28 research outputs found
Effect of Rossby and Alfv\'{e}n waves on the dynamics of the tachocline
To understand magnetic diffusion, momentum transport, and mixing in the
interior of the sun, we consider an idealized model of the tachocline, namely
magnetohydrodynamics (MHD) turbulence on a plane subject to a large
scale shear (provided by the latitudinal differential rotation). This model
enables us to self-consistently derive the influence of shear, Rossby and
Alfv\'{e}n waves on the transport properties of turbulence. In the strong
magnetic field regime, we find that the turbulent viscosity and diffusivity are
reduced by magnetic fields only, similarly to the two-dimensional MHD case
(without Rossby waves). In the weak magnetic field regime, we find a crossover
scale () from a Alfv\'{e}n dominated regime (on small scales) to a Rossby
dominated regime (on large scales). For parameter values typical of the
tachocline, is larger that the solar radius so that Rossby waves are
unlikely to play an important role in the transport of magnetic field and
angular momentum. This is mainly due to the enhancement of magnetic
back-reaction by shearing which efficiently generates small scales, thus strong
currents
Dynamo quenching due to shear flow
We provide a theory of dynamo (α effect) and momentum transport in three-dimensional magnetohydrodynamics. For the first time, we show that the α effect is reduced by the shear even in the absence of magnetic field. The α effect is further suppressed by magnetic fields well below equipartition (with the large-scale flow) with different scalings depending on the relative strength of shear and magnetic field. The turbulent viscosity is also found to be significantly reduced by shear and magnetic fields, with positive value. These results suggest a crucial effect of shear and magnetic field on dynamo quenching and momentum transport reduction, with important implications for laboratory and astrophysical plasmas, in particular, for the dynamics of the Sun
Non-Fickian dispersion in porous media : 1. Multiscale measurements using single-well injection withdrawal tracer tests
International audienceWe present a set of single-well injection withdrawal tracer tests in a paleoreef porous reservoir displaying important small-scale heterogeneity. An improved dual-packer probe was designed to perform dirac-like tracer injection and accurate downhole automatic measurements of the tracer concentration during the recovery phase. By flushing the tracer, at constant flow rate, for increasing time duration, we can probe distinctly different reservoir volumes and test the multiscale predictability of the (non-Fickian) dispersion models. First we describe the characteristics, from microscale to meter scale, of the reservoir rock. Second, the specificity of the tracer test setup and the results obtained using two different tracers and measurement methods (salinity-conductivity and fluorescent dyeÂoptical measurement, respectively) are presented. All the tracer tests display strongly tailed breakthrough curves (BTC) consistent with diffusion in immobile regions. Conductivity results, measured over 3 orders of magnitude only, could have been easily interpreted by the conventional mobile-immobile (MIM) diffusive mass transfer model of asymptotic log-log slope of 2. However, the fluorescent dye sensor, which allows exploring much lower concentration values, shows that a change in the log-log slope occurs at larger time with an asymptotic value of 1.5, corresponding to the double-porosity model. These results suggest that the conventional, one-slope MIM transfer rate model is too simplistic to account for the real multiscale heterogeneity of the diffusion-dominant fraction of the reservoir
Analytical theory of forced rotating sheared turbulence: The parallel case
Forced turbulence combined with the effect of rotation and shear flow is studied. In a previous paper [N. Leprovost and E. J. Kim, Phys. Rev. E 78, 016301 (2008)], we considered the case where the shear and the rotation are perpendicular. Here, we consider the complementary case of parallel rotation and shear, elucidating how rotation and flow shear influence the generation of shear flow (e.g., the direction of energy cascade), turbulence level, transport of particles, and momentum. We show that turbulence amplitude and transport are always quenched due to strong shear (Ο=Îœky2âAâȘĄ1, where A is the shearing rate, Îœ is the molecular viscosity, and ky is a characteristic wave number of small-scale turbulence), with stronger reduction in the direction of the shear than those in the perpendicular directions. In contrast with the case where rotation and shear are perpendicular, we found that rotation affects turbulence amplitude only for very rapid rotation (ΩâȘąA) where it reduces slightly the anisotropy due to shear flow. Also, concerning the transport properties of turbulence, we find that rotation affects only the transport of particle and only for rapid rotation, leading to an almost isotropic transport (whereas, in the case of perpendicular rotation and shear, rotation favors isotropic transport even for slow rotation). Furthermore, the interaction between the shear and the rotation is shown to give rise to nondiffusive flux of angular momentum (Î effect), even in the absence of external sources of anisotropy, which can provide a mechanism for the creation of shearing structures in astrophysical and geophysical systems
Dynamics and thermodynamics of axisymmetric flows: I. Theory
We develop new variational principles to study stability and equilibrium of
axisymmetric flows. We show that there is an infinite number of steady state
