Boundary layer control

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.

Frank E. Fish states that dolphins appear to have a turbulent boundary layer to reduce the likelihood of separation and minimize drag, and that mechanisms for maintaining a laminar boundary layer to reduce skin friction have not been demonstrated for dolphins.

The wings of birds have a leading edge feature called the Alula which delays wing stalling at low speeds in a similar manner to the leading edge slat on an aircraft wing.

Thin membrane wings found on bats and insects have features which appear to cause favourable roughening at the Reynolds numbers involved, thereby enabling these creatures to fly better than would otherwise be the case.

Balls may be given features which roughen the surface and extend the hit or throw distance. Roughening causes the boundary layer to become turbulent and remain attached farther round the back before breaking away with a smaller wake than would otherwise be the case. Balls may be struck in different ways to give them spin which makes them follow a curved path. The spin causes boundary layer separation to be biased to one side which produces a side force.

BL control (roughening) was applied to golf balls in the 19th century. The stitching on cricket balls and baseballs acts as a boundary layer control structure.

In the case of a freestream flow past a cylinder, three methods may be employed to control the boundary layer separation that occurs due to the adverse pressure gradient. Rotation of the cylinder can reduce or eliminate the boundary layer that is formed on the side which is moving in the same direction as the freestream. The side moving against the flow also exhibits only partial separation of the boundary layer. Suction applied through a slit in the cylinder near a separation point can also delay the onset of separation by removing fluid particles that have been slowed in the boundary layer. Alternatively, fluid can be blown from a faired slit such that the slowed fluid is accelerated and thus the point of separation is delayed.

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