554 research outputs found
Dispersion mécanique de l'or dans les matériaux de surface : exemple du site aurifÚre de Piéla (Burkina-Faso)
A Piéla, l'or primaire est associé à des filons de quartz, dans une zone de cisaillement des formations volcano-sédimentaires birimiennes (Protérozoïque inférieur). La présence d'or a également été mise en évidence dans les matériaux superficiels latéritiques bien que les roches sous-jacentes et leurs altérites soient en général stériles. L'étude de la répartition de l'or sur les différentes surfaces aplanies montre que l'or est concentré par des transferts mécaniques dans les cailloutis et graviers de cuirasse ferrugineuse et de quartz de la plaine alluviale et dans des portions du moyen-glacis cuirassé assimilables à une ancienne terrasse. Ailleurs sur les surfaces cuirassées (haut-glacis, moyen-glacis), l'or est dispersé à faible teneur ou absent. Les caractÚres morphologiques et chimiques des particules d'or évoluent depuis la zone minéralisée jusqu'à la plaine alluviale distale ainsi que sur les glacis. Cette évolution se marque par une augmentation de l'émoussé, par des traits morphologiques spécifiques du transport mécanique (stries, bordures repliées, aplatissement) et par un lessivage de l'argent jusqu'au coeur des particules. Dans les matériaux superficiels du site de Piéla, les particules d'or présentent des degrés d'usure différents et ne montrent pas l'évolution progressive avec diminution de la taille des particules et lessivage de l'argent, souvent décrite de bas en haut des profils cuirassés sur roche mÚre minéralisée. (Résumé d'auteur
Détermination du début de la période thermosensible pour le déterminisme du sexe chez le tilapia (Oreochromis niloticus L.)
Lâobjectif de la prĂ©sente Ă©tude a Ă©tĂ© de prĂ©ciser davantage le âtimingâ de la pĂ©riode thermosensible ainsi que lâeffet du moment dâapplication de la tempĂ©rature de âmasculinisationâ sur la sex-ratio chez le tilapia du Nil Oreochromis niloticus. Lâanalyse des rĂ©sultats a montrĂ© que la rĂ©duction de la pĂ©riode dâexposition aux hautes tempĂ©ratures (21 au lieu de 28 jours) nâa pas dâeffet nĂ©gatif sur la sex-ratio. Dans tous les lots exposĂ©s Ă 37°C entre 0 et 11 jours post-rĂ©sorption vitelline, les populations sont presque exclusivement monosexes mĂąles (> 97%), alors que dans les lots qui ont Ă©tĂ© soumis le plus tard au traitement thermique, la proportion des mĂąles est plus faible (82%) et une forte proportion dâindividus intersexuĂ©s (18%) a Ă©tĂ© observĂ©e. Ainsi, la limite infĂ©rieure de thermosensibilitĂ© dâOreochromis niloticus pourrait se situer entre le 11Ăšme et le 14Ăšme jour aprĂšs la rĂ©sorption de la vĂ©sicule vitelline
Spurious diffusion in particle simulations of the Kolmogorov flow
Particle simulations of the Kolmogorov flow are analyzed by the
Landau-Lifshitz fluctuating hydrodynamics. It is shown that a spurious
diffusion of the center of mass corrupts the statistical properties of the
flow. The analytical expression for the corresponding diffusion coefficient is
derived.Comment: 10 pages, no figure
Behaviour and passage of European silver eels (Anguilla anguilla) at a small hydropower plant during their downstream migration
Between 2004 and 2007, 116 downstream migrant silver eels (Anguilla anguilla) were monitored at a hydropower plant on the Gave de Pau river in South-West France using radio and PIT telemetry. The objectives of the study were: (i) to determine the environmental conditions when eels arrived and passed the facility; (ii) to determine the rate of eel escapement (passage other than via the turbines); (iii) to describe the behaviour of eels faced with the intake structure and the permeability of the intake trashracks for the different sizes of eel; and (iv) to determine whether surface bypasses originally designed for salmon could be effective for eels. Five types of behaviour of silver eels in the forebay and at the plant intakes were identified. The study showed the key factor influencing both eel behaviour and the route taken through the plant was variation in river discharge. Escapement rate was related to eel length and the spill flow to river flow ratio, which could be described by a logistic regression model. The surface bypasses originally designed for salmon were found to aid downstream eel migration significantly. At velocities < 0.40 m·sâ1, no eels, even the largest, for which the racks are a physical barrier, were found impinged
on the trashracks
Lifespan theorem for constrained surface diffusion flows
We consider closed immersed hypersurfaces in and evolving by
a class of constrained surface diffusion flows. Our result, similar to earlier
results for the Willmore flow, gives both a positive lower bound on the time
for which a smooth solution exists, and a small upper bound on a power of the
total curvature during this time. By phrasing the theorem in terms of the
concentration of curvature in the initial surface, our result holds for very
general initial data and has applications to further development in asymptotic
analysis for these flows.Comment: 29 pages. arXiv admin note: substantial text overlap with
arXiv:1201.657
Advanced Orbiting Systems Data Generator/Simulator: A Functional Description of the Software (Version 3)
