PROLOGUE

In contrast to the early chapters, in the present chapter I will emphasize the frontiers of the field of flow control, pondering mostly the control of turbulent flows. I will review the important advances in the field that have taken place during the past few years and are anticipated to dominate progress in the future, essentially covering the fifth era outlined in Section 1.2. By comparison with laminar flow control or separation prevention, the control of turbulent flow remains a very challenging problem. Flow instabilities quickly magnify near critical flow regimes, and therefore delaying transition or separation is a relatively easier task. In contrast, classical control strategies are often ineffective for fully turbulent flows. Newer ideas for turbulent flow control to achieve skin-friction drag reduction, for example, focus on the direct onslaught on coherent structures. Spurred by the recent developments in chaos control, microfabrication, and soft computing tools, reactive control of turbulent flows through which sensors detect oncoming coherent structures and actuators attempt to modulate these quasi-periodic events favorably is now in the realm of the possible for future practical devices.