PROLOGUE

Among the goals of external flow control are separation postponement, lift enhancement, transition delay or advancement, and drag reduction. These objectives are not necessarily mutually exclusive. For low-Reynolds-number lifting surfaces, where the formation of a laminar separation bubble may have a dominant effect on the flow- field, the interrelation between the preceding goals is particularly salient, presenting an additional degree of complexity when flow control is attempted to achieve, say, maximum lift-to-drag ratio. This chapter discusses the aerodynamics of low- Reynolds-number lifting surfaces—particularly the formation and control of separation bubbles.

Introduction

Insects, birds and bats have perfected the art of flight through millions of years of evolution. Man's dream of flying dates back to the early Greek myth of Daedalus and his son Icarus, but the first successful heavier-than-air flight took place less than a century ago. Today, the Reynolds numbers for natural and man-made fliers span the amazing range from 102 to 109, insects being at the low end of this spectrum and huge airships occupying the high end (Carmichael 1981).