Experiments on the instabilities in swirling and non-swirling free jets

Instabilities present in a free-swirling jet in the Reynolds number range from 20,000 to 60,000 and swirl number of 0.45 were studied experimentally, using smoke visualization and hot-wire measurements. A non-swirling jet of Reynolds number 60,000 produced from the same facility was also studied for validation of the experimental procedures and direct comparison with the swirling jet. To the author's knowledge this represents the first study of the instabilities of a swirling free jet reported in the open literature.; Time-mean velocity components and turbulence intensity were surveyed using a single hot-wire (non-swirling jet) and a V-wire (swirling jet) probe. Like the non-swirling jet, the young shear layer from the nozzle exit of the swirling jet develops Kelvin-Helmholtz instability waves, which roll up into large scale organized motion.; To enhance our understanding of these instabilities, axisymmetric and helical waves were excited in the Strouhal number range 0.75-1.5 (swirling jet) and 0.9-3.75 (non-swirling jet) by acoustic excitation. Phase-locked multiple exposure photographs confirmed the existence of such structures. Velocity spectra and ensemble averaged measurements (phase referenced to the excitation) were used to determined the axial evolution of the large-scale structures. These measurements were compared with the instability wave evolution in non-swirling jets obtained using Michalke's calculation. In the swirling jets, the growth rates of the helical mode are higher than those of the axisymmetric mode. However, the growth rates of all instability waves are considerably smaller in a swirling jet than in a non-swirling jet. In general, swirl stabilizes the organized motions.; Another important phenomenon appearing at the core of a free-swirling jet is vortex bursting. Both spiral and bubble type vortex breakdown are observed at the jet core. Vortex bursting induces an asymmetric flowfield and instantly replaces the potential core with a large amount of turbulence. Upon interacting with the vortex breakdown, the shear layer along with the jet periphery loses its organized structure and, in general, "random turbulence" follows.