There are many physical phenomena and laws discovered by man quite by accident. Starting with the legendary apple that fell on the head of Isaac Newton and peacefully taking a bath of Archimedes to the latest discoveries in the field of creating new materials and biochemistry. The Coanda effect belongs to the same series of discoveries. Oddly enough, but its practical application in technology is still still at the very initial stage. So, the Coanda effect - what is it?
Discovery story
Romanian engineer Henri Coanda during the tests of his experimental aircraft equipped with a jet engine, but having a wooden hull, in order to avoid ignition of the hull from a jet stream, installed protective metal plates on the sides of the engines. However, the effect of this turned out to be the opposite of what was expected. Expiring jet streams for some unknown reason began to be attracted to these protective plates and the wooden structures of the airframe located in the area of their location could ignite. The tests ended in an accident, but the inventor himself was not injured. All this happened at the very beginning of the 20th century.
Experimental verification
The Coanda effect is a phenomenon that can be checked without leaving the kitchen. If you open the water in the tap and bring a flat plate to the water stream, you can see this effect with your own eyes. Water will be slightly noticeably deviated towards the plate. At the same time, the water flow rate may not be very high. In principle, this phenomenon is observed in any environment: water or air. The main thing is the presence of a medium flow and the presence of a surface adjacent to this stream on one side.
By the way, this phenomenon has another name - the effect of the teapot. Due to this effect, when the kettle is tilted, water does not fall into the cup, but flows down the spout, filling the tablecloth, and sometimes the knees of others. Since the laws of hydrodynamics and aerodynamics as a whole, with few exceptions, are practically identical so as not to be repeated, in the future the Coanda effect will be considered for the air environment.
Physics of the phenomenon
The Coanda effect is based on the resulting pressure difference in the flow in the presence of a wall restricting this flow, preventing the free access of air from one side. Any air stream consists of layers having different speeds. Moreover, it was experimentally proved that the friction force between the air layer and the adjacent solid surface is less than between the individual layers of air. Thus, the speed of the layer of air passing near the surface is higher than the speed of the layer of air distant from this surface.
Moreover, at a sufficiently large distance, the velocity of one of the layers of air relative to the surface will generally be zero. It turns out an inhomogeneous velocity field along the height of the stream. In accordance with the laws of gas dynamics, there arises a transverse difference in pressure, which deflects the flow in the direction of lower pressure, that is, where the velocity of the air layer is higher - towards the bounding wall. By choosing the shape of the nozzle and surface, experimenting with distances and speeds, you can change the direction of flow in a fairly wide range.
Maths
For a very long time, the described phenomenon was not recognized at all, despite its obviousness and the relative ease of experimental verification. Then the need arose for theoretical calculations of the force and the vector of this force, that is, to make the calculation of the Coanda effect. Such calculations were made for different types of jets.
The derived formulas are rather cumbersome and represent a combination of differential calculus with trigonometry. But these complex and multi-stage calculations can only give an approximate result. Of course, all this is considered not on paper, but using modern algorithms embedded in computers. However, real values can only be obtained experimentally. Too many factors affect this effect, and not all of them can be described using mathematical formulas.
What does this phenomenon depend on
If you miss a thorough analysis of formulas that require outstanding skill, the strength of the Coanda effect depends on the flow rate, the ratio of the flow diameter and the wall curvature. The experiments showed that the location and diameter of the nozzle, the roughness of the wall surface, the distance between the flow and the wall bounding it, and also the shape of the wall itself are of great importance. It is also noted that the Coanda effect is more pronounced in a turbulent flow.
What else did the discoverer come up with
After the discovery of the phenomenon, A. Coanda began to develop it and search for practical application. The result of his efforts was a patent for the invention of a flying umbrella. If nozzles emitting a gas stream are installed in the center of a hemisphere similar to an umbrella, then, in accordance with the Coanda effect, this stream will be pressed to the surface of the hemisphere and flow down, creating a region of reduced pressure above the umbrella, pushing it up. The inventor himself called it the wing of an airplane, rolled into a ring.
Attempts to put this invention into practice were unsuccessful. The reason is the instability of the device in the air. However, the latest achievements in the field of intelligent control of unstable structures in the air, the so-called Fly by Wire principle, gives hope for the appearance of this exotic aircraft.
What did you manage to implement
Although the inventor’s umbrella could not be raised into the air, the Coanda effect in aviation is used, but, relatively speaking, in secondary areas. Of the most prominent examples, a helicopter without a tail rotor developed in the 1940s, the functions of which to compensate for the rotation of the rotor, was carried out by a fan and nozzles with special guides. The same system made it possible to control the helicopter in yaw and pitch. This has been applied on MD 520N, MD 600N and MD Explorer helicopters.
On airplanes, the Coanda effect is, first of all, an increase in the lifting force by additional blowing from the engine of the upper surface of the wing, which gives the maximum effect when mechanization is launched, that is, when the wing has the most “convex” profile, allowing the flow to go almost plumb down. This is implemented on Soviet aircraft An-72, An-74, as well as An-70. All these machines have improved takeoff and landing characteristics, allowing the use of short strips for takeoff and landing.
Of American technology, you can call the Boeing S-7, using the same principle, as well as a number of experimental machines. In the post-war period, many attempts were made to create an aircraft based on the principles of the Coanda effect. All of them had the shape of a flying saucer, and all of them, after a certain time, were closed due to technical difficulties. Perhaps these works in a strictly guarded form are underway at the present time.
From heaven to earth and under water
To increase the adhesion of the wheels to the track, the Coanda effect began to be used in the design of Formula 1 cars. The machines are equipped with diffusers and fairings, to which the flow of exhaust gases is pressed, providing the desired effect. The picture above shows the movement of exhaust gases adhering to the contours, despite the fact that the exhaust pipe itself is looking up.
In addition to land transport, experimental work was carried out and is being carried out related to the use of this phenomenon in submarines. In particular, a rather exotic underwater bicycle was created in St. Petersburg, for some reason named in English - Blue Space, translated as “blue space”. What he uses for movement is the Coanda effect. In the front of the "underwater bicycle" fairings are installed, in which rowing rollers are mounted, which suck water through special slots. Then the water is pushed to the surface of the machine body, creating traction on its surface. Water flows around the entire hull, being again absorbed into the slot in the stern, it is pushed out.