Acoustic scattering and time-frequency analysis for probing turbulent ows

RECEIVED : Shortened title : Acoustic probing of a vorticity eld. Abstract It is generally known that vorticity is characterized by rapid change and inter-mittency. However, this intermittency has generally been described in terms of probability density functions in a statistical framework. The present report introduces a new methodology which couples an advanced experimental design which utilizes ultrasonic probes to obtain information at diierent scales, with a method of time-frequency analysis which is particularly eeective in this setting. By these means, the discrete events which characterize vorticity can be rather precisely observed in terms of their time of occurence, their lifetime and their Doppler shifts as a function of time. In addition, it is demonstrated that these events exhibit a striking similarity in terms of their time-frequency representations obtained at two closely spaced scales.