493 research outputs found
Onset of Patterns in an Ocillated Granular Layer: Continuum and Molecular Dynamics Simulations
We study the onset of patterns in vertically oscillated layers of
frictionless dissipative particles. Using both numerical solutions of continuum
equations to Navier-Stokes order and molecular dynamics (MD) simulations, we
find that standing waves form stripe patterns above a critical acceleration of
the cell. Changing the frequency of oscillation of the cell changes the
wavelength of the resulting pattern; MD and continuum simulations both yield
wavelengths in accord with previous experimental results. The value of the
critical acceleration for ordered standing waves is approximately 10% higher in
molecular dynamics simulations than in the continuum simulations, and the
amplitude of the waves differs significantly between the models. The delay in
the onset of order in molecular dynamics simulations and the amplitude of noise
below this onset are consistent with the presence of fluctuations which are
absent in the continuum theory. The strength of the noise obtained by fit to
Swift-Hohenberg theory is orders of magnitude larger than the thermal noise in
fluid convection experiments, and is comparable to the noise found in
experiments with oscillated granular layers and in recent fluid experiments on
fluids near the critical point. Good agreement is found between the mean field
value of onset from the Swift-Hohenberg fit and the onset in continuum
simulations. Patterns are compared in cells oscillated at two different
frequencies in MD; the layer with larger wavelength patterns has less noise
than the layer with smaller wavelength patterns.Comment: Published in Physical Review
Crucial role of sidewalls in velocity distributions in quasi-2D granular gases
Our experiments and three-dimensional molecular dynamics simulations of
particles confined to a vertical monolayer by closely spaced frictional walls
(sidewalls) yield velocity distributions with non-Gaussian tails and a peak
near zero velocity. Simulations with frictionless sidewalls are not peaked.
Thus interactions between particles and their container are an important
determinant of the shape of the distribution and should be considered when
evaluating experiments on a tightly constrained monolayer of particles.Comment: 4 pages, 4 figures, Added reference, model explanation charified,
other minor change
Kink-induced transport and segregation in oscillated granular layers
We use experiments and molecular dynamics simulations of vertically
oscillated granular layers to study horizontal particle segregation induced by
a kink (a boundary between domains oscillating out of phase). Counter-rotating
convection rolls carry the larger particles in a bidisperse layer along the
granular surface to a kink, where they become trapped. The convection
originates from avalanches that occur inside the layer, along the interface
between solidified and fluidized grains. The position of a kink can be
controlled by modulation of the container frequency, making possible systematic
harvesting of the larger particles.Comment: 4 pages, 5 figures. to appear in Phys. Rev. Let
Novel Technique for Ultra-sensitive Determination of Trace Elements in Organic Scintillators
A technique based on neutron activation has been developed for an extremely
high sensitivity analysis of trace elements in organic materials. Organic
materials are sealed in plastic or high purity quartz and irradiated at the
HFIR and MITR. The most volatile materials such as liquid scintillator (LS) are
first preconcentrated by clean vacuum evaporation. Activities of interest are
separated from side activities by acid digestion and ion exchange. The
technique has been applied to study the liquid scintillator used in the KamLAND
neutrino experiment. Detection limits of <2.4X10**-15 g 40K/g LS, <5.5X10**-15
g Th/g LS, and <8X10**-15 g U/g LS have been achieved.Comment: 16 pages, 3 figures, accepted for publication in Nuclear Instruments
and Methods
Rhombic Patterns: Broken Hexagonal Symmetry
Landau-Ginzburg equations derived to conserve two-dimensional spatial symmetries lead to the prediction that rhombic arrays with characteristic angles slightly differ from 60 degrees should form in many systems. Beyond the bifurcation from the uniform state to patterns, rhombic patterns are linearly stable for a band of angles near the 60 degrees angle of regular hexagons. Experiments conducted on a reaction-diffusion system involving a chlorite-iodide-malonic acid reaction yield rhombic patterns in good accord with the theory.Energy Laboratory of the University of HoustonOffice of Naval ResearchU.S. Department of Energy Office of Basic Energy SciencesRobert A. Welch FoundationCenter for Nonlinear Dynamic
Hexagons, Kinks and Disorder in Oscillated Granular Layers
Experiments on vertically oscillated granular layers in an evacuated
container reveal a sequence of well-defined pattern bifurcations as the
container acceleration is increased. Period doublings of the layer center of
mass motion and a parametric wave instability interact to produce hexagons and
more complicated patterns composed of distinct spatial domains of different
relative phase separated by kinks (phase discontinuities). Above a critical
acceleration, the layer becomes disordered in both space and time.Comment: 4 pages. The RevTeX file has a macro allowing various styles. The
appropriate style is "myprint" which is the defaul
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