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 in tense vorticity events, localized in time and space, also known as coherent structures. We describe here an experimental technique based on the acoust ic scattering phenomenon allowing the direct probing of the vorticity field in a turbulent flow. In addition, as in any scattering experiment, the inf ormation is in the Fourier domain: the scattered pressure signal is a direc t image of the time evolution of a well-specified spatial Fourier mode of t he vorticity field. Using time-frequency distributions (TFD), recently intr oduced in signal analysis theory for the analysis of the scattered acoustic signals, we show how the legibility of these signals is significantly impr oved (time-resolved spectroscopy). The method is illustrated on data extrac ted 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 an d reveals a time continuous transfer of energy from the largest scales towa rds smaller scales (turbulent cascade). We claim that the recourse to TFD l eads to an operational definition of coherent structures associated with ph ase stationarity in the time-frequency plane. (C)1999 Elsevier Science B.V. All rights reserved.