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On the mechanism of the Gent–McWilliams instability of a columnar vortex in stratified rotating fluids
No full textInternational audienceIn stably stratified and rotating fluids, an axisymmetric columnar vortex can be unstable to a special instability with an azimuthal wavenumber m = 1 which bends and slices the vortex into pancake vortices (Gent & McWilliams Geophys. Astrophys. Fluid Dyn., vol. 35 (1-4), 1986, pp. 209-233). This bending instability, called the 'Gent-McWilliams instability' herein, is distinct from the shear, centrifugal or radiative instabilities. The goals of the paper are to better understand the origin and properties of this instability and to explain why it operates only in stratified rotating fluids. Both numerical and asymptotic stability analyses of several velocity profiles have been performed for wide ranges of Froude number Fr-h = Omega(0)/N and Rossby number R-0 = 2 Omega(0)/f, where Omega(0) is the angular velocity on the vortex axis, N the Brunt-Vaisala frequency and f the Coriolis parameter. Numerical analyses restricted to the centrifugally stable range show that the maximum growth rate of the Gent-McWilliams instability increases with R-0 and is independent of Frh for Fr-h 1, the maximum growth rate decreases dramatically with Fr-h. Long axial wavelength asymptotic analyses for isolated vortices prove that the Gent-McWilliams instability is due to the destabilization of the long-wavelength bending mode by a critical layer at the radius r(c) where the angular velocity Omega is equal to the frequency omega : Omega(r(c)) = omega. A necessary and sufficient instability condition valid for long wavelengths, finite Rossby number and Fr-h 0. Such a critical layer r(c) exists for finite Rossby and Froude numbers because the real part of the frequency of the long-wavelength bending mode is positive instead of being negative as in a homogeneous non-rotating fluid (R-0 = Fr-h = infinity). When Fr-h > 1, the instability condition zeta' (r(c)) > 0 is necessary but not sufficient because the destabilizing effect of the critical layer r(c) is strongly reduced by a second stabilizing critical layer r(c2) existing at the radius where the angular velocity is equal to the Brunt-Vaisala frequency. For non-isolated vortices, numerical results show that only finite axial wavenumbers are unstable to the Gent-McWilliams instability
Methane storms as a driver of Titan's dune orientation
No full textInternational audienceThe equatorial regions of Saturn's moon Titan are covered by linear dunes that propagate eastwards 1-3. Global climate models (GCMs), however, predict westward mean surface winds at low latitudes on Titan, similar to the trade winds on Earth 1,4. This apparent contradiction has been attributed to Saturn's gravitational tides 1 , large-scale topography 4 and wind statistics 5 , but none of these hypotheses fully explains the global eastward propagation of dunes in Titan's equatorial band. However, above altitudes of about 5 km, Titan's atmosphere is in eastward super-rotation, suggesting that this momentum may be delivered to the surface. Here we assess the influence of equatorial tropical methane storms-which develop at high altitudes during the equinox-on Titan's dune orientation, using mesoscale simulations of convective methane clouds 6,7 with a GCM wind profile that includes super-rotation 8. We find that these storms produce fast eastward gust fronts above the surface that exceed the normal westward surface winds. These episodic gusts generated by tropical storms are expected to dominate aeolian transport, leading to eastward propagation of dunes. We therefore suggest a coupling between super-rotation, tropical methane storms and dune formation on Titan. This framework, applied to GCM predictions and analogies to some terrestrial dune fields, explains the linear shape, eastward propagation and poleward divergence of Titan's dunes, and implies an equatorial origin of dune sand
Revisiting the identification of generalized Maxwell models from experimental results
No full textInternational audienceLinear viscoelastic material behavior is often modeled using a generalized Maxwell model. The material parameters, i.e. relaxation times and elastic moduli, of the Maxwell elements are determined from either a relaxation or a Dynamical Mechanical Analysis (DMA) experiments. The underlying mathematical problem is known to be ill-posed, which means that uniqueness of the identification is not assured and that small errors in the initial data will conduct to high discrepancies in the identified parameters. The standard technique to remove the ill-posedness is to chose a priori a series of relaxation times and to identify only the moduli. The aim of this paper is to propose two techniques to identify an optimal series of relaxation times. In the case of the relaxation experiment relaxation times will be optimized from the numerical integration of the measured relaxation spectrum. In the case of the DMA experiments we show that mathematical results obtained by Krein and Nudelmann can be used to determine the complete series of relaxation times. The methods are illustrated by identification examples using both artificial and experimental data. The results show that the methods provide a good match of the identified models in term of relaxation or complex moduli
Évolutions de courbes et surfaces pour le traitement d'images.
