Absolute and convective instability of a round jet emerging into an ambient medium of different viscosity

We perform linear stability calculations for a jet emerging into an ambient medium of a different viscosity but the same density. These calculations are intended to isolate the effects of viscosity variation alone. We conduct a systematic study of the effect of ambient-to-jet viscosity ratio, jet Reynolds number, and velocity profile specified by the shear layer thickness, thickness over which the viscosity change occurs, and radial shifts in velocity profiles on the growth of axisymmetric and helical modes. Additional terms in the disturbance kinetic energy equation that represent the coupling between the velocity fluctuations and the viscosity field are shown to be responsible for the additional destabilization. Radial shifts in velocity profile that represent real effects likely to be encountered in experiments are shown to be strongly destabilizing. In all cases the temporal growth rates of axisymmetric and helical mode are very close, except at low Reynolds numbers. Spatiotemporal analysis suggests that for sufficiently large ambient viscosity, low-viscosity jets become absolutely unstable. Over a wide range of parameters, two modes of absolute instability exist simultaneously, with an axisymmetric mode predicted to dominate a helical mode. Over a certain narrower space, the helical mode dominates. The transition boundary for absolute and convective instability is compared with recent experiments, and the results are found to be in reasonable agreement for the transition of the helical mode, when velocity profiles are used that correspond to the similarity solution for development of the boundary layer under a spatially variable viscosity.