369 research outputs found
Mineralogical and geochemical characteristics of two bauxitic profiles, Fria, Guinea Republic
Bauxite deposits of the Fria district, Guinea, have been exploited since 1960. These lateritic bauxites, located on the upper parts of plateaus, result from weathering of paleozoic schists. The ores are composed of gibbsite associated with pyrophyllite, Al-substituted goethite, and kaolinite. Pyrophyllite and Al-substituted goethite may contain up to 9% of the total Al2O3 content of the bauxite ; this cannot be recovered through the Bayer process because these phases are insoluble in the leaching solutions. Kaolinite is soluble under Bayer leaching but this dissolution induces precipitation of sodium alumino-silicates, which apart from loss of further alumina results in decreasing efficiency of the process through scale formation. Detailed knowledge of the distribution of the different ore types and their mineralogical composition is necessary for efficient processing. (Résumé d'auteur
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
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
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
The role of perceived source location in auditory stream segregation: separation affects sound organization, common fate does not
The human auditory system is capable of grouping sounds originating from different sound sources into coherent auditory streams, a process termed auditory stream segregation. Several cues can inïŹuence auditory stream segregation, but the full set of cues and the way in which they are integrated is still unknown. In the current study, we tested whether auditory motion can serve as a cue for segregating sequences of tones. Our hypothesis was that, following the principle of common fate, sounds emitted by sources moving together in space along similar trajectories will be more likely to be grouped into a single auditory stream, while sounds emitted by independently moving sources will more often be heard as two streams. Stimuli were derived from sound recordings in which the sound source motion was induced by walking humans. Although the results showed a clear effect of spatial separation, auditory motion had a negligible inïŹuence on stream segregation. Hence, auditory motion may not be used as a primitive cue in auditory stream segregation
Is Random Close Packing of Spheres Well Defined?
Despite its long history, there are many fundamental issues concerning random
packings of spheres that remain elusive, including a precise definition of
random close packing (RCP). We argue that the current picture of RCP cannot be
made mathematically precise and support this conclusion via a molecular
dynamics study of hard spheres using the Lubachevsky-Stillinger compression
algorithm. We suggest that this impasse can be broken by introducing the new
concept of a maximally random jammed state, which can be made precise.Comment: 6 pages total, 2 figure
Granular Flows in a Rotating Drum: the Scaling Law between Velocity and Thickness of the Flow
The flow of dry granular material in a half-filled rotating drum is studied.
The thickness of the flowing zone is measured for several rotation speeds, drum
sizes and beads sizes (size ratio between drum and beads ranging from 47 to
7400). Varying the rotation speed, a scaling law linking mean velocity vs
thickness of the flow, , is deduced for each couple (beads, drum).
The obtained exponent is not always equal to 1, value previously reported
in a drum, but varies with the geometry of the system. For small size ratios,
exponents higher than 1 are obtained due to a saturation of the flowing zone
thickness. The exponent of the power law decreases with the size ratio, leading
to exponents lower than 1 for high size ratios. These exponents imply that the
velocity gradient of a dry granular flow in a rotating drum is not constant.
More fundamentally, these results show that the flow of a granular material in
a rotating drum is very sensible to the geometry, and that the deduction of the
``rheology'' of a granular medium flowing in such a geometry is not obvious
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