Coherent structures in subsonic coaxial jets

This paper describes a detailed study of the initial region of coaxial jets and the large scale coherent structures within this region. The coaxial jets investigated have an area ratio of 2·67 and were run at three different velocity ratios. A simple model in terms of two arrays of vortex-rings in the mixing region of the coaxial jet is proposed, and its use verified by comparisons with experimental results. Overall and spectral results of hot wire and microphone signals, obtained in the flow, support the model proposed. Evidence supporting the concept of a preferred mode, which governs the growth or decay of the vortices in the intermediate zone, is also presented.