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The effect of small-amplitude convective disturbances on the size and bursting of a laminar separation bubble
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.ORCID iD: 0000-0001-7864-3071
2011 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 671, 1-33 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2011. Vol. 671, 1-33 p.
Keyword [en]
boundary layer separation, boundary layer stability, transition to turbulence
National Category
Mechanical Engineering
URN: urn:nbn:se:kth:diva-31866DOI: 10.1017/S0022112010004957ISI: 000288100100001ScopusID: 2-s2.0-79952793152OAI: diva2:408329
Swedish eā€Science Research Center
QC 20110404Available from: 2011-04-04 Created: 2011-03-28 Last updated: 2012-05-23Bibliographically approved

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