Dual-tracer PLIF measurements of entrainment and mixing in a driven axisymmetric jet

The time-dependent mixing state during vortex formation and merging is 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 PLIF are used to obtain instantaneous images of molecularly mixed jet fluid fraction. The time varying scalar dissipation field and area-averaged mixedness of the vortex core region are also used to investigate various stages of vortex interaction. These mixing properties are analyzed in conjunction with the observed vortex dynamics, such as unsteady vortex convection velocity. The results indicate that there are three phases of the pairing event with distinct mixing characteristics, including the vortex roll-up, approach of the trailing vortex, and co-rotation/homogenization. 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. The co-rotation phase is marked by gross deformation and stretching of the trailing vortex, and rapid homogenization. These stages of vortex pairing coincide with the temporal evolution of vorticity observed in previous studies.