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

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

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

Aeolian sans ripples: experimental study of saturated states

We report an experimental investigation of aeolian sand ripples, performed both in a wind tunnel and on stoss slopes of dunes. Starting from a flat bed, we can identify three regimes: appearance of an initial wavelength, coarsening of the pattern and finally saturation of the ripples. We show that both initial and final wavelengths, as well as the propagative speed of the ripples, are linear functions of the wind velocity. Investigating the evolution of an initially corrugated bed, we exhibit non-linear stable solutions for a finite range of wavelengths, which demonstrates the existence of a saturation in amplitude. These results contradict most of the models.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Lett. Title changed, figures corrected and simplified, more field data included, text clarifie

Block to granular-like transition in dense bubble flows

We have experimentally investigated 2-dimensional dense bubble flows underneath inclined planes. Velocity profiles and velocity fluctuations have been measured. A broad second-order phase transition between two dynamical regimes is observed as a function of the tilt angle $\theta$. For low $\theta$ values, a block motion is observed. For high $\theta$ values, the velocity profile becomes curved and a shear velocity gradient appears in the flow.Comment: Europhys. Lett. (2003) in pres

What medium-term growth rates after the crisis?

The financial crisis originated in the United States in 2007 and, then, spread in all the economies of the world. After a drop in activity of historic magnitude, first signs of recovery were recorded in 2009. However, our study of previous banking crises that have taken place in OECD countries over the last forty years leads one to expect a very gradual return of growth rates to their pre-crisis values, with long-lasting losses for the level of GDP. According to our estimations, average losses observed during past banking crises occurred through a reduction in capital stocks, an increase in unemployment rates and a drop in participation rates. Conversely, past banking crises seemed to have had little impact on total factor productivity. In France, the 1992-1993 crisis, which shares some common features with the current crisis, had also long-lasting negative impacts on the employment and unemployment rates. Employment-rate losses were fairly similar for men and women. Ten years passed before the overall unemployment rate returned to its pre-crisis level. Based on various post-crisis scenarios, we illustrate the mechanical medium-term impact of the current crisis on public finances, whithout any fiscal adjustment after 2012. Even in the scenario of a complete recovery by 2018 of GDP losses recorded in 2008 and 2009, the impact on the public debt would exceed 20 % of GDP ten years after the crisis as a result of declining revenues and increasing interest payments. The impact would be still higher in less favourable scenarios.financial crisis, medium-term growth

Shear bands in granular flow through a mixing length model

We discuss the advantages and results of using a mixing-length, compressible model to account for shear banding behaviour in granular flow. We formulate a general approach based on two function of the solid fraction to be determined. Studying the vertical chute flow, we show that shear band thickness is always independent from flowrate in the quasistatic limit, for Coulomb wall boundary conditions. The effect of bin width is addressed using the functions developed by Pouliquen and coworkers, predicting a linear dependence of shear band thickness by channel width, while literature reports contrasting data. We also discuss the influence of wall roughness on shear bands. Through a Coulomb wall friction criterion we show that our model correctly predicts the effect of increasing wall roughness on the thickness of shear bands. Then a simple mixing-length approach to steady granular flows can be useful and representative of a number of original features of granular flow.Comment: submitted to EP

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 influence 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 influence on stream segregation. Hence, auditory motion may not be used as a primitive cue in auditory stream segregation