206 research outputs found
On the decrease of intermittency in decaying rotating turbulence
The scaling of the longitudinal velocity structure functions, , is analyzed up to order in a
decaying rotating turbulence experiment from a large Particle Image Velocimetry
(PIV) dataset. The exponent of the second-order structure function, ,
increases throughout the self-similar decay regime, up to the Ekman time scale.
The normalized higher-order exponents, , are close to those
of the intermittent non-rotating case at small times, but show a marked
departure at larger times, on a time scale ( is the
rotation rate), although a strictly non-intermittent linear law is not reached.Comment: 5 pages, 5 figures. In revision for Phys. Fluids Letter
Inertial waves and modes excited by the libration of a rotating cube
We report experimental measurements of the flow in a cubic container
submitted to a longitudinal libration, i.e. a rotation modulated in time.
Velocity fields in a vertical and a horizontal plane are measured in the
librating frame using a corotating particle image velocimetry system. When the
libration frequency is smaller than twice the mean rotation rate
, inertial waves can propagate in the interior of the fluid. At
arbitrary excitation frequencies , the oscillating flow
shows two contributions: (i) a basic flow induced by the libration motion, and
(ii) inertial wave beams propagating obliquely upward and downward from the
horizontal edges of the cube. In addition to these two contributions, inertial
modes may also be excited at some specific resonant frequencies. We
characterize in particular the resonance of the mode of lowest order compatible
with the symmetries of the forcing, noted [2,1,+]. By comparing the measured
flow fields to the expected inviscid inertial modes computed numerically
[L.R.M. Maas, Fluid Dyn. Res. \textbf{33}, 373 (2003)], we show that only a
subset of inertial modes, matching the symmetries of the forcing, can be
excited by the libration.Comment: Phys. Fluids (in press
An inertial range length scale in structure functions
It is shown using experimental and numerical data that within the traditional
inertial subrange defined by where the third order structure function is linear
that the higher order structure function scaling exponents for longitudinal and
transverse structure functions converge only over larger scales, , where
has scaling intermediate between and as a function of
. Below these scales, scaling exponents cannot be determined for any
of the structure functions without resorting to procedures such as extended
self-similarity (ESS). With ESS, different longitudinal and transverse higher
order exponents are obtained that are consistent with earlier results. The
relationship of these statistics to derivative and pressure statistics, to
turbulent structures and to length scales is discussed.Comment: 25 pages, 9 figure
Molecular solids of actinide hexacyanoferrate: Structure and bonding
The hexacyanometallate family is well known in transition metal chemistry because the remarkable electronic delocalization along the metal-cyano-metal bond can be tuned in order to design systems that undergo a reversible and controlled change of their physical properties. We have been working for few years on the description of the molecular and electronic structure of materials formed with [Fe(CN)6]n- building blocks and actinide ions (An = Th, U, Np, Pu, Am) and have compared these new materials to those obtained with lanthanide cations at oxidation state
+III. In order to evaluate the influence of the actinide coordination polyhedron on the three- dimensional molecular structure, both atomic number and formal oxidation state have been varied : oxidation states +III, +IV. EXAFS at both iron K edge and actinide LIII edge is the dedicated structural probe to obtain structural information on these systems. Data at both edges have been combined to obtain a three-dimensional model. In addition, qualitative electronic information has been gathered with two spectroscopic tools : UV-Near IR spectrophotometry and low energy XANES data that can probe each atom of the structural unit : Fe, C, N and An. Coupling these spectroscopic tools to theoretical calculations will lead in the future to a better description of bonding in these molecular solids. Of primary interest is the actinide cation ability to form ionic – covalent bonding as 5f orbitals are being filled by modification of oxidation state and/or atomic number
Etude du comportement paramagnétique des actinides (IV) (Th, U, Np et Pu) en solution en présence de ligands
International audienceLa spectroscopie RMN est une technique capable de fournir des informations structurales sur des complexes métalliques en solution. Elle peut être appliquée à l’analyse de petites molécules en chimie organique, de protéines (macros molécules) en biologie mais aussi en chimie nucléaire avec la nucléarisation des spectromètres [1]. La présence d’un Actinide (An), avec des électrons 5f non appariés, génère une modification du spectre RMN (un élargissement et/ou une variation du déplacement chimique des pics) caractéristique du paramagnétisme du cation.Dans le cas des complexes paramagnétiques de Ln (éléments 4), le déplacement chimique total est modélisé par l’équation de Bleaney (équations (a) et (b)) [2]. Il dépend du déplacement chimique paramagnétique induit qui peut être décomposé en deux contributions : un terme de contact (délocalisation de l’électron sur l’atome donneur du ligand) et un terme dipolaire (interaction entre le spin électronique et nucléaire à travers l’espace). A partir de ce dernier terme, on peut déduire des informations structurales en utilisant les constantes de Bleaney
Management des services dans l’entreprise touristique mondiale en réseau
Ce rapport rédigé en 2012 s\u27intitule Management des services dans l\u27entreprise touristique mondiale en réseau. Il a été rédigé par plusieurs enseignants chercheurs de l\u27UFR ESTHUA.
Comparing the statistics of interstellar turbulence in simulations and observations: Solenoidal versus compressive turbulence forcing
We study two limiting cases of turbulence forcing in numerical experiments:
solenoidal (divergence-free) forcing, and compressive (curl-free) forcing, and
compare our results to observations reported in the literature. We solve the
equations of hydrodynamics on grids with up to 1024^3 cells for purely
solenoidal and purely compressive forcing. Eleven lower-resolution models with
mixtures of both forcings are also analysed. We find velocity dispersion--size
relations consistent with observations and independent numerical simulations,
irrespective of the type of forcing. However, compressive forcing yields
stronger turbulent compression at the same RMS Mach number than solenoidal
forcing, resulting in a three times larger standard deviation of volumetric and
column density probability distributions (PDFs). We conclude that the strong
dependence of the density PDF on the type of forcing must be taken into account
in any theory using the PDF to predict properties of star formation. We supply
a quantitative description of this dependence. We find that different observed
regions show evidence of different mixtures of compressive and solenoidal
forcing, with more compressive forcing occurring primarily in swept-up shells.Comment: 28 pages, 20 figures, published as Highlight Paper in A&A, 512, A81
(2010); simulation movies available at
http://www.ita.uni-heidelberg.de/~chfeder/videos.shtml?lang=e
Dynamics of Vulmar/VulMITE group of transposable elements in Chenopodiaceae subfamily Betoideae
Transposable elements are important factors driving plant genome evolution. Upon their mobilization, novel insertion polymorphisms are being created. We investigated differences in copy number and insertion polymorphism of a group of Mariner-like transposable elements Vulmar and related VulMITE miniature inverted-repeat transposable elements (MITEs) in species representing subfamily Betoideae. Insertion sites of these elements were identified using a modified transposon display protocol, allowing amplification of longer fragments representing regions flanking insertion sites. Subsequently, a subset of TD fragments was converted into insertion site-based polymorphism (ISBP) markers. The investigated group of transposable elements was the most abundant in accessions representing the section Beta, showing intraspecific insertion polymorphisms likely resulting from their recent activity. In contrast, no unique insertions were observed for species of the genus Beta section Corollinae, while a set of section-specific insertions was observed in the genus Patellifolia, however, only two of them were polymorphic between P. procumbens and P. webbiana. We hypothesize that Vulmar and VulMITE elements were inactivated in the section Corollinae, while they remained active in the section Beta and the genus Patellifolia. The ISBP markers generally confirmed the insertion patterns observed with TD markers, including presence of distinct subsets of TE insertions specific to Beta and Patellifolia
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