Coandă effect

With the collective term Coandă effect different causally unrelated phenomena are known which suggest a tendency of a gas jet or a liquid flow, " run along " on a convex surface, rather than to peel off and to move on in the original direction of flow.

A precise definition and demarcation for Bernoulli effect are difficult. In the scientific literature the term is often used.

History

Henri Coandă built in 1910 his first aircraft, the Coandă - 1910. It should be a Thermojet, a combination of kolbengetriebenem compressor and two combustion chambers, driven. The engine was mounted on the fuselage nose and should expel the two push beams obliquely backwards / outwards. In the first trial, however, Coandă observed that the hot gases of the fuselage contour followed and streamed along it. The aircraft was destroyed in this work, more practical tests with jet engines were not made until about 30 years later.

The Coandă effect as the Avrocar also used for configuration of " flying saucers ", which earned him an aura of mystery. Today, there are technical applications of the effect in the aerospace, Formula 1 and other fields.

Liquid flows

The adhesion of the liquid to flow around solids is due to the molecular interactions, such as Van -der- Waals interaction. These cause the adherence of liquids, also on the underside of horizontal surfaces. The deflection of the jet of water is therefore not due to the Coanda effect, which is based on an interaction of the fluid jet by the surrounding fluid in relation to an adjacent surface.

Gas jets

In a " moving " surface can conversely show how they are " attached" to a gas jet. The following simple experiment shows this:

The experiment is very simple to implement and is erroneously often consulted for explanation of lift on the wings. However, it does not explain the origin of the lift to the wings, as these flows around different.

A side effect is the ping-pong ball, which "hangs" in an oblique air jet: Due to the Coanda effect, the flow of the air jet does not solve the ball from but surrounded him ( almost) completely without detachment. Since the ball slightly depends beneath the center of the air jet, the flow around is performed not symmetrical. It is deflected more air downwardly, as the flow velocity, and the beam cross-section relative to the upper surface is lower at the bottom of the ball. In response, the ball receives an upward force. This is done in superposition with the Magnus effect (the ball rotates). Both effects - every man for himself - let the ball do not fall down, but " slip " only at the bottom of the air jet along. The resistance that the ball the flow opposes keeps him at a distance to the nozzle and prevents gravity that it is simply blown away. Thus, the ball floating in a more or less stable position.

Explanation

Coandă described the following: a gas - jet - ie a spatially very limited flow, the environment is quite different from the (mostly dormant ) - flows along a surface. If the area receding from the original direction of flow of the jet not flow straight on, but follows the surface. Coandă had (ie not with rays) already prior to that observation extensive experiments with "normal" thought and was certainly aware that normal currents of a convex rounding can follow only limited and then peel off.

It is ( for brevity referred to as the " Coanda flow " only here in the following ) with a normal flow, for example, compared to an airfoil just be here a flow according to the Coanda effect. What is striking is the much greater ability of the Coanda flow to follow a convex wall and not to delaminate. Therefore, it is useful to work out especially the differences between Coandă and normal flow in addition to the similarities.

Both types of flow exist near the wall of a very thin frictional boundary layer (in the graphic dark gray) and further out from a prospective of the wall friction flow. In the boundary layer decides the conditions under which the flow along the wall near the wall comes to a stop and then consequently can leave the wall (see stall ).

In a normal flow applies outside the boundary layer Bernoulli's law. It is here applied as follows: Convex receding wall → more space for the flow → deceleration of the flow due to the mass conservation law → pressure rise due to Bernoulli's law. The graph is the pressure gradient in its braking effect - ie negative - outlines (). In the barrier layer the increase in pressure of undisturbed flow is passed unchanged to the wall, where the flow is therefore slowed down not only by the friction, but also by the pressure increase, which leads to a standstill and to detach soon.

