On the need to reassess the design tools for active flow control

The pressure distribution on a surface, over which a wall-jet is blowing, is altered by the wall jet's entrainment. It renders the boundary layer approximation – that justifies the use of an inviscid flow solution to determine the pressure over the surface – invalid. Thus, in order for Active Flow Control (AFC) by blowing to become a viable technology, some of the preconceptions associated with Boundary Layer Control (BLC) for many decades must be discarded. In particular, the momentum coefficient used to characterize BLC should be replaced by another variable that represents a conserved quantity that is independent of specific installations. Injected momentum is a vector quantity whose effect on a surface like a wing depends on its specific design, location, and orientation. Therefore, a new approach is proposed based on the AFC system's power consumption and its mass flowrate. Moreover, all flow installations suffer from unavoidable losses, which must be determined in an unambiguous manner, allowing for an impartial comparison of AFC systems. The present article provides examples from tests carried out at various universities and at NASA, exposing some popular misconceptions. It does not provide a design tool due to the complexity of the needed approach, but a method to assess the efficacy and efficiency of an evolving platform that includes AFC is suggested.