Numerical study of Bingham flow in macrosopic two dimensional heterogenous porous media

The flow of non-Newtonian fluids is ubiquitous in many applications in the geological and industrial context. We focus here on yield stress fluids (YSF), i.e. a material that requires minimal stress to flow. We study numerically the flow of yield stress fluids in 2D porous media on a macroscopic scale in the presence of local heterogeneities. As with the microscopic problem, heterogeneities are of crucial importance because some regions will flow more easily than others. As a result, the flow is characterized by preferential flow paths with fractal features. These fractal properties are characterized by different scale exponents that will be determined and analyzed. One of the salient features of these results is that these exponents seem to be independent of the amplitude of heterogeneities for a log-normal distribution. In addition, these exponents appear to differ from those at the microscopic level, illustrating the fact that, although similar, the two scales are governed by different sets of equations

Frozen Fronts Selection in flow against self-sustained chemical waves

Autocatalytic reaction fronts between two reacting species in the absence of fluid flow, propagate as solitary waves. The coupling between autocatalytic reaction front and forced hydrodynamic flow may lead to stationary front whose velocity and shape depend on the underlying flow field. We focus on the issue of the chemo-hydrodynamic coupling between forced advection opposed to self-sustained chemical waves which can lead to static stationary fronts, i.e Frozen Fronts, $FF$. Towards that purpose, we perform experiments, analytical computations and numerical simulations with the autocatalytic Iodate Arsenious Acid reaction ($IAA$) over a wide range of flow velocities around a solid disk. For the same set of control parameters, we observe two types of frozen fronts: an upstream $FF$ which avoid the solid disk and a downstream $FF$ with two symmetric branches emerging from the solid disk surface. We delineate the range over which we do observe these Frozen Fronts. We also address the relevance of the so-called eikonal, thin front limit to describe the observed fronts and select the frozen front shapes.Comment: draf

Impact of internal gravity waves on the rotation profile inside pre-main sequence low-mass stars

We study the impact of internal gravity waves (IGW), meridional circulation, shear turbulence, and stellar contraction on the internal rotation profile and surface velocity evolution of solar metallicity low-mass pre-main sequence stars. We compute a grid of rotating stellar evolution models with masses between 0.6 and 2.0Msun taking these processes into account for the transport of angular momentum, as soon as the radiative core appears and assuming no more disk-locking from that moment on.IGW generation along the PMS is computed taking Reynolds-stress and buoyancy into account in the bulk of the stellar convective envelope and convective core (when present). Redistribution of angular momentum within the radiative layers accounts for damping of prograde and retrograde IGW by thermal diffusivity and viscosity in corotation resonance. Over the whole mass range considered, IGW are found to be efficiently generated by the convective envelope and to slow down the stellar core early on the PMS. In stars more massive than ~ 1.6Msun, IGW produced by the convective core also contribute to angular momentum redistribution close to the ZAMS. Overall, IGW are found to significantly change the internal rotation profile of PMS low-mass stars.Comment: Accepted for publication in A&A (15 pages

Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars

(Abridged) We present a detailed analysis of the main physical processes responsible for the transport of angular momentum and chemical species in the radiative regions of rotating stars. We focus on cases where meridional circulation and shear-induced turbulence only are included in the simulations. Our analysis is based on a 2-D representation of the secular hydrodynamics, which is treated using expansions in spherical harmonics. We present a full reconstruction of the meridional circulation and of the associated fluctuations of temperature and mean molecular weight along with diagnosis for the transport of angular momentum, heat and chemicals. In the present paper these tools are used to validate the analysis of two main sequence stellar models of 1.5 and 20 Msun for which the hydrodynamics has been previously extensively studied in the literature. We obtain a clear visualization and a precise estimation of the different terms entering the angular momentum and heat transport equations in radiative zones. This enables us to corroborate the main results obtained over the past decade by Zahn, Maeder, and collaborators concerning the secular hydrodynamics of such objects. We focus on the meridional circulation driven by angular momentum losses and structural readjustements. We confirm quantitatively for the first time through detailed computations and separation of the various components that the advection of entropy by this circulation is very well balanced by the barotropic effects and the thermal relaxation during most of the main sequence evolution. This enables us to derive simplifications for the thermal relaxation on this phase. The meridional currents in turn advect heat and generate temperature fluctuations that induce differential rotation through thermal wind thus closing the transport loop.Comment: 16 pages, 18 figures. Accepted for publication in A&

Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 - New test for hydrodynamical stellar models

[abridged] Previous analyses of lithium abundances in main sequence and red giant stars have revealed the action of mixing mechanisms other than convection in stellar interiors. Beryllium abundances in stars with lithium abundance determinations can offer valuable complementary information on the nature of these mechanisms. Our aim is to derive beryllium abundances along the whole evolutionary sequence of an open cluster, IC 4651. These Be abundances are used together with previously determined Li abundances, in the same sample stars, to investigate the mixing mechanisms in a range of stellar masses and evolutionary stages. New beryllium abundances are determined from high-resolution, high signal-to-noise UVES spectra using spectrum synthesis and model atmospheres. The careful synthetic modelling of the Be lines region is used to calculate reliable abundances in rapidly rotating stars. The observed behavior of Be and Li is compared to theoretical predictions from stellar models including rotation-induced mixing, internal gravity waves, atomic diffusion, and thermohaline mixing. Beryllium is detected in all the main sequence and turn-off sample stars, both slow- and fast-rotating stars, including the Li-dip stars, but was not detected in the red giants. Confirming previous results, we find that the Li dip is also a Be dip, although the depletion of Be is more modest than that of Li in the corresponding effective temperature range. For post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung gap than expected from classical predictions as does the Li dilution. A clear dispersion in the Be abundances is also observed. Theoretical stellar models including the hydrodynamical transport processes mentioned above are able to reproduce well all the observed features.Comment: 12 pages, accepted for publication in A&A, revised final versio

