Boundary layer control
Boundary layer control (BLC) refers to methods of controlling the behaviour of fluid flow boundary layers. This holds particular interest in aeronautical engineering because drag may be reduced whilst achieving high lift.
In the case of a freestream flow past a cylinder, a number of 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.
In aeronautical engineering, boundary layer control refers to a number of methods of controlling the boundary layer of air on the main wing of an aircraft. In doing so, parasitic drag can be greatly reduced and performance likewise increased, while the usable angle of attack can be greatly increased, thereby dramatically improving lift at slow speeds. An aircraft with a boundary layer control system thus has greatly improved performance over a similar plane without such a system, often offering the otherwise contradictory features of STOL performance and high cruising speeds.
Much research was conducted to study the lift performance enhancement due to suction for aerofoils in the 1920s and 1930s at the Aerodynamische Versuchsanstalt in Goettingen. An example of an aircraft which uses BLC is the Japanese sea plane the ShinMaywa US-1. This large four-engined aircraft is used for anti-submarine warfare (ASW) and search and rescue (SAR). It is capable of STOL operation and very low air speeds, useful for both ASW and SAR.
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- Blown flap
- Coandă effect
- High-lift device
- Circulation control wing
- Leading edge slot
- Boundary layer suction
- Vortex generator
- Hermann Schlichting, Klaus Gersten, E. Krause, H. Jr. Oertel, C. Mayes "Boundary-Layer Theory" 8th edition Springer 2004 ISBN 3-540-66270-7
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