280 research outputs found
Long surface wave instability in dense granular flows
In this paper we present an experimental study of the long surface wave
instability that can develop when a granular material flows down a rough
inclined plane. The threshold and the dispersion relation of the instability
are precisely measured by imposing a controlled perturbation at the entrance of
the flow and measuring its evolution along the slope. The results are compared
with the prediction of a linear stability analysis conducted in the framework
of the depth-averaged or Saint-Venant equations. We show that when the friction
law proposed in Pouliquen (1999a) is introduced in the Saint-Venant equations,
the theory is able to predict quantitatively the stability threshold and the
phase velocity of the waves but fails in predicting the observed cutoff
frequency. The instability is shown to be of the same nature as the long wave
instability observed in classical fluids but with characteristics that can
dramatically differ due to the specificity of the granular rheology.Comment: 29 pages, 20 figures, to be published in Journal of Fluid Mechanic
Velocity correlations in dense granular flows
Velocity fluctuations of grains flowing down a rough inclined plane are
experimentally studied. The grains at the free surface exhibit fluctuating
motions, which are correlated over few grains diameters. The characteristic
correlation length is shown to depend on the inclination of the plane and not
on the thickness of the flowing layer. This result strongly supports the idea
that dense granular flows are controlled by a characteristic length larger than
the particle diameter
Compaction of a granular material under cyclic shear
In this paper we present experimental results concerning the compaction of a
granular assembly of spheres under periodic shear deformation. The dynamic of
the system is slow and continuous when the amplitude of the shear is constant,
but exhibits rapid evolution of the volume fraction when a sudden change in
shear amplitude is imposed. This rapid response is shown to be to be
uncorrelated with the slow compaction process.Comment: 7 pages, 9 figures, accepted for publication in European Physical
Journal
Oil price and potential output growth in the long run
The goal of this paper is to gauge the impact of the expected oil price increase on the potential output growth of the French economy in the long run. This potential output exercise is conducted using CES (Constant Elasticity of Substitution) production functions featuring three factors: capital, labour and energy. Moreover, the sectoral composition of the economy is taken into account through a breaking down of the economy into four sectors (manufacturing industry, construction, market services, and agriculture). The model yields a potential output of growth of about 2 % per year in the absence of oil price variations. The various scenarios of oil price increases result in a shortage of growth between 0.1 and 0.6 % per year in the medium run with respect to the constant oil price scenario. The major part of this growth shortage channels through a negative impact on the manufacturing sector, which is highly energy-intensive and also the engine of technical progress.Potential output growth, Unbalanced growth, Oil price
VLSI implementation of an energy-aware wake-up detector for an acoustic surveillance sensor network
We present a low-power VLSI wake-up detector for a sensor network that uses acoustic signals to localize ground-base vehicles. The detection criterion is the degree of low-frequency periodicity in the acoustic signal, and the periodicity is computed from the "bumpiness" of the autocorrelation of a one-bit version of the signal. We then describe a CMOS ASIC that implements the periodicity estimation algorithm. The ASIC is functional and its core consumes 835 nanowatts. It was integrated into an acoustic enclosure and deployed in field tests with synthesized sounds and ground-based vehicles.Fil: Goldberg, David H.. Johns Hopkins University; Estados UnidosFil: Andreou, Andreas. Johns Hopkins University; Estados UnidosFil: Julian, Pedro Marcelo. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Universidad Nacional del Sur. Departamento de IngenierĂa ElĂ©ctrica y de Computadoras; ArgentinaFil: Pouliquen, Philippe O.. Johns Hopkins University; Estados UnidosFil: Riddle, Laurence. Signal Systems Corporation; Estados UnidosFil: Rosasco, Rich. Signal Systems Corporation; Estados Unido
Transverse Instability of Avalanches in Granular Flows down Incline
Avalanche experiments on an erodible substrate are treated in the framework
of ``partial fluidization'' model of dense granular flows. The model identifies
a family of propagating soliton-like avalanches with shape and velocity
controlled by the inclination angle and the depth of substrate. At high
inclination angles the solitons display a transverse instability, followed by
coarsening and fingering similar to recent experimental observation. A primary
cause for the transverse instability is directly related to the dependence of
soliton velocity on the granular mass trapped in the avalanche.Comment: 3 figures, 4 pages, submitted to Phys Rev Let
A constitutive law for dense granular flows
A continuum description of granular flows would be of considerable help in
predicting natural geophysical hazards or in designing industrial processes.
However, the constitutive equations for dry granular flows, which govern how
the material moves under shear, are still a matter of debate. One difficulty is
that grains can behave like a solid (in a sand pile), a liquid (when poured
from a silo) or a gas (when strongly agitated). For the two extreme regimes,
constitutive equations have been proposed based on kinetic theory for
collisional rapid flows, and soil mechanics for slow plastic flows. However,
the intermediate dense regime, where the granular material flows like a liquid,
still lacks a unified view and has motivated many studies over the past decade.
The main characteristics of granular liquids are: a yield criterion (a critical
shear stress below which flow is not possible) and a complex dependence on
shear rate when flowing. In this sense, granular matter shares similarities
with classical visco-plastic fluids such as Bingham fluids. Here we propose a
new constitutive relation for dense granular flows, inspired by this analogy
and recent numerical and experimental work. We then test our three-dimensional
(3D) model through experiments on granular flows on a pile between rough
sidewalls, in which a complex 3D flow pattern develops. We show that, without
any fitting parameter, the model gives quantitative predictions for the flow
shape and velocity profiles. Our results support the idea that a simple
visco-plastic approach can quantitatively capture granular flow properties, and
could serve as a basic tool for modelling more complex flows in geophysical or
industrial applications.Comment: http://www.nature.com/nature/journal/v441/n7094/abs/nature04801.htm
Effective boundary conditions for dense granular flows
We derive an effective boundary condition for granular flow taking into
account the effect of the heterogeneity of the force network on sliding
friction dynamics. This yields an intermediate boundary condition which lies in
the limit between no-slip and Coulomb friction; two simple functions relating
wall stress, velocity, and velocity variance are found from numerical
simulations. Moreover, we show that this effective boundary condition
corresponds to Navier slip condition when GDR MiDi's model is assumed to be
valid, and that the slip length depends on the length scale that characterises
the system, \emph{viz} the particle diameter.Comment: 4 pages, 5 figure
On the dependence of the avalanche angle on the granular layer thickness
A layer of sand of thickness h flows down a rough surface if the inclination
is larger than some threshold value theta which decreases with h. A tentative
microscopic model for the dependence of theta with h is proposed for rigid
frictional grains, based on the following hypothesis: (i) a horizontal layer of
sand has some coordination z larger than a critical value z_c where mechanical
stability is lost (ii) as the tilt angle is increased, the configurations
visited present a growing proportion $_s of sliding contacts. Instability with
respect to flow occurs when z-z_s=z_c. This criterion leads to a prediction for
theta(h) in good agreement with empirical observations.Comment: 6 pages, 2 figure
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