Comparisons of initially turbulent, low-velocity-ratio circular and square coaxial jets

A qualitative and quantitative comparison between unforced flows emanating from equivalent geometries of axisymmetric (circular) and square coaxial nozzles is presented. The initial state of the jets is turbulent, as in nozzles relevant to practical applications. Flow visualization and velocity measurements were performed at a coflow-jet Reynolds number of 1.9 x 10(4) and at an inner-to-outer jet velocity ratio of 0.3. Scaling of mean velocity and turbulence profiles has been examined for the three shear layers formed in the near field of axisymmetric and square coaxial flows. Visualizations and local velocity measurements have indicated modest mixing enhancement when square nozzles are used compared to the axisymmetric ones, and this is largely attributed to differences in initial velocity profiles between these two configurations. Low-coherence, large-scale periodic structures were observed for both nozzle configurations in the midfield of the inner mixing region, which has wake characteristics. The outer mixing region, which is initially highly turbulent, shows no signs of an "indigenous" organized structure. The spectral characteristics of the circular and square nozzle combinations are qualitatively similar. However, the discrete frequency peaks associated with the wake of the inner mixing region are much broader in the case of the square nozzles. More important, the dominant frequency at the end of the midfield within the inner mixing region is lower in the case of the square nozzles compared to that of the circular ones. Spectra from the turbulent shear layers of the middle mixing region in the near field indicate the presence of a short-lived shear layer preferred mode at a much higher frequency than the one associated with the subsequent development of the wake downstream. The existence of axis switching, a phenomenon observed in single nonaxisymmetric nozzles, is not evident from visualizations and measurements in the square coaxial nozzle, presumably due to the initial low coherence of large-scale structures in the shear layers.