Dynamic and instability of submarine avalanches

We perform a laboratory-scale experiment of submarine avalanches on a rough inclined plane. A sediment layer is prepared and thereafter tilted up to an angle lower than the spontaneous avalanche angle. The sediment is scrapped until an avalanche is triggered. Based on the stability diagram of the sediment layer, we investigate different structures for the avalanche front dynamics. First we see a straight front descending the slope, and then a transverse instability occurs. Eventually, a fingering instability shows up similar to rivulets appearing for a viscous fluid flowing down an incline. The mechanisms leading to this new instability and the wavelength selection are discussed.Comment: 4 pages, 6 figures, to appear in the proceedings of Powders and Grains 200

Evidence of Raleigh-Hertz surface waves and shear stiffness anomaly in granular media

Due to the non-linearity of Hertzian contacts, the speed of sound in granular matter increases with pressure. Under gravity, the non-linear elastic description predicts that acoustic propagation is only possible through surface modes, called Rayleigh-Hertz modes and guided by the index gradient. Here we directly evidence these modes in a controlled laboratory experiment and use them to probe the elastic properties of a granular packing under vanishing confining pressure. The shape and the dispersion relation of both transverse and sagittal modes are compared to the prediction of non-linear elasticity that includes finite size effects. This allows to test the existence of a shear stiffness anomaly close to the jamming transition.Comment: 4 pages 4 figure

Micro-Brillouin spectroscopy mapping of the residual density field induced by Vickers indentation in a soda-lime silicate glass

High-resolution Brillouin scattering is used to achieve 3-dimensional maps of the longitudinal acoustic mode frequency shift in soda-lime silicate glasses subject to Vickers indentations. Assuming that residual stress-induced effects are simply proportional to density changes, residual densification fields are obtained. The density gradient is nearly isotropic, confirming earlier optical observations made on a similar glass. The results show that Brillouin micro-spectroscopy opens the way to a fully quantitative comparison of experimental data with predictions of mechanical models for the identification of a constitutive law.Comment: 4 pages, 3 figures, revised version, to appear in Appl. Phys. Let

Wormhole cosmic strings

We construct regular multi-wormhole solutions to a gravitating $\sigma$ model in three space-time dimensions, and extend these solutions to cylindrical traversable wormholes in four and five dimensions. We then discuss the possibility of identifying wormhole mouths in pairs to give rise to Wheeler wormholes. Such an identification is consistent with the original field equations only in the absence of the $\sigma$-model source, but with possible naked cosmic string sources. The resulting Wheeler wormhole space-times are flat outside the sources and may be asymptotically Minkowskian.Comment: 17 pages, LaTeX, 4 figures (hard copy available on request

Gravitating Chern-Simons vortices

The construction of self-dual vortex solutions to the Chern-Simons-Higgs model (with a suitable eighth-order potential) coupled to Einstein gravity in (2 + 1) dimensions is reconsidered. We show that the self-duality condition may be derived from the sole assumption $g_{00} = 1$. Next, we derive a family of exact, doubly self-dual vortex solutions, which interpolate between the symmetrical and asymmetrical vacua. The corresponding spacetimes have two regions at spatial infinity. The eighth-order Higgs potential is positive definite, and closed timelike curves are absent, if the gravitational constant is chosen to be negative.Comment: 11 pages, LaTe

Quantification des polyphénols présents dans des coupes histologiques de baies de raisin par analyse automatique d'images couleur

Quantification des polyphénols présents dans des coupes histologiques de baies de raisin par analyse automatique d\u27images couleu

Unsupervised segmentation of scenes containing vegetation (Forsythia) and soil by hierarchical analysis of bi-dimensional histograms

An unsupervised algorithm for the segmentation of scenes containing vegetation and soil is presented. It is based on a hierarchical analysis of bi-dimensional color histograms. Its performances reveal as good as those obtained from an expert (manual) segmentation, and from a neural network approach

Noise robustness of a new algorithm used for classifying pixels in color images by hierarchical analysis of bi-dimensional histograms

This paper evaluates the noise robustness of a new unsupervised algorithm used for classifying pixels in color images, by means of a hierarchical analysis of bi-dimensional histograms. Two types of additional noise are considered : gaussian noise and uniform noise, with varying correlation between the three colorimetric planes. Quantitative results show the influence of the noise level on the classification

Caractérisation des composés polyphénoliques dans des coupes histologiques de baies de raisin par analyse d'images couleur

Caractérisation des composés polyphénoliques dans des coupes histologiques de baies de raisin par analyse d\u27images couleu

L’analyse d’images au service du végétal

Détection précoce du stress hydrique du forsythia. Automatisation de l\u27analyse des paramètres morphométriques indicateurs précoces du stress. Automatisation de la quantification des agrégats polyphénoliques dans des coupes histologiques de baies de raisin. Relations avec la typicitéMesure automatisée de la cinétique de germination de lots de semences dans un incubateur de Jacobsen