Coandă effect

    The Coandă effect (IPA: ['kwandə]), 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 discoverer Henri Coandă, who was the first to understand the practical importance of the phenomenon for aircraft development.

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Discovery

Henri Coandă made the discovery during experiments with his Coandă-1910 aircraft, which is the first aircraft to use a motorjet (an early type of jet engine). In 1934 he obtained a patent in France for a "Method and apparatus for deviation of a fluid into another fluid". What is today known as the Coandă effect was described by its discoverer as the "Deviation of a plan jet of a fluid that penetrates another fluid in the vicinity of a convex wall."

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.

Causes

In the instance of a stream of water on a spoon, the Coandă effect can be explained largely on the basis of surface tension or Van der Waals forces. In the instance of a gas flow against a surface with ambient gas or liquid flow in ambient liquid, then the Coandă effect can be explained on the basis of momentum and entrainment of the fluid. As a gas flows over an airfoil, the gas is drawn down to adhere to the airfoil by a combination of the greater pressure above the gas flow and the lower pressure below the flow caused by an evacuating effect of the flow itself, which as a result of shear, entrains the slow-moving fluid trapped between the flow and the down-stream end of the upper surface of the airfoil. The effect of a spoon apparently attracting a flow of water is caused by this effect as well, since the flow of water entrains gases to flow down along the stream, and these gases are then pulled, along with the flow of water, in towards the spoon, as a result of the pressure differential. Supersonic flows have a different response.

Applications

The Coandă effect 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 a high speed jet engine mounted in a pod over the wing 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. The NOTAR helicopter replaces the conventional propeller tail rotor with a Coandă effect tail.

An important practical use of the Coandă effect is for inclined hydropower screens, which separate debris, fish, etc., otherwise in the input flow to the turbines. Due to the slope, the debris falls from the screens without mechanical clearing, and due to the wires of the screen optimising the Coandă effect, the water flows though the screen to the penstock leading the water to the turbine.

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. The effect can also be seen by placing a can in front of a lit candle- if one blows directly at the can, the air will bend around it and extinguish the candle.

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 farther 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

• Aerodynamics
• Fluid dynamics
• Boundary layer
• Fluid friction
• Circulation control wing