No full textThe goal of this manuscript was to study several problems which appear in image processing and which involves hypersurfaces of the Euclidian space R^n. Denoising a image basically consists in smoothing its lines. This smoothing can appear either as a minimizer of a suitable functional or results from a regularizing flow on the level sets of the image. In this thesis, we study two examples of these approaches.In the first chapter, we smooth by minimization. More precisely, we work on generalizations of the procedure suggested by Rudin Osher and Fatemi, which penalizes the total variation. We prove that under different assumptions on the domain, on the way to link the image to the data and on the choice of the total variation (isotropic, anisotropic,...), the continuity of the source image is preserved by the minimizing procedure.In Chapter 2, we study Mean curvature flow and add some obstacles which constraint the evolution. We choose the level-set approach: the surface is the preimage of 0 by a function which therefore satisfies a PDE. We prove existence and uniqueness of a (viscosity) solution for this equation. and study its asymptotic in time using comparison with a discrete minimizing scheme.In Chapter 3 (with M. Novaga), we add some information to the result of Chapter 2 by focusing on the geometric formulation of the mean curvature flow with obstacles. We follow the approach by Ecker and Huisken to show that there exists a unique solution of the motion in short times.Finally, in the last chapter (with M. Novaga and P. Pozzi), we make a first step towards the understanding of crystalline motion. Restricted to the planar framework, we show (using an approximation by a smooth motion) that there exists a short time of existence for an anisotropic curvature motion of an immersed curve.L'objectif de cette thèse a été d'étudier différents problèmes apparaissant naturellement en traitement d'images et mettant en jeu des hypersurfaces de l'espace euclidien à n dimensions. Débruiter une image consiste essentiellement à en lisser les lignes. Ce lissage peut apparaître soit comme le résultat d'une minimisation d'une fonctionnelle, soit comme l'application d'un flot régularisant sur les lignes de l'image. Dans ce manuscrit, nous étudions deux exemples de ces deux approches.Dans le chapitre 1, on lisse par minimisation et on s'intéresse à la régularité de la solution. Plus précisément, on travaille sur des généralisations de la minimisation proposée par Rudin, Osher et Fatemi qui pénalise la variation totale. On cherche à montrer que sous différentes hypothèses sur le domaine, les conditions d'attaches aux données ainsi que le choix de la variation totale (isotrope, anisotrope,...), la continuité de l'image observée se transmet forcément au minimiseur, ce qui montre que le débruitage par minimisation ne vas pas faire apparaître de discontinuité non observée.Dans le capitre 2, on étudie le flot par courbure moyenne (éventuellement anisotrope), qui est connu pour avoir un effet régularisant cite{alvarez93}. On y ajoute des obstacles. L'approche choisie est celle des lignes de niveau : la surface est l'image réciproque de 0 par une fonction qu'on fait évoluer. On démontre existence et unicité d'une fonction solution (de viscosité) de l'équation du mouvement par ligne de niveau et on étudie son asymptotique en temps en la comparant à un mouvement minimisant discret.Dans le chapitre 3 (travail en collaboration avec M. Novaga), on précise le résultat du chapitre 2 en étudiant le même problème mais sous forme géométrique (ce qui est nettement plus précis que l'approche ligne de niveau). On suit l'approche de Ecker et Huisken pour montrer qu'il existe une unique solution au mouvement par courbure moyenne avec obstacles en temps court.Enfin, dans le dernier chapitre (travail en collaboration avec M. Novaga et P. Pozzi), on fait un premier pas vers l'étude géométrique du mouvement anisotrope (on pourra en particulier traiter les anisotropies cristallines). Uniquement restreints à la dimension deux, on montre, en l'approchant par un mouvement lisse, l'existence d'un mouvement par courbure anisotrope d'une courbe immergée pour un temps petit
The Ugi Reaction of Cyanoacetic Acid as a Route to Tetramic Acid Derivatives
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Fast, uniform, and compact scalar multiplication for elliptic curves and genus 2 Jacobians with applications to signature schemes
