Absolute instability in variable density round jets

This is a brief report on the properties of round jets of different densities issuing into the ambient air. Different densities were obtained by premixing helium and air in various proportions. We show that these jets have two types of behaviour depending on the density ratio between the jet and the ambient fluids, one characterized by very sharp peaks in the power spectral density of the velocity in the near field of the jet, and another with broadened and much less prominant spectral peaks. We examine the possibility that the first state corresponds to absolute instability, and the second to convective instability. It appears that the nature of instability can be changed from absolute to convective by very simple means reminiscent of similar possibilities in low Reynolds number wakes of circular cylinders. Flow visualization reveals that the low-density jets intermittently breakdown, and spread spectacularly, beyond a certain small axial distance.