Au-Cu/SBA(Ti) based catalysts for photocatalytic applications

Comunicación a congresoIn this work, it has been synthesized several Au and Au-Cu alloy photocatalysts supported on two different mesoporous supports: a non-commercial SBA-15 and a post-synthesis TiO2 modified SBA-15 (TiSBA-15), with which a high dispersion of TiO2 species have been achieved maintaining the SBA-15 structure. In addition, it has also been obtained highly dispersed Au nanoparticles confined in SBA-15 pore channels, as can be observed in Figure 1. The photocatalysts have been preliminary tested in the preferential CO oxidation in a H2-rich stream (CO-PROX) at room temperature and atmospheric pressure under simulated solar light irradiation. In spite of the very low gold and copper loading (1.5 wt% and 0.5wt% respectively), the catalysts resulted active and selective in the low temperature photo-CO-PROX.Universidad de Málaga, Campus de Excelencia Internacional Andalucía Tec

On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: II- Application to light elements in population I main-sequence stars

In the two previous papers of this series, we have discussed the importance of t he $\mu$-gradients due to helium settling on rotation-induced mixing, first in a n approximate analytical way, second in a 2D numerical simulation. We have found that, for slowly rotating low mass stars, a process of ``creeping paralysis" in which the circulation and the diffusion are nearly frozen may take place below the convective zone. Here we apply this theory to the case of lithium and beryll ium in galactic clusters and specially the Hyades. We take into account the rota tional braking with rotation velocities adjusted to the present observations. We find that two different cells of meridional circulation appear on the hot side of the "lithium dip" and that the "creeping paralysis" process occurs, not dir ectly below the convective zone, but deeper inside the radiative zone, at the to p of the second cell. As a consequence, the two cells are disconnected, which ma y be the basic reason for the lithium increase with effective temperature on thi s side of the dip. On the cool side, there is just one cell of circulation and t he paralysis has not yet set down at the age of the Hyades; the same modelisatio n accounts nicely for the beryllium observations as well as for the lithium ones .Comment: 13 printed pages, 10 figures. ApJ, in press (April 20, 2003

Relation Between First Arrival Time and Permeability in Self-Affine Fractures with Areas in Contact

We demonstrate that the first arrival time in dispersive processes in self-affine fractures are governed by the same length scale characterizing the fractures as that which controls their permeability. In one-dimensional channel flow this length scale is the aperture of the bottle neck, i.e., the region having the smallest aperture. In two dimensions, the concept of a bottle neck is generalized to that of a minimal path normal to the flow. The length scale is then the average aperture along this path. There is a linear relationship between the first arrival time and this length scale, even when there is strong overlap between the fracture surfaces creating areas with zero permeability. We express the first arrival time directly in terms of the permeability.Comment: EPL (2012)

International effort toward a SSR-based linkage map for C. clementina : [P128]

Following the difficulties encountered for assembling a 1.2 x sequencing of the highly heterozygous sweet orange genome, the International Citrus Genomic Consortium (ICOC) decided to estab1ish the first reference sequence of a whole nuclear citrus genome from a haploid Clementine. A saturated genetic linkage map of Clementine based on sequence-characterized markers was considered by the ICGC as an important too1 for genome sequence assemb1y. In this framework, CIRAD proposed to use an interspecific population C. maxima x C. clementina to implement the reference Clementine genetic map. A population of 250 hybrids of Chandler pummelo x Clementine was established in Corsica and 190 hybrids were used in this first phase of mapping. Collaboration was established between two French organizations (CIRAD and INRA), two groups from United States (UF-CREC and UCR), one Spanish institute (IVIA), INRA Morocco and Cukurova University from Turkey. Forty markers were found heterozygous in Clementine among a previous set of 90 SSR markers developed by CIRAD from microsatellite-enriched genomic libraries. With the objective to integrate the physical and genetic maps of Clementine, CIRAD and IVIA have developed new SSR markers from microsatellite sequences identified in BAC End Sequences (BES) of diploid Clementine. On hundred and 10 of these new markers were found heterozygous for Clementine or Chandler pummelo and were used for genotyping. INRA France deve1oped 500 SSR markers from ESTs databases and found 170 markers heterozygous for Clementine. INRA Morocco contributed to the genotyping of 112 SSR markers developed from EST databases and genomic libraries, while 50 ESTs SSR were analysed by Cukurova University. SSR markers mainly developed from EST databases and already mapped for sweet orange were genotyped by UF-CREC (70 markers) and UCR (60 markers) to allow comparisons among the C. sinensis. C. maxima and C. c1ementina maps. lndeed, taking advantage of the important allelic differentiation between Clementine and Chandler, two parallel linkage maps can be developed from this population. As perspective, in the framework of the global haploid Clementine sequencing project, a collaboration between the French and Spanish groups plans: (i) to extend the population size to 380 hybrids between Clementine and pummelo. and (ii) to develop an array from SNPs identified in Clementine BES for High- Throughput Genotyping. All genotyping data will be stored in the online TropGene database (http://tropgenedb.cirad.fr/). Additional international groups are very welcome to join the project, using these progenies for genotyping their own markers. This should contribute to a very high density map of Clementine and to comparative mapping studies between citrus species. (Texte intégral