1,319 research outputs found
The Universal Aspect Ratio of Vortices in Rotating Stratified Flows: Theory and Simulation
We derive a relationship for the vortex aspect ratio (vertical
half-thickness over horizontal length scale) for steady and slowly evolving
vortices in rotating stratified fluids, as a function of the Brunt-Vaisala
frequencies within the vortex and in the background fluid outside the
vortex , the Coriolis parameter , and the Rossby number of the
vortex: . This relation is valid for
cyclones and anticyclones in either the cyclostrophic or geostrophic regimes;
it works with vortices in Boussinesq fluids or ideal gases, and the background
density gradient need not be uniform. Our relation for has many
consequences for equilibrium vortices in rotating stratified flows. For
example, cyclones must have ; weak anticyclones (with . We verify our relation for with numerical simulations of
the three-dimensional Boussinesq equations for a wide variety of vortices,
including: vortices that are initially in (dissipationless) equilibrium and
then evolve due to an imposed weak viscous dissipation or density radiation;
anticyclones created by the geostrophic adjustment of a patch of locally mixed
density; cyclones created by fluid suction from a small localised region;
vortices created from the remnants of the violent breakups of columnar
vortices; and weakly non-axisymmetric vortices. The values of the aspect ratios
of our numerically-computed vortices validate our relationship for ,
and generally they differ significantly from the values obtained from the
much-cited conjecture that in quasi-geostrophic vortices.Comment: Submitted to the Journal of Fluid Mechanics. Also see the companion
paper by Aubert et al. "The Universal Aspect Ratio of Vortices in Rotating
Stratified Flows: Experiments and Observations" 201
The Universal Aspect Ratio of Vortices in Rotating Stratifi?ed Flows: Experiments and Observations
We validate a new law for the aspect ratio of vortices in a
rotating, stratified flow, where and are the vertical half-height and
horizontal length scale of the vortices. The aspect ratio depends not only on
the Coriolis parameter f and buoyancy (or Brunt-Vaisala) frequency of
the background flow, but also on the buoyancy frequency within the vortex
and on the Rossby number of the vortex such that . This law for is obeyed precisely by the
exact equilibrium solution of the inviscid Boussinesq equations that we show to
be a useful model of our laboratory vortices. The law is valid for both
cyclones and anticyclones. Our anticyclones are generated by injecting fluid
into a rotating tank filled with linearly-stratified salt water. The vortices
are far from the top and bottom boundaries of the tank, so there is no Ekman
circulation. In one set of experiments, the vortices viscously decay, but as
they do, they continue to obey our law for , which decreases over time.
In a second set of experiments, the vortices are sustained by a slow continuous
injection after they form, so they evolve more slowly and have larger |Ro|, but
they also obey our law for . The law for is not only validated
by our experiments, but is also shown to be consistent with observations of the
aspect ratios of Atlantic meddies and Jupiter's Great Red Spot and Oval BA. The
relationship for is derived and examined numerically in a companion
paper by Hassanzadeh et al. (2012).Comment: Submitted to the Journal of Fluid Mechanics. Also see the companion
paper by Hassanzadeh et al. "The Universal Aspect Ratio of Vortices in
Rotating Stratifi?ed Flows: Theory and Simulation" 201
Dinoflagellate bioluminescence in response to mechanical stimuli in water flows
Bioluminescence of plankton organisms induced by water movements has long been observed and is still under investigations because of its great complexity. In particular, the exact mechanism occurring at the level of the cell has not been yet fully understood. This work is devoted to the study of the bioluminescence of the dinoflagellates plankton species Pyrocystis noctiluca in response to mechanical stimuli generated by water flows. Several experiments were performed with different types of flows in a Couette shearing apparatus. All of them converge to the conclusion that stationary homogeneous laminar shear does not trigger massive bioluminescence, but that acceleration and shear are both necessary to stimulate together an intense bioluminescence response. The distribution of the experimental bioluminescence thresholds is finally calculated from the light emission response for the Pyrocystis noctiluca species
Genomic Organization and Expression of Iron Metabolism Genes in the Emerging Pathogenic Mold
The ubiquitous mold is increasingly recognized as an emerging pathogen, especially among patients with underlying disorders such as immunodeficiency or cystic fibrosis (CF). Indeed, it ranks the second among the filamentous fungi colonizing the respiratory tract of CF patients. However, our knowledge about virulence factors of this fungus is still limited. The role of iron-uptake systems may be critical for establishment of infections, notably in the iron-rich environment of the CF lung. Two main strategies are employed by fungi to efficiently acquire iron from their host or from their ecological niche: siderophore production and reductive iron assimilation (RIA) systems. The aim of this study was to assess the existence of orthologous genes involved in iron metabolism in the recently sequenced genome of . At first, a tBLASTn analysis using iron-related proteins as query revealed orthologs of almost all relevant loci in the genome. Whereas the genes putatively involved in RIA were randomly distributed, siderophore biosynthesis and transport genes were organized in two clusters, each containing a non-ribosomal peptide synthetase (NRPS) whose orthologs in have been described to catalyze hydroxamate siderophore synthesis. Nevertheless, comparative genomic analysis of siderophore-related clusters showed greater similarity between and phylogenetically close molds than with species. The expression level of these genes was then evaluated by exposing conidia to iron starvation and iron excess. The expression of several orthologs of genes involved in siderophore-based iron uptake or RIA was significantly induced during iron starvation, and conversely repressed in iron excess conditions. Altogether, these results indicate that possesses the genetic information required for efficient and competitive iron uptake. They also suggest an important role of the siderophore production system in iron uptake by
