Etude numérique de la focalisation aléatoire d'une onde se propageant dans une turbulence thermique 2D

We consider the propagation of acoustic waves, in either the geometric or the parabolic approximation, through an ensemble of realizations of a random field modeling a 2D homogeneous isotropic thermal turbulence. Our results for a plane wave demonstrate that the occurrence of caustics as well as the maximum peak in the scintillation index are governed by the same universal parameter related to the correlation function of the temperature fluctuations

Intensity fluctuations of spherical acoustic waves propagating through thermal turbulence*

International audienc

Influence d'une turbulence thermique sur la propagation du son dans l'atmosphère en présence d'une zone d'ombre

A numerical technique for simulating the acoustic wave propagation through a turbulent atmosphere is introduced. We first generate a random homogeneous and isotropic medium by creating N independent realizations in terms of discrete Fourier modes. Then, for each realization, we solve a parabolic equation using a Split step Padé algorithm. Finally, we calculate the mean intensity of the acoustic field and its phase and amplitude fluctuations. Our numerical results are compared with outdoor measurements

Etude expérimentale de l'influence de la turbulence sur une methode d'imagerie haute resolution

On présente les résultats d'une expérience réalisée en laboratoire et simulant la propagation des ondes acoustiques dans l'atmosphère. Nous mettons en évidence l'influence d'une turbulence cinématique bien contrôlée sur la localisation de sources ponctuelles. Cette localisation est effectuée par une méthode d'imagerie haute-résolution fondée sur le traitement de la matrice interspectrale. Le caractère aléatoire du milieu dégrade les performances de la méthode, mais dans des conditions proches de celles rencontrées en propagation aérienne il est encore possible de distinguer deux sources écartées de 1° avec une antenne d'envergure 14 longueurs d'onde. L'analyse expérimentale est complétée par une étude numérique développée à partir de l'approximation parabolique de l'équation stochastique de Helmholtz. La modélisation du spectre des fluctuations d'indice du milieu par une forme de Von Karman permet de prédire de façon correcte la décorrélation spatiale le long de l'antenne de réception, et ainsi d'estimer la matrice interspectrale. Avec ce modèle nous avons étudié de façon systématique l'influence de la vitesse moyenne sur le problème type de la localisation de 2 sources avec une antenne de 3 capteurs

Occurrence of caustics for high-frequency acoustic waves propagating through turbulent fields

International audienc

RECONSTRUCTION DE FRONTS D'ONDE DISTORDUS PAR LA TURBULENCE

The wave front distortions of a spherical wave that propagates through a turbulent thermal field are investigated under a variety of propagation conditions. Our measurements of phase difference demonstrate the limitation of the Rytov's method and point out some difficulties related to the occurrence of random caustics in the saturation region

Analysis of the dissipation and dispersion properties of the multi-domain Chebyshev pseudospectral method

The accuracy of the multi-domain Chebyshev pseudospectral method is investigated for wave propagation problems by examining the properties of the method in the wavenumber space theoretically in terms of dispersion and dissipation errors. For a number of (N+1) points in the subdomains used in the literature, with N typically between 8 to 32, significant errors can be obtained for waves discretized by more than p points per wavelength. The dispersion and dissipation errors determined from the analysis in the wavenumber space are found to be in good agreement with those obtained in test cases. Accuracy limits based on arbitrary criteria are proposed, yielding minimum resolutions of 7.7, 5.2 and 4.0 points per wavelength for N=8, 16 and 32 respectively. The numerical efficiency of the method is estimated, showing that it is preferable to choose N between 16 and 32 in practice. The stability of the method is also assessed using the standard fourth-order Runge–Kutta algorithm. Finally, 1-D and 2-D problems involving long-range wave propagation are solved to illustrate the dissipation and dispersion errors for short waves. The error anisotropy is studied in the 2-D case, in particular for a hybrid Fourier–Chebyshev configuration

Simulation of the propagation of an acoustic wave through a turbulent velocity field: A study of phase variance

International audienc

"Estimating long-term representative spl in complex environments using a coupling method based on acoustic (mw-wape) and micrometeorological (submeso) numerical predictions "

International audienc