Proper Orthogonal Decomposition (P.O.D.) is a technique used for analysis of vortex structures in a turbulent flow. In this study, complex shear flows are observed by P.I.V. measurements (Particle Image Velocimetry) of a large diameter ratio annular jet. The annular jet is an example of complex shear flow situations. Two axisymmetric shear layers, originating at the jet exit, one at the nozzle lip and the other at the centre body, eventually meet downstream or interact with each other. The main aim of this study is to observe and analyze the effects of active control using acoustic waves on an annular jet with a great diameter ratio (r= 0.91), in order to find a new way to reduce jet instabilities. This contribution discusses the application of Proper Orthogonal Decomposition to the P.I.V. (Particle Image Velocimetry) velocity fields of an annular jet and on a statistic of time resolved tomographic images of the initial zone of the annular jet. Acoustic waves are then applied on the annular jet with different frequencies (fundamental, first harmonic). Measurements are conducted with a Reynolds number ReDo=107800. The fluctuation frequency of the stagnation point is known for this Reynolds number. The Strouhal number corresponding to this frequency is StDo = 0.27. The P.O.D. analysis applied on a natural annular jet and an excited annular jet enables us to see the importance of the triggering of the acoustic wave with the stagnation point motion. An active control is therefore necessary to use acoustic excitation to reduce instabilities in the initial zone of these turbulent jets. Active control has already been used with round jets and has given promising results, but only a few studies have been conducted on annular jets in this field. This work will permit us essentially to have a better knowledge of annular jets and to meet manufacturers' needs. 德國LaVision PIV/PLIF粒子成像測速場儀