Coandă effect

"[2] It is named after Romanian inventor Henri Coandă, who was the first to recognize the practical application of the phenomenon in aircraft design around 1910.

The motor-driven turbine pushed hot air rearward, and Coandă noticed that the airflow was attracted to nearby surfaces.

[4][5] This name was accepted by the leading aerodynamicist Theodore von Kármán, who had a long scientific relationship with Coandă on aerodynamics problems.

The resultant forces from this low pressure tube end up balancing any perpendicular flow instability, which stabilises the jet in a straight line.

[10] Early sources provide theoretical and experimental information needed to derive a detailed explanation of the effect.

On the left image of the preceding section: "The mechanism of Coandă effect", the effect as described, in the terms of T. Young as "the lateral pressure which eases the inflection of a current of air near an obstacle", represents a free jet emerging from an orifice and an obstacle in the surroundings.

It includes the tendency of a free jet emerging from an orifice to entrain fluid from the surroundings confined with limited access, without developing any region of lower pressure when there is no obstacle in the surroundings, as is the case on the opposite side where turbulent mixing occurs at ambient pressure.

[11][12] Above a critical ⁠h/r⁠ ratio of 0.5 only local effects at the origin of the jet are seen extending over a small angle of 18° along the curved wall.

A calculation made by Woods in 1954[13] 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 was recently obtained,[14][self-published source?]

and shows inertial effects represented by the inviscid solution: the calculated pressure field is similar to the experimental one described above, outside the nozzle.

According to Van Dyke,[15] as quoted in Lift, 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.

Moreover, in the case of a free jet the equation can be solved in closed form, giving the distribution of velocity along the circular wall.

In this last case which is the geometry proposed by Coandă, the claim of the inventor is that the quantity of fluid entrained by the jet from the surroundings is increased when the jet is deflected, a feature exploited to improve the scavenging of internal combustion engines, and to increase the maximum lift coefficient of a wing, as indicated in the applications below.

Closely following the work of Coandă on applications of his research, and in particular the work on his "Aerodina Lenticulară,"[19] 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".

Two prototypes were built as "proof-of-concept" test vehicles for a more advanced U.S. Air Force fighter and also for a U.S. Army tactical combat aircraft requirement.

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.

A better understanding of Coandă effect was provided by the scientific literature produced by ACHEON EU FP7 project.

[29][30] A practical use of the Coandă effect is for inclined hydropower screens,[31] which separate debris, fish, etc., otherwise in the input flow to the turbines.

[38] 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.

[39] 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 not have bodywork intended to contribute to aerodynamic effect situated directly behind it.

[41][42] The Coandă effect can be demonstrated by directing a small jet of air upwards at an angle over a ping pong ball.

This demonstration can be performed using a hairdryer on the lowest setting or a vacuum cleaner if the outlet can be attached to the pipe and aimed upwards at an angle.

Another demonstration is to direct the air flow from, e.g., a vacuum cleaner operating in reverse, tangentially past a round cylinder.

A ping pong ball is held in a diagonal stream of air. This is a demonstration of the Coandă effect. The ball "sticks" to the lower side of the air stream, which stops the ball from falling down. The jet as a whole keeps the ball some distance from the jet exhaust, and gravity prevents it from being blown away.
A diagram of a generic engine that harnesses the Coandă effect to generate lift (or forward motion if tilted 90° on its side). The engine is approximately bullet or inverted bowl shaped, with fluid being expelled horizontally from a circular slit near the top of the bullet. A small step at the lower edge of the slit ensures that a low pressure vortex develops immediately below the point where the fluid exits the slit (see Diagram 5). From there on the Coandă effect causes the sheet of fluid to cling to the curved outer surface of the engine. The entrainment of the ambient fluid into the stream flowing over the bullet, causes a low pressure area above the bullet (Diagrams 1–5) . This, together with the ambient ("high") pressure below the bullet causes lift, or, if mounted horizontally, forward motion in the direction of the apex of the bullet. [ 7 ]
Measurements of surface pressure along a circularly curved wall of radius ( r = 12 cm), deflecting a turbulent jet of air ( Reynolds number = 10 6 ) of width ( h ). The pressure begins to fall before the origin of the jet, due to local effects at the point of exit of the air from the nozzle which creates the jet. If the ⁠ h / r ⁠ ratio (ratio of the width of the jet to the radius of curvature of the wall) is less than 0.5, a true Coandă effect is observed, with the wall pressures along the curved wall remaining at this low (sub-ambient pressure) level until the jet reaches the end of the wall (when the pressure rapidly returns to ambient pressure). If the ⁠ h / r ⁠ ratio is more than 0.5, only the local effects occur at the origin of the jet, after which the jet immediately separates from the wall, and there is no Coandă effect. Experiments by Kadosch and Liermann in Kadosch's laboratory, SNECMA. [ 11 ]
Pressure distribution along the circular wall of a wall jet
The first Avrocar being readied at the Avro Canada factory in 1958
A Coandă engine (items 3,6–8) replaces the tail rotor in the NOTAR helicopter. 1 Air intake. 2 Variable pitch fan. 3 Tail boom with Coandă Slots. 4 Vertical stabilizers. 5 Direct jet thruster. 6 Downwash. 7 Circulation control tailboom cross-section. 8 Anti-torque lift.
A depiction of the Blackburn Buccaneer aircraft. Blowing slots at the leading edges of the wing , tailplane and trailing edge flaps / ailerons are highlighted. These aerodynamic features contribute to the Coandă airflow over the wing.
The C-17 Globemaster III has externally blown flaps with part of the engine flow passing through the flap slots to be turned over the top surfaces by the Coandă effect.