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La propagation acoustique des ondes de souffle est affectée par la topographie, la végétation et les conditions météorologiques. Ces effets comportent une part importante d'incertitudes du fait de la méconnaissance et de la nature aléatoire du milieu de propagation. Afin de quantifier ces incertitudes, des enregistrements acoustiques ont été réalisés autour d'un site pyrotechnique industriel jusqu'à 20 km pour environ 600 explosions d'énergies comprises entre 300 et 500 kg d'équivalent TNT. Ces mesures sont classifiées en fonction des conditions de propagation puis comparées à des simulations numériques. Les incertitudes associées à la source explosive sont analysées par des méthodes de clustering. Les incertitudes dues à la méconnaissance des grandes échelles de l'atmosphère sont analysées à l'aide d'un modèle de propagation unidirectionnel couplé avec des polynômes de chaos. Les résultats préliminaires montrent que l'incertitude associée à l'atmosphère est dominante et que son amplitude est en partie retrouvée par les modèles.

La Plateforme numérique du Laboratoire d'Étude et de Modélisation en Acoustique (PLetma) regroupe les codes de calcul scientifique développés par les institutions partenaires du LETMA depuis 2015. Elle contient quatre codes différents: tracé de rayons, méthode unidirectionnelle non-linéaire, modes normaux et méthode Navier-Stokes directe. L'objectif de la plateforme est de pourvoir un ensemble d'outils complémentaires pour la modélisation de signaux infrasonores large bande dans une atmosphère inhomogène et en mouvement. Elle permet de couvrir un large éventail de problèmes de propagation basse fréquence issus de la géophysique ou de l'ingénierie. Implémentée en MPI et OpenMP, la plateforme tire profit du calcul parallèle pour réduire le coût des simulations de grande taille. PLetma donne une infrastructure pour l'inclusion d'outils de modélisation de la propagation et de la variabilité du problème. Des comparaisons ont été effectuées entre les différentes méthodes sur un cas test (Sabatini et al., JASA 2016). Des analyses de sensibilité ont été effectuées pour comprendre l'effet des paramètres environnementaux sur la propagation des infrasons sur de longues distances. [Ce travail a été effectué dans le cadre du LETMA (Laboratoire ETudes et Modélisation Acoustique), un laboratoire de recherche contractuel entre le CEA, le CNRS, l'École Centrale de Lyon, C-Innov, et Sorbonne Université.]

International audience; In application of the Comprehensive nuclear-Test-Ban Treaty (CTBT), micro-barometric stations of the International Monitoring System (IMS) network detect various powerful natural and artificial infrasound sources. Sources such as explosions and meteorites produce similar multi-arrival pressure signatures. In order to distinguish these sources from one another, it is necessary to model the long range propagation of infrasound. Operational three dimensional method is ray tracing. However, in relatively frequent cases, micro-barometric stations are located into the geometrical shadow zone where only diffracted waves are recorded. To simulate infrasound propagation in this zone, the ray tracing method is generalized to complex ray theory. The sources, the media and the ground parameters are extended to complex values using analytic continuation. Two analytical cases are used to validate this method. The first one concerns the diffraction at a fold caustic in the vicinity of the ground. The second one concerns creeping waves at the ground. The analytical solutions of these cases show that complex ray theory is able to obtain infrasound wave arrival times, azimuths and apparent velocities in shadow zones. Our documented case study is the Carancas meteorite which has impacted the ground in Peru on September 15th, 2007. Comparisons between Carancas records and simulations of its sonic boom are thus performed.

International audience; Outdoor blast wave propagation is affected by numerous physical parameters such as the source energy, the topography, the ground nature, or the atmospheric conditions. Part of these effects are deterministic and well explained using numerical simulation. A wide angle parabolic equation code with Arome meteorological data and SRTM topography is used to model waveform signatures up to 20km for 500kg explosion. These waveform signatures are uncertain due to random nature of the media not included in the model. To quantify this uncertainty and validate the model, comparisons between simulations and measurements are performed for numerous blast wave records of well calibrated ground explosions. A metamodel of the maximum overpressure is proposed.

