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Modélisation par une approche temporelle de la propagation acoustique en milieu extérieur : traitement de frontières complexes et validation sur site ferroviaire

Abstract : In this work, a numerical model to treat outdoor sound propagation in the time domain is proposed. In the context of railway noise, extended acoustic sources in motion have to be considered. The typical frequency band of interest goes up to 8000 Hz. Finite-Difference Time-Domain(FDTD) methods which are used to solve the linearized Euler equations are then well-adapted tothe problem. To do so, finite-difference techniques developed in the computational aeroacoustics community are employed. Meteorological effects (mean wind and temperature profiles) as well as ground effects (impedance and topography) are taken into account. In the first chapter, finite-difference techniques and the time-domain impedance boundary condition, based on a recursive convolution, are presented in the general case of the tridimensional problem. Propagation of acoustic waves in a stratified atmosphere over an impedance ground is then considered. In homogeneous conditions, waveforms are compared to those obtained with an analytical solution. In downward-refracting conditions, arrival times of the different contributions are analysed using a geometrical acoustic approach. In both cases, presence of surface waves is highlighted. At last, in a first analysis, effects of a non-compact source in rectilinear motion on the acoustic pressure field are studied. In the second chapter, the coupling between near-field calculations and far-field calculations is considered. Indeed, the FDTD model currently needs large computational time and memory to handle large propagation distances. A coupling strategy using parabolic equation methods can then be employed. A split-step Padé method is used in order to choose the angular validity of the parabolic approximation. A study on the starting field adapted to the order of the Padé approximant is proposed. The third chapter deals with the modeling of the topography in the FDTD solver. Curvilinear coordinates are introduced, and numerical resolution is similar to the cartesian case. Several applications are proposed. Propagation over a cylinder is first studied ; presence of surface waves is highlighted. Then, influence of the topography of a railway site on the measure of sound pressure levels is analysed. In near-field, significant differences are obtained at low frequencies. In far-field, the results depend strongly on the meteorological conditions. At last, comparisons of sound pressure levels calculated with the propagation model and measured during an experimental campaign carried out in october 2001 in Saint-Berthevin are realised. In a last study, the propagation model is validated thanks to measurements performed in may2010 on a railway site in La Veuve. Measurement of the topography, of the surface impedances and of meteorological parameters have been realised. Sound pressure levels and waveforms computed with the propagation model are in close agreement with those obtained experimentally in the case of a pulse source.
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Didier Dragna. Modélisation par une approche temporelle de la propagation acoustique en milieu extérieur : traitement de frontières complexes et validation sur site ferroviaire. Autre. Ecole Centrale de Lyon, 2011. Français. ⟨NNT : 2011ECDL0035⟩. ⟨tel-00687519⟩

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