A low-density axisymmetric jet is defined as a stream of low-density fluid discharging into a higher density ambient fluid. It is encountered in many industries, from fuel injection in combustors to plasma spraying in thermal coatings. A low-density jet can transition from a steady state (a fixed point) to self-excited oscillations (a limit cycle) via a Hopf bifurcation when its three control parameters, the jet-to-ambient density ratio S, the initial momentum thickness θ₀, and the Reynolds number Re fall into a certain range. In the literature, this bifurcation is often regarded as supercritical. However, we present experimental evidence showing that this Hopf bifurcation can also be subcritical. A universal scaling which incorporates the three control parameters is proposed to...[ Read more ]

A low-density axisymmetric jet is defined as a stream of low-density fluid discharging into a higher density ambient fluid. It is encountered in many industries, from fuel injection in combustors to plasma spraying in thermal coatings. A low-density jet can transition from a steady state (a fixed point) to self-excited oscillations (a limit cycle) via a Hopf bifurcation when its three control parameters, the jet-to-ambient density ratio S, the initial momentum thickness θ₀, and the Reynolds number Re fall into a certain range. In the literature, this bifurcation is often regarded as supercritical. However, we present experimental evidence showing that this Hopf bifurcation can also be subcritical. A universal scaling which incorporates the three control parameters is proposed to predict the onset of global instability in low-density jets. Apart from the conventional supercritical and subcritical Hopf bifurcations, we also find that at one unique condition, the jet shows intermittency. This intermittency is further characterized to be type-II Pomeau-Manneville intermittency. The physical cause of this phenomenon is conjectured to be either a variation of the global instability threshold or a change of the size of the absolutely unstable region. Lastly, coherence resonance phenomenon is observed and investigated in the subthreshold region when the jets are externally forced with Gaussian white noise. This phenomenon can be used as a precursor to predict the onset of global instabilities in such jets.