758 research outputs found
Universal velocity distributions in an experimental granular fluid
We present experimental results on the velocity statistics of a uniformly
heated granular fluid, in a quasi-2D configuration. We find the base state, as
measured by the single particle velocity distribution , to be universal
over a wide range of filling fractions and only weakly dependent on all other
system parameters. There is a consistent overpopulation in the distribution's
tails, which scale as . More
importantly, the high probability central region of , at low velocities,
deviates from a Maxwell-Boltzmann by a second order Sonine polynomial with a
single adjustable parameter, in agreement with recent theoretical analysis of
inelastic hard spheres driven by a stochastic thermostat. To our knowledge,
this is the first time that Sonine deviations have been measured in an
experimental system.Comment: 13 pages, 15 figures, with minor corrections, submitted to Phys. Rev.
Minutes 1875
https://place.asburyseminary.edu/freemethodistminutesyearbooks/1013/thumbnail.jp
Shocks in supersonic sand
We measure time-averaged velocity, density, and temperature fields for steady
granular flow past a wedge and calculate a speed of granular pressure
disturbances (sound speed) equal to 10% of the flow speed. The flow is
supersonic, forming shocks nearly identical to those in a supersonic gas.
Molecular dynamics simulations of Newton's laws and Monte Carlo simulations of
the Boltzmann equation yield fields in quantitative agreement with experiment.
A numerical solution of Navier-Stokes-like equations agrees with a molecular
dynamics simulation for experimental conditions excluding wall friction.Comment: 4 pages, 5 figure
Minutes 1877
https://place.asburyseminary.edu/freemethodistminutesyearbooks/1015/thumbnail.jp
A Continuum Description of Vibrated Sand
The motion of a thin layer of granular material on a plate undergoing
sinusoidal vibrations is considered. We develop equations of motion for the
local thickness and the horizontal velocity of the layer. The driving comes
from the violent impact of the grains on the plate. A linear stability theory
reveals that the waves are excited non-resonantly, in contrast to the usual
Faraday waves in liquids. Together with the experimentally observed continuum
scaling, the model suggests a close connection between the neutral curve and
the dispersion relation of the waves, which agrees quite well with experiments.
For strong hysteresis we find localized oscillon solutions.Comment: paper has been considerably extended (11 instead of 6 pages; 6
instead of 4 figures) much better agreement with experiment. obtain now
oscillons in 1 dimensio
Phase Bubbles and Spatiotemporal Chaos in Granular Patterns
We use inelastic hard sphere molecular dynamics simulations and laboratory
experiments to study patterns in vertically oscillated granular layers. The
simulations and experiments reveal that {\em phase bubbles} spontaneously
nucleate in the patterns when the container acceleration amplitude exceeds a
critical value, about , where the pattern is approximately hexagonal,
oscillating at one-fourth the driving frequency (). A phase bubble is a
localized region that oscillates with a phase opposite (differing by ) to
that of the surrounding pattern; a localized phase shift is often called an
{\em arching} in studies of two-dimensional systems. The simulations show
that the formation of phase bubbles is triggered by undulation at the bottom of
the layer on a large length scale compared to the wavelength of the pattern.
Once formed, a phase bubble shrinks as if it had a surface tension, and
disappears in tens to hundreds of cycles. We find that there is an oscillatory
momentum transfer across a kink, and this shrinking is caused by a net
collisional momentum inward across the boundary enclosing the bubble. At
increasing acceleration amplitudes, the patterns evolve into randomly moving
labyrinthian kinks (spatiotemporal chaos). We observe in the simulations that
and subharmonic patterns emerge as primary instabilities, but that
they are unstable to the undulation of the layer. Our experiments confirm the
existence of transient and patterns.Comment: 6 pages, 12 figures, submitted to Phys. Rev. E on July 1st, 2001. for
better quality figures, visit http://chaos.ph.utexas.edu/research/moo
Speech rhythm: a metaphor?
Is speech rhythmic? In the absence of evidence for a traditional view that languages strive to coordinate either syllables or stress-feet with regular time intervals, we consider the alternative that languages exhibit contrastive rhythm subsisting merely in the alternation of stronger and weaker elements. This is initially plausible, particularly for languages with a steep ‘prominence gradient’, i.e. a large disparity between stronger and weaker elements; but we point out that alternation is poorly achieved even by a ‘stress-timed’ language such as English, and, historically, languages have conspicuously failed to adopt simple phonological remedies that would ensure alternation. Languages seem more concerned to allow ‘syntagmatic contrast’ between successive units and to use durational effects to support linguistic functions than to facilitate rhythm. Furthermore, some languages (e.g. Tamil, Korean) lack the lexical prominence which would most straightforwardly underpin prominence alternation. We conclude that speech is not incontestibly rhythmic, and may even be antirhythmic. However, its linguistic structure and patterning allow the metaphorical extension of rhythm in varying degrees and in different ways depending on the language, and that it is this analogical process which allows speech to be matched to external rhythms
Transport Coefficients for Granular Media from Molecular Dynamics Simulations
Under many conditions, macroscopic grains flow like a fluid; kinetic theory
pred icts continuum equations of motion for this granular fluid. In order to
test the theory, we perform event driven molecular simulations of a
two-dimensional gas of inelastic hard disks, driven by contact with a heat
bath. Even for strong dissipation, high densities, and small numbers of
particles, we find that continuum theory describes the system well. With a bath
that heats the gas homogeneously, strong velocity correlations produce a
slightly smaller energy loss due to inelastic collisions than that predicted by
kinetic theory. With an inhomogeneous heat bath, thermal or velocity gradients
are induced. Determination of the resulting fluxes allows calculation of the
thermal conductivity and shear viscosity, which are compared to the predictions
of granular kinetic theory, and which can be used in continuum modeling of
granular flows. The shear viscosity is close to the prediction of kinetic
theory, while the thermal conductivity can be overestimated by a factor of 2;
in each case, transport is lowered with increasing inelasticity.Comment: 14 pages, 17 figures, 39 references, submitted to PRE feb 199
Stability of the Autism Diagnostic Interview—Revised from Pre-School to Elementary School Age in Children with Autism Spectrum Disorders
This study examined the stability of scores on the ADI-R from pre-school to elementary school age in children with autism spectrum disorders (ASD). Participants were 35 children who, at T1, all had a clinical diagnosis of ASD. On initial assessment (mean age 3.5 years; SD 0.6), all met ADI-R algorithm criteria for autism. ADI-R assessments were repeated at follow up (FU; mean age 10.5 years; SD 0.8). Changes in ADI-R total, domain and ADI-R algorithm item scores were assessed. Twentyeight children continued to score above the ADI-R cut-off for autism at FU, although significant decreases in ADI-R domain and item scores were also found. In conclusion, while classification of children according to ADI-R criteria, generally remained stable between pre-school and elementary school age, many children demonstrated significant improvements in symptom severity
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