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Manipulation of turbulent boundary layers by outer-layer devices: skin-friction and flow-visualization results

Published online by Cambridge University Press:  21 April 2006

A. M. Savill  and
J. C. Mumford
A. M. Savill
Affiliation:
Fluid Dynamics Section, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK Present address: Engineering Department, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK.
J. C. Mumford
Affiliation:
Fluid Dynamics Section, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK Present address: Engineering Department, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK.
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Abstract

Parametric studies have been made of devices introduced into the outer region of a low-Reynolds-number turbulent boundary layer (Reθ = 1000-3500) with a view to understanding the manner in which such ‘manipulators’ reduce the surface drag. The devices considered were single flat plates, a cylinder with the same drag as one of these, and two plates stacked, staggered, and in tandem, with chord Reynolds numbers Rec in the range 1000-100000. Direct measurements of local skin friction using a floating-plate drag balance are reported together with the results of laser- sheet smoke flow visualization. The skin-friction results are in good agreement with other floating-element data while the visual and photographic studies in both stationary and convected frames completely support the hairpin description of the boundary-layer structure, and reveal that the wake of the device may play a more important role than has previously been suggested. A picture is presented of the interaction of the devices and their wakes with the hairpin eddy structure which could explain the magnitudes and shapes of the skin-friction distributions observed downstream; their dependence on the height, thickness, and if appropriate, length and spacing of the device(s); the optimum values for these parameters; and the existence of a preferred, tandem plate configuration. The results suggest that such plates do not act primarily as large-eddy break-up systems (LEBUs). Instead several ‘active’ mechanisms are identified which supplement the ‘passive’ effect of the imposed momentum defect. The suppression of large-scale motions is one of these, but, a t least for our relatively thick devices, it would appear that it is an interaction between the vortices, introduced into the layer via the wake, and the near-wall structure that provides the principal mechanism for reducing the skin friction. The observation that the maximum skin-friction reduction always occurred close to the position where these vortices reached the sublayer provides strong evidence for such a view.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 191 , June 1988 , pp. 389 - 418
Copyright
© 1988 Cambridge University Press

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