High-speed PIV measurements of the near-field of coaxial, round jets*

A coaxial jet is characterized by two mixing layers, i.e. between inner and outer jet, and between outer jet and quiescent ambient. For a given nozzle geometry, the ratio between outer and inner jet velocities, r_u, as well as absolute jet exit velocities are important. They strongly affect the flow field and in particular the structure of vortices generated by wake and shear instabilities. Here, we measured the near-field flow dynamics of a coaxial, round water jet using high-speed particle image velocimetry (PIV). The inner and outer jets were issued from a long tube and a constant area annulus, respectively. Vortex generation, convection velocities, and vortex interaction between the mixing layers were studied. Measurements were performed for r_u = 0.66, 1, and 1.32 for which wake instabilities due to finite lip thickness become important. For each case, the dominating instability mode was investigated and spatial maps of governing Strouhal numbers indicated the extent of coherent vortices prior to their break-up. Vortex strengths and sizes based on instantaneous swirling strength maps are presented. In addition, mean velocity and Reynolds shear stress jumps across ensemble averaged instantaneous internal and external interfaces were determined.