Dislocations in the wave front of acoustic waves propagating through thermal turbulent fields

Abstract

Acoustic waves traversing a turbulent medium develop random changes in phase and amplitude. Here the wavefront distortions of a spherical wave that traverses a medium exhibiting temperature fluctuations are investigated under well controlled laboratory conditions. A heated grid in air is placed horizontally in a large anechoic room and the mixing of the free convected plumes above the grid generates an homogeneous isotropic thermal turbulent field. By varying the frequency of the ultrasonic source and the distance of propagation, all the regimes from weak scattering to strong scattering can be explored. The presentation will concentrate on the measurements of phase variations along a linear array of microphones. Measurements of these phase differences demonstrate the limitation of the Rytov’s method for remote sensing techniques due to the existence of phase jumps between neighboring elements of the array. Numerical simulations based on a wide-angle parabolic equation are used to explain these rapid phase variations in terms of random caustics and phase dislocations.