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The preferred mode of incompressible jets: linear frequency response analysis

Published online by Cambridge University Press:  25 January 2013

X. Garnaud*
Affiliation:
Laboratoire d’Hydrodynamique (LadHyX), Ecole Polytechnique – CNRS, 91128 Palaiseau, France
L. Lesshafft
Affiliation:
Laboratoire d’Hydrodynamique (LadHyX), Ecole Polytechnique – CNRS, 91128 Palaiseau, France
P. J. Schmid
Affiliation:
Laboratoire d’Hydrodynamique (LadHyX), Ecole Polytechnique – CNRS, 91128 Palaiseau, France
P. Huerre
Affiliation:
Laboratoire d’Hydrodynamique (LadHyX), Ecole Polytechnique – CNRS, 91128 Palaiseau, France
*
Email address for correspondence: [email protected]

Abstract

The linear amplification of axisymmetric external forcing in incompressible jet flows is investigated within a fully non-parallel framework. Experimental and numerical studies have shown that isothermal jets preferably amplify external perturbations for Strouhal numbers in the range $0. 25\leq {\mathit{St}}_{D} \leq 0. 5$, depending on the operating conditions. In the present study, the optimal forcing of an incompressible jet is computed as a function of the excitation frequency. This analysis characterizes the preferred amplification as a pseudo-resonance with a dominant Strouhal number of around $0. 45$. The flow response at this frequency takes the form of a vortical wavepacket that peaks inside the potential core. Its global structure is characterized by the cooperation of local shear-layer and jet-column modes.

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Papers
Copyright
©2013 Cambridge University Press

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