PROLOGUE

Under certain conditions, wall-bounded flows separate. To improve the performance of natural or man-made flow systems, it may be beneficial to delay or advance this detachment process. This chapter reviews the status and outlook of separation control for steady and unsteady flows. Passive and active techniques to prevent or to provoke flow detachment are considered, and suggestions are made for further research.

Introduction

The Phenomenon of Separation

Fluid particles in a boundary layer are slowed down by wall friction. If the flow is sufficiently retarded, for example, owing to the presence of an adverse pressure gradient, the momentum of those particles will be reduced by both the wall shear and the pressure gradient. In terms of energy principles, the kinetic energy gained at the expense of potential energy in the favorable-pressure-gradient region is depleted by viscous effects within the boundary layer. In the adverse-pressure-gradient region, the remaining kinetic energy is converted to potential energy but is too small to surmount the pressure hill, and the motion of near-wall fluid particles is eventually arrested. At some point (or line), the viscous layer departs or breaks away from the bounding surface. The surface streamline nearest to the wall leaves the body at this point, and the boundary layer is said to separate (Maskell 1955).