1,563 research outputs found
Some aspects of electrical conduction in granular systems of various dimensions
We report on measurements of the electrical conductivity in both a 2D
triangular lattice of metallic beads and in a chain of beads. The
voltage/current characteristics are qualitatively similar in both experiments.
At low applied current, the voltage is found to increase logarithmically in a
good agreement with a model of widely distributed resistances in series. At
high enough current, the voltage saturates due to the local welding of
microcontacts between beads. The frequency dependence of the saturation voltage
gives an estimate of the size of these welded microcontacts. The DC value of
the saturation voltage (~ 0.4 V per contact) gives an indirect measure of the
number of welded contact carrying the current within the 2D lattice. Also, a
new measurement technique provides a map of the current paths within the 2D
lattice of beads. For an isotropic compression of the 2D granular medium, the
current paths are localized in few discrete linear paths. This
quasi-onedimensional nature of the electrical conductivity thus explains the
similarity between the characteristics in the 1D and 2D systems.Comment: To be published in The European Physical Journal
Effects of electromagnetic waves on the electrical properties of contacts between grains
A DC electrical current is injected through a chain of metallic beads. The
electrical resistances of each bead-bead contacts are measured. At low current,
the distribution of these resistances is large and log-normal. At high enough
current, the resistance distribution becomes sharp and Gaussian due to the
creation of microweldings between some beads. The action of nearby
electromagnetic waves (sparks) on the electrical conductivity of the chain is
also studied. The spark effect is to lower the resistance values of the more
resistive contacts, the best conductive ones remaining unaffected by the spark
production. The spark is able to induce through the chain a current enough to
create microweldings between some beads. This explains why the electrical
resistance of a granular medium is so sensitive to the electromagnetic waves
produced in its vicinity.Comment: 4 pages, 5 figure
Single Stellar Populations in the Near-Infrared - I. Preparation of the IRTF spectral stellar library
We present a detailed study of the stars of the IRTF spectral library to
understand its full extent and reliability for use with Stellar Population (SP)
modeling. The library consist of 210 stars, with a total of 292 spectra,
covering the wavelength range of 0.94 to 2.41 micron at a resolution R = 2000.
For every star we infer the effective temperature (Teff), gravity (logg) and
metallicity ([Z/Zsun]) using a full-spectrum fitting approach in a section of
the K band (2.19 to 2.34 micron) and temperature-NIR colour relations. We test
the flux calibration of these stars by calculating their integrated colours and
comparing them with the Pickles library colour-temperature relations. We also
investigate the NIR colours as a function of the calculated effective
temperature and compared them in colour-colour diagrams with the Pickles
library. This latter test shows a good broad-band flux calibration, important
for the SP models. Finally, we measure the resolution R as a function of
wavelength. We find that the resolution increases as a function of lambda from
about 6 angstrom in J to 10 angstrom in the red part of the K-band. With these
tests we establish that the IRTF library, the largest currently available
general library of stars at intermediate resolution in the NIR, is an excellent
candidate to be used in stellar population models. We present these models in
the next paper of this series.Comment: 17 pages, 19 figures. Accepted for publication in Astronomy and
Astrophysic
Capillary wave turbulence on a spherical fluid surface in low gravity
We report the observation of capillary wave turbulence on the surface of a
fluid layer in a low-gravity environment. In such conditions, the fluid covers
all the internal surface of the spherical container which is submitted to
random forcing. The surface wave amplitude displays power-law spectrum over two
decades in frequency, corresponding to wavelength from to a few . This
spectrum is found in roughly good agreement with wave turbulence theory. Such a
large scale observation without gravity waves has never been reached during
ground experiments. When the forcing is periodic, two-dimensional spherical
patterns are observed on the fluid surface such as subharmonic stripes or
hexagons with wavelength satisfying the capillary wave dispersion relation
On the origin of intermittency in wave turbulence
Using standard signal processing tools, we experimentally report that
intermittency of wave turbulence on the surface of a fluid occurs even when two
typical large-scale coherent structures (gravity wave breakings and bursts of
capillary waves on steep gravity waves) are not taken into account. We also
show that intermittency depends on the power injected into the waves. The
dependence of the power-law exponent of the gravity-wave spectrum on the
forcing amplitude cannot be also ascribed to these coherent structures.
Statistics of these both events are studied.Comment: To be published in EP
Experimental study of the inverse cascade in gravity wave turbulence
We perform experiments to study the inverse cascade regime of gravity wave
turbulence on the surface of a fluid. Surface waves are forced at an
intermediate scale corresponding to the gravity-capillary wavelength. In
response to this forcing, waves at larger scales are observed. The spectrum of
their amplitudes exhibits a frequency-power law at high enough forcing. Both
observations are ascribed to the upscale wave action transfers of gravity wave
turbulence. The spectrum exponent is close to the value predicted by the weak
turbulence theory. The spectrum amplitude is found to scale linearly with the
mean injected power. We measure also the distributions of the injected power
fluctuations in the presence of upscale (inverse) transfers or in the presence
of a downscale (direct) cascade in gravity wave turbulence.Comment: in press in EPL (2011
Observation of intermittency in wave turbulence
We report the observation of intermittency in gravity-capillary wave
turbulence on the surface of mercury. We measure the temporal fluctuations of
surface wave amplitude at a given location. We show that the shape of the
probability density function of the local slope increments of the surface waves
strongly changes across the time scales. The related structure functions and
the flatness are found to be power laws of the time scale on more than one
decade. The exponents of these power laws increase nonlinearly with the order
of the structure function. All these observations show the intermittent nature
of the increments of the local slope in wave turbulence. We discuss the
possible origin of this intermittency.Comment: new version to Phys. Rev. Let
- …