Coaxial round water jet at velocity ratios close to unity, Part I: Mean and turbulent flow characteristics

In this paper, planar time-resolved particle image velocimetry measurements are reported in the near-field of a coaxial jet at velocity ratios, r_u = 0.66, 1.0 and 1.32. At each r_u, at least two different Reynolds numbers, Re, were investigated. Spatial distributions of mean velocities, Reynolds stresses, vorticity and swirling strength strongly depend on r_u. Re effects in the normalized, mean velocity distributions were mainly visible in the wake associated with the finite lip thickness. In addition, increased normalized Reynolds stresses were obtained at x/D_o≈ 1.5 for the lowest Re at a given r_u. Farther downstream, Re effects became negligible as inner and outer jets merged. A comparison between different terms in the turbulent kinetic energy (TKE) budget showed that TKE is predominantly produced in the outer shear layer, while turbulent diffusion results in radial transport. Mean advection of TKE (a “sink” term) is mainly significant close to the nozzle exit.