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The initial region of subsonic coaxial jets

This paper describes part of a detailed study of the initial region of coaxial jets of three different mean-velocity ratios. Similarity of the mean-velocity and turbulent-intensity profiles within the two mixing regions inside the initial merging zone, and within the mixing region inside the fully-merged zone, has been observed. Similarity with single-jet results has been obtained. Overall pressure measurements and hot-wire and microphone spectra inside coaxial jets yield two pronounced peaks, suggesting the existence of two types of vortices at different frequencies. The higher-frequency or primary vortices are found to be generated in the primary or inner mixing region, and the lower-frequency or secondary ones in the secondary or outer mixing region. It seems that the former are generated further upstream than the latter. Both types of vortices are found in the initial merging zone. However, their growth or decay and their dominance depend on the mean-velocity ratio. Low mean-velocity ratios indicate the dominance of the high-frequency vortices. At high mean-velocity ratios, the low-frequency vortices are dominant. The Strouhal number of the vortices plays an important part in their growth and decay. The complicated flow structure of coaxial jets can be very simply related to, and described by, the much simpler structure of single jets.