solutions. We show that these steady states maximize a (non-universal)
-function. We derive relaxation equations which can be used as numerical
algorithm to construct stable stationary solutions of axisymmetric flows. In a
second part, we develop a thermodynamical approach to the equilibrium states at
some fixed coarse-grained scale. We show that the resulting distribution can be
divided in a universal part coming from the conservation of robust invariants
and one non-universal determined by the initial conditions through the fragile
invariants (for freely evolving systems) or by a prior distribution encoding
non-ideal effects such as viscosity, small-scale forcing and dissipation (for
forced systems). Finally, we derive a parameterization of inviscid mixing to
describe the dynamics of the system at the coarse-grained scale
Analytical theory of forced rotating sheared turbulence: The perpendicular case
Rotation and shear flows are ubiquitous features of many astrophysical and geophysical bodies. To understand their origin and effect on turbulent transport in these systems, we consider a forced turbulence and investigate the combined effect of rotation and shear flow on the turbulence properties. Specifically, we study how rotation and flow shear influence the generation of shear flow (e.g., the direction of energy cascade), turbulence level, transport of particles and momentum, and the anisotropy in these quantities. In all the cases considered, turbulence amplitude is always quenched due to strong shear (Ο=Îœky2/AâȘĄ1, where A is the shearing rate, Îœ is the molecular viscosity, and ky is a characteristic wave number of small-scale turbulence), with stronger reduction in the direction of the shear than those in the perpendicular directions. Specifically, in the large rotation limit (ΩâȘąA), they scale as Aâ1 and Aâ1|lnâΟ|, respectively, while in the weak rotation limit (ΩâȘĄA), they scale as Aâ1 and Aâ2/3, respectively. Thus, flow shear always leads to weak turbulence with an effectively stronger turbulence in the plane perpendicular to shear than in the shear direction, regardless of rotation rate. The anisotropy in turbulence amplitude is, however, weaker by a factor of Ο1/3|lnâΟ| (âAâ1/3|lnâΟ|) in the rapid rotation limit (ΩâȘąA) than that in the weak rotation limit (ΩâȘĄA) since rotation favors almost-isotropic turbulence. Compared to turbulence amplitude, particle transport is found to crucially depend on whether rotation is stronger or weaker than flow shear. When rotation is stronger than flow shear (ΩâȘąA), the transport is inhibited by inertial waves, being quenched inversely proportional to the rotation rate (i.e., âΩâ1) while in the opposite case, it is reduced by shearing as Aâ1. Furthermore, the anisotropy is found to be very weak in the strong rotation limit (by a factor of 2) while significant in the strong shear limit. The turbulent viscosity is found to be negative with inverse cascade of energy as long as rotation is sufficiently strong compared to flow shear (ΩâȘąA) while positive in the opposite limit of weak rotation (ΩâȘĄA). Even if the eddy viscosity is negative for strong rotation (ΩâȘąA), flow shear, which transfers energy to small scales, has an interesting effect by slowing down the rate of inverse cascade with the value of negative eddy viscosity decreasing as |ÎœT|âAâ2 for strong shear. Furthermore, the interaction between the shear and the rotation is shown to give rise to a nondiffusive flux of angular momentum (Î effect), even in the absence of external sources of anisotropy. This effect provides a mechanism for the existence of shearing structures in astrophysical and geophysical systems
The Cultural Project : Formal Chronological Modelling of the Early and Middle Neolithic Sequence in Lower Alsace
Starting from questions about the nature of cultural diversity, this paper examines the pace and tempo of change and the relative importance of continuity and discontinuity. To unravel the cultural project of the past, we apply chronological modelling of radiocarbon dates within a Bayesian statistical framework, to interrogate the Neolithic cultural sequence in Lower Alsace, in the upper Rhine valley, in broad terms from the later sixth to the end of the fifth millennium cal BC. Detailed formal estimates are provided for the long succession of cultural groups, from the early Neolithic Linear Pottery culture (LBK) to the Bischheim Occidental du Rhin Supérieur (BORS) groups at the end of the Middle Neolithic, using seriation and typology of pottery as the starting point in modelling. The rate of ceramic change, as well as frequent shifts in the nature, location and density of settlements, are documented in detail, down to lifetime and generational timescales. This reveals a Neolithic world in Lower Alsace busy with comings and goings, tinkerings and adjustments, and relocations and realignments. A significant hiatus is identified between the end of the LBK and the start of the Hinkelstein group, in the early part of the fifth millennium cal BC. On the basis of modelling of existing dates for other parts of the Rhineland, this appears to be a wider phenomenon, and possible explanations are discussed; full reoccupation of the landscape is only seen in the Grossgartach phase. Radical shifts are also proposed at the end of the Middle Neolithic
DĂ©tection des radionuclĂ©ides artificiels dans les produits laitiers aprĂšs lâaccident de Tchernobyl
DĂšs les premiers jours du mois de mai 1986, le contrĂŽle du niveau de radioactivitĂ© artificielle dans les denrĂ©es alimentaires a Ă©tĂ© intensifiĂ© et de nombreuses mesures dâactivitĂ© ont Ă©tĂ© rĂ©alisĂ©es dans les laboratoires des services vĂ©tĂ©rinaires.
