Coaxial round water jet at velocity ratios close to unity, Part II: Vortex shedding and spectral analysis

In part I of this study, planar time-resolved particle image velocimetry measurements were reported in the near-field of a coaxial jet at velocity ratios, r_u = 0.66, 1.0 and 1.32. At each r_u, at least two different Reynolds numbers, Re, were investigated. In part II of this study, instantaneous vortex characteristics are studied. Instantaneous swirling strength maps revealed “trains” of alternating sign vortices as a result of shear and wake instabilities. Their spatial distribution strongly depended on r_u. In contrast, Re effects on their size and number were small, but their circulation increased with Re as well as with r_u. The circulation of outer shear layer vortices collapsed when normalized by the outer jet's diameter and jet exit velocity for all investigated conditions. No such scaling was found for inner shear layer vortices. Spectral analysis revealed distinct “islands” of high and low peak passage frequencies associated with the inner and outer shear layers, respectively, indicating that “lock-in” between inner and outer shear layers was absent. Strouhal numbers were similar for all Re and r_u and those based on momentum thicknesses indicated that inner and outer jets did not develop independently.