On the problem of large-scale magnetic field generation in rotating compressible convection

Mean-field dynamo theory suggests that turbulent convection in a rotating layer of electrically-conducting fluid produces a significant alpha-effect, which is one of the key ingredients in any mean-field dynamo model. Provided that this alpha-effect operates more efficiently than (turbulent) magnetic diffusion, such a system should be capable of sustaining a large-scale dynamo. However, in the Boussinesq model that was considered by Cattaneo&Hughes (2006) the dynamo produced small-scale, intermittent magnetic fields with no significant large-scale component. In this paper, we consider the compressible analogue of the rotating convective layer that was considered by Cattaneo&Hughes (2006). Varying the horizontal scale of the computational domain, we investigate the dependence of the dynamo upon the rotation rate. Our simulations indicate that these turbulent compressible flows can drive a small-scale dynamo but, even when the layer is rotating very rapidly (with a mid-layer Taylor number of Ta=10^8), we find no evidence for the generation of a significant large-scale component of the magnetic field on a dynamical timescale. Like Cattaneo&Hughes(2006), we measure a negligible (time-averaged) alpha-effect when a uniform horizontal magnetic field is imposed across the computational domain. Although the total horizontal magnetic flux is a conserved quantity in these simulations, the (depth-dependent) horizontally-averaged magnetic field always exhibits strong fluctuations. If these fluctuations are artificially suppressed within the code, we measure a significant mean electromotive force that is comparable to that found in related calculations in which the alpha-effect is measured using the test-field method, even though we observe no large-scale dynamo action.Comment: 25 pages, 10 figures, to appear in J. Fluid Mec

Rayleigh-B\'enard convection with a melting boundary

We study the evolution of a melting front between the solid and liquid phases of a pure incompressible material where fluid motions are driven by unstable temperature gradients. In a plane layer geometry, this can be seen as classical Rayleigh-B\'enard convection where the upper solid boundary is allowed to melt due to the heat flux brought by the fluid underneath. This free-boundary problem is studied numerically in two dimensions using a phase-field approach, classically used to study the melting and solidification of alloys, which we dynamically couple with the Navier-Stokes equations in the Boussinesq approximation. The advantage of this approach is that it requires only moderate modifications of classical numerical methods. We focus on the case where the solid is initially nearly isothermal, so that the evolution of the topography is related to the inhomogeneous heat flux from thermal convection, and does not depend on the conduction problem in the solid. From a very thin stable layer of fluid, convection cells appears as the depth -- and therefore the effective Rayleigh number of the layer increases. The continuous melting of the solid leads to dynamical transitions between different convection cell sizes and topography amplitudes. The Nusselt number can be larger than its value for a planar upper boundary, due to the feedback of the topography on the flow, which can stabilize large-scale laminar convection cells.Comment: 36 pages, 16 figure

Directional solidification of Al2-Cu-Al and Al3-Ni-Al eutectics during TEXUS rocket flight

One lamellar eutectic sample and one fiber-like eutectic sample were solidified directionally during the TEXUS-6 rocket flight. The microstructures and the results of the thermal analysis, obtained from the temperatures recorded on the cartridge skin, are compared. No appreciable modifications of the regularity of the eutectic structures were observed by passing from 1 g to 0.0001 g in these experiments. No steady state growth conditions were achieved in these experiments

On the effect of rotation on magnetohydrodynamic turbulence at high magnetic Reynolds number

This article is focused on the dynamics of a rotating electrically conducting fluid in a turbulent state. As inside the Earth's core or in various industrial processes, a flow is altered by the presence of both background rotation and a large scale magnetic field. In this context, we present a set of 3D direct numerical simulations of incompressible decaying turbulence. We focus on parameters similar to the ones encountered in geophysical and astrophysical flows, so that the Rossby number is small, the interaction parameter is large, but the Elsasser number, defining the ratio between Coriolis and Lorentz forces, is about unity. These simulations allow to quantify the effect of rotation and thus inertial waves on the growth of magnetic fluctuations due to Alfv\'en waves. Rotation prevents the occurrence of equipartition between kinetic and magnetic energies, with a reduction of magnetic energy at decreasing Elsasser number {\Lambda}. It also causes a decrease of energy transfer mediated by cubic correlations. In terms of flow structure, a decrease of {\Lambda} corresponds to an increase in the misalignment of velocity and magnetic field.Comment: 18 pages, 12 figure

Mesogranulation and small-scale dynamo action in the quiet Sun

Regions of quiet Sun generally exhibit a complex distribution of small-scale magnetic field structures, which interact with the near-surface turbulent convective motions. Furthermore, it is probable that some of these magnetic fields are generated locally by a convective dynamo mechanism. In addition to the well-known granular and supergranular convective scales, various observations have indicated that there is an intermediate scale of convection, known as mesogranulation, with vertical magnetic flux concentrations accumulating preferentially at mesogranular boundaries. Our aim is to investigate the small-scale dynamo properties of a convective flow that exhibits both granulation and mesogranulation, comparing our findings with solar observations. Adopting an idealised model for a localised region of quiet Sun, we use numerical simulations of compressible magnetohydrodynamics, in a 3D Cartesian domain, to investigate the parametric dependence of this system (focusing particularly upon the effects of varying the aspect ratio and the Reynolds number). In purely hydrodynamic convection, we find that mesogranulation is a robust feature of this system provided that the domain is wide enough to accommodate these large-scale motions. The mesogranular peak in the kinetic energy spectrum is more pronounced in the higher Reynolds number simulations. We investigate the dynamo properties of this system in both the kinematic and the nonlinear regimes and we find that the dynamo is always more efficient in larger domains, when mesogranulation is present. Furthermore, we use a filtering technique in Fourier space to demonstrate that it is indeed the larger scales of motion that are primarily responsible for driving the dynamo. In the nonlinear regime, the magnetic field distribution compares very favourably to observations, both in terms of the spatial distribution and the measured field strengths.Comment: 12 pages, 11 figures, accepted for publication in Astronomy & Astrophysic

