Flow control is a major rapidly evolving field of fluid dynamics. It implies a small change of a configuration serving an ideally large engineering benefit, like drag reduction, lift increase, mixing enhancement or noise reduction. This change may be accomplished by passive or active devices. Passive devices, like turbulators or roughness elements, are steady and require no energy by definition. Active control requires actuators that may be driven in a time-dependent manner and require energy. Examples are valves and plasma actuators. The actuation command may be pre-determined (open-loop control) or be dependent on sensors monitoring the flow state (closed-loop control).