Impact of thixotropy on flow patterns induced in a stirred tank : numerical and experimental studies

Agitation of a thixotropic shear-thinning fluid exhibiting a yield stress is investigated both experimentally and via simulations. Steady-state experiments are conducted at three impeller rotation rates (1, 2 and 8 s−1) for a tank stirred with an axial-impeller and flow-field measurements are made using particle image velocimetry (PIV) measurements. Threedimensional numerical simulations are also performed using the commercial CFD code ANSYS CFX10.0. The viscosity of the suspension is determined experimentally and is modelled using two shear-dependant laws, one of which takes into account the flow instabilities of such fluids at low shear rates. At the highest impeller speed, the flow exhibits the familiar outward pumping action associated with axial-flow impellers. However, as the impeller speed decreases, a cavern is formed around the impeller, the flow generated in the vicinity of the agitator reorganizes and its pumping capacity vanishes. An unusual flow pattern, where the radial velocity dominates, is observed experimentally at the lowest stirring speed. It is found to result from wall slip effects. Using blades with rough surfaces prevents this peculiar behaviour and mainly resolves the discrepancies between the experimental and computational results

Semi-quantitative analysis of bulk chondritic material using X-Ray fluorescence spectroscopy

73rd Annual Meeting of the Meteoritical-Society, New York, NY, July 26-30, 2010International audienceSynchrotron radiation X-ray fluorescence (SR-XRF) is a method of choice to analyze fragile, unique meteoritic samples, requiring no sample preparation. It is a nondestructive, multielemental, quantitative method, easily coupled to diffraction and speciation for a detailed sample characterization. The composition of samples thicker than a few microns is however difficult to obtain due to the high attenuation of the characteristic X-rays resulting in non-detection of low-Z elements (Z≤ 14)

Hyperspectral non-destructive analyses of Martian return samples under quarantine

International audienceIn preparation for the upcoming sample return missions containing potential biohazards which may have withstood the rigors of space travel we present a hyperspectral method of in-situ analysis of grains combining several non-destructive imaging diagnostics, performed in BSL4 quarantine conditions. This offers an alternative to the analyses in facilities at large, using optimized experimental setups while keeping the samples in conditions of quarantine. Our methodology was tested during analyses of meteorites and cometary and interstellar grains from the recent NASA Stardust mission

Non-desctructive X-Ray, Raman and IR Imaging of quarantined Mars return samples

73rd Annual Meeting of the Meteoritical-Society : New York, NY, July 26-30, 2010International audienceIn preparation for the upcoming international Mars Sample Return mission, bringing to Earth samples containing potential biohazards, we have implemented a hyperspectral method of analysis of grains performed in BSL4 quarantine conditions,by combining several non-destructive imaging diagnostics. This methodology was tested on meteorites [1, 2] and cometary grains from the recent NASA Stardust mission [3-6]

Isotopic composition of carbon and nitrogen in ureilitic fragments of the Almahata Sitta meteorite

This study characterizes carbon and nitrogen abundances and isotopic compositions in ureilitic fragments of Almahata Sitta. Ureilites are carbon-rich (containing up to 7 wt% C) and were formed early in solar system history, thus the origin of carbon in ureilites has significance for the origin of solar system carbon. These samples were collected soon after they fell, so they are among the freshest ureilite samples available and were analyzed using stepped combustion mass spectrometry. They contained 1.2–2.3 wt% carbon; most showed the major carbon release at temperatures of 600–700 °C with peak values of δ13C from −7.3 to +0.4‰, similar to literature values for unbrecciated (“monomict”) ureilites. They also contained a minor low temperature (≤500 °C) component (δ13C = ca −25‰). Bulk nitrogen contents (9.4–27 ppm) resemble those of unbrecciated ureilites, with major releases mostly occurring at 600–750 °C. A significant lower temperature release of nitrogen occurred in all samples. Main release δ15N values of −53 to −94‰ fall within the range reported for diamond separates and acid residues from ureilites, and identify an isotopically primordial nitrogen component. However, they differ from common polymict ureilites which are more nitrogen-rich and isotopically heavier. Thus, although the parent asteroid 2008TC3 was undoubtedly a polymict ureilite breccia, this cannot be deduced from an isotopic study of individual ureilite fragments. The combined main release δ13C and δ15N values do not overlap the fields for carbonaceous or enstatite chondrites, suggesting that carbon in ureilites was not derived from these sources

Mineralogy and petrology of comet 81P/wild 2 nucleus samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk

Formation et transformations de la matière extraterrestre (grains comètaires et météorites choquées)

Au cours de cette thèse, nous avons pu participer à l'étude chimique et minéralogique des grains de la mission Stardust comètaires par des techniques basées sur le rayonnement synchrotron. L'importance de ces études tient au fait que les comètes sont parmi les objets les plus primitifs et les plus intacts de notre système solaire. Les météorites font aussi partie des premières briques du système solaire. Nous avons pu étudier les transformations qui se produisent au cours du choc dans ces roches. Nous avons notamment montré comment une partie de l'atmosphère martienne peut être capturée par les météorites lors de leur éjection mais aussi les différentes transformations qui peuvent se produire au cours du choc. Pour ceci, nous nous sommes intéressés à trois des grands systèmes minéralogiques qui existent : les olivines, les pyroxènes et le carbone. Nous avons ainsi pu mettre en évidence différents modes et mécanismes de transition de phase au sein de ces ensembles et montrer que le passage par un intermédiaire liquide dans deux des trois cas change la vision que nous avons des transitions de basse pression en polymorphe de haute pression dans les météorites. Par ailleurs, nous avons pu mettre en évidence l'existence d'un nouveau polytype du diamant et d'un nouvelle phase carbonée ultra dure dans les ureilites. Enfin, nous avons aussi étudié le comportement des feldspaths potassiques à haute pression par cellule à enclume de diamant couplée à la diffraction des rayons X et montré que la structure de haute pression, la hollandite, bien que subissant une transition de phase que nous avons découverte, semble stable jusqu'à la base du manteau terrestre.During this Ph. D, we were member of the Preliminary Examination Team which studied the chemical ans mineralogical compositions of Stardust mission cometary grains using synchrotron radiation techniques. These studies are of great importance since comets are the most primitives and unmodified bodies of our Solar System. Apart from comets, meteorites are also part of the first solar system stones. Using a very wide range of analytical techniques, we studied the different transformations which take place during a shock event undergone by these rocks. We particulary studied how part of the martian atmosphere was trapped in martian meteorites during their ejection from their parental body. We also studied the different phase transformations during the shock events on three different mineralogical system : olivines, pyroxenes and carbon. We showed that different transformation mechanisms were active in these systems and that a melting step in two of the three cases greatly change our vision in the transformation mechanism from low pressure polymorphs to high pressure polymorphs in meteorites. Moreover, we described a new diamond polytype and a new ultra-hard phase of carbon in ureilites. Finally , we also made in situ studies of the behavior of K-feldspar at high pressure using diamond anvil cell coupled with X-ray diffraction techniques. We showed that the high pressure polymorph, K-hollandite, over goes a phase transition which we discovered and that it seems to be stable as deep as the bottom of the Earth mantle.LYON-ENS Sciences (693872304) / SudocSudocFranceF

Shock-induced compaction, melting, and entrapment of atmospheric gases in Martian meteorites.

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