Quantitative investigation of vortical structures in the near-exit region of an axisymmetric turbulent jet

Abstract

High-resolution particle image velocimetry (PIV) measurements were made at the exit of an axisymmetric free jet at a Reynolds number of 21 900 with a top-hat, low-turbulence exit velocity profile. The data were analysed using the proper orthogonal decomposition (POD) technique to identify the main energy-containing vortices. The vortices so identified have been further quantified by computing their size, position, circulation and direction of rotation. The technique employed resolved vortices of radius 0.055 jet exit diameters and larger. This quantitative information has also been related to observations from flow visualisation images and measurements of the downstream evolution of the jet. The data clearly show the formation of alternating direction toroidal vortices identified in the fluctuating fields which begin to be resolved at a streamwise location of one-half the jet exit diameter. The vortices grow in number approximately linearly in the downstream direction until about one jet diameter. After this, the number of vortices decreases and their size and circulation increase as they move downstream. This is indicative of the vortex pairing process. The high spread rate of the current jet is attributed to the vigorous production of large-scale vortical structures in this near-exit region due to the very near top-hat exit velocity profile and the resulting strong shear layer near the exit.