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The two classes of primary modal instability in laminar separation bubbles

Published online by Cambridge University Press:  10 October 2013

Daniel Rodríguez ,
Elmer M. Gennaro  and
Matthew P. Juniper
Daniel Rodríguez*
Affiliation:
School of Aeronautics, Universidad Politécnica de Madrid, Pza. Cardenal Cisneros 3, E-28040 Madrid, Spain Department of Aeronautics, São Carlos School of Engineering, Universidade de São Paulo, Rua Jõao Dagnone 1100, 13563-120, São Carlos, Brazil
Elmer M. Gennaro
Affiliation:
Department of Aeronautics, São Carlos School of Engineering, Universidade de São Paulo, Rua Jõao Dagnone 1100, 13563-120, São Carlos, Brazil
Matthew P. Juniper
Affiliation:
Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK
*
†Email address for correspondence: dani@torroja.dmt.upm.es
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Abstract

The self-excited global instability mechanisms existing in flat-plate laminar separation bubbles are studied here, in order to shed light on the causes of unsteadiness and three-dimensionality of unforced, nominally two-dimensional separated flows. The presence of two known linear global mechanisms, namely an oscillator behaviour driven by local regions of absolute inflectional instability and a centrifugal instability giving rise to a steady three-dimensionalization of the bubble, is studied in a series of model separation bubbles. These results indicate that absolute instability, and consequently a global oscillator behaviour, does not exist for two-dimensional bubbles with a peak reversed-flow velocity below $12\hspace{0.167em} \% $ of the free-stream velocity. However, the three-dimensional instability becomes active for recirculation levels as low as ${u}_{rev} \approx 7\hspace{0.167em} \% $. These findings suggest a route to the three-dimensionality and unsteadiness observed in experiments and simulations substantially different from that usually found in the literature of laminar separation bubbles, in which two-dimensional vortex shedding is followed by three-dimensionalization.

JFM classification

Instability: Absolute/convective instability Boundary Layers: Boundary layer separation
Type
Rapids
Information
Journal of Fluid Mechanics , Volume 734 , 10 November 2013 , R4
Copyright
©2013 Cambridge University Press 

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