**1. Introduction**

The Boundary Layer (BL) dynamic performance greatly affects the forces acting on any Bluff body. Ideally, the boundary layer should be attached to the surface but when separation occurs, the vortical structures and the dynamic forces' amplitude rapidly increase. In many aerodynamic applications, the Bluff bodies are shaped in such a way that the boundary separation is delayed as much as possible. Nevertheless, using novel technologies, it is possible to reattach the previously separated BL, or at least further delay its separation. One of the novel techniques which allows for the modification of the separation point of the BL is Active Flow Control (AFC). This consists of injecting/sucking fluid in pre-defined locations. In the vast majority of AFC applications, it is essential to perform an energy assessment in order to make sure that the energy saved by the reduction/increase of the forces due to the modification of the BL separation point is much larger than the energy employed for the actuation. In order to achieve this goal, it is essential to properly tune the five parameters associated to any AFC implementation, groove position, groove width, momentum coefficient, jet inclination angle and jet frequency. Such tuning can be carried out via a parametric optimization or using any optimizer. In other words, AFC is always associated with optimization methodologies; otherwise, the energy assessment cannot be successfully accomplished. The present book is based on a set of published articles that highlight some novel applications of flow control.
