Modelling the palaeo-evolution of the geodynamo

International audienceAlthough it is known that the geodynamo has been operating for at least 3.2 Ga, it remains difficult to infer the intensity, dipolarity and stability (occurrence of reversals) of the Precam-brian magnetic field of the Earth. In order to assist the interpretation of palaeomagnetic data, we produce models for the long-term evolution of the geodynamo by combining core ther-modynamics with a systematic scaling analysis of numerical dynamo simulations. We update earlier dynamo scaling results by exploring a parameter space, which has been extended in order to account for core aspect ratios and buoyancy source distributions relevant to Earth in the Precambrian. Our analysis highlights the central role of the convective power, which is an output of core thermodynamics and the main input of our updated scalings. As the thermal evolution of the Earth's core is not well known, two end-member models of heat flow evolution at the core–mantle boundary (CMB) are used, respectively, terminating at present heat flows of 11 TW (high-power scenario) and 3 TW (low power scenario). The resulting models predict that until the appearance of the inner core, a thermal dynamo driven only by secular cooling, and without any need for radioactive heating, can produce a dipole moment of strength comparable to that of the present field, thus precluding an interpretation of the oldest palaeomagnetic records as evidence of the inner core presence. The observed lack of strong long-term trends in palaeointensity data throughout the Earth's history can be rationalized by the weakness of palaeointensity variations predicted by our models relatively to the data scatter. Specifically, the most significant internal magnetic field increase which we predict is associated to the sudden power increase resulting from inner core nucleation, but the dynamo becomes deeper-seated in the core, thus largely cancelling the increase at the core and Earth surface, and diminishing the prospect of observing this event in palaeointensity data. Our models additionally suggest that the geodynamo has lied close to the transition to polarity reversals throughout its history. In the Precambrian, we predict a dynamo with similar dipolarity and less frequent reversals than at present times, due to conditions of generally lower convective forcing. Quantifying the typical CMB heat flow variation needed for the geodynamo to cross the transition from a reversing to a non-reversing state, we find that it is unlikely that such a variation may have caused superchrons in the last 0.5 Ga without shutting down dynamo action altogether

Sensitivity of the aerodynamics damping coefficient prediction to the turbulence modelling conjugated with the vibration mode shape

International audienceThe flutter corresponds to an aerodynamic loading of the structure which amplifies the natural blade vibration. In this paper, a modern design of a high pressure compressor is investigated using a time-linearized RANS solver on 2D blade to blade channel. Two operating points at part speed have been selected, the first with only small supersonic pockets and the second with the interblade channel blocked. Two vibration modes are investigated, the first torsion mode (with a nodal diameter at 2) and the first flexion mode (with a nodal diameter at 2, 4 and 6). Two different two equations turbulence models, k-l and k-ω have been used to resolve the steady state. The unsteady resolution is based on the previous steady state field. Turbulent variables are calculated over time based on a k-ω turbulence model. It was found that for some mode shapes, but not for all, the work exchange between the flow and the blade presents a large disparity depending on the turbulence model used primarily in the steady calculation. This paper proposes a parametric study in terms of rotor velocities, nodal diameters and vibration mode shapes to determine which flow phenomena are sensitive to the turbulence modelling. Main results point to the effect of the shockwave motion, and its interaction with the boundary layer and its separation

Turbulent Couette-Taylor flows with endwall effects: a numerical benchmark

International audienceThe accurate prediction of fluid flow within rotating systems has a primary role for the reliability and performance of rotating machineries. The selection of a suitable model to account for the effects of turbulence on such complex flows remains an open issue in the literature. This paper reports a numerical benchmark of different approaches available within commercial CFD solvers together with results obtained by means of in-house developed or open-source available research codes exploiting a suitable Reynolds Stress Model (RSM) closure, Large Eddy Simulation (LES) and a direct numerical simulation (DNS). The predictions are compared to the experimental data of Burin et al. (2010) in an original enclosed Couette-Taylor apparatus with endcap rings. The results are discussed in details for both the mean and turbulent fields. A particular attention has been turned to the scaling of the turbulent angular momentum G with the Reynolds number Re. By DNS, G is found to be proportional to Rea, the exponent a = 1.9 being constant in our case for the whole range of Reynolds numbers. Most of the approaches predict quite well the good trends apart from the k-w SST model, which provides relatively poor agreement with the experiments even for the mean tangential velocity profile. Among the RANS models, even though no approach appears to be fully satisfactory, the RSM closure offers the best overall agreement

