PROLOGUE

Drag is the force by which a fluid resists the relative motion of a solid. An equal and opposite reaction force acts on the body surface as a result of the fluid deformation, and drag is the component of this force parallel to the direction of the relative velocity vector. The fluid can be external or internal to the solid boundaries, and the solid surface can be rigid or compliant. Billions of gallons of fossil fuel are used annually to overcome the drag encountered by vehicles moving in air or water and the fluid resistance in gas, water, or oil pipelines. Flow control aims at minimizing this drag force, and the subject is explicitly or implicitly interwoven in every chapter of this book. Delaying laminar-to-turbulence transition (Chapter 6) is usually sought in order to benefit from the much lower skin friction associated with laminar boundary layers. Preventing separation (Chapter 8) means reducing the pressure drag, and when separation is provoked, as for example in delta wings, it is desirable to keep the associated drag penalty to a minimum.