The baroclinic forcing of the shear-layer three-dimensional instability

It has been demonstrated that, within the context of variable-density shear flows, the generation-destruction of vorticity by the baroclinic torque may substantially alter the transition dynamics of shear flows. The focus of the present contribution is on baroclinic effects beyond the Boussinesq approximation but uncorrelated to compressibility. The baroclinic torque results from the inertial component of the pressure gradient only. The vorticity evolves within a quasi-solenoidal velocity field without suffering from strong dilatationnal effects that scale with any relevant Mach number. This purely inertial influence of density variations is likely to occur in high Reynolds number mixing of fluids of different densities or in thermal mixing. The vorticity is redistributed to the benefits of the light-side vorticity braid, the other being vorticity depleted in a first stage and feeded with an opposite sign vorticity afterwards, as stressed by Reinaud et al. (1999). These two opposite-sign vorticity sheets are lying around the vanishing primary structure core, still figuring the center of this two-layers system. In three-dimensions the vorticity dynamics is also affected by the vortex stretching mechanism that enable circulation to travel among vorticity components through 3D instability modes. The consequences of the baroclinic redistribution of spanwise vorticity on the development of three-dimensionnal modes is the focus point of the present proposition. The interference with the pairing process and further subharmonics emergence is not yet considered

The structure of a statistically steady turbulent boundary layer near a free-slip surface

The interaction between a free-slip surface with unsheared but sustained turbulence is investigated in a series of direct numerical simulations. By changing (i) the distance between the (plane) source of turbulence and the surface, and (ii) the value of the viscosity, a set of five different data sets has been obtained in which the value of the Reynolds-number varies by a factor of 4. The observed structure of the interaction layer is in agreement with current knowledge, being made of three embedded sublayers: a blockage layer, a slip layer, and a Kolmogorov layer. Practical measures of the different thicknesses are proposed that lead to a new Reynolds-number scaling based on easy-to-evaluate surface quantities. This scaling is consistent with previous proposals but makes easier the comparison between free-surface flows when they differ by the characteristics of the distant turbulent field. Its use will be straightforward in a turbulence-modeling framework

Approximation of the surface impedance for a stratified medium

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Onde guidées par une perturbation du demi-espace élastique homogène

Projet IDENTNous nous intéressons au problème de l'existence et du calcul numérique de modes guidés par la perturbation géométrique locale d'un demi-espace élastique 3D homogène. La géométrie du milieu est infinie et invariante dans la direction de propagation. Ce problème se ramène à un problème linéaire de valeurs propres, posé dans la section 2D transverse du guide. Nous présentons une méthode numérique permettant de calculer les relations de dispersion des modes ainsi que de les représenter. Cette méthode a comme principale caractéristique que le domaine de calcul est limité à la seule perturbation, grâce à l'introduction d'une condition aux limites transparentes à l'interface entre le demi-espace et la perturbation. Nous présentons les résultats numériques obtenus avec différents guides d'ondes étudiés par cette méthode

Etude par simulation directe de l'interaction turbulence/surface plane sans cisaillement en vue de l'analyse des transferts intercomposantes

Cette communication présente une simulation directe de l'interaction turbulence/surface plane sans cisaillement. L'originalité de la configuration étudiée tient au fait que la turbulence est entretenue à distance par un forçage aléatoire localisé au voisinage d'un plan de l'espace parallèle à la surface. L'écoulement est donc statistiquement stationnaire et la couche de surface continûment alimentée par diffusion turbulente. On donne les premiers résultats de simulation obtenus. On discute de la pertinence de ces résultats pour l'interprétation des mécanismes de transfert énergétique intercomposantes au voisinage d'une surface de blocage

Second order absorbing boundary conditions for the wave equation :A solution for the corner problem

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A New nonconforming finite element method for the computation of electromagnetic guided waves (I) mathematical analysis

We are concerned with the problem of computing electromagnetic guided waves in a closed, inomogeneous, cylindrical wave guide. These waves correspond to solutions of an eigenvalue problem and classical methods produce in addition to approximations to the solutions, spurious modes which are particularly troublesome because they correspond to nonzero approximations of the 0 eigenvalue. A nonconforming finite element method for the calculation of guided waves without spurious modes is introduced and analyzed

On a Convolution Operator Arising in a Double Porosity Model

Contaminant transport in a fractured porous medium can be modeled, under appropriate conditions, with a double porosity model. Such a model consists of a parabolic equation with a coupling term describing contaminant exchange between the fractures, which have high permeability, and the matrix block, which has low permeability. Interpreting the coupling term as an operator, pseudo-differential in time, we obtain a method for calculating this term which is rapid in comparison with standard discretization methods

Absorbing boundary conditions for Rayleigh waves

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Baroclinic instabilities

1. Introduction - Illustrative examples from experiments and simulations 2. The baroclinic torque in high Froude number flows, its organization, scale and order of magnitude 3. Stability of the inhomogeneous mixing-layer 4. Transition of the inhomogeneous mixing-layer and the 2D secondary baroclinic instability 5. The strain field of 2D light jets 6. Transition to three-dimensionality in light jets and the question of side-jets 7. Baroclinic instability of heavy vortices and some elements on vortex interaction in inhomogeneous 2D turbulenc