Everything about Coand Effect totally explained
The
Coandă effect is the tendency of a
fluid jet to stay attached to an adjacent curved surface. 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'll 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' 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 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.
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.
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.
Further Information
Get more info on 'Coand Effect'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://coand___effect.totallyexplained.com">Coandă effect Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
