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

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.

Effect of Free-Stream Turbulence on the Reduction of Surface Friction in a Turbulent Boundary Layer Behind LEBU-Devices

“…The friction reduction behind the LEBU-device first increases downstream, attains a maximum, and then decreases and vanishes [15,16]. Accordingly, if in each section of the reduced-friction region the flow pattern is similar to that in the tensiometer region, it follows that an increase in the turbulence rate does not affect the relative reduction in friction behind the LEBU-device.…”

Large-Eddy BreakUp Devices – a 40 Years Perspective from a Stockholm Horizon

“…Of the journal publications one is about negative results [14] whereas the others that are published in journals are of more general character, i.e. discussing structural changes and skin-friction reductions without claiming drag reduction [29]. Hence the notion that LEBUs could produce drag reduction never entered the reviewed scientific literature; maybe a good sign that journal editors actually act as gatekeepers of scientific rigour.…”

Direct intervention of hairpin structures for turbulent boundary-layer control

“…It has been proposed that LEBUs interrupt the energy production cycle of the turbulent boundary layer by directly interacting with the large-eddy structures, leading to skin-friction drag reductions in the downstream. 42 As the turbulent flow passes through a LEBU device, vortices are shed from its trailing edge, modifying the velocity field behind the device. It is also believed that the interaction between the device wake and large-scale structures leads to reductions in skin-friction drag by LEBUs.…”