The Advanced Orbiting System (AOS) Data Generator/Simulator is a software implementation of the transmitter (data generation) section of the CCSDS Recommendation 701.0-B-2 for Advanced Orbiting Systems: Networks and Data Links. An object-oriented approach to the simulation of a complex, high-performance communication protocol, it makes full use of the concepts of data-encapsulation and inheritance to ease implementation. The backbone of the software is a general-purpose packet description and generation module that may be used as part of any packet- based simulation software. The user-interface to the program is in the form of a command-language, designed to ease the process of generation of large, multiple data-streams. The output of the program may be configured for interpretation by a graphical user interface (for visual inspection of the data), or as a bit-stream suitable for further processing. This paper consists of three sections. The first two sections provide a brief, yet comprehensive description of the above CCSDS Recommendation. The various kinds and qualities of user-services, data units involved, and data-paths defined by the protocol are discussed. The different qualities of service (in terms or data reliability) available to the user (and the error-control schemes used to provide them) are also discussed. The last section describes the structure and user-interfaces of the AOS Data Generator/Simulator
Advanced Orbiting Systems Data Generator/Simulator: A Functional Description of the Software (Version 3)
The Advanced Orbiting System (AOS) Data Generator/Simulator is a software implementation of the transmitter (data generation) section of the CCSDS Recommendation 701.0-B-2 for Advanced Orbiting Systems: Networks and Data Links. An object-oriented approach to the simulation of a complex, high-performance communication protocol, it makes full use of the concepts of data-encapsulation and inheritance to ease implementation. The backbone of the software is a general-purpose packet description and generation module that may be used as part of any packet- based simulation software. The user-interface to the program is in the form of a command-language, designed to ease the process of generation of large, multiple data-streams. The output of the program may be configured for interpretation by a graphical user interface (for visual inspection of the data), or as a bit-stream suitable for further processing. This paper consists of three sections. The first two sections provide a brief, yet comprehensive description of the above CCSDS Recommendation. The various kinds and qualities of user-services, data units involved, and data-paths defined by the protocol are discussed. The different qualities of service (in terms or data reliability) available to the user (and the error-control schemes used to provide them) are also discussed. The last section describes the structure and user-interfaces of the AOS Data Generator/Simulator
Analysis of the Reaction Rate Coefficients for Slow Bimolecular Chemical Reactions
Simple bimolecular reactions are analyzed
within the framework of the Boltzmann equation in the initial stage of a
chemical reaction with the system far from chemical equilibrium. The
Chapman-Enskog methodology is applied to determine the coefficients of the
expansion of the distribution functions in terms of Sonine polynomials for
peculiar molecular velocities. The results are applied to the reaction
, and the influence of the non-Maxwellian
distribution and of the activation-energy dependent reactive cross sections
upon the forward and reverse reaction rate coefficients are discussed.Comment: 11 pages, 5 figures, to appear in vol.42 of the Brazilian Journal of
Physic
Gas phase Elemental abundances in Molecular cloudS (GEMS) VI. A sulphur journey across star-forming regions: study of thioformaldehyde emission
In the context of the IRAM 30m Large Program GEMS, we present a study of
thioformaldehyde in several starless cores located in star-forming filaments of
Taurus, Perseus, and Orion. We investigate the influence of the environmental
conditions on the abundances of these molecules in the cores, and the effect of
time evolution. We have modelled the observed lines of H2CS, HDCS, and D2CS
using the radiative transfer code RADEX. We have also used the chemical code
Nautilus to model the evolution of these species depending on the
characteristics of the starless cores. We derive column densities and
abundances for all the cores. We also derive deuterium fractionation ratios,
Dfrac, to determine and compare the evolutionary stage between different parts
of each star-forming region. Our results indicate that the north region of the
B213 filament in Taurus is more evolved than the south, while the north-eastern
part of Perseus presents an earlier evolutionary stage than the south-western
zone. Model results also show that Dfrac decreases with the cosmic-ray
ionisation rate, while it increases with density and with the degree of sulphur
depletion. In particular, we only reproduce the observations when the initial
sulphur abundance in the starless cores is at least one order of magnitude
lower than the solar elemental sulphur abundance. The progressive increase in
HDCS/H2CS and D2CS/H2CS with time makes these ratios powerful tools for
deriving the chemical evolutionary stage of starless cores. However, they
cannot be used to derive the temperature of these regions, since both ratios
present a similar evolution at two different temperature ranges (7-11 K and
15-19 K). Regarding chemistry, (deuterated) thioformaldehyde is mainly formed
through gas-phase reactions (double-replacement and neutral-neutral
displacement reactions), while surface chemistry plays an important role as a
destruction mechanism.Comment: 31 pages, 26 figure
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