Collective dynamics in coupled maps on a lattice with quenched disorder

It is investigated how a spatial quenched disorder modifies the dynamics of coupled map lattices. The disorder is introduced via the presence or absence of coupling terms among lattice sites. Two nonlinear maps have been considered embodying two paradigmatic dynamics. The Miller and Huse map can be associated with an Ising-like dynamics, whereas the logistic coupled maps is a prototype of a non trivial collective dynamics. Various indicators quantifying the overall behavior, demonstrates that even a small amount of spatial disorder is capable to alter the dynamics found for purely ordered cases.Comment: 27 pages, 15 figures, accepted for publication in Phys. Rev.

A merging criterion for two-dimensional co-rotating vortices

10 p.We propose a quantitative criterion for the merging of a pair of equal two-dimensional co-rotating vortices. A cross-validation between experimental and theoretical analyses is performed. Experimental vortices are generated by the roll-up of a vortex sheet originating from the identical and impulsive rotation of two plates. The phenomenon is then followed up in time until a rapid pairing transition occurs for which critical parameters are measured. In the theoretical approach, the nonlinear Euler solution representing a pair of equal vortices is computed for various nonuniform vorticity distributions. The stability analysis of such a configuration then provides critical values for the onset of merging. From this data set, a criterion depending on global impulse quantities is extracted for different shapes of the vorticity distribution. This theoretical statement agrees well with our experimentally based criterion

Subcritical instability of liquid metal channel flow in the presence of a spanwise magnetic field

The linear and nonlinear evolution of perturbations is investigated in a magnetohydrodynamic channel flow with electrically insulating walls. The applied magnetic field is parallel to the walls and orthogonal to the stream. Linear optimal perturbations and their maximum amplifications over finite time intervals are computed using a scheme based on the direct and adjoint governing equations. It is shown that dominant optimal perturbations are no more the classical streamwise modes and how the flow is two-dimenzionalized for high enough Hartmann numbers. For fixed Reynolds and Hartmann numbers, direct numerical simulations are applied to investigate how the transition to turbulence is affected by the magnetic field. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/60899/1/4140005_ftp.pd

Interaction vorticité/déformation des monopôles et des dipôles

Par des approches théoriques et numériques, nous généralisons aux vortex non uniformes la relation établie par Moore & Saffman (1971) entre l'ellipticité d'un patch de vorticité et le champ de déformation extérieure qu'il subit. Nous en montrons les implications dans le cadre des monopôles et celui des dipôles

Reconnexion de vortex 3D : simulation et modelisation

Deux tourbillons initialement quasi rectilignes et tournant en contra-rotation se reconnectent de proche en proche pour former des anneaux. Dans cette étude, on simule une reconnexion unique par simulation numérique directe à Re=1500 (basé sur la circulation des vortex). A cette simulation numérique est associé un modèle 2D par lequel les effets 3D, dus à la courbure des vortex, sont pris en compte à travers la superposition d'un simple champ de déformation 3D uniforme instationnaire issu de la DNS. Des différences avec la description de Saffman (1990) sont mises en évidence, en particulier une éjection de vorticité au moment de la reconnexion, qui affaiblit considérablement les deux vortex dans la région de reconnexion

On the three-dimensional temporal spectrum of stretched vortices

The three-dimensional stability problem of a stretched stationary vortex is addressed in this letter. More specifically, we prove that the discrete part of the temporal spectrum is only associated with two-dimensional perturbations.Comment: 4 pages, RevTeX, submitted to PR

Resuspension du sédiment due au déferlement d' ondes solitaires internes

On étudie la resuspension du sédiment en raison du déferlement des ondes internes (OI) sur la pente d'une plage. Nous proposons un modèle non-hydrostatique-Boussinesq dans un système stratifié avec deux stratifications différentes. Nous reproduisons quelques expériences de Michallet et Ivey (1) et Helfrich (2). La concentration de sédiments est modélisée par une équation d'un scalaire passive avec une vitesse de sédimentation ws. Le flux de sédiment est nul sur la surface libre. Sur la pente, on impose une loi phénoménologique qui dépend du cisaillement τb exercé par l'écoulement. L'érosion se produit lorsque le cisaillement τb dépasse un coefficient critique τce. La déposition de sédiments se produit lorsque le cisaillement τb est inférieur au coefficient de cisaillement critique τcd. Des simulations numériques sont effectuées en utilisant le code Gerris (3). Deux cas sont étudiés: lorsque seule l'érosion est prise en compte (ND) et quand l'érosion et la déposition sont toutes deux présentes (D). On discute l'influence des paramètres τce et τcd et on compare la masse totale du sédiment Φ et sa distribution

