A study has been made of the instability and the subsequent breakdown of axisymmetric jets of helium/air mixtures emerging into ambient air. Although the density of the nozzle gas is less than that of the ambient fluid, the jet is essentially non-buoyant. Two kinds of instability are observed in the near field, depending upon the mean flow parameters. When the ratio of the exiting nozzle fluid density to ambient fluid density is ρe/ρ∞ > 0.6, shear-layer fluctuations evolve in a fashion similar to that observed in constant-density jets: the power spectrum near the nozzle is determined by weak background disturbances whose subsequent spatial amplification agrees closely with the spatial stability theory. When the density ratio is less than 0.6, an intense oscillatory instability may also arise. The overall behaviour of this latter mode (to be called the ‘oscillating’ mode) is shown to depend solely upon the density ratio and upon D/θ, where D is the nozzle diameter and θ is the momentum thickness of the boundary layer at the nozzle exit. The behaviour of this mode is found to be independent of the Reynolds number, within the range covered by the present experiments. This is even true in the immediate vicinity of the nozzle where, unlike in the case of shear-layer modes, the intensity of the oscillating mode is independent of background disturbances. The streamwise growth rate associated with the oscillating mode is not abnormally large, however. The frequency of the oscillating mode compares well with predictions based on a spatio-temporal theory, but not with those of the standard spatial theory.
From high-speed films it is found that the overall structure of the oscillating mode repeats itself with extreme regularity. The high degree of repeatability of the oscillating mode, in association with a strong pairing process, leads to abnormally large centreline velocity fluctuation, with its root-mean-square value being about 30 % of the nozzle exit velocity. Energetic and highly regular pairing is found also to lead to the early and abrupt breakdown of the potential core. The regularity often extends even to the finer structure immediately downstream of the breakdown. An attempt is made to explain these special features both in terms of the large-amplitude vorticity field, and in terms of the theoretically predicted space–time evolution of wave packets.