Experimental investigations on secondary structures in a fully developed turbulent jet

The effect of streamwise vorticity on entrainment in jet flows has been the subject of various works and is related to many applications involving effective fluid mixing. In this work the near-field of a fully developed turbulent round jet was investigated at different Reynolds numbers ranging from ≈ 3000 to ≈ 30000 by means of time-resolved particle image velocimetry. Acquisitions were carried out on transverse planes at different downstream positions, in order to visualize the evolution of vortical structures and to assess their features. Results from average velocity fields and rms show a strong impact of crosswise vortical structures on radial velocity distributions as downstream distance increases. Velocity correlation functions analysis exhibits a decrease of crosswise structures size as the Reynolds number increases, reaching an asymptotic behaviour which may be also observed in TKE distributions, which feature high levels in the outer radial area. Entrainment rates calculations supported by spectral analysis confirm the decisive role of streamwise vorticity in the control of the entrainment process as well as its dependence on Reynolds number. Entrainment rates approaching an asymptotic state versus Reynolds number suggests a connection between the effectiveness of this process and crosswise vortical structures size.