Laminar Flow Control (LFC) describes technical means to control the boundary layer development. Laminar flow control consists in a boundary layer suction (that is removal of some flow through surface holes), or a wall cooling. A more unusual technique consists in using resonant walls.
A boundary layer usually changes from laminar to turbulent, due to disturbances in the viscous layer (Tollmien-Schlichting waves) that amplify. Amplification, though, occurs only if certain stability criteria (depending on Reynolds number, free-stream turbulence, surface conditions, forced vibrations, etc.) are not satisfied.
Boundary layer stability is a large topic on its own. We will limit the following considerations to a few practical aspects of LFC.
Boundary layer suction requires a propulsion system that consumes energy. The method is effective if the power required to activate the LFC system is less the power saved thanks to the boundary layer control.
The normal velocity required to suck part of the boundary layer are very small and have no macroscopic effect on the surface pressure distribution.
The surface must be of superior quality and with minimum roughness, the normal suction must be as uniform as possible, to avoid further distortions in the flow structure.
Table 1 lists a number of results achieved with LFC wings (Pfenninger, 1977).