our earlier
work [Uehara {\it et al.} Phys. Rev. Lett. {\bf 90}, 194301 (2003)] in three
regards. First, we explore the behavior vs sphere diameter and density more
systematically, by holding one of these parameters constant while varying the
other. Second, we prepare the granular medium more reproducibly and, third, we
measure the penetration depth more accurately. The new data support our
previous conclusion that the penetration depth is proportional to the 1/2 power
of sphere density, the 2/3 power of sphere diameter, and the 1/3 power of total
drop distance
Interstitial gas and density-segregation in vertically-vibrated granular media
We report experimental studies of the effect of interstitial gas on
mass-density-segregation in a vertically-vibrated mixture of equal-sized bronze
and glass spheres. Sufficiently strong vibration in the presence of
interstitial gas induces vertical segregation into sharply separated bronze and
glass layers. We find that the segregated steady state (i.e., bronze or glass
layer on top) is a sensitive function of gas pressure and viscosity, as well as
vibration frequency and amplitude. In particular, we identify distinct regimes
of behavior that characterize the change from bronze-on-top to glass-on-top
steady-state.Comment: 4 pages, 5 figures, submitted to PRL; accepted in PRE as rapid
communication, with revised text and reference
Statistical characterization of the forces on spheres in an upflow of air
The dynamics of a sphere fluidized in a nearly-levitating upflow of air were
previously found to be identical to those of a Brownian particle in a
two-dimensional harmonic trap, consistent with a Langevin equation [Ojha {\it
et al.}, Nature {\bf 427}, 521 (2004)]. The random forcing, the drag, and the
trapping potential represent different aspects of the interaction of the sphere
with the air flow. In this paper we vary the experimental conditions for a
single sphere, and report on how the force terms in the Langevin equation scale
with air flow speed, sphere radius, sphere density, and system size. We also
report on the effective interaction potential between two spheres in an upflow
of air.Comment: 7 pages, experimen
Geometry dependence of the clogging transition in tilted hoppers
We report the effect of system geometry on the clogging of granular material
flowing out of flat-bottomed hoppers with variable aperture size D. For such
systems, there exists a critical aperture size Dc at which there is a
divergence in the time for a flow to clog. To better understand the origins of
Dc, we perturb the system by tilting the hopper an angle Q and mapping out a
clogging phase diagram as a function of Q and D. The clogging transition
demarcates the boundary between the freely-flowing (large D, small Q) and
clogging (small D, large Q) regimes. We investigate how the system geometry
affects Dc by mapping out this phase diagram for hoppers with either a circular
hole or a rectangular narrow slit. Additionally, we vary the grain shape,
investigating smooth spheres (glass beads), compact angular grains (beach
sand), disk-like grains (lentils), and rod-like grains (rice). We find that the
value of Dc grows with increasing Q, diverging at pi-Qr where Qr is the angle
of repose. For circular apertures, the shape of the clogging transition is the
same for all grain types. However, this is not the case for the narrow slit
apertures, where the rate of growth of the critical hole size with tilt angle
depends on the material
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
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