No full textWe give a general framework for uniform, constant-time one-and two-dimensional scalar multiplication algorithms for elliptic curves and Jacobians of genus 2 curves that operate by projecting to the x-line or Kummer surface, where we can exploit faster and more uniform pseudomultiplication, before recovering the proper " signed " output back on the curve or Jacobian. This extends the work of López and Dahab, Okeya and Sakurai, and Brier and Joye to genus 2, and also to two-dimensional scalar multiplication. Our results show that many existing fast pseudomultiplication implementations (hitherto limited to applications in Diffie–Hellman key exchange) can be wrapped with simple and efficient pre-and post-computations to yield competitive full scalar multiplication algorithms, ready for use in more general discrete logarithm-based cryptosystems, including signature schemes. This is especially interesting for genus 2, where Kummer surfaces can outperform comparable elliptic curve systems. As an example, we construct an instance of the Schnorr signature scheme driven by Kummer surface arithmetic
Search for a light charged Higgs boson decaying to c-sbar in pp collisions at sqrt(s) = 8 TeV
No full textsee paper for full list of authorsInternational audienceA search for a light charged Higgs boson, originating from the decay of a top quark and subsequently decaying into a charm quark and a strange antiquark, is presented. The data used in the analysis correspond to an integrated luminosity of 19.7 inverse-femtobarns recorded in proton-proton collisions at sqrt(s) = 8 TeV by the CMS experiment at the LHC. The search is performed in the process t tbar to W+/- b H-/+ bbar, where the W boson decays to a lepton (electron or muon) and a neutrino. The decays lead to a final state comprising an isolated lepton, at least four jets and large missing transverse energy. No significant deviation is observed in the data with respect to the standard model predictions, and model-independent upper limits are set on the branching fraction BF( t to H+ b ), ranging from 1.2 to 6.5% for a charged Higgs boson with mass between 90 and 160 GeV, under the assumption that BF( H+ to c sbar ) = 100%
Cell cycle progression is regulated by intertwined redox oscillators
No full textInternational audienceThe different phases of the eukaryotic cell cycle are exceptionally well-preserved phenomena. DNA decompaction, RNA and protein synthesis (in late G1 phase) followed by DNA replication (in S phase) and lipid synthesis (in G2 phase) occur after resting cells (in G0) are committed to proliferate. The G1 phase of the cell cycle is characterized by an increase in the glycolytic metabolism, sustained by high NAD+/NADH ratio. A transient cytosolic acidification occurs, probably due to lactic acid synthesis or ATP hydrolysis, followed by cytosolic alkalinization. A hyperpolarized transmembrane potential is also observed, as result of sodium/potassium pump (NaK-ATPase) activity. During progression of the cell cycle, the Pentose Phosphate Pathway (PPP) is activated by increased NADP+/NADPH ratio, converting glucose 6-phosphate to nucleotide precursors. Then, nucleic acid synthesis and DNA replication occur in S phase. Along with S phase, unpublished results show a cytosolic acidification, probably the result of glutaminolysis occurring during this phase. In G2 phase there is a decrease in NADPH concentration (used for membrane lipid synthesis) and a cytoplasmic alkalinization occurs. Mitochondria hyperfusion matches the cytosolic acidification at late G1/S transition and then triggers ATP synthesis by oxidative phosphorylation. We hypothesize here that the cytosolic pH may coordinate mitochondrial activity and thus the different redox cycles, which in turn control the cell metabolism
Controlling Redox Status for Stem Cell Survival, Expansion, and Differentiation
No full textInternational audienceReactive oxygen species (ROS) have long been considered as pathological agents inducing apoptosis under adverse culture conditions. However, recent findings have challenged this dogma and physiological levels of ROS are now considered as secondary messengers, mediating numerous cellular functions in stem cells. Stem cells represent important tools for tissue engineering, drug screening, and disease modeling. However, the safe use of stem cells for clinical applications still requires culture improvements to obtain functional cells. With the examples of mesenchymal stem cells (MSCs) and pluripotent stem cells (PSCs), this review investigates the roles of ROS in the maintenance of self-renewal, proliferation, and differentiation of stem cells. In addition, this work highlights that the tight control of stem cell microenvironment, including cell organization, and metabolic and mechanical environments, may be an effective approach to regulate endogenous ROS generation. Taken together, this paper indicates the need for better quantification of ROS towards the accurate control of stem cell fate
Accessing transversity GPDs in neutrino-production of a charmed meson
No full textInternational audienceWe demonstrate that the transversity chiral odd generalized parton distributions can be accessed through the azimuthal dependence of the nu N -> l D N' or nubar N -> l D N' differential cross sections, in the framework of the colinear QCD approach, where GPDs factorize from perturbatively calculablecoefficient functions calculated up to order mc/Q