Global instability of low-density jets

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...

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Bibliographic Details
Published in:Journal of fluid mechanics 2017-06, Vol.820, p.187-207
Main Authors: Coenen, W., Lesshafft, L., Garnaud, X., Sevilla, A.
Format: Article
Language:English
Subjects:
Approximation
Buoyancy
Colleges & universities
Composition effects
Density
Dynamics
Experiments
Feedback
Fluids
Hypotheses
Instability
Jets
Mach number
Mathematical analysis
Molecular weight
Oscillations
Outflow
Permits
Stability
Stability analysis
Velocity
Viscosity
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Summary: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.
ISSN:0022-1120
1469-7645
DOI:10.1017/jfm.2017.203