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Boundary layer control refers to methods of controlling the behaviour of fluid flow boundary layers.
It may be desirable to reduce flow separation on fast vehicles to reduce the size of the wake (streamlining), which may reduce drag. Boundary layer separation is generally undesirable in aircraft high lift coefficient systems and jet engine intakes.
Laminar flow produces less skin friction than turbulent but a turbulent boundary layer transfers heat better. Turbulent boundary layers are more resistant to separation.
The energy in a boundary layer may need to be increased to keep it attached to its surface. Fresh air can be introduced through slots or mixed in from above. The low momentum layer at the surface can be sucked away through a perforated surface or bled away when it is in a high pressure duct. It can be scooped off completely by a diverter or internal bleed ducting. Its energy can be increased above that of the free stream by introducing high velocity air.