The stability of ducted compound flows and consequences for the geometry of coaxial injectors

A two-dimensional wake-like compound flow, formed by a low-speed stream embedded within a high-speed flow, is examined in this article. It is shown that the range of absolutely unstable flow in parameter space greatly increases when such a flow is confined within a duct. Parameters studied here are: the density ratio, which is from 0.1 to 1000; the velocity ratio, which varies from co-flow to counter-flow; and the ratio of the duct width to the width of the central jet. Absolutely unstable flows permit perturbations to propagate upstream, and can lead to self-sustained global oscillations similar to the vortex shedding process which takes place in the wake of a bluff body. This theoretical situation models the wake-like behaviour of a two-fluid coaxial injector with a recessed central tube. The aerodynamic destabilizing mechanism is of primary importance whereas the stabilizing mechanisms, which are not considered here, are of secondary importance. The conclusions from this analysis of a ducted compound flow can explain why one observes self-sustained oscillations in recessed coaxial injectors. The presence of a recirculation bubble in the central flow, which is the basis of other proposed explanations, is not required.