Absolute Instability in a Confined Low-Density Axisymmetric Jet

Previous research showed that absolute instability could occur in unconfined low-density jets, which was linked to the onset of self-sustained oscillation that aids mixing. This paper performs spatio-temporal analysis to study the absolute stability of a confined low-density axisymmetric jet. Multiple spatial eigenvalues are present for a prescribed real angular frequency, including the downstream-traveling spatially amplifying Kelvin-Helmholtz mode. Using Briggs-Bers criteria, absolute instabilities are identified in weakly confined and strongly confined low-density jets. In addition, the relation between the absolute growth rate and the confinement ratio is also explored. The results may be relevant to the design of facilities such as coaxial injectors in aircraft engines.