We report an experimental study of sound propagation in the bulk of a homogeneous flow made of a single intense vortex in air. By using an adaptive-coherent average technique, the mean amplitude and phase of the distorted wave fronts are reconstructed. A large phase shift is observed on the wave fronts on either side of the “shadow” region behind the vortex. The shape of the averaged amplitude, as a function of space coordinates, shows that caustics are also formed in the acoustic field. In a nonstationary flow, these effects make it possible to monitor the vortex strength and position.

• Received 3 April 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.1413