Evolution of the turbulent/non-turbulent interface of an axisymmetric turbulent jet

Measurements of a turbulent round air jet, using particle image velocimetry, were made to investigate the dynamics and transport processes at the continuous bounding interface between the turbulent and non-turbulent regions (T/NT) of the jet flow. The Reynolds number of the jet was in the range Red = 3,000–6,500 based on jet diameter with measurements taken between 0 and 40 jet diameters from the jet nozzle exit, which covers the developing and self-similar region of the jet. A velocity thresholding technique was used to define the T/NT interface. The results based on this methodology were found to agree well with previously published results (Westerweel et al. in Phys Rev Lett 95:174501, 2005; Westerweel et al. in J Fluid Mech 631:199–230, 2009; and Anand and Boersma in Exp Fluids 47:995–1007, 2009). The evolution of the turbulence characteristics at the interface was investigated using conditionally averaged axial velocity, azimuthal vorticity, turbulent intensity, and Reynolds shear stress. A distinct change in behavior is observed between the near-field region, 0 ≤ x/d ≤ 8, and the far-field, self-similar region, x/d > 20. Profiles of the conditional mean axial velocity demonstrate no clear jump in the conditional mean axial velocity at the interface for x/d < 8. However, the emergence of the jump is observed at x/d ≈ 8, and the magnitude of the jump continues to increase between 8 < x/d < 20 before reaching a steady value in the self-similar region, x/d > 20. All the scaled statistics show self-similar behavior downstream of x/d > 20. Similar behavior is observed for the peak in the azimuthal vorticity and the jump in the Reynolds shear stress profiles. The evolution of the rate of entrainment also confirms the creation and growth of the jump in the mean conditional velocity profiles in the developing region of the jet.