899 research outputs found
CARACTERISATION D'UN LASER Nd : YAG CONTINU A MODES COUPLES
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Vortex migration in protoplanetary disks
We consider the radial migration of vortices in two-dimensional isothermal
gaseous disks. We find that a vortex core, orbiting at the local gas velocity,
induces velocity perturbations that propagate away from the vortex as density
waves. The resulting spiral wave pattern is reminiscent of an embedded planet.
There are two main causes for asymmetries in these wakes: geometrical effects
tend to favor the outer wave, while a radial vortensity gradient leads to an
asymmetric vortex core, which favors the wave at the side that has the lowest
density. In the case of asymmetric waves, which we always find except for a
disk of constant pressure, there is a net exchange of angular momentum between
the vortex and the surrounding disk, which leads to orbital migration of the
vortex. Numerical hydrodynamical simulations show that this migration can be
very rapid, on a time scale of a few thousand orbits, for vortices with a size
comparable to the scale height of the disk. We discuss the possible effects of
vortex migration on planet formation scenarios.Comment: 13 pages, 13 figures, accepted for publication in Ap
Formation of ZnâCa phyllomanganate nanoparticles in grass roots
International audienceIt is now well established that a number of terrestrial and aquatic microorganisms have the capacity to oxidize and precipitate Mn as phyllomanganate. However, this biomineralization has never been shown to occur in plant tissues, nor has the structure of a natural Mn(IV) biooxide been characterized in detail. We show that the graminaceous plant Festuca rubra (red fescue) produces a Zn-rich phyllomanganate with constant Zn:Mn and Ca:Mn atomic ratios (0.46 and 0.38, respectively) when grown on a contaminated sediment. This new phase is so far the Zn-richest manganate known to form in nature (chalcophanite has a Zn:Mn ratio of 0.33) and has no synthetic equivalent. Visual examination of root fragments under a microscope shows black precipitates about ten to several tens of microns in size, and their imaging with backscattered and secondary electrons demonstrates that they are located in the root epidermis. In situ measurements by Mn and Zn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray diffraction (XRD) with a micro-focused beam can be quantitatively described by a single-phase model consisting of Mn(IV) octahedral layers with 22% vacant sites capped with tetrahedral and octahedral Zn in proportions of 3:1. The layer charge deficit is also partly balanced by interlayer Mn and Ca. Diffracting crystallites have a domain radius of 33 Ă
in the ab plane and contain only 1.2 layers (not, vert, similar8.6 Ă
) on average. Since this biogenic Mn oxide consists mostly of isolated layers, basal 00l reflections are essentially absent despite its lamellar structure. Individual Mn layers are probably held together in the MnâZn precipitates by stabilizing organic molecules. Zinc biomineralization by plants likely is a defense mechanism against toxicity induced by excess concentrations of this metal in the rhizosphere
Prévalence des lombalgies et de leurs facteurs de risque professionnels chez les travailleurs de l'agriculture en France en 2010
Introduction
La phase pilote du programme Coset-MSA a permis dâĂ©valuer la prĂ©valence des lombalgies et de leurs facteurs de risque professionnels chez les travailleurs de lâagriculture, et notamment les agriculteurs exploitants et les ouvriers agricoles.
MĂ©thodes
Les travailleurs ont Ă©tĂ© recrutĂ©s, dans cinq dĂ©partements, par tirage au sort dans les bases de donnĂ©es de la MSA et invitĂ©s Ă remplir un auto-questionnaire portant sur les expositions et les lombalgies au cours des 12 derniers mois. LâĂ©chantillon dâanalyse Ă©tait constituĂ© de 1130 travailleurs, ĂągĂ©s de 18 à 65 ans, exerçant dans le secteur agriculture, sylviculture et pĂȘche, et en activitĂ© au moment de lâenquĂȘte.
RĂ©sultats
La prĂ©valence des lombalgies au cours des 12 derniers mois Ă©tait Ă©levĂ©e, chez les hommes (65 %) comme chez les femmes (64 %). Chez ces derniĂšres, la prĂ©valence diminuait significativement avec lâĂąge (de 95 % chez les moins de 30 ans Ă 56 % chez les 50 ans et plus, p < 0,01). Des douleurs quotidiennes Ă©taient rapportĂ©es par 10 % des hommes et 11 % des femmes, avec une prĂ©valence qui augmentait avec lâĂąge, de façon significative chez les hommes (p < 0,001). La prĂ©valence ne diffĂ©rait pas significativement selon la catĂ©gorie socioprofessionnelle (CSP). Cependant, soulignons la forte prĂ©valence des lombalgies parmi les hommes artisans, commerçants et chefs dâentreprise (surtout des jardiniers, paysagistesâŠ) malgrĂ© des effectifs faibles. Chez les femmes, les agricultrices exploitantes et les ouvriĂšres agricoles Ă©taient plus nombreuses Ă souffrir de lombalgies, quelle que soit la durĂ©e des symptĂŽmes.
Le port de charges de plus de 25 kg plus de 2 heures par jour concernait 16 % des hommes et 5 % des femmes, se pencher en avant ou sur le cÎté plus de 4 heures par jour 15 % et 16 % respectivement et la conduite plus de 4 heures par jour 27 % et 10 %. Chez les hommes, les ouvriers non agricoles étaient les plus exposés aux trois facteurs (17 % au port de charges [p < 0,05], 22 % au fait de se pencher [p < 0,01], 32 % à la conduite [NS]). Chez les femmes, les ouvriÚres agricoles rapportaient les plus fortes expositions : 29 % se penchaient plus de 4 h/j (p < 0,001) et 16 % conduisaient plus de 4 h/j (NS).
Conclusions
La prĂ©valence des lombalgies est particuliĂšrement Ă©levĂ©e dans lâagriculture, comparativement Ă des Ă©tudes portant sur tous secteurs dâactivitĂ© (par exemple, lâĂ©tude Cosali menĂ©e auprĂšs de salariĂ©s des Pays de la Loire). De plus, cette Ă©tude permet dâidentifier les CSP les plus exposĂ©es aux facteurs de risque professionnels de lombalgie, qui devraient ĂȘtre ciblĂ©es prioritairement par les actions de prĂ©vention
Critical point network for drainage between rough surfaces
In this paper, we present a network method for computing two-phase flows between two rough surfaces with significant contact areas. Low-capillary number drainage is investigated here since one-phase flows have been previously investigated in other contributions. An invasion percolation algorithm is presented for modeling slow displacement of a wetting fluid by a non wetting one between two rough surfaces. Short-correlated Gaussian process is used to model random rough surfaces.The algorithm is based on a network description of the fracture aperture field. The network is constructed from the identification of critical points (saddles and maxima) of the aperture field. The invasion potential is determined from examining drainage process in a flat mini-channel. A direct comparison between numerical prediction and experimental visualizations on an identical geometry has been performed for one realization of an artificial fracture with a moderate fractional contact area of about 0.3. A good agreement is found between predictions and observations
Nonuniversal scaling behavior of Barkhausen noise
We simulate Barkhausen avalanches on fractal clusters in a two-dimensional
diluted Ising ferromagnet with an effective Gaussian random field. We vary the
concentration of defect sites and find a scaling region for moderate
disorder, where the distribution of avalanche sizes has the form . The exponents for size
and for length distribution, and the fractal dimension of
avalanches satisfy the scaling relation .
For fixed disorder the exponents vary with driving rate in agreement with
experiments on amorphous Si-Fe alloys.Comment: 5 pages, Latex, 4 PostScript figures include
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