Reynolds number influence on statistical behaviors of turbulence in a circular free jet

The present paper examines the effect of Reynolds number on turbulence properties in the transition region of a circular jet issuing from a smoothly contracting nozzle. Hot-wire measurements were performed for this investigation through varying the jet-exit Reynolds number Re-d (equivalent to U(j)d/nu, where U-j, d, and nu are the jet-exit mean velocity, nozzle diameter, and kinematic viscosity) approximately from Re-d approximate to 4 x 10(3) to Re-d approximate to 2 x 10(4). Results reveal that the rates of the mean flow decay and spread vary with Reynolds number for Re-d < 10(4) and tend to become Reynolds-number independent at Re-d >= 10(4). Even more importantly, the small-scale turbulence properties, e. g., the mean rate of dissipation of kinetic energy (epsilon), the Kolmogorov and Taylor microscales, are found to vary in different forms over the Re-d ranges of Re-d > 10(4) and Re-d < 10(4). Namely, the critical Reynolds number appears to occur at Re-d,Re-cr approximate to 10(4) across which the jet turbulence behaves distinctly. Two turbulence regimes are therefore identified: (i) developing or partially developed turbulence at Re-d < Re-d,Re-cr and (ii) fully developed turbulence at Re-d >= Re-d,Re-cr. It is suggested that the energy dissipation rate (DR) can be expressed as epsilon similar to nu U-c(2)/R-2 in regime (i) and epsilon similar to U-c(3)/R in regime (ii), where U-c and R are the centerline (or maximum) mean velocity and half-radius at which the mean velocity is 0.5U(c). In addition, the critical Reynolds number appears to vary from flow to flow. (C) 2013 AIP Publishing LLC.