Determinants of Pulmonary Hypertension in Left Ventricular Dysfunction

AbstractObjectives. This study sought to analyze the determinants of pulmonary hypertension in patients with left ventricular dysfunction.Background. Pulmonary hypertension in patients with left ventricular dysfunction is a predictor of poor outcome. The independent role of cardiac functional abnormalities in the genesis of pulmonary hypertension is unclear.Methods. In 102 consecutive patients with primary left ventricular dysfunction (ejection fraction <50%), systolic pulmonary artery pressure was prospectively measured by Doppler echocardiography (using tricuspid regurgitant velocity), and left ventricular systolic and diastolic function, functional mitral regurgitation, cardiac output and left atrial volume were quantified.Results. Systolic pulmonary artery pressure was elevated in patients with left ventricular dysfunction (51 ± 14 mm Hg [mean ± SD]), but the range was wide (23 to 87 mm Hg). Of the numerous variables correlating significantly with systolic pulmonary artery pressure, the strongest were mitral deceleration time (r = −0.61, p = 0.0001; odds ratio of pulmonary pressure ≥50 mm Hg [95% confidence interval] if <150 ms, 48.8 [14.8 to 161]) and mitral effective regurgitant orifice (r = 0.50, p = 0.0001; odds ratio [95% confidence interval] if ≥20 mm2, 5.9 [2.3 to 15.5]). In multivariate analysis, these two variables were the strongest predictors of systolic pulmonary artery pressure in association with age (p = 0.005). Ejection fraction or end-systolic volume was not an independent predictor of pulmonary artery pressure.Conclusions. Pulmonary hypertension is frequent and highly variable in patients with left ventricular dysfunction. It is not independently related to the degree of left ventricular systolic dysfunction but is strongly associated with diastolic dysfunction (shorter mitral deceleration time) and the degree of functional mitral regurgitation (larger effective regurgitant orifice). These results emphasize the importance of assessing diastolic function and quantifying mitral regurgitation in patients with left ventricular dysfunction.(J Am Coll Cardiol 1997;29:153–9)

Helical vortex systems: linear analysis and nonlinear dynamics

The near wake behind helicopter rotors, wind turbines and more generally behind rotating devices are dominated by helical vortices. Investigating their stability properties is a necessary step to predict their dynamics. Instabilities in such vortex systems have mainly been studied theoretically (Widnall 1972 [1], Okulov [2] and Sørensen 2007 [3]) in an inviscid framework for small core size vortices. The aim of the present study is to generalize these works to the viscous framework for arbitrary core sizes and vorticity profiles.The base flows considered here are helically symmetric: fields are invariant through combined axial translation of distance z and rotation of angle θ= Δz/L around the z-axis, where 2π L denotes the helix pitch.We first perform a linear temporal stability analysis of these base flows, using an Arnoldi [4] procedure coupled to two different codes : (i) a linearised version of the helical DNS code HELIX [5], (ii) another linear code called HELIKZ, which computes the dynamics of arbitrary perturbations in the vicinity of a helically symmetric base flow. These two codes permit the investigation of different types of instability modes: (i) modes having the same helical symmetry as the base flow which generalize the Okulov modes ; (ii) modes depending on z as exp ikz which generalize the Widnall modes. In the first case (i), instabilities are found to be dominated by displacement modes of the type presented in figure 1 for the case of two vortices. In the second case (ii), modes will be compared to those observed in recent experimental work (Leweke et al. 2014 [6]). We then compute the nonlinear dynamics of a basic flow perturbed with a linear mode of type (i) set at a small initial amplitude. In the helical framework, the displacement mode is shown to be responsible for leap-frog dynamics (cf. figure 2) and/or vortex merging (cf. figure 3) with characteristics depending on the various parameters