Experimental and theoretical study of the elliptic instability in a rotating stratified flow
The combined effects of Coriolis force and buoyancy effects on the dynamics of a weakly elliptical bounded vortex are treated theoretically as well as experimentally. As predicted theoretically, stratification and rotation have antagonist contributions to the stability of an elliptical vortex. Thus if the stratification is strong enough (Nb>Omega_c, Nb and Omega_c being respectively the Brunt-Väisälä frequency and the rotation rate of the flow in a frame rotating with the elliptical deformation at angular velocity Omega_t), we have observed that only anticyclones (such that |Wa|<Omega_c with Wa=2(Omega_c+Omega_t)) are unstable, whereas the cyclones are always stable. In addition if the stratification is weak, instability areas over change. These instability thresholds found theoretically have been observed experimentally with a good accuracy and the measured growth rate are in a good agreement with those predicted by a linear stability analysis in the limit of small deformation
Magnetic field induced by elliiptical instability in a rotating tidally distorded sphere
It is usually believed that the geo-dynamo of the Earth or more generally of
other planets, is created by the convective fluid motions inside their molten
cores. An alternative to this thermal or compositional convection can however
be found in the inertial waves resonances generated by the eventual precession
of these planets or by the possible tidal distorsions of their liquid cores. We
will review in this paper some of our experimental works devoted to the
elliptical instability and present some new results when the experimental fluid
is a liquid metal. We show in particular that an imposed magnetic field is
distorted by the spin- over mode generated by the elliptical instability. In
our experiment, the field is weak (20 Gauss) and the Lorenz force is negligible
compared to the inertial forces, therefore the magnetic field does not modify
the fluid flow and the pure hydrodynamics growth rates of the instability are
recovered through magnetic measurements
Forme et persistance de tourbillons lenticulaires dans les écoulements stratifiés tournants : du laboratoire à la Tâche Rouge de Jupiter !
La Grande Tâche Rouge de Jupiter et les Meddies de l'océan Atlantique sont les exemples les plus connus de tourbillons lenticulaires existant dans les écoulements stratifiés tournants. Grâce à l'équilibre des différentes forces agissant sur le fluide à l'intérieur du tourbillon, il est possible de comprendre la persistance de ces tourbillons et de prédire le rapport d'aspect vertical de ces anticyclones. Nos expériences montrent que cette loi d'échelle est respectée par ces tourbillons depuis l'échelle du laboratoire jusqu'à la Tâche Rouge de Jupiter
Chlorine-bearing molecules in molecular absorbers at intermediate redshifts
We use observations of chlorine-bearing species in molecular absorbers at intermediate redshifts to investigate chemical properties and Cl-35/Cl-37 isotopic ratios in the absorbing sightlines. Chloronium (H2Cl+) is detected along three independent lines of sight in the z = 0.89 and z = 0.68 molecular absorbers located in front of the lensed quasars PKS 1830-211 and B 0218+357, respectively. Hydrogen chloride (HCl) was observed only toward PKS 1830-211, and is found to behave differently from H2Cl+. It is detected in one line of sight with an abundance ratio [H2Cl+]/[HCl] similar to 1, but remains undetected in the other, more diffuse, line of sight, with a ratio [H2Cl+] / [HCl] > 17. The absorption profiles of these two chlorine-bearing species are compared to other species and discussed in terms of the physical properties of the absorbing gas. Our findings are consistent with the picture emerging from chemical models where different species trace gas with different molecular hydrogen fraction. The Cl-35/Cl-37 isotopic ratios are measured in the different lines of sight and are discussed in terms of stellar nucleosynthesis
A new approach towards ferromagnetic conducting materials based on TTF-containing polynuclear complexes
International audienceFive complexes containing binuclear cation [Cu2(LH)2]2+ (LH2 = 1 : 2 Schiff base of 1,3-diaminobenzene and butanedione monoxime) were prepared and characterized. Metathesis of one perchlorate anion in [Cu2(LH)2(H2O)2](ClO4)2 (1) by anionic TTF-carboxylate (TTF-CO2−) leads to the complex [Cu2(LH)2(CH3OH)2](TTF-CO2)(ClO4)*H2O (2). Reactions of 1 with substituted pyridines bipy, dpe and TTF-CH = CH-py result in formation of the complexes {[Cu2(LH)2(bipy)](ClO4)2}n*2nH2O (3), [Cu2(LH)2(dpe)2](ClO4)2*2CH3OH (4) and [Cu2(LH)2(TTF-CH = CH-py)(H2O)](ClO4)2*1.5H2O (5), where bipy = 4,4′-bipyridine, dpe = trans-(4-pyridyl)-1,2-ethylene and TTF-CH = CH-py = 1-(2-tetrathiafulvalenyl)-2-(4-pyridyl)ethylene. Whereas complex 2 is built from discrete ionic particles (with rather long Cu-S contacts), compounds 1 and 3 contain 1D polymeric chains, in which structural units are bonded through Cu-O bonds or through bridging bipy molecule, respectively. Dinuclear complexes 4 and 5 are linked though π-stacking of dpe or TTF-CH = CH-py, respectively. All complexes are characterized by dominating ferromagnetic behavior with J values in the range from +9.92(8) cm−1 to +13.4(2) cm−1 for Hamiltonian H = -JS1S2. Magnetic properties of the compounds, containing stacks of aromatic molecules in crystal structures (4 and 5), correspond to ferromagnetic intradimer and antiferromagnetic intermolecular interactions (zJ′ = −0.158(3) and −0.290(2) cm−1, respectively). It was found that TTF-CH = CH-py ligand in [Cu2(LH)2(TTF-CH = CH-py)(H2O)]2+ could be electrochemically oxidized to cation-radical form in the solution
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