Coanda effect
The collective term Coandă effect denotes various causally unrelated phenomena that suggest a tendency of a gas jet or a liquid flow to "run along" a convex surface instead of detaching and moving on in the original direction of flow.
A precise definition and the differentiation from the Bernoulli effect are difficult. The term is rarely used in scientific literature.
history
Henri Coandă built his first aircraft in 1910, the Coandă-1910 . It was supposed to be powered by a Thermojet , a combination of a piston-driven compressor and two combustion chambers. The motor was attached to the tip of the fuselage and should eject the two thrust jets diagonally backwards / outwards . During the first test, however, Coandă observed that the hot gases followed the contour of the fuselage and flowed along it. The aircraft was destroyed during this work; further practical tests with jet propulsion were not carried out until about 30 years later.
The Coandă effect was also used to project “flying saucers” like the Avrocar . Today there are technical applications of the effect in aviation, Formula 1 and other areas.
Fluid flows
The adhesion of the liquid to solid bodies in a flow is due to the molecular interactions such as the Van-der-Waals interaction . These also cause liquids to adhere to the underside of horizontal surfaces. The deflection of the water jet is therefore not due to the Coanda effect, which is based on an interaction of the fluid jet with the surrounding fluid in connection with an adjacent surface.
Gas jets
In the case of a “moving” surface, conversely, you can also show how it “hangs” on a gas jet. The following simple experiment shows this:
- Bubble along the top (convex) of a piece of paper. It is lifted, "sticks" to the air jet and is kept in suspension. Since the paper is only lightly weighted, an equilibrium is established: the wall is movable, so it can "give way" to the deflection of the flow, and the flow is only deflected slightly - only as much as is necessary to overcome the weight of the paper.
The experiment is very easy to carry out and is mistakenly used to explain the lift on wings . However, it does not explain the origin of lift on wings, because the flow around them is different.
A side effect is the ping-pong ball, which "hangs" in an inclined air jet: Due to the Coandă effect, the flow of the air jet does not detach from the ball, but circles it (almost) completely without detachment. Since the ball hangs slightly below the center of the air jet, the flow around it is not symmetrical. More air is deflected downwards, since the flow velocity and the jet cross-section are lower on the underside of the ball compared to the top. In response, the ball experiences an upward force. This is done in superposition with the Magnus effect (the ball spins). Both effects - each for itself - do not let the ball fall down, but only "slide" along the underside of the air jet. The resistance the ball offers to the flow keeps it at a distance from the nozzle, and gravity prevents it from simply being blown away. This allows the ball to float in a more or less stable position.
Explanation
Coandă described the following: A gas jet - i.e. a spatially narrowly limited flow that differs significantly from the (mostly stationary) environment - flows along a surface. If the surface deviates from the original direction of flow, the jet does not continue to flow in a straight line, but follows the surface. Before this observation, Coandă had already undertaken extensive experiments with “normal” flows (i.e. not with jets) and was certainly aware that normal flows can only follow a convex curve to a limited extent and then separate.
A flow corresponding to the Coandă effect (hereinafter referred to as “Coandă flow” for the sake of brevity) is briefly compared with a normal flow, for example on an airfoil profile. What is striking is the significantly greater ability of the Coandă flow to follow a convex wall and not detach itself. Therefore, it makes sense, in addition to the similarities, to work out the differences between Coandă and normal flow.
Both types of flow consist of a very thin frictional boundary layer near the wall (dark gray in the graphic) and further outside of a flow that is not influenced by wall friction. In the boundary layer it is decided under which conditions the flow along the wall in the immediate vicinity of the wall comes to a standstill and can then leave the wall (see flow separation ).
In a normal flow, Bernoulli's law applies outside the boundary layer . It is to be used here as follows: convex receding wall → more space for the flow → slowing down of the flow due to the law of mass conservation → pressure increase due to Bernoulli's law. In the graphic, the braking effect of the pressure gradient - i.e. negative - is sketched ( ). In the boundary layer, the pressure increase is passed on unchanged from the undisturbed flow to the wall, so there the flow is not only slowed down by the friction but also by the pressure increase, which very soon leads to standstill and separation.
