Hairpin Vortices in Turbulent Boundary Layers: The Implications for Reducing Surface Drag

Abstract

Ultimately, control of turbulent shear flows must center around the selected modification of the flow structure of the turbulence, or the conditions which gives rise to the flow structure. Consequently, in order to develop rational methods of control, it is necessary to understand the basic dynamical elements of the turbulent boundary layer. In this paper, a model of the turbulent boundary layer, based upon a series of complimentary computational and experimental studies, is described in which the hairpin vortex is suggested to be a basic flow structure of boundary-layer turbulence. The paper discusses the development of single hairpin vortices, how such vortices interact with a shear flow and with one another, and how they interact with the flow near a wall to produce new vortices through a viscous interaction. It is demonstrated how key features of turbulent flow structure (e.g. low-speed streaks, bursting, pockets, etc.) can be explained in terms of the motion and effects of convecting hairpin vortices. The implications of this model in reducing drag are also discussed.