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Self-sustained oscillations in variable-density round jets

Published online by Cambridge University Press:  14 June 2007

JOSEPH W. NICHOLS
Affiliation:
Department of Mechanical Engineering, University of Washington, Seattle, WA 98195, USA
PETER J. SCHMID
Affiliation:
Laboratoire d'Hydrodynamique (LadHyX), CNRS–École Polytechnique, 91128 Palaiseau, France
JAMES J. RILEY
Affiliation:
Department of Mechanical Engineering, University of Washington, Seattle, WA 98195, USA
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Abstract

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The stability properties of round variable-density low-Mach-number jets are studied by means of direct numerical simulation (DNS) and linear stability analysis. Fully three-dimensional DNS of variable-density jets, with and without gravity, demonstrate that the presence of buoyancy causes a more abrupt transition to turbulence. This effect helps to explain differences between normal gravity and microgravity jet diffusion flames observed in the laboratory.

The complete spectrum of spatial eigenmodes of the linearized low-Mach-number equations is calculated using a global matrix method. Also, an analytic form for the continuous portion of this spectrum is derived, and used to verify the numerical method. The absolute instability of variable-density jets is confirmed using Brigg's method, and a comprehensive parametric study of the strength and frequency of this instability is performed. Effects of Reynolds number, the density ratio of ambient-to-jet fluid (S1), shear-layer thickness and Froude number are considered. Finally, a region of local absolute instability is shown to exist in the near field of the jet by applying linear stability analysis to mean profiles measured from DNS.

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Papers
Copyright
Copyright © Cambridge University Press 2007