The main difference to the Coandă flow lies in the fact that there a jet flows along the wall (“wall jet effect”). The Coandă flow thus consists of the boundary layer, a relatively thin undisturbed layer (the jet), but then of a further friction layer to the air masses “outside” (outlined in light gray in the graphic). The air outside is at rest , so there is no pressure increase according to Bernoulli's law, so there is no essential cause for the separation in the boundary layer to the wall. The Coandă flow lasts longer than a normal flow. What ultimately leads to the replacement of a Coandă flow is the friction ( shear stress), the centrifugal force ( which is usually not important in normal flow) and in corresponding cases also the force of gravity .
Since in normal flight operations it is not a jet that flows along the wings, but a normal flow in which Bernoulli's law applies everywhere outside the boundary layer and pressure increases are produced, the Coandă flow cannot be used to explain the generation of lift.
Circumstances that stand in the way of understanding the Coandă effect:
- The Coandă flow becomes more difficult to understand in three-dimensional space, because the separation is not only caused by standing still ("from behind"), but also takes place on the sides of the jet by lateral acceleration, the jet becomes narrower and thicker.
- The Coandă effect becomes difficult to understand when it works together with other effects in a wide variety of experiments, such as in the ping-pong-ball experiment mentioned above, in which the Magnus effect also delays the detachment further.
Applications
The Coandă effect is used in aircraft construction to increase lift in two variants:
The engine is positioned just above the wing and its thrust jet is deflected downwards by a flap system on the wing - this is of course only possible 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 Soviet Antonov An-32 , Antonov An-72 , Antonov An-74 and a candidate for the " AMST project " of the US Air Force (Advanced Medium STOL Transport), the YC-14 . If this arrangement is to be of benefit, it requires enormous engine power, and the wing flaps in the area of the thrust jet must be particularly strong and protected. There are also major problems with controllability and safety (for example in the event of an 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 in order to let it flow further around flaps etc. than would otherwise be possible. This is no longer a “pure Coandă flow”, because the flow in the area should only be “improved”: At the outer shear layer, the high speed of the jet should be transferred to the already slow boundary layer of the outer flow.
This principle has been successfully used on conventional hydrofoils in the area of nose and end valves ( boundary layer blow-out ), for example in the large flying boats made by the manufacturer Shin Meiwa used by the Japanese Navy and Coast Guard . This application also requires very high engine performance, because the powerful jets have to be generated. The “normal” flow can be improved with such measures in special flight conditions (slow flight during take-off and landing), but a normal flight condition cannot be influenced for cost reasons.
A spectacular application is the NOTAR helicopter , on which the tail rotor can be saved: On the tubular round boom in the area of the rotor downdraft, the downdraft is directed around the boom by blowing out air so that it partially compensates for the rotor counter-torque . In addition, a variable control nozzle is required at the end of the boom. The advantages lie in the saving of heavy and complex mechanics and in the considerable gain in safety. The price: An additional internal fan to generate the air flow on the tail boom. One executed pattern is the MD Explorer .
In 2012, this principle also found its way into Formula 1 : The exhaust systems use this effect to generate more contact pressure by guiding the exhaust gases to the gaps between the rear wheels and the base plate and thus shielding the diffuser from airflow from the side.
There are other applications in heating and ventilation as well as in the kitchen and laboratory area. Drip-free pouring of liquid - especially from a full beaker (with a spout) - takes place along a glass rod. There are pourers on beverage cans and for beverage packs and (alcohol) bottles. Old cork stopper bottles from the drugstore had a collar that was easy to pour, medicine and laboratory glass bottles usually have pouring rings made of plastic or made from the edge of the glass itself. The long list of Coandă's US patents also includes nozzles for carburetors.
In water catchments, maintenance-free filters use the Coandă effect via inclined inlet sieves.
The Coandă effect is also used for pneumatic or hydraulic control systems. ( Fluidics )
literature
- Alexander Sauberer: Experimental Studies on the Coanda Effect. Dipl.-Arb., Vienna University of Technology 1998.
- Anton Felder: Investigations into the Coanda effect-possible application in civil engineering. Diss., TU Munich 1993.
- Heribert Martinides: Measurements of the turbulent free jet and the Coanda effect. Dipl.-Arb., TU Vienna 1958.
Web links
swell
- ^ Formula 1 exhaust comparison: A problem called Coanda of the magazine AUTO MOTOR UND SPORT
- ↑ Maintenance-free fine sieves in Inox - Thaler system