Self-excited oscillations and mixing in a heated round jet

An axisymmetric hot-air jet discharging into cold ambient air is investigated experimentally. We consider the transitional regime, that is, Reynolds numbers at which the jet is initially laminar. In the first part of the paper it is demonstrated by several different experiments that, for sufficiently low Reynolds number and a ratio of jet exit to ambient density below approximately 0.7, global oscillations of the ‘jet column’ become self-excited, a behaviour which is related to local absolute instability in the potential core region. The onset of the global oscillations is identified as a Hopf bifurcation and two axisymmetric global modes are observed below the critical density ratio. Finally, it is shown that in the (self-excited) limit-cycle regime the spreading of the hot jet is intermittently quite spectacular, with half-angles in excess of 45°. Using flow visualization, this large spreading of low-density jets is related to the generation of strong ‘side jets’ emanating from the jet column.