Vortex interaction and mixing in a driven gaseous axisymmetric jet

Vortex formation and merging are investigated in the near field of a driven axisymmetric jet. Acoustic forcing is used to obtain repeatable vortex pairing events, and simultaneous passive scalar and cold-chemistry planar laser-induced flourescence are used to obtain instantaneous images of molecularly mixed jet fluid fraction. The time-varying scalar dissipation field and area-averaged stirredness of the vortex core region are measured at various stages of vortex interaction. These mixing properties are analyzed in conjunction with the observed vortex dynamics, such as the time-dependent vortex convection velocity. The results indicate that there are several phases of the pairing event with distinct mixing characteristics, including vortex roll-up, interaction, coalescence, and reentrainment. Vortex roll-up is nearly laminar with molecular diffusion between the layers of jet and co-flow fluid. The most dramatic change in the mixing state of the leading vortex, which includes the appearance of a uniformly mixed core region, occurs as the trailing vortex approaches and interferes with co-flow fluid entrainment. Vortex coalescence is marked by gross deformation and stretching of the trailing vortex, and rapid homogenization of the diffusion layers. Finally, re-entrainment of pure fluid after the pairing event results in an elongated, nonrotating structure. These stages of vortex pairing correspond to the temporal evolution of vorticity observed in previous studies.