Vorticity measurements with an acoustic time-reversal mirror

Abstract

Time-reversal invariance for acoustic propagation is broken when the propagating medium contains a flow. The authors take advantage of the violation of this symmetry to achieve a new way of characterizing the vorticity field of a flow with a double time-reversal mirror (TRM). The double TRM is made up of 2 arrays of piezo-electric transducers placed in front of each other and on either side of the flow. The time-reversal process is performed between the 2 arrays: when 1 array emits, the other 1 receives, time-reverses and re-emits the acoustic wave. In the presence of a flow, the acoustic wavefront is distorted at a crossing through the flow. However this distortion is related to the amplitude and size of the vorticity field of the flow and is often too small to be observed experimentally after 1 crossing. In order to amplify this wavefront distortion, the authors make the acoustic wave do several round trips through the flow between the 2 arrays. The greater number of crossings through the vorticity field induces an amplification of the effect of vorticity on the acoustic wavefront. This effect can then be easily measured and allows the authors to characterize vorticity even for small vorticity fields.