Identification des mécanismes physiques à l'origine du flottement de blocage en soufflante à fort taux de dilution

Les écoulements rencontrés en tête des soufflantes aéronautiques à fort taux de dilution sont transsoniques et turbulents. Une onde de choc apparaît alors sur l'extrados des aubes et interagit avec la couche limite. La position stationnaire de l'onde de choc dépend de la forme du profil et du point de fonctionnement. A haut débit, l'incidence à l'amont des profils est négative, une poche supersonique se forme dans le canal inter-aubes et se termine par une onde de choc droite bloquant le canal. Un tel écoulement est généralement associé au décollement de la couche limite à l'aval de l'onde de choc. Lorsque le profil entre en vibration, l'onde de choc oscille et génère une fluctuation du chargement aérodynamique. L'échange d'énergie entre le fluide et la structure peut être stabilisant ou conduire à la rupture des aubes. Une telle instabilité aéroélastique est connue sous le nom de flottement. L'objectif de ce travail est l'identification des mécanismes physiques à l'origine du flottement de blocage en soufflante. La modélisation de la réponse instationnaire de l'interaction onde de choc / couche limite s'appuie sur la résolution des équations de Navier-Stokes moyennées au sens de Reynolds (RANS) et la fermeture des équations par l'hypothèse de Boussinesq et le modèle k-? de Wilcox. Une méthode linéarisée en temps permettant la résolution des équations RANS dans le domaine fréquentiel est utilisée. Les méthodes et modèles utilisés ont été validés à partir de résultats expérimentaux. Pour identifier les mécanismes à l'origine du flottement de blocage, un dessin provisoire d'une soufflante transsonique à fort taux de dilution, l'ECL5, a été réalisé. L'étude aéroélastique repose sur la simulation bidimensionnelle d'une coupe de tête où l'échange d'énergie est maximal. Le mode vibratoire étudié consiste en une rotation du profil autour du bord d'attaque. Une étude de sensibilité à la fréquence réduite permet, pour différents diamètres nodaux, d'analyser l'écoulement à partir de l'hypothèse quasi-stationnaire puis d'augmenter la fréquence d'excitation. Le diamètre nodal nul se caractérise par la stabilité aéroélastique du profil indépendamment de la fréquence d'excitation alors que le flottement apparaît pour le diamètre nodal +4ND (correspondant à un déphasage inter-aubes de 90°) pour une fréquence réduite k=0.15. La décomposition de la vibration du profil en la somme de sources locales permet d'approfondir l'analyse et d'identifier sans ambiguïté la source du flottement. Les ondes progressives émises au bord d'attaque et l'excitation locale de la zone d'interaction onde de choc / couche limite ont une contribution stabilisante. A l'inverse, une contribution fortement déstabilisante des ondes acoustiques émises au bord de fuite est observée. En remontant l'écoulement, ces ondes se rapprochent de l'onde de choc, leur longueur d'onde diminue et leur amplitude augmente (phénomène documenté sous le nom de blocage acoustique). Ces ondes pilotent alors le mouvement d'oscillation de l'onde de choc et contribuent négativement à la stabilité. La contribution des ondes acoustiques rétrogrades étant supérieure à celle des ondes progressives et de l'excitation locale de l'onde de choc, on observe finalement une instabilité aéroélastique de type flottement

Transferts de chaleur et de masse dans de ecoulements turbulents de Taylor-Couette avec flux axial

