Words are food for thought; utter the right one and you will have created a delicacy to be savored. And so will begin every chapter in this book with the rich words spoken through the ages by the wisest of all men.

The ability to actively or passively manipulate a flow field to effect a desired change is of immense technological importance, and this undoubtedly accounts for the subject's being hotly pursued at present by more scientists and engineers than any other topic in fluid mechanics. The art of flow control is as old as prehistoric man, whose sheer perseverance resulted in the invention of streamlined spears, sickle-shaped boomerangs, and fin-stabilized arrows. The German engineer Ludwig Prandtl pioneered the science of flow control at the beginning of the twentieth century.

The potential benefits of realizing efficient flow-control systems range from saving billions of dollars in fuel cost for land, air, and sea vehicles to achieving economically and environmentally more competitive industrial processes involving fluid flows. The purpose of this book is to provide an up-to-date view of the fundamentals of some basic flows and control practices that can be employed to produce needed effects. Understanding of some basic mechanisms in free and wall-bounded turbulence has increased substantially in the last few years. This understanding suggests that taming of turbulence—the quintessential challenge in the field of flow control—is possible so as to eliminate some of its deleterious effects while enhancing its useful traits.