Parameters Defining Active Flow Control

There is a conceptual difference between Boundary Layer Control (BLC) and Active Flow Control (AFC). The former aims to compensate for frictional losses in the boundary layer, while the latter seeks to exploit instabilities in the flow to alter its character. The momentum coefficient Cμ has been the parameter of choice for energizing the boundary layer and enhancing the circulation by blowing. While it has proven useful in cataloging some data, it is not a universally valid single parameter describing the complex flow over wings. Furthermore, the jet velocity and density required for its calculation are difficult to measure in situ, necessitating unrealistic assumptions or extensive calibrations. The best quantity to be used is one that defines the energy requirements of the entire AFC system while realizing its lack of uniqueness. Experimental investigations on a thick flapped airfoil and a simple swept wing demonstrate that such a quantity is more suitable for design purposes because it shifts the perspective from detailed jet flow to system needs. Although this figure-of-merit approach black boxes flow interaction details between the jet and the freestream, system-based coefficients can still provide various physical insights in their capacity as cataloguing parameters.