The main difference from the Coanda flow lies in the fact that there is a jet flowing along the wall. The Coanda flow thus consists of the boundary layer, a relatively thin layer undisturbed ( the beam ), but then made ​​a further friction layer to the air masses 'outside' (in the graphic light gray outlines ). The air outside is at rest, so there is no increase in pressure according to the Bernoulli law, in the boundary layer to the wall thus missing a significant cause of the detachment. So the Coanda flow adheres longer than a normal flow. What ultimately leads to the detachment of a Coanda flow, the friction ( shear stress ), the centrifugal force ( in the normal flow mostly not significant ) and in appropriate cases, the force of gravity.

Since in normal flight operations not a jet flowing along the wings, but a normal flow in which outside the boundary layer everywhere Bernoulli's law is valid and pressure rise is produced, the Coanda flow can not be used to explain the emergence of buoyancy.

Circumstances, contrary to the understanding of the Coandă effect:

• The Coanda flow is difficult to understand in three-dimensional space because the detachment not only by stoppage ( "from behind" ) is caused, but also occurs at the sides of the beam by lateral acceleration, the beam is narrower and thicker.
• Heavy transparent is the Coandă effect if it acts in various experiments together with other effects, such as in the above ping-pong ball experiment in which further delayed and the Magnus effect the separation.

Applications

The Coandă effect is used technically successful in aircraft to increase the buoyancy in two versions:

The engine is positioned just above the wing and deflected the thrust jet by a flap system on the wing "suction" down - this is of course possible only in a very small area of the wing, the rest of the wing works in a "normal" flow. One of its first applications was the effect on the Soviet Antonov An-72 An-32/Antonow and a candidate of the " AMST project" of the U.S. Air Force (Advanced Medium STOL Transport ), the YC- 14th If this arrangement is to bring benefit, they require tremendous power settings, also the wing flaps must be built and protected particularly strong in the area of the shear beam. Similarly, there are major problems in the controllability and safety ( for example, assuming engine failure ).

The second application is a mixture of Coandă and "normal" flow: the jet is blown into the already strongly developed boundary layer of a "normal " flow, to let them continue to flow to valves, etc., than would otherwise be possible. This is not a "pure Coandă flow " more, because the flow in the area is only to be "improved": At the outer shear layer, the high speed of the beam to be transmitted to the already slow boundary layer of the outer flow.

Successful applications of this principle, there are at conventional wings in the area of the nose and end flaps ( boundary-layer blow-out ), for example in the large used by the Japanese Navy and Coast Guard flying boats of the manufacturer Shin Meiwa. Also, this application requires very high power settings, because the strong rays must indeed be generated. The "normal" flow can be improved with such measures in special flight modes ( slow flight during takeoff and landing ), but a normal flight condition is not so for cost reasons influenced.

A spectacular application is the NOTAR helicopter on which the tail rotor can be saved: On running a tube round boom is guided around the boom in the area of the rotor downdraft by blowing air from the downdraft that it partially offsets the rotor anti-torque. In addition, however, a variable steering is still needed at the end of the boom. The advantages are the saving serious and complex mechanics and the significant safety benefits. The Prize: An additional inboard fan for generating the air flow to the tail boom. A running pattern is the MD Explorer.

In 2012, this principle has held one way into the formula: The exhaust systems utilize this effect to create more downforce by the exhaust gases are led to the split between rear wheels and base plate and thus shield the diffuser against lateral Einströmungen.

Other applications are in the Heating and ventilation as well as in kitchens and laboratories. Drip-free pouring of liquids - especially from a highly filled beaker ( with spout ) - is applied along a glass rod. Spout there is to beverage cans and beverage packages and ( alcoholic ) bottles. Korkverstoppelte old bottles from the drugstore had a ausgießgünstigen collar, medical and laboratory glass bottles have formed mostly pouring rings made ​​of plastic or even from the edge of the glass itself. The long list of Coanda's U.S. patents now also includes jets for the carburetor.

In water intakes use maintenance-free filter the Coandă effect on inclined inlet screens.