Analysis of Acoustic Wave Propagation in a Thin Moving Fluid

We study the propagation of acoustic waves in a fluid that is contained in a thin two-dimensional tube, and that it is moving with a velocity profile that only depends on the transversal coordinate of the tube. The governing equations are the Galbrun equations, or, equivalently, the linearized Euler equations. We analyze the approximate model that was recently derived by Bonnet-Bendhia, Durufl\'e and Joly to describe the propagation of the acoustic waves in the limit when the width of the tube goes to zero. We study this model for strictly monotonic stable velocity profiles. We prove that the equations of the model of Bonnet-Bendhia, Durufl\'e and Joly are well posed, i.e., that there is a unique global solution, and that the solution depends continuously on the initial data. Moreover, we prove that for smooth profiles the solution grows at most as $t^3$ as $t \to \infty$, and that for piecewise linear profiles it grows at most as $t^4$. This establishes the stability of the model in a weak sense. These results are obtained constructing a quasi-explicit representation of the solution. Our quasi-explicit representation gives a physical interpretation of the propagation of acoustic waves in the fluid and it provides an efficient way to compute numerically the solution.Comment: 35 pages, 4 figure

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

Density fluctuation correlations in free turbulent binary mixing

This paper is devoted to the analysis of the turbulent mass flux and, more generally, of the density fluctuation correlation (d.f.c.) effects in variable-density fluid motion. The situation is restricted to the free turbulent binary mixing of an inhomogeneous round jet discharging into a quiescent atmosphere. Based on conventional (Reynolds) averaging, a ternary regrouping of the correlations occurring in the statistical averaging of the open equations is first introduced. Then an exact algebraic relationship between the d.f.c. terms and the second-order moments is demonstrated. Some consequences of this result on the global behaviour of variable-density jets are analytically discussed. The effects of the d.f.c. terms are shown to give a qualitative explanation of the influence of the ratio of the densities of the inlet jet and ambient fluid on the centerline decay rates of mean velocity and mass fraction, the entrainment rate and the restructuring of the jet. Finally, the sensitivity of second-order modelling to the d.f.c. terms is illustrated and it is suggested that such terms should be considered as independent variables in the closing procedure

The Rayleigh–Taylor instability of two-dimensional high-density vortices

We investigate the stability of variable-density two-dimensional isolated vortices in the frame of incompressible mixing under negligible gravity. The focus on a single vortex flow stands as a first step towards vortex interactions and turbulent mixing. From heuristic arguments developed on a perturbed barotropic vortex, we find that highdensity vortices are subject to a Rayleigh–Taylor instability. The basic mechanism relies on baroclinic vorticity generation when the density gradient is misaligned with the centripetal acceleration field. For Gaussian radial distributions of vorticity and density, the intensity of the baroclinic torque due to isopycnic deformation is shown to increase with the ratio δ/δρ of the vorticity radius to the density radius. Concentration of mass near the vortex core is confirmed to promote the instability by the use of an inviscid linear stability analysis. We measure the amplification rate for the favoured azimuthal wavenumbers m=2, 3 on the whole range of positive density contrasts between the core and the surroundings. The separate influence of the density-contrast and the radius ratio is detailed for modes up to m=6. For growing azimuthal wavenumbers, the two-dimensional structure of the eigenmode concentrates on a ring of narrowing radial extent centred on the radius of maximum density gradient. The instability of the isolated high-density vortex is then explored beyond the linear stage based on high-Reynolds-number numerical simulations for modes m=2,3 and a moderate density contrast Cρ =0.5. Secondary roll-ups are seen to emerge from the nonlinear evolution of the vorticity and density fields. The transition towards m smaller vortices involves vorticity exchange between initially-rotating dense fluid particles and the irrotational less-dense medium. It is shown that baroclinic enstrophy production is associated with the centrifugal mass ejection away from the vortex centre

Mathematical models for dispersive electromagnetic waves: an overview

In this work, we investigate mathematical models for electromagnetic wave propagation in dispersive isotropic media. We emphasize the link between physical requirements and mathematical properties of the models. A particular attention is devoted to the notion of non-dissipativity and passivity. We consider successively the case of so-called local media and general passive media. The models are studied through energy techniques, spectral theory and dispersion analysis of plane waves. For making the article self-contained, we provide in appendix some useful mathematical background.Comment: 46 pages, 16 figure

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

Transparent Boundary Conditions for Wave Propagation in Fractal Trees: Approximation by Local Operators

This work is dedicated to the construction and analysis of high-order transparent boundary conditions for the weighted wave equation on a fractal tree, which models sound propagation inside human lungs. This article follows the works [10, 9], aimed at the analysis and numerical treatment of the model, as well as the construction of low-order and exact discrete boundary conditions. The method suggested in this article is based on the truncation of the meromorphic series that approximate the symbol of the Dirichlet-to-Neumann operator, similarly to the absorbing boundary conditions of B. En-gquist and A. Majda. We analyze its stability, convergence and complexity. The error analysis is largely based on spectral estimates of the underlying weighted Laplacian. Numerical results confirm the efficiency of the method

Approximate Models for Wave Propagation Across Thin Periodic Interfaces

This work deals with the scattering of acoustic waves by a thin ring which contains many regularly-spaced heterogeneties. We provide a complete description of the asymptotic of the solution with respect to the period and the thickness of the heterogeneities. Then, we build a simplified model replacing the thin perforated ring by an effective transmission condition. We pay particular attention to the stabilization of the effective transmission condition. Error estimates and numerical simulations are carried out to validate the accuracy of the model

On the Well-Posedness , Stability And Accuracy Of An Asymptotic Model For Thin Periodic Interfaces In Electromagnetic Scattering Problems

International audienceWe analyze the well-posedness and stability properties of a parameter dependent problem that models the reflection and transmission of electromagnetic waves at a thin and rapidly oscillating interface. The latter is modeled using approximate interface conditions that can be derived using asymptotic expansion of the exact solution with respect to the small parameter (proportional to the periodicity length of oscillations and the width of the interface). The obtained uniform stability results are then used to analyze the accuracy (with respect to the small parameter) of the proposed model

Solving the Homogeneous Isotropic Linear Elastodynamics Equations Using Potentials and Finite Elements. The Case of the Rigid Boundary Condition

International audienceIn this article, elastic wave propagation in a homogeneous isotropic elastic medium with rigid boundary is considered. A method based on the decoupling of pressure and shear waves via the use of scalar potentials is proposed. This method is adapted to a finite elements discretization, which is discussed. A stable, energy preserving numerical scheme is presented, as well as 2D numerical results.Dans cet article, on s'intéresse à la propagation d'ondes élastiques dans un matériau élastique homogène et isotrope avec une condition d'encastrement. On propose une méthode basée sur le découplage des ondes de pression et de cisaillement via l'utilisation de potentiels scalaires. Cette méthode est adaptée à une discrétisation éléments finis, et on présente un schéma stable préservant une énergie discrète et des résultats numériques en 2D