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The effect of small-amplitude convective disturbances on the size and bursting of a laminar separation bubble

Published online by Cambridge University Press:  07 March 2011

OLAF MARXEN  and
DAN S. HENNINGSON
OLAF MARXEN*
Affiliation:
Linné Flow Centre, Department of Mechanics, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden Center for Turbulence Research, Stanford University, Stanford, CA 94305-3035, USA
DAN S. HENNINGSON
Affiliation:
Linné Flow Centre, Department of Mechanics, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden
*
†Email address for correspondence: olaf.marxen@stanford.edu
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Abstract

Short laminar separation bubbles can develop on a flat plate due to an externally imposed pressure gradient. Here, these bubbles are computed by means of direct numerical simulations. Laminar–turbulent transition occurs in the bubble, triggered by small disturbance input with fixed frequency, but varying amplitude, to keep the bubbles short. The forcing amplitudes span a range of two orders of magnitude. All resulting bubbles differ with respect to their mean flow, linear-stability characteristics and distance between transition and mean reattachment locations. Mechanisms responsible for these differences are analysed in detail. Switching off the disturbance input or reducing it below a certain, very small threshold causes the short bubble to grow continuously. Eventually, it no longer exhibits typical characteristics of a short laminar separation bubble. Instead, it is argued that bursting has occurred and the bubble displays characteristics of a long-bubble state, even though this state was not a statistically steady state. This hypothesis is backed by a comparison of numerical results with measurements. For long bubbles, the transition to turbulence is not able to reattach the flow immediately. This effect can lead to the bursting of a short bubble, which remains short only when sufficiently large disturbances are convected into the bubble. Large-scale spanwise-oriented vortices at transition are observed for short but not for long bubbles. The failure of the transition process to reattach the flow in the long-bubble case is ascribed to this difference in transitional vortical structures.

JFM classification

Boundary Layers: Boundary layer separation Boundary Layers: Boundary layer stability Instability: Transition to turbulence

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Type
Papers
Information
Journal of Fluid Mechanics , Volume 671 , 25 March 2011 , pp. 1 - 33
Copyright
Copyright © Cambridge University Press 2011

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