Near-field dynamics of a turbulent round jet with moderate swirl

The near-field characteristics of a turbulent jet with moderate swirl generated by a fully developed, axially rotating pipe flow are investigated with LDV, time-resolved stereoscopic PIV measurements, as well as with large-eddy simulations. Large-scale vortical structures in either double, triple or even quadruple-helix configuration are found at the pipe exit but rapidly break down or amalgamate after two jet diameters. Further downstream, the swirling jet is dominated by large-scale sweeping motions not present at such a scale and strength in the non-swirling case. Of special interest is the recently discovered counter-rotating core (in the mean) which develops about six jet diameters downstream the jet exit. Data for all six Reynolds stresses is reported at this position and it is argued that the counter-rotation is the result of the transport of angular momentum radially outward by the radial-azimuthal Reynolds shear stress. The mechanisms behind this transport are discussed by qualitative analysis of the time-resolved PIV and LES data and comparisons with the non-swirling case are made.