Experimental study of the mixing transition in a gaseous axisymmetric jet

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 Re_D = 16,000 - 30,000. Instantaneous images, ensemble-averaged statistics, and probability density functions (PDFs) of mixed jet fluid fraction were used to identify sudden and dramatic changes in the mixing layer composition that marked the onset and completion of the transition to small-scale mixing. These changes included the appearance of a dual-layered mixing layer structure, a 20-25% drop in the mixed jet fluid volume fraction, a sudden change in the growth rate of the shear layer, a 30-35% drop in the preferred mixed jet fluid fraction, and a shift from stationary to stationary/marching radial PDFs. The mixing transition for all Reynolds numbers in this regime was found to begin after the first vortex pairing and was completed by the second vortex pairing. The axial location of the first vortex pairing for all Reynolds numbers was found to occur at a “pairing parameter” value of Rx/λ ≈ 8, where R = (l-r)/(l+r), r is the low- to high- speed velocity ratio, and λ is the initial 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.