Near-field flow characteristics of a free coaxial jet

Detailed planar PIV measurements are reported of the flow field in the near-field of a free coaxial air jet. Measurements were performed at outer to inner jet velocity ratios of r_u(=U_o/U_i)=∞, 2, 1, and 0.5, while keeping U_o constant. Mean velocity distributions and turbulent stress distributions compared well to those reported in the literature at similar r_u. The spatial organization of the generated vortices, analyzed based on the instantaneous vorticity and swirling strength distributions, revealed distinct spatial wavelengths. Associated Strouhal numbers (assuming vortex convection velocities) were higher than those reported in the literature for different nozzle geometries. Our results point at the importance of the nozzle geometry and associated flow exit conditions on the vortex generation frequency. In addition, for the present nozzle geometry and jet exit flow conditions, the outer and inner jet are strongly coupled.