Independent confirmation of {\beta} Pictoris b imaging with NICI

Context. {\beta} Pictoris b is one of the most studied objects nowadays since it was identified with VLT/NaCo as a bona-fide exoplanet with a mass of about 9 times that of Jupiter at an orbital separation of 8-9 AU. The link between the planet and the dusty disk is unambiguously attested and this system provides an opportunity to study the disk/planet interactions and to constrain formation and evolutionary models of gas giant planets. Still, {\beta} Pictoris b had never been confirmed with other telescopes so far. Aims. We aimed at an independent confirmation using a different instrument. Methods. We retrieved archive images from Gemini South obtained with the instrument NICI, which is designed for high contrast imaging. The observations combine coronagraphy and angular differential imaging and were obtained at three epochs in Nov. 2008, Dec. 2009 and Dec. 2010. Results. We report the detection with NICI of the planet {\beta} Pictoris b in Dec. 2010 images at a separation of 404 \pm 10 mas and P A = 212.1 \pm 0.7{\deg} . It is the first time this planet is observed with a telescope different than the VLT.Comment: Letter accepted for publication in Astronomy and Astrophysics on Feb. 21, 2013. 4 pages, 2 figure

France: Fin de l'échappée. Perspectives 2002-2003 pour l'économie française.

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Refractive-index and density matching in concentrated particle suspensions: a review

Optical measurement techniques such as particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) are now routinely used in experimental fluid mechanics to investigate pure fluids or dilute suspensions. For highly concentrated particle suspensions, material turbidity has long been a substantial impediment to these techniques, which explains why they have been scarcely used so far. A renewed interest has emerged with the development of specific methods combining the use of iso-index suspensions and imaging techniques. This review paper gives a broad overview of recent advances in visualization techniques suited to concentrated particle suspensions. In particular, we show how classic methods such as PIV, LDV, particle tracking velocimetry, and laser induced fluorescence can be adapted to deal with concentrated particle suspension

Positionnement et influence du service sécurité dans les industries à risques

National audienceLe contexte économique et social de ces dernières décennies, caractérisé par une forte compétitivité entre les entreprises, un développement rapide des technologies et des pressions des systèmes réglementaires et du public, a entraîné une importante complexification des systèmes sociotechniques. Nos sociétés deviennent ainsi particulièrement vulnérables. Dans ce contexte le risque n’est pas une notion simple à appréhender. L’approche technique, rationnelle (par les normes et les règlementations) reste majoritaire dans les organisations par rapport aux approches par les sciences humaines et sociales. Ces dernières sont cependant essentielles pour capturer la subjectivité de la notion de risque. La thèse présentée ici porte sur le risque industriel dans les installations à risque en fonctionnement quotidien en se basant sur le cadre théorique développé en ergonomie et en sociologie des organisations. Suite à l’identification d’un manque de données empiriques à son sujet malgré sa centralité, l’objet d’étude sera le service sécurité. Son fonctionnement, ses stratégies et son contexte seront analysés à l'aide d'une étude de terrain en vue de comprendre son positionnement et son influence dans l’organisation

modèle France de l'OFCE. La nouvelle version: e-mod.fr.

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Projections macroéconomiques à moyen et long terme : rapport pour le Conseil d'orientation des retraites.

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Contre vent et marées : perspectives 2001-2002 de l'économie française.

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The dam-break problem for concentrated suspensions of neutrally buoyant particles

This paper addresses the dam-break problem for particle suspensions, that is, the flow of a finite volume of suspension released suddenly down an inclined flume. We were concerned with concentrated suspensions made up of neutrally-buoyant non-colloidal particles within a Newtonian fluid. Experiments were conducted over wide ranges of slope, concentration, and mass. The major contributions of our experimental study are the simultaneous measurement of local flow properties far from the sidewalls (velocity profile and, with lower accuracy, particle concentration) and macroscopic features (front position, flow depth profile). To that end, the refractive index of the fluid was adapted to closely match that of the particles, enabling data acquisition up to particle volume fractions of 60%. Particle migration resulted in the blunting of the velocity profile, in contrast to the parabolic profile observed in homogeneous Newtonian fluids. The experimental results were compared with predictions from lubrication theory and particle migration theory. For solids fractions as large as 45%, the flow behaviour did not differ much from that of a homogeneous Newtonian fluid. More specifically, we observed that the velocity profiles were closely approximated by a parabolic form and there was little evidence of particle migration throughout the depth. For particle concentrations in the 52%–56% range, the flow depth and front position were fairly well predicted by lubrication theory, but taking a closer look at the velocity profiles revealed that particle migration had noticeable effects on the shape of the velocity profile (blunting), but had little impact on its strength, which explained why lubrication theory performed well. Particle migration theories (such as the shear-induced diffusion model) successfully captured the slow evolution of the velocity profiles. For particle concentrations in excess of 56%, the macroscopic flow features were grossly predicted by lubrication theory (to within 20% for the flow depth, 50% for the front position). The flows seemed to reach a steady state, i.e., the shape of the velocity profile showed little time dependence

Granular suspension avalanches. II. Plastic regime

We present flume experiments showing plastic behavior for perfectly density-matched suspensions of non-Brownian particles within a Newtonian fluid. In contrast with most earlier experimental investigations (carried out using coaxial cylinder rheometers), we obtained our rheological information by studying thin films of suspension flowing down an inclined flume. Using particles with the same refractive index as the interstitial fluid made it possible to measure the velocity field far from the wall using a laser-optical system. At long times, a stick-slip regime occurred as soon as the fluid pressure dropped sufficiently for the particle pressure to become compressive. Our explanation was that the drop in fluid pressure combined with the surface tension caused the flow to come to rest by significantly increasing flow resistance. However, the reason why the fluid pressure diffused through the pores during the stick phases escaped our understanding of suspension rheology

Granular suspension avalanches. I. Macro-viscous behavior

We experimentally studied the flow behavior of a fixed volume of granular suspension, initially contained in a reservoir and released down an inclined flume. Here “granular suspension” refers to a suspension of non-Brownian particles in a viscous fluid. Depending on the solids fraction, density mismatch, and particle size distribution, a wealth of behaviors can be observed. Here we report and interpret results obtained with granular suspensions, which consisted of neutrally buoyant particles with a solids fraction (ϕ = 0.575–0.595) close to the maximum random packing fraction (estimated at ϕm = 0.625). The particles had the same refractive index as the fluid, which made it possible to measure the velocity profiles inside the moving bulk and far from the sidewalls. Additional information such as the front position and the flow depth was also recorded. Three regimes were observed. At early times, the flow features were reminiscent of homogeneous Newtonian fluids (e.g., the same dependence of the front position on time). At later times, the free surface became more and more bumpy as fractures developed within the bulk. This fracture process ultimately gave rise to a stick-slip regime, in which the suspension moved intermittently. In this paper, we focus on the first regime referred to as the macro-viscous regime. Although the bulk flow properties looked like those of Newtonian fluids, the internal dynamics were much richer