20,915 research outputs found
Deformation-Driven Diffusion and Plastic Flow in Two-Dimensional Amorphous Granular Pillars
We report a combined experimental and simulation study of deformation-induced
diffusion in compacted two-dimensional amorphous granular pillars, in which
thermal fluctuations play negligible role. The pillars, consisting of
bidisperse cylindrical acetal plastic particles standing upright on a
substrate, are deformed uniaxially and quasistatically by a rigid bar moving at
a constant speed. The plastic flow and particle rearrangements in the pillars
are characterized by computing the best-fit affine transformation strain and
non-affine displacement associated with each particle between two stages of
deformation. The non-affine displacement exhibits exponential crossover from
ballistic to diffusive behavior with respect to the cumulative deviatoric
strain, indicating that in athermal granular packings, the cumulative
deviatoric strain plays the role of time in thermal systems and drives
effective particle diffusion. We further study the size-dependent deformation
of the granular pillars by simulation, and find that different-sized pillars
follow self-similar shape evolution during deformation. In addition, the yield
stress of the pillars increases linearly with pillar size. Formation of
transient shear lines in the pillars during deformation becomes more evident as
pillar size increases. The width of these elementary shear bands is about twice
the diameter of a particle, and does not vary with pillar size.Comment: 14 pages, 11 figure
Resonator-Aided Single-Atom Detection on a Microfabricated Chip
We use an optical cavity to detect single atoms magnetically trapped on an
atom chip. We implement the detection using both fluorescence into the cavity
and reduction in cavity transmission due to the presence of atoms. In
fluorescence, we register 2.0(2) photon counts per atom, which allows us to
detect single atoms with 75% efficiency in 250 microseconds. In absorption, we
measure transmission attenuation of 3.3(3)% per atom, which allows us to count
small numbers of atoms with a resolution of about 1 atom.Comment: 4.1 pages, 5 figures, and submitted to Physical Review Letter
Decay of weak solutions to the 2D dissipative quasi-geostrophic equation
We address the decay of the norm of weak solutions to the 2D dissipative
quasi-geostrophic equation. When the initial data is in only, we prove
that the norm tends to zero but with no uniform rate, that is, there are
solutions with arbitrarily slow decay. For the initial data in ,
with , we are able to obtain a uniform decay rate in . We
also prove that when the norm of the initial data
is small enough, the norms, for have uniform
decay rates. This result allows us to prove decay for the norms, for , when the initial data is in .Comment: A paragraph describing work by Carrillo and Ferreira proving results
directly related to the ones in this paper is added in the Introduction. Rest
of the article remains unchange
Nonlinear Discrete Systems with Nonanalytic Dispersion Relations
A discrete system of coupled waves (with nonanalytic dispersion relation) is
derived in the context of the spectral transform theory for the Ablowitz Ladik
spectral problem (discrete version of the Zakharov-Shabat system). This 3-wave
evolution problem is a discrete version of the stimulated Raman scattering
equations, and it is shown to be solvable for arbitrary boundary value of the
two radiation fields and initial value of the medium state. The spectral
transform is constructed on the basis of the D-bar approach.Comment: RevTex file, to appear in Journ. Math. Phy
A simple method for enhanced vibration-based structural health monitoring
This study suggests a novel method for structural vibration-based health monitoring for beams which only utilises the first natural frequency of the beam in order to detect and localise a defect. The method is based on the application of a static force in different positions along the beam. It is shown that the application of a static force on a damaged beam induces stresses at the defect which in turn cause changes in the structural natural frequencies. A very simple procedure for damage detection is suggested which uses a static force applied in just one point, in the middle of the beam. Localisation is made using two additional application points of the static force. Damage is modelled as a small notch through the whole width of the beam. The method is demonstrated and validated numerically, using a finite element model of the beam, and experimentally for a simply supported beam. Our results show that the frequency variation with the change of the force application point can be used to detect and in the same time localize very precisely even a very small defect. The method can be extended for health monitoring of other more complicated structures
A simple method for enhanced vibration-based structural health monitoring
This study suggests a novel method for structural vibration-based health monitoring for beams which only utilises the first natural frequency of the beam in order to detect and localise a defect. The method is based on the application of a static force in different positions along the beam. It is shown that the application of a static force on a damaged beam induces stresses at the defect which in turn cause changes in the structural natural frequencies. A very simple procedure for damage detection is suggested which uses a static force applied in just one point, in the middle of the beam. Localisation is made using two additional application points of the static force. Damage is modelled as a small notch through the whole width of the beam. The method is demonstrated and validated numerically, using a finite element model of the beam, and experimentally for a simply supported beam. Our results show that the frequency variation with the change of the force application point can be used to detect and in the same time localize very precisely even a very small defect. The method can be extended for health monitoring of other more complicated structures
An Hedonic Analysis of the Effects of Lake Water Clarity on New Hampshire Lakefront Properties
Policy makers often face the problem of evaluating how water quality affects a region's economic well-being. Using water clarity as a measure of the degree of eutrophication levels (as a lake becomes inundated with nutrients, water clarity decreases markedly), analysis is performed on sales data collected over a six-year period. Our results indicate that water clarity has a significant effect on prices paid for residential properties. Effects of a one-meter change in clarity on property value are also estimated for an average lake in four real estate market areas in New Hampshire, with effects differing substantially by area. Our findings provide state and local policy makers a measure of the cost of water quality degradation as measured by changes in water clarity, and demonstrate that protecting water quality may have a positive effect on property tax revenues.Resource /Energy Economics and Policy,
Gas Purity effect on GEM Performance in He and Ne at Low Temperatures
The performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne, He+H2
and Ne+H2 was studied at temperatures in the range of 3-293 K. This paper
reports on previously published measurements and additional studies on the
effects of the purity of the gases in which the GEM performance is evaluated.
In He, at temperatures between 77 and 293 K, triple-GEM structures operate at
rather high gains, exceeding 1000. There is an indication that this high gain
is achieved through the Penning effect as a result of impurities in the gas. At
lower temperatures the gain-voltage characteristics are significantly modified
probably due to the freeze-out of these impurities. Double-GEM and single-GEM
structures can operate down to 3 K at gains reaching only several tens at a gas
density of about 0.5 g/l; at higher densities the maximum gain drops further.
In Ne, the maximum gain also drops at cryogenic temperatures. The gain drop in
Ne at low temperatures can be re-established in Penning mixtures of Ne+H2: very
high gains, exceeding 104, have been obtained in these mixtures at 30-77 K, at
a density of 9.2 g/l which corresponds to saturated Ne vapor density at 27 K.
The addition of small amounts of H2 in He also re-establishes large GEM gains
above 30 K but no gain was observed in He+H2 at 4 K and a density of 1.7 g/l
(corresponding to roughly one-tenth of the saturated vapor density). These
studies are, in part, being pursued in the development of two-phase He and Ne
detectors for solar neutrino detection.Comment: 4 pages, 7 figure
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