Instability of a Low Viscosity Jet Emerging into a High Viscosity Medium: Linear Stability Analysis

TL;DR

This study uses linear stability analysis to investigate how viscosity gradients affect the stability of a jet in a different viscosity medium, revealing conditions for absolute instability and the dominance of certain modes.

Contribution

It isolates the effects of viscosity variation on jet stability and identifies the destabilizing influence of viscosity-velocity coupling and velocity profile shifts.

Findings

Viscosity gradients can cause absolute instability in jets.

Radial velocity profile shifts strongly destabilize the flow.

Two modes of absolute instability can coexist depending on parameters.

Abstract

Many natural and engineering systems involve the mixing of two fluid streams, in which the effects of density and viscosity gradients play important roles in determining flow stability. 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 the velocity profile specified by the shear layer thickness, the 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. Spatio-temporal analysis in the complex wavenumber plane 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/convective instability is compared with recent experiments, and the results are found to 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.