Modes, transient dynamics and forced response of circular jets

Abstract : The linear stability of axisymmetric jets is studied in a global frame- work, such that the geometry and the non-parallelism of the base flow are fully accounted for. In order to characterize the "preferred mode" of the jet, which is consistently observed in experiments and numerical simulations, different types of analyses are carried out. As a well-known conjecture ascribes the existence of this "preferred mode" to a resonance between external noise and the least-stable eigenmode, a modal study is first performed. This analysis prompted the development of a new numerical method allowing an efficient treatment of compressible flows; however, the results indicate that a modal representation is not well-suited for a description of the dynamics of convection-dominated stable flows. Studies of simplified model problems are presented that underpin this conclusion. Instead, the instability dynamics of such flows are characterized in a consistent manner in terms of optimal initial perturbations and optimal harmonic forcing. The latter approach robustly reproduces experimental observations with regard to the frequency and the spatial structure of the jet "preferred mode". Furthermore, the results allow an interpretation of this global wavepacket structure as a cooperation of different families of local instability modes. The present analysis demonstrates that the "preferred mode" does not arise from resonance with an eigenmode, but rather is to be interpreted as a pseudo-resonance.
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Submitted on : Tuesday, October 9, 2012 - 3:02:24 PM
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Xavier Garnaud. Modes, transient dynamics and forced response of circular jets. Fluids mechanics [physics.class-ph]. Ecole Polytechnique X, 2012. English. ⟨tel-00740133⟩

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