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Global modes and transient response of a cold supersonic jet

Published online by Cambridge University Press:  31 January 2011

JOSEPH W. NICHOLS  and
SANJIVA K. LELE
JOSEPH W. NICHOLS*
Affiliation:
Center for Turbulence Research, Stanford University, Stanford, CA 94305-3035, USA
SANJIVA K. LELE
Affiliation:
Center for Turbulence Research, Stanford University, Stanford, CA 94305-3035, USA Department of Aeronautics and Astronautics, Stanford University, Stanford, CA 94305-4035, USA Department of Mechanical Engineering, Stanford, CA 94305-3030, USA
*
†Email address for correspondence: [email protected]
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Abstract

Global-mode analysis is applied to a cold, M = 2.5 laminar jet. Global modes of the non-parallel jet capture directly both near-field dynamics and far-field acoustics which, in this case, are coupled by Mach wave radiation. In addition to type (a) modes corresponding to Kelvin–Helmholtz instability, it is found that the jet also supports upstream-propagating type (b) modes which could not be resolved by previous analyses of the parabolized stability equations. The locally neutrally propagating part of a type (a) mode consists of the growth and decay of an aerodynamic wavepacket attached to the jet, coupled with a beam of acoustic radiation at a low angle to the jet downstream axis. Type (b) modes are shown to be related to the subsonic family of modes predicted by Tam & Hu (1989). Finally, significant transient growth is recovered by superposing damped, but non-normal, global modes, leading to a novel interpretation of jet noise production. The mechanism of optimal transient growth is identified with a propagating aerodynamic wavepacket which emits an acoustic wavepacket to the far field at an axial location consistent with the peaks of the locally neutrally propagating parts of type (a) modes.

JFM classification

Instability: Absolute/convective instability Acoustics: Jet noise Wakes/Jets: Jets
Type
Papers
Information
Journal of Fluid Mechanics , Volume 669 , 25 February 2011 , pp. 225 - 241
Copyright
Copyright © Cambridge University Press 2011

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References

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