Une Ă©valuation des activitĂ©s en iode a Ă©tĂ© faite Ă partir de 6 prĂ©lĂšvements de lait fournis par chaque dĂ©partement. Les activitĂ©s moyennes en iode 131 sont restĂ©es infĂ©rieures Ă 30 Bq/kg dans toute la moitiĂ© Ouest de la France. Des niveaux plus Ă©levĂ©s ont Ă©tĂ© dĂ©tectĂ©s dans lâEst. Les valeurs maximales enregistrĂ©es ont Ă©tĂ© de 700 Bq/kg dans des Ă©chantillons du Nord-Est de la France, au cours de la premiĂšre semaine du mois de mai 1986. Les valeurs les plus Ă©levĂ©es en cĂ©sium 137 et 134 ont Ă©tĂ© mises en Ă©vidence dans le Sud-Est oĂč des pluies diluviennes sont survenues au dĂ©but du mois de mai. Ces prĂ©cipitations ont entraĂźnĂ© des dĂ©pĂŽts dâisotopes radioactifs plus importants dans ces rĂ©gions. Cependant, une trĂšs grande hĂ©tĂ©rogĂ©nĂ©itĂ© des activitĂ©s a Ă©tĂ© observĂ©e dans des zones trĂšs proches. Les valeurs moyennes ont toujours Ă©tĂ© trĂšs infĂ©rieures aux niveaux de tolĂ©rance maximale fixĂ©es par la CEE. Aucun produit animal ou dâorigine animale nâa Ă©tĂ© retirĂ© Ă la consommation
Origin identification of maritime pine stands in France using chloroplast simple-sequence repeats
Des lots de graines du nord ouest de la péninsule ibérique (Portugal et Galice) ont été introduits dans les années 1950 dans le sud-ouest de la France (Aquitaine), et les peuplements issus de ces graines ont fortement souffert des gelées. Un test variétal basé sur les marqueurs terpéniques fut développé dans les années 1980 afin d'identifier l'origine géographique des peuplements adultes en Aquitaine sur lesquels des graines étaient récoltées puis commercialisées. Dans cet article nous décrivons un nouveau test qui utilise des marqueurs microsatellites chloroplastiques (cpSSRs) et simulations pour identifier l'origine des peuplements. Une étude comparative des tests biochimique (terpÚnes) et cpSSR a été menée sur cinq peuplements adultes. Les résultats obtenus sont identiques, mais le test ADN s'est avéré plus rapide et plus précis. L'utilisation de ce nouveau test devrait permettre de garantir l'origine géographique des peuplements de pin maritime du sud-ouest de la France (Aquitaine).Maritime pine seed-lots from north-western Iberian regions (Portugal and Galicia) were introduced in the 1950s to the south-west of France (Aquitaine region), and the stands they formed suffered considerable frost damage. In the mid 1980s, a biochemical test was developed to test the putative origin of adult stands in Aquitaine, before seeds could be distributed for commercial purposes in France. In this paper, we describe a new test employing chloroplast simple-sequence repeats (cpSSRs) to facilitate identification of stand origin based on randomisation tests. The origin of five stands of unknown origin was determined with both the cpSSR and biochemical (terpene profile analysis) tests. The results from the two tests were concordant, but the DNA-based test gave faster and more accurate results. Use of this test should help when determining the origin of maritime pine stands in the Aquitaine region of France
Geodetic fixing of tide gauge bench marks : technical report
Under the auspices of the International Association for Physical Sciences of the Ocean (IAPSO) a committee was established to
identify the oceanographic and geophysical requirements for fixing Tide Gauge Bench Maries (TGBM's) in an absolute terrestrial
coordinate system; to evaluate the technology for fixing TGBM's; and to malce recommendations to the Commission on Mean Sea Level
and Tides (IAPSO) of a strategy for coordinated global fixing ofTGBM's and for making the results centrally·available.
To meet these goals, the committee met for a several day session at the Woods Hole Oceanographic Institution in November, 1988.
From this workshop came a series of technical conclusions, plus specific recommendations to achieve the goals of the committee.
Included in these discussions were Very Long Baseline Interferometry, Satellite Laser Ranging, Lunar Laser Ranging, Global Positioning
System, and Absolute Gravity Meters, as well as mechanisms for logging and distributing the results from these systems, perhaps via the
Permanent Service for Mean Sea Level (PSMSL) in Britain.Funding was provided by the International Association for Physical Sciences of the Ocean
(IAPSO), the National Oceanic and Atmospheric Administration (NOAA),
and the Sea Grant Program through the Woods Hole Oceanographic Institution