Colloque avec actes et comité de lecture. Internationale.; International audience; Des simulations numériques directes des équations de Navier-Stokes bidimensionnelles sont effectuées pour déterminer le champ de pression produit par une source infrasonore dans une atmosphère réaliste. L'algorithme numérique repose sur une méthode aux différences finies dans le domaine temporel développée pour des applications en aéroacoustique. La distorsion de la signature acoustique due aux non linéarités ainsi que les effets de dissipation visqueuse sont analysés et discutés.

International audience; Using ray theory, long range propagation of infrasound through the atmosphere is modeled in the framework of the Comprehensive Nuclear-Test-Ban Treaty. In atmospheric propagation, the high frequency hypothesis is based on the assumption that space and time scales of atmospheric properties are much larger than acoustic wave scales. A 3D nonlinear ray tracing code is developed to compute the temporal pressure signature at receivers. These signatures are obtained by solving a generalized Burgers' equation along each ray taking into account nonlinear effects, shear and bulk viscosity absorption and molecular vibrational relaxation mechanisms. The propagation of infrasound emitted by a motionless point source in a realistic atmosphere is analysed. To quantify the validity limits of our approach, we investigate effects of the wind, atmospheric absorption, nonlinearities, refraction and scattering by small atmospheric scales on observed phase kinds, their travel time and their waveform. Nonlinear mechanisms are important to model the evolution of waveform signatures especially to find the N-wave and U-wave shapes of, respectively, thermospheric and stratospheric paths. The relative importance of nonlinear effects are compared for several source energies. Sound exposure level and characteristic frequency are analysed in order to develop scaling laws. Comparisons are made with measurements and results available in the literature.

International audience; In the framework of the Comprehensive Nuclear-Test-Ban Treaty, long range propagation of infrasound through the atmosphere is investigated using numerical models. The study problem is a cylindrical blast in an inhomogeneous atmosphere. Vertical profiles of temperature, wind and density obtained during the Misty Picture High explosive experiment conducted in May 1987 are used. The propagation is modeled using two codes. First, the full 2-D Navier-Stokes equations are solved with a low-dispersion and low-dissipation finite-difference algorithm initially developed for aeroacoustics. Second, a nonlinear ray tracing model based on weak shock and locally plane wave assumptions is used. In this code, the waveform evolves along rays following Burgers' equation. The initialisation of the cylindrical blast wave is described for both codes. A benchmark case of propagation is defined and numerical comparisons are performed. Absorption and nonlinear effects are investigated in detail for both stratospheric and thermospheric paths. Some limits of the ray tracing model are shown and quantified.

National audience; Using ray theory, long range propagation of infrasound through the atmosphere is modeled in the framework of the Comprehensive Nuclear-Test-Ban Treaty. In atmospheric propagation, the high frequency hypothesis is based on the assumption that space and time scales of atmospheric properties (temperature, wind, density) are much larger than acoustic wave scales. An operational 3D nonlinear ray tracing code is developed to compute the temporal pressure signature at receivers. The global pressure signature at the receiver is the sum of eigenray contributions that link the source to the receiver. They are obtained by solving a generalized Burgers equation along each eigenray taking into account nonlinear effects, shear and bulk viscosity absorption and molecular vibrational relaxation mechanisms. This equation is solved using a Fourier Galerkin spectral scheme. Specific developments are performed to pass through caustics and take into account ground reflection. The propagation of infrasound emitted by a motionless point source in a realistic atmosphere will illustrate the analysis. To quantify the validity limits of our approach we investigate effects of the wind, atmospheric absorption, nonlinearities, refraction and scattering by small atmospheric scales on observed phase kinds, their travel time and their waveform. To estimate the nonlinearity effects relative to the linear dissipative effects we evaluate the Gol'dberg number. We note that nonlinear mechanisms are quite important to model the evolution of infrasonic waveform signatures. The 'N' and the 'U' measured waveform shape of, respectively, thermospheric and stratospheric paths are associated with nonlinear mechanisms. Nonlinearities are weak but the development of nonlinear models is necessary in order to characterize the source yield. Comparisons will be made with results available in the literature and recent numerical simulations based on Navier-Stokes equations.