Coandă

Coandă effect

The Coandă effect ('kwandə) is the tendency of a fluid jet to stay attached to an adjacent curved surface that is very well shaped . The principle was named after Romanian Henri Coandă, who was the first to recognize the practical application of the phenomenon in aircraft development.

Discovery

An early description of the Coanda effect was provided by Thomas Young in a lecture given to the The Royal Society in 1800:

The lateral pressure which urges the flame of a candle towards the stream of air from a blowpipe is probably exactly similar to that pressure which eases the inflexion of a current of air near an obstacle. Mark the dimple which a slender stream of air makes on the surface of water. Bring a convex body into contact with the side of the stream and the place of the dimple will immediately show the current is deflected towards the body; and if the body be at liberty to move in every direction it will be urged towards the current.

A hundred years later, Henri Coandă identified an application of the effect 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 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 his "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 attracting a spoon as it flows over the bowl, the Coandă effect is a result of surface tension or Van der Waals forces plus Newton's second and third laws. The effect, in this case, is actually caused by attractive forces. In the instance of a gas flow over a convex curved surface in ambient gas, however, the Coandă effect is a result of the momentum of the gas and entrainment of ambient gas and has nothing to do with attractive forces. As a gas flows over a convex 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 flow, rarefies the slow-moving fluid trapped between the flow and the upper surface of the airfoil. Supersonic flows have a different behavior and gas flows in a liquid, for instance in submarine propulsion using the Coandă effect, are also more complicated.

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 sheet blowing over the curved surface of the top of the wing. The bending of the flow results in its acceleration and as a result of Newton's Third Law pressure is decreased; aerodynamic lift is increased. The flow from a high speed jet engine mounted in a pod over the wing produces enhanced lift by dramatically increasing the velocity gradient in the shear flow in the boundary layer. In this velocity gradient particles are blown away from the surface, thus lowering the pressure there.

The effect 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 optimizing the Coandă effect, the water flows though the screen to the penstocks leading the water to the turbines.

The Coandă effect is also used to make automotive windshield washers which function without moving parts and to create pneumatic logic circuits.

The operation principle of oscillatory flowmeters also relies on the Coanda phenomenon. The incoming liquid enters a chamber that contains 2 "islands". Due to the Coanda effect the main stream splits up and goes under one of the islands. This flow then feeds itself back into the main stream making it split up again, but in the direction of the second isle. This process repeats itself as long as the liquid circulates the chamber, resulting in a self induced oscillation that is directly proportional to the velocity of the liquid and consequently the volume of substance flowing through the meter. A sensor picks up the frequency of this oscillations and transforms it into an analog signal yielding volume passing through.

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.

In cardiovascular medicine, the Coanda effect has been used to explain why eccentric mitral regurgitation jets are attracted and dispersed along adjacent left atrial wall surfaces (so called "wall-hugging jets" as seen on echocardiographic color-doppler interrogation). This is clinically relevant because the visual area (and thus severity) of these eccentric wall-hugging jets is often underestimated compared to the more readily apparent central jets. In these cases, volumetric methods such as the proximal isovelocity surface area (PISA) method are preferred to quantify the severity of mitral regurgitation.

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 to run over the back of the spoon. 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.

A third demonstration is to direct the air flow from, e.g., a vacuum cleaner operating in reverse, tangentially past a round cylinder. A waste basket works well. The air flow seems to "wrap around" the cylinder and can be detected at more than 180° from the incoming flow. Under the right conditions, flow rate, weight of the cylinder, smoothness of the surface it sits on, the cylinder will actually move. Note that the cylinder will not move directly into the flow as a misapplication of the Bernoulli effect would predict, but at a diagonal.

References

See also

External links

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