International audienceNous nous intéressons ici aux transferts de chaleur et de masse dans unsys eme de Taylor-Couette avec flux axial. Un dispositif permettant des mesures de vitesse et de coefficients de transferts á eté développé. Les régime etudiés sont turbulents : nombre de Reynolds axial entre 5600 et 11200 et entre 7900 et 79 millios pour le nombre de Taylor. Nous reportons le nombre de Nusselt en fonction du nombre de Reynolds axial et du nombre de Taylor ainsi que des mesures de vitesse. La présence de structures organisées proche du rotor á eté observée, ce qui est confirmé par un calcul DNS

Multifunctional hybrid silica nanoparticles based on [Mo₆Br₁₄]²⁻ phosphorescent nanosized clusters, magnetic γ-Fe₂O₃ and plasmonic gold nanoparticles

International audienceWe report on the synthesis, characterization and photophysical study of new luminescent and magnetic hybrid silica nanoparticles. Our method is based on the co-encapsulation of single maghemite γ-Fe2O3 nanoparticles and luminescent molybdenum cluster units [Mo6Br(i)8Br(a)6](2-) through a water-in-oil (W/O) microemulsion technique. The as-prepared core-shell [Cs2Mo6Br14-γFe2O3]@SiO2 nanoparticles (45-53 nm) possess a single magnetic core (6, 10.5 or 15 nm) and the cluster units are dispersed in the entire volume of the silica sphere. The [Cs2Mo6Br14-γFe2O3]@SiO2 nanoparticles have a perfect spherical shape with a good monodispersity and they display red and near-infrared (NIR) emission in water under UV excitation, whose intensity depends on the magnetic core size. The hybrid nanoparticles have been characterized by transmission electron microscopy (TEM), high annular angular dark field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDX), UV-Vis-NIR spectroscopy and magnetometer SQUID analysis. Small gold nanoparticles were successfully nucleated at the surface of the hybrid silica nanoparticles in order to add plasmonic properties

Short-Term Long Chain Omega3 Diet Protects from Neuroinflammatory Processes and Memory Impairment in Aged Mice

Regular consumption of food enriched in omega3 polyunsaturated fatty acids (ω3 PUFAs) has been shown to reduce risk of cognitive decline in elderly, and possibly development of Alzheimer's disease. Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are the most likely active components of ω3-rich PUFAs diets in the brain. We therefore hypothesized that exposing mice to a DHA and EPA enriched diet may reduce neuroinflammation and protect against memory impairment in aged mice. For this purpose, mice were exposed to a control diet throughout life and were further submitted to a diet enriched in EPA and DHA during 2 additional months. Cytokine expression together with a thorough analysis of astrocytes morphology assessed by a 3D reconstruction was measured in the hippocampus of young (3-month-old) and aged (22-month-old) mice. In addition, the effects of EPA and DHA on spatial memory and associated Fos activation in the hippocampus were assessed. We showed that a 2-month EPA/DHA treatment increased these long-chain ω3 PUFAs in the brain, prevented cytokines expression and astrocytes morphology changes in the hippocampus and restored spatial memory deficits and Fos-associated activation in the hippocampus of aged mice. Collectively, these data indicated that diet-induced accumulation of EPA and DHA in the brain protects against neuroinflammation and cognitive impairment linked to aging, further reinforcing the idea that increased EPA and DHA intake may provide protection to the brain of aged subjects

Optimal design of multi-channel microreactor for uniform residence time distribution

Multi-channel microreactors can be used for various applications that require chemical or electrochemical reactions in either liquid, gaseous or multi phase. For an optimal control of the chemical reactions, one key parameter for the design of such microreactors is the residence time distribution of the fluid, which should be as uniform as possible in the series of microchannels that make up the core of the reactor. Based on simplifying assumptions, an analytical model is proposed for optimizing the design of the collecting and distributing channels which supply the series of rectangular microchannels of the reactor, in the case of liquid flows. The accuracy of this analytical approach is discussed after comparison with CFD simulations and hybrid analytical-CFD calculations that allow an improved refinement of the meshing in the most complex zones of the flow. The analytical model is then extended to the case of microchannels with other cross-sections (trapezoidal or circular segment) and to gaseous flows, in the continuum and slip flow regimes. In the latter case, the model is based on second-order slip flow boundary conditions, and takes into account the compressibility as well as the rarefaction of the gas flow