The taming of the shrew: Why is it so difficult to control turbulence?

Abstract

In the present chapter I shall emphasize the frontiers of the field of flow control, pondering mostly the control of turbulent flows. I shall review the important advances in the field that took place during the past few years and are anticipated to dominate progress in the future. By comparison with laminar flow control or separation prevention, the control of turbulent flow remains a very challenging problem. Flow instabilities magnify quickly near critical flow regimes, and therefore delaying transition or separation are relatively easier tasks. In contrast, classical control strategies are often ineffective for fully turbulent flows. Newer ideas for turbulent flow control to achieve, for example, skin-friction drag reduction 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, where sensors detect oncoming coherent structures and actuators attempt to favorably modulate those quasi-periodic events, is now in the realm of the possible for future practical devices. In this chapter, I shall provide estimates for the number, size, frequency and energy consumption of the sensor/ actuator arrays needed to control the turbulent boundary layer on a full-scale aircraft or submarine. © 2007 Springer-Verlag Berlin Heidelberg.