L'histoire sociale des risques naturels en questions. Sources et problématiques nouvelles

Jusqu'à une date récente également, les interrogations sur les catastrophes naturelles (inondations, séismes, avalanches, accidents climatiques majeurs) n'ont servi pour l'essentiel qu'à illustrer un double paradigme, culturel et politique. Elles étaient d'une part le témoignage de la passivité et de la superstition des populations face à ces événements, et qui renvoyaient à l'église le soin de les protéger contre l'ire de Dieu, et furent d'autre part un lieu où se manifesta progressivement la capacité grandissante de l'Etat à prendre en charge ces risques et à assurer, grâce à son administration et ses ingénieurs, les secours aux victimes et la protection aux populations. Pour toute une tradition historiographique, face aux catastrophes, les attitudes des sociétés anciennes seraient restées marquées par le fatalisme et la superstition. Les acceptions les plus communes évoquent « la résignation devant ce qui dépasse les victimes », « le fatalisme quand la terre tremble », « l'attitude d'acceptation d'une sorte de fatalité naturelle ». Face aux catastrophes qui les accablaient, les populations seraient restées passives et résignées jusqu'à ce que le pouvoir royal et l'esprit des Lumières commencent à apporter les éclairages scientifiques et les réponses techniques nécessaires.Ce sont ces approches qui sont revisitées dans cette contribution. Le re-questionnement nécessaire passe par l'exploitation de nouvelles sources, souvent d'origine locale, qui montrent que, loin des clichés sur les populations résignées, écrasées par la colère divine et sans autre secours que celui de l'Etat, celles-ci ont su au contraire faire preuve de réactivité face aux événements, et élaborer localement des dispositifs de protection et d'entraide, repris peu à peu par les services de l'Éta

Naissance de la Franc-maçonnerie en Dauphiné au XVIIIe siècle

C'est en 1747 qu'apparurent simultanément les trois premières loges attestées en Dauphiné. Mais ce sont les années 1760 qui marquèrent le véritable début de la maçonnerie provinciale avec la multiplication des ateliers, et les premières demandes d'affiliation à la Grande Loge d'Angleterre.Ce mouvement maçonnique qui resta modeste au regard d'autres province ne resta cependant pas inactif dans la période prérévolutionnaire, mais cette participation resta essentiellement grenobloise et peu radicale. Par delà les différences sociales, les loges maçonniques incarnèrent, durant la deuxième moitié du XVIIIe siècle des aspirations communes des élites à contribuer à un changement qui ne remette pas en cause les structures du passé. Les bouleversements radicaux n'étaient pas leur propo

Le roi et les épizooties : l'indemnisation des sinistrés en Dauphiné dans la seconde moitié du XVIIIe siècle

Au rang des calamités, les pertes de bétail figuraient sans doute sous l'Ancien Régime parmi les plus fréquentes, et les accidents les plus graves commencèrent à partir du XVIIIe siècle, comme d'autres catastrophes (au premier rang desquelles les inondations), à mobiliser l'attention et les aides royales

Impact of experimental conditions on material response during forming of steel in semi-solid state

Semi-solid forming is an effective near-net-shape forming process to produce components with complex geometry and in fewer forming steps. It benefits from the complex thixotropic behaviour of semi-solids. However, the consequences of such behaviour on the flow during thixoforming, is still neither completely characterized and nor fully understood, especially for high melting point alloys. The study described in this paper investigates thixoextrusion for C38 low carbon steel material using dies at temperatures much lower than the slug temperature. Four different process parameters were studied: the initial slug temperature, the die temperature, the ram speed and the presence of a ceramic layer at the tool/material interface. The extruded parts were found to have an exact shape and a good surface state only if the temperature was below a certain value. This critical temperature is not an intrinsic material property since its value depends on die temperature and the presence of the Ceraspray©layer. Two kinds of flow were highlighted: a homogeneous flow controlled by the behaviour of the solid skeleton characterized by a positive strain rate sensitivity, and a non homogeneous flow (macro liquid/solid phase separation) dominated by the flow of the free liquid. With decreasing ram speed, heat losses increase so that the overall consistency of the material improves, leading to apparent negative strain rate sensitivity. Finally, some ways to optimise thixoforming are proposed

L'enjeu de la mixité dans un autre mouvement de jeunesse catholique : JOC et JOCF. Une approche historique.

Soirée d'étude organisé par les Scouts et Guides de France au Conseil Economique Social et Environnemental dans le cadre de la présentation de leur nouveau projet éducatif et une réflexion nationale sur l'enjeu de la mixité dans l'éducation.Réflexions historiques autour du surgissement de la mixité dans un mouvement de jeunesse d'action catholique spécialisée (JOC-F)