Instantaneous, quantitative, planar images of molecularly mixed-jet fluid fraction were obtained for the purpose of studying the mixing transition in a gaseous axisymmetric jet from ReD=16 200–29 200. By using a simultaneous nitric oxide and acetone planar laser-induced fluorescence technique, the mixing transition was detected from sudden changes in the molecularly mixed-jet fluid volume fraction, the growth rate of the shear layer, the preferred mixed-jet fluid fraction, and the character of axial/radial probability density functions. The mixing transition for all Reynolds numbers in this regime was found to begin after the first vortex pairing near Rx/λ=6 and was completed by the second vortex pairing near Rx/λ=12, where R=(1−r)/(1+r),r is the low- to high-speed freestream velocity ratio, and λ is the natural instability wavelength. The statistical quantities at all Reynolds numbers were found to collapse when scaled with Rx/λ, with the exception of the mixing layer width. The latter collapsed for all Reynolds numbers when scaled by Rx/λ prior to the mixing transition, and by x/D beyond the mixing transition, as expected for turbulent jets for which r≈0.

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