Structures in a Turbulent Boundary Layer

Abstract

Drawn by the prospect of net drag reduction, the manipulation of turbulent boundary layers has attracted much attention. Many aspects of a turbulent boundary layer are relatively easy to alter (e.g., the boundary layer can be artificially thickened, the local skin-friction distribution can be altered, etc.). However, very few manipulation techniques have been shown to produce a net drag reduction. One successful manipulation technique involves suspending thin ribbons or airfoils in the outer portion of the boundary layer, either individually or in pairs. This device, known either by the acronym BLADE (Boundary Layer Alteration DEvice) or by the acronym LEBU (Large Eddy Break Up) device, was devised and first researched at the Illinois Institute of Technology)1 and at NASA Langley.2 The latter name derives from the proposed mechanism by which a net drag reduction is achieved and which will be justified in the following presentation. Summaries of the results obtained using BLADE type devices have been compiled by Anders,3 Mumford & Savill4 and Bandyopadhyay.5 We have investigated in detail the changes in the turbulent flat-plate boundary layer produced by a BLADE device. Presented here are the results showing the effect of manipulation on the coherent structures in the boundary layer. (For a complete account of this investigation, see Lynn.6) To isolate the effects of introducing a simple momentum defect, the effects of a wire device on the boundary layer were also investigated. The wire device, consisting of a wire replacing the ribbons, was chosen to have the same device drag as the optimized BLADE device. Wire devices have not been shown to produce net drag reductions.