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 nearly self-preserving region of an axisymmetric free air jet, a 2.54 cm air jet at a Reynolds number Re_D = 6.8 × 10⁴ has been investigated for x/D≥40 via both long and short time-averaged space-time correlation measurements. Conventional space-time correlation data with probe separations in the stream wise direction by as much as 25 diameters suggest the passage of large-scale organized structures. The radial extent of these structures is about one local jet diameter, and the azimuthal extent is about a quadrant of the cross-section. Recurring quasi-periodic patterns are observed in the time series of short time-averaged correlations between longitudinal velocity fluctuations obtained with two arrays of hot-wires separated in the streamwise direction. These orderly patterns provide direct evidence for the existence of these structures. Quantitative details of these structures are now being investigated.