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The Coandă effect (IPA: *), also known as "boundary layer attachment", is the tendency of a stream of fluid to stay attached to a convex surface, rather than follow a straight line in its original direction. The principle was named after Romanian inventor Henri Coandă, who was the first to understand the practical importance of the phenomenon for aircraft development. He made the discovery during experiments with his Coandă-1910 aircraft, which is the first aircraft based on an early type of jet engine.
It has important applications in various high-lift devices on aircraft, where air moving over the wing can be "bent down" towards the ground using flaps and a jet blowing over a curved surface. The flow from high speed jet produces enhanced lift through turbulent mixing that does not occur above a normal wing. It was first implemented in a practical sense during the U.S. Air Force's AMST project. Several aircraft, notably the Boeing YC-14 (the first modern type to exploit the effect), have been built to take advantage of this effect, by mounting turbofans on the top of wing to provide high-speed air even at low flying speeds, but to date only one aircraft has gone into production using this system to a major degree, the Antonov An-72 'Coaler'. The McDonnell Douglas YC-15 and its successor, the Boeing C-17 Globemaster III, also employ the effect, though to a less substantial degree.
Closely following the work of Coandă on applications of his research, and in particular the work on Aerodina Lenticulara, John Frost of Avro Canada also spent considerable time researching the effect, leading to a series of "inside out" hovercraft-like aircraft where the air exited in a ring around the outside of the aircraft and was directed by being "attached" to a flap-like ring. This is as opposed to a traditional hovercraft design, in which the air is blown into a central area, the plenum, and directed down with the use of a fabric "skirt". Only one of Frost's designs was ever built, the Avrocar.
Demonstration
If one holds the back of a spoon in the edge of a stream of water running freely out of a tap (faucet), the stream of water will deflect from the vertical in order to run over the back of the spoon. This is the Coandă effect in action.
This demonstration is the combination of the Venturi effect and the Coandă effect. The Venturi effect would cause a drop in pressure between the spoon and the stream of water, which would then be drawn towards the spoon. Once the surface of the stream hits the spoon, the Coandă effect keeps it running over the convex surface.
Lift from an airfoil
Some people have attempted to explain how a wing generates lift, by invoking the Coandă effect. However, this theory does not produce quantifiable data, and so it is unable to predict such things as the thickness of the boundary layer. Professional aerodynamicists regard this theory as a fallacy. For example, the theory states that air “sticks” to the surface because of its viscosity. This implies that if the viscosity of the fluid changes, the amount of lift an airfoil produces should change in proportion. Experiments show that the amount of lift produced by a real wing is independent of viscosity over a wide range. The real Coandă effect requires turbulence, so it occurs only if the viscosity is sufficiently low. Furthermore, the air speeds up above a wing's upper surface. The theory assumes that the relative air-flow meets the wing at the same velocity as in free air and then follows the curve. This understates the pressure gradients by an order of magnitude.
Air conditioning
In air conditioning the Coandă effect is exploited to increase the throw of a ceiling mounted diffuser. Because the Coandă effect causes air discharged from the diffuser to "stick" to the ceiling, it travels further before dropping for the same discharge velocity than it would if the diffuser was mounted in free air, without the neighbouring ceiling. Lower discharge velocity means lower noise levels and, in the case of variable air volume (VAV) air conditioning systems, permits greater turn-down ratios. Linear diffusers and slot diffusers that present a greater length of contact with the ceiling exhibit greater Coandă effect.
See also
External links
- The Coandă fallacy
- An explanation of lift invoking the Coandă effect
- A page with a video of the Coandă effect in action
Fluid dynamics | Aerodynamics | Physical phenomena
Coandă-Effekt | Efecto Coanda | Effet Coanda | Effetto Coanda | Coandă-effect | コアンダ効果 | Efekt Coandy | Coandaeffekt | Coanda etkisi | 康达效应
"Coandă effect"