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

Marxen, O and Henningson, DS (2011) The effect of small-amplitude convective disturbances on the size and bursting of a laminar separation bubble Journal of Fluid Mechanics, 671. pp. 1-33.

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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. © 2011 Cambridge University Press.

Item Type: Article
Divisions : Surrey research (other units)
Authors :
Henningson, DS
Date : 25 March 2011
DOI : 10.1017/S0022112010004957
Depositing User : Symplectic Elements
Date Deposited : 17 May 2017 13:28
Last Modified : 24 Jan 2020 23:59

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