On the effects of a low-density weak co-flow on the near field of an axisymmetric jet

The near-field flow characteristics of an axisymmetric air jet issuing into ambient air were studied, with a focus on the structure and growth of the instabilities and vortices within the shear layer. Changes in the jet's stability characteristics and spreading properties were observed with the application of small levels of a helium co-flow shroud. The addition of the low-density gas served to introduce density gradients within the shear layer, altering the density-weighted vorticity profile, and a subsequent suppression of the Kelvin-Helmholtz instability. Density gradients, visualized by the schlieren technique, indicated an increased delay in the formation of vortex roll-ups and a greater suppression of the vortex pairing process for increased low-density co-flow. Power spectra plots, obtained using hot-wire anemometry, suggest that the initial instability frequency of the air jet remains constant throughout all conditions examined, however, they are made more stable with increased annular flow. The effects of a more stable jet and changes to the vortex dynamics brought on by a density gradient in the shear layer were revealed in mean and instantaneous flow data obtained using particle image velocimetry. Co-flowing mass flow rates of 2.5% resulted in a potential core length increase of 41% due to a 20% reduction in the shear layer spreading rate. Reduced mixing and entrainment, and an overall change of the turbulence profile also resulted from the applied annular flow.