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Global instability of low-density jets

Published online by Cambridge University Press:  05 May 2017

W. Coenen Open the ORCID record for W. Coenen [Opens in a new window] ,
L. Lesshafft Open the ORCID record for L. Lesshafft [Opens in a new window] ,
X. Garnaud  and
A. Sevilla Open the ORCID record for A. Sevilla [Opens in a new window]
W. Coenen*
Affiliation:
Grupo de Mecánica de Fluidos, Universidad Carlos III de Madrid, Av. Universidad 30, 28911 Leganés (Madrid), Spain Department of Mechanical and Aerospace Engineering, University of California San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0411, USA
L. Lesshafft
Affiliation:
Laboratoire d’Hydrodynamique (LadHyX), École polytechnique – CNRS, 91128 Palaiseau, France
X. Garnaud
Affiliation:
Laboratoire d’Hydrodynamique (LadHyX), École polytechnique – CNRS, 91128 Palaiseau, France
A. Sevilla
Affiliation:
Grupo de Mecánica de Fluidos, Universidad Carlos III de Madrid, Av. Universidad 30, 28911 Leganés (Madrid), Spain
*
†Email address for correspondence: [email protected]
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Abstract

The global stability of laminar axisymmetric low-density jets is investigated in the low Mach number approximation. The linear modal dynamics is found to be characterised by two features: a stable arc branch of eigenmodes and an isolated eigenmode. Both features are studied in detail, revealing that, whereas the former is highly sensitive to numerical domain size and its existence can be linked to spurious feedback from the outflow boundary, the latter is the physical eigenmode that is responsible for the appearance of self-sustained oscillations in low-density jets observed in experiments at low Mach numbers. In contrast to previous local spatio-temporal stability analyses, the present global analysis permits, for the first time, the determination of the critical conditions for the onset of global instability, as well the frequency of the associated oscillations, without additional hypotheses, yielding predictions in fair agreement with previous experimental observations. It is shown that under the conditions of those experiments, viscosity variation with composition, as well as buoyancy, only have a small effect on the onset of instability.

JFM classification

Instability: Instability Wakes/Jets: Jets
Type
Papers
Information
Journal of Fluid Mechanics , Volume 820 , 10 June 2017 , pp. 187 - 207
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Copyright
© 2017 Cambridge University Press 

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Footnotes

‡

Present address: Safran Tech, Rue des Jeunes Bois, 78772, Magny-Les-Hameaux, France.

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