The Coanda Effect @·AIRCRAFTUBE

  • The Boeing C-17 Globemaster III uses the Coanda effect at low speeds The Boeing C-17 Globemaster III uses the Coanda effect at low speeds
    The Boeing C-17 Globemaster III uses the Coanda effect at low speeds
  • The Avrocar in 1958 The Avrocar in 1958
    The Avrocar in 1958
  • The Boeing YC-14 used the Coanda effect The Boeing YC-14 used the Coanda effect
    The Boeing YC-14 used the Coanda effect
  • A spinning ping pong ball A spinning ping pong ball
    A spinning ping pong ball

The Coandă effect

The Coandă effect is the tendency of a fluid jet to stay attached to a convex surface. The principle was named after Romanian aerodynamics pioneer Henri Coandă, who was the first to recognize the practical application of the phenomenon in aircraft development.

Discovery

An early description of this phenomenon was provided by Thomas Young in a lecture given to 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 inflection 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 mounted an unusual engine he designed. The motor-driven turbine pushed hot air rearward, and Coandă noticed that the airflow was attracted to nearby surfaces. He discussed this matter with leading aerodynamicist Theodore von Kármán who named it the Coandă effect. In 1934 Coandă obtained a patent in France for a "Method and apparatus for deviation of a fluid into another fluid". The effect was described as the "Deviation of a plain jet of a fluid that penetrates another fluid in the vicinity of a convex wall."

Conditions of existence

Early sources provide information from which a detailed explanation of Coandă effect and its limits can be derived.

Coandă effect may occur along a curved wall either in a free jet or a wall jet.

To compare experience with a calculation we refer to a two-dimensional plane wall jet of width h along a circular wall of radius r. A wall jet follows a flat horizontal wall, say of infinite radius, or better whose radius is that of the Earth, without separation because the surface pressure as well as the external pressure in the mixing zone is everywhere equal to the atmospheric pressure and the boundary layer does not separate from the wall.

With a much smaller radius (12 centimeters in the image) a transverse difference arises between external and wall surface pressure, creating a pressure field depending upon h/r, the relative curvature. This pressure field can appear between a zone around and after the origin of the jet where it gradually arises, and a zone before the point where the jet boundary layer separates at atmospheric pressure where it gradually decreases.

Experiments made in 1956 at a Reynolds number 106 and various widths h show the pressure measured along a circular wall, entered at a horizontal distance from the origin of the jet.

Above a critical h/r ratio = 0.5 only a local effect formed by these two zones each extending over a small angle 9° is observed. This is not a Coandă effect. If the h/r ratio is smaller than the critical value 0.5, an additional deflection that can validly be called a true Coandă effect occurs in between at a nearly constant pressure, as in a conventional wall jet.

A calculation made by L.C. Woods in 1954 of an inviscid flow along a circular wall shows that an inviscid solution exists with any curvature h/r and any given deflection angle up to a separation point on the wall, where a singular point appears with an infinite slope of the surface pressure curve.

Introducing in the calculation the angle at separation found in the preceding experiments for each value of the relative curvature h/r, the image here against was recently obtained which displays the inertial effects represented by the inviscid solution: the calculated pressure field is very similar to the experimental one as described above, outside the nozzle. It is clear that the flow curvature is caused exclusively by the transverse pressure gradient as described by T. Young; then viscosity only produces a boundary layer along the wall and turbulent mixing with ambient air as in a conventional wall jet, except that this boundary layer separates, under the action of the difference between the finally ambient pressure and a smaller surface pressure along the wall. According to Van Dyke, quoted in: Lift (force), the derivation of his equation (4c) also shows that the contribution of viscous stress to flow turning is negligible.

An alternative way would be to calculate the deflection angle at which the boundary layer subjected to the inviscid pressure field separates. A rough calculation has been tried which gives the separation angle as a function of h/r and the Reynolds number: results are reported on image, e.g. 54° calculated instead of 60° measured for h/r = 0.25. More experiments and a more accurate boundary layer calculation would be desirable.

Other experiments made in 2004 with a wall jet along a circular wall show that Coandă effect does not occur in a laminar flow, and the critical h/r ratio for small Reynolds number is much smaller: down to h/r= 0.14 if Re= 500 and h/r= 0.05 if Re=100.

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 aerodynamic lift. The flow from a high speed jet engine mounted in a pod over the wing produces increased 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. Closely following the work of Coandă on applications of his research, and in particular the work on his "Aerodina Lenticulară," John Frost of Avro Canada also spent considerable time researching the effect, leading to a series of "inside out" hovercraft-like aircraft from which 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.

