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Global instabilities of subsonic jets
The stability of spatially developing subsonic jets is investigated. If the density ratio between the jet and the outside is sufficiently low, arbitrarily small perturbations grow in time until self sustained oscillation appear in the non-linear regime. Both the apparition of these structures and their frequency is well explained by a local instability argument.
In an isothermal setting on the contrary all eigenmodes are found to be stable, meaning that perturbations eventually decay at long times. While this observation is in agreement with classical local stability results, a stable eigen-mode spectrum seems inappropriate for the description of the convective instability dynamics of jets, which are known to be highly receptive to external perturbations. Instead, we propose to characterize jet instability in terms of the linear flow response to time-harmonic forcing. External perturbations inside the nozzle duct are identified that give rise to the most amplified flow response at a prescribed frequency.
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