Search for large-scale coherent structures in the nearly self-preserving region of a turbulent axisymmetric jet

In an attempt to explore the existence of large-scale coherent structures in the self-preserving region of an axisymmetric free air jet, a 2.54 cm air jet at a Reynolds number of 46,000 has been investigated for distances in excess of 40 diameters using both long and short time-averaged space-time correlation measurements. Conventional space-time correlation data with probe separations in the streamwise direction by as much as 25 diameters suggest the existence of large-scale coherent structures centered off the jet axis. The radial extent of these structures is about one local jet diameter, and the azimuthal extent is about a quadrant of the cross-section. Time series of short time-averaged correlations between longitudinal velocity fluctuations obtained with two arrays of hot-wires separated in the streamwise direction strongly support the existence of these structures. Nearly periodic peaks and valleys were intermittently observed. A qualitative picture of the spatial characteristics of these structures was inferred from the short time-averaged cross correlation of these time series with a sinusoidal model function. Traces of instantaneous transverse velocity and Reynolds stress as well as the instantaneous velocity vector patterns obtained from a radial array of X-wires also depict existence of large-scale organized motions.