There are many physical phenomena and laws discovered by man quite by accident. Starting from the legendary apple that fell on the head of Isaac Newton, and Archimedes peacefully taking a bath, 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 at the very initial stage. So, what is the Coanda effect?
Discovery history
Romanian engineer Henri Coanda, while testing his experimental aircraft, equipped with a jet engine, but having a wooden body, in order to prevent ignition of the body 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. The expiring jets, for unknown reasons, began to be attracted to these protective plates and the wooden structures of the airframe located in the area of \u200b\u200btheir placement could ignite. The tests ended in an accident, but the inventor himself did notSuffered. All this happened at the very beginning of the 20th century.
Experimental verification
The Coanda effect is a phenomenon that you can test from the comfort of your kitchen. If you open the water in the tap and bring a flat plate to the stream of water, you can see this effect with your own eyes. Water will barely noticeably deviate towards the plate. At the same time, the flow rate of water may not be very high. In principle, this phenomenon is observed in any medium: water or air. The main thing is the presence of a medium flow and the presence of a surface adjacent to this flow on one side.
By the way, this phenomenon has another name - the kettle effect. It is thanks to this effect that when the teapot is tilted, the water from it does not fall into the cup, but flows down the spout, flooding 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 free access of air from one side. Any air flow consists of layers with different speeds. At the same time, it has been experimentally proved that the friction force between the air layer and the adjacent solid surface is less than between individual air layers. Thus, the velocity of the air layer passing close to the surface turns out to beabove the speed of the layer of air distant from this surface.
Moreover, at a sufficiently large distance, the speed of one of the layers of air relative to the surface will generally be equal to zero. It turns out a non-uniform field of velocities along the flow height. In accordance with the laws of gas dynamics, a transverse pressure difference arises here, which deflects the flow towards lower pressure, that is, to where the speed of the air layer is higher - towards the bounding wall. By choosing the shape of the nozzle and surface, experimenting with distances and speed, it is possible to change the flow direction in a fairly wide range.
Math
For a very long time, the described phenomenon was not recognized at all, despite its obviousness and the relative ease of experimental verification. Then there was a need for theoretical calculations of the force and the vector of this force, that is, to calculate the Coanda effect. Such calculations were made for different types of jets.
The derived formulas are quite cumbersome and represent a combination of differential calculus with trigonometry. But these complex and multi-step calculations can only give an approximate result. Of course, all this is not calculated on paper, but using modern algorithms embedded in computers. However, real values can only be obtained experimentally. Too many factors contribute to this effect, and not all of them can be described using mathematical formulas.
What does this phenomenon depend on
Leaving aside the thorough analysis of the formulas, which requires extraordinary skill, the strength of the Coanda effect depends on the flow velocity, the ratio of the flow diameter and the curvature of the wall. Experiments have shown that the location and diameter of the nozzle, the roughness of the wall surface, the distance between the flow and the wall limiting it, as well as the shape of the wall itself, are of great importance. It is also noted that the Coanda effect is more pronounced in 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 applications. The result of his efforts was a patent for the invention of a flying umbrella. If nozzles are installed in the center of the hemisphere similar to an umbrella, ejecting a stream of gases, then, in accordance with the Coanda effect, this stream will be pressed against the surface of the hemisphere and flow downward, creating a region of low pressure above the umbrella, pushing it up. The inventor himself called it the wing of an aircraft, rolled into a ring.
Attempts to put this invention into practice have not been successful. The reason is the instability of the apparatus in the air. However, recent advances in the field of intelligent control of unstable structures in the air, the so-called Fly by Wire principle, give hope for the emergence of this exotic aircraft.
What was accomplished
Although it was not possible to lift the inventor's umbrella into the air, the Coanda effect inaviation is used, but, relatively speaking, in secondary areas. Of the most outstanding examples, one can cite a helicopter without a tail rotor developed in the 40s, whose functions to compensate for the rotation of the main rotor were performed by a fan installed in the rear and nozzles with special guides. The same system made it possible to control the helicopter in yaw and pitch. This has been applied on the MD 520N, MD 600N and MD Explorer.
On airplanes, the Coanda effect is, first of all, an increase in lift by additional airflow from the engine to the upper surface of the wing, which gives the maximum effect when the mechanization is released, that is, when the wing has the most "convex" profile, allowing the flow to leave almost vertically down. This has been implemented on Soviet An-72, An-74, and An-70 aircraft. All of these machines have improved takeoff and landing characteristics, allowing the use of short takeoff and landing lanes.
From American technology, we can name the "Boeing C-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. It is possible that these works are being carried out in a strictly guarded form at the present time.
From heaven to earth and under water
To increase the grip of the wheels with the track, the Coanda effect began to be usedand in the designs of Formula 1 cars. The machines are equipped with diffusers and fairings, against which the flow of exhaust gases is pressed, providing the desired effect. The picture above shows the movement of exhaust gases sticking to the contours, despite the fact that the exhaust pipe itself is pointing up.
In addition to land transport, experimental work has been and is being carried out related to the use of this phenomenon on submarines. In particular, a rather exotic underwater bike was created in St. Petersburg, for some reason called in English - Blue Space, translated as "blue space". What he uses to move around is the Coanda effect. Fairings are installed in front of the “underwater bike”, in which rowing rollers are mounted, sucking water through special slots. The water is then pushed onto the surface of the machine body, creating thrust on its surface. Water flows around the entire hull, being sucked