Detection of coherent vorticity structures using time-scale resolved acoustic spectroscopy

It seems widely accepted by the turbulence community that the intermittency observed in fully turbulent hows is closely related to the existence of intense vorticity events, localized in time and space, also known as coherent structures. We describe here an experimental technique based on the acoustic scattering phenomenon allowing the direct probing of the vorticity field in a turbulent flow. In addition, as in any scattering experiment, the information is in the Fourier domain: the scattered pressure signal is a direct image of the time evolution of a well-specified spatial Fourier mode of the vorticity field. Using time-frequency distributions (TFD), recently introduced in signal analysis theory for the analysis of the scattered acoustic signals, we show how the legibility of these signals is significantly improved (time-resolved spectroscopy). The method is illustrated on data extracted from a highly-turbulent jet how: discrete vorticity events are clearly evidenced. The definition of a generalized time-scale correlation function allows the measurement of the spatial correlation length of these events and reveals a time continuous transfer of energy from the largest scales towards smaller scales (turbulent cascade). We claim that the recourse to TFD leads to an operational definition of coherent structures associated with phase stationarity in the time-frequency plane. (C)1999 Elsevier Science B.V. All rights reserved.