Onset of absolute instability on a pitching aerofoil

A global transient linear stability analysis of the three-dimensional time-dependent flow around an aerofoil undergoing small-amplitude pitching motion is performed using the optimally time-dependent (OTD) framework. The most salient linear instabilities associated with the instantaneous basic state...

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Bibliographic Details
Published in:Journal of fluid mechanics 2024-05, Vol.988, Article A8
Main Authors: Kern, J.S., Negi, P.S., Hanifi, A., Henningson, D.S.
Format: Article
Language:English
Subjects:
absolute/convective instability
Airfoils
boundary layer stability
Breakdown
Bubbles
Evolution
Instability
JFM Papers
Pitching
Pitching motion
Reynolds number
Separation
Stability
Stability analysis
Three dimensional analysis
Three dimensional flow
Time dependence
Turbulence
Velocity distribution
Velocity profiles
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Summary:A global transient linear stability analysis of the three-dimensional time-dependent flow around an aerofoil undergoing small-amplitude pitching motion is performed using the optimally time-dependent (OTD) framework. The most salient linear instabilities associated with the instantaneous basic state are computed and tracked over time. The resulting OTD modes reflect the variations in the basic state and can be used as predictors of its spatial and temporal evolution, including the formation of a laminar separation bubble and its gradual spanwise modulation via primary global instability, leading to secondary instability and finally rapid breakdown to turbulence. The study confirms and expands upon earlier stability analyses of the same case based on the local properties of spanwise averaged velocity profiles in the bubble that predicted the onset of absolute instability soon followed by rapid breakdown of the separation bubble. The three-dimensional structure of the most unstable OTD mode is extracted, which compares well with both the locally absolutely unstable mode and the evolution of the basic state itself.
ISSN:0022-1120
1469-7645
1469-7645
DOI:10.1017/jfm.2024.407