TURBULENT STRUCTURE DURING TRANSITION TO SELF-SIMILARITY IN A ROUND JET

The developing turbulent region of a round jet was investigated using an improved implementation of digital particle image velocimetry (DPIV ). The two-dimensional flow field in planes normal and parallel to the axial velocity was measured at locations between 15 and 30 diameters downstream, for two Reynolds numbers of 5500 and 16,000. The study con sisted of instantaneous snapshots of the velocity and vorticity fields as well as measurements of velocity correlations up to third order. I n this regime, the Reynolds number had a significant effect on both th e instantaneous flow structure and the profiles of mean velocity acros s the jet. Coherent streamwise structures were present in the jet core for the lower Reynolds number. Additional structures whose evolution was governed by time scales two orders of magnitude larger than the co nvective scale inside the jet were observed in the entrainment field. The velocity correlations provided further support for the validity of DPIV turbulence measurements. The data was consistent with the equati ons of motion and momentum was conserved. DPIV measurements of turbule nt kinetic energy components agreed with the hot-wire measurements of previous studies.