The VZ-9 AV Avrocar (often listed as VZ-9) was a Canadian vertical takeoff and landing (VTOL) aircraft developed by Avro Aircraft Ltd. as part of a secret United States military project carried out in the early years of the Cold War. The Avrocar intended to exploit the Coandă effect to provide lift and thrust from a single "turborotor" blowing exhaust out the rim of the disk-shaped aircraft to provide anticipated VTOL-like performance. In the air, it would have resembled a flying saucer. Two prototypes were built as "proof-of-concept" test vehicles for a more advanced USAF fighter and also for a U.S. Army tactical combat aircraft requirement.

Avro's 1956 Project 1794 for the US military designed a larger-scale flying saucer based on the Coandă effect and intended to reach speeds between Mach 3 and Mach 4. Project documents remained classified until 2012.

The effect was also implemented during the U.S. Air Force's AMST project. Several aircraft, notably the Boeing YC-14 (the first modern type to exploit the effect), NASA's Quiet Short-Haul Research Aircraft, and NAL's Asuka research aircraft have been built to take advantage of this effect, by mounting turbofans on the top of the wings 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 Shin Meiwa US-1A flying boat utilizes a similar system, only it directs the propwash from its four turboprop engines over the top of the wing to generate low-speed lift. More uniquely, it incorporates a fifth turboshaft engine inside of the wing center-section solely to provide air for powerful blown flaps. The addition of these two systems gives the aircraft an impressive STOL capability.

The McDonnell Douglas YC-15 and its successor, the Boeing C-17 Globemaster III, also employ the effect. 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 used in dual-pattern fluid dispensers in automobile windshield washers.

The operation principle of oscillatory flowmeters also relies on the Coandă phenomenon. The incoming liquid enters a chamber that contains 2 "islands". Due to the Coandă 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 oscillation 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 turndown ratios. Linear diffusers and slot diffusers that present a greater length of contact with the ceiling exhibit a greater Coandă effect.

In cardiovascular medicine, the Coandă effect accounts for the separate streams of blood in the fetal right atrium. It also explains 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.

In medicine, the Coandă effect is used in ventilators.

In meteorology, the Coandă effect theory has also been applied to some air streams flowing out of mountain ranges such as the Carpathian Mountains and Transylvanian Alps, where effects on agriculture and vegetation have been noted. It also appears to be an effect in the Rhone Valley in France and near Big Delta in Alaska.

In Formula One automobile racing, the Coandă effect has been exploited by the McLaren, Sauber, Ferrari and Lotus teams, after the first introduction by Adrian Newey (Red Bull Team) in 2011, to help redirect exhaust gases to run through the rear diffuser with the intention of increasing downforce at the rear of the car. Due to changes in regulations set in place by the FIA from the beginning of the 2014 Formula One season, the intention of redirecting exhaust gases to use the Coandă effect have been negated, due to the mandatory requirement that the car exhaust must not have bodywork directly behind the exit for use of aerodynamic effect.

Demonstration

The Coandă effect can be demonstrated by directing a small jet of air upwards at an angle over a ping pong ball. The jet is drawn to and follows the upper surface of the ball curving around it, due to the (radial) acceleration (slowing and turning) of the air around the ball. With enough airflow, this change in momentum is balanced by the equal and opposite force on the ball supporting its weight. This demonstration can be performed using a vacuum cleaner if the outlet can be attached to the pipe and aimed upwards at an angle.

A common misconception is that Coandă effect is demonstrated when a stream of tap water flows over the back of a spoon held lightly in the stream and the spoon is pulled into the stream (for example, Massey in "Mechanics of Fluids" uses the Coandă effect to explain the deflection of water around a cylinder). While the flow looks very similar to the air flow over the ping pong ball above (if one could see the air flow), the cause is not really the Coandă effect. Here, because it is a flow of water into air, there is little entrainment of the surrounding fluid (the air) into the jet (the stream of water). This particular demonstration is dominated by surface tension. (McLean in "Understanding Aerdynamics" states that the water deflection "actually demonstrates molecular attraction and surface tension").

Another 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.

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.

Problems caused

The engineering use of Coandă effect has disadvantages as well as advantages.

In marine propulsion, the efficiency of a propeller or thruster can be severely curtailed by the Coandă effect. The force on the vessel generated by a propeller is a function of the speed, volume and direction of the water jet leaving the propeller. Under certain conditions (e.g., when a ship moves through water) the Coandă effect changes the direction of a propeller jet, causing it to follow the shape of the ship's hull. The side force from a tunnel thruster at the bow of a ship decreases rapidly with forward speed. The side thrust may completely disappear at speeds above about 3 knots.

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