Propagation des ondes acoustiques à travers un milieu turbulent (Etudes théorique et expériementale des effets de diffraction et des effets non linéaire)

Propagation of nonlinear acoustic signals in randomly inhomogeneous moving media is one of the most important problems in many modern applications of theoretical and experimental acoustics. For example, a detailed knowledge of the acoustic field structure is necessary in order to predict peak positive pressure levels caused by supersonic flights near the ground surface. Up to date, distortions of the acoustic wave in media with turbulent flows have been studied only in the linear parabolic approximation or in the approximation of nonlinear geometrical acoustics. In this context, investigation of nonlinear acoustic fields taking into account wave diffraction effect and influence of both longitudinal and transverse components of medium motion is of great importance. In this work, propagation of nonlinear acoustic signals in randomly inhomogeneous moving media is studied numerically and experimentally. The nonlinear evolution equation of Khokhlov-Zabolotskaya-Kuznetsov type, which accounts for both longitudinal and transverse to the wave propagation direction components of the inhomogeneous velocity field, is derived. An effective numerical algorithm is developed to simulate the propagation of acoustic shock waves with narrow fronts in inhomogeneous media. Advantages of the derived parabolic equation compared to the geometrical acoustics approximation are shown based on numerical modeling. It is shown that the characteristic structure of the acoustic field in turbulent media is mainly determined by the longitudinal component of the velocity field. However, under certain conditions, the transverse velocity fluctuations lead to essential distortions of the acoustic field. The influence of nonlinear effects on acoustic wave random focusing in turbulent medium is also studied. The laboratory scale experimental setup is designed to investigate the multiple focusing effects on the intense N-wave propagation in turbulent flow. Statistical distributions of acoustic wave parameters are measured up to distances longer than the distance of first caustic occurrence, determined by the outer turbulence scale. In order to interpret correctly the distortion introduced by the measuring system, a method of wide band high frequency microphone calibration based on nonlinear lengthening of the acoustic pulse in absorptive media is developed. It is shown that in turbulent medium, acoustic wave mean peak positive pressure decreases and mean rise time increases faster than in homogeneous air. However, in turbulent medium acoustic pressure amplitudes, 3-4 times higher than that measured in homogeneous air, are observed. Results of numerical modeling appear to be in a good agreement with the experimental data that confirms the validity of the developed theoretical model.LYON-Ecole Centrale (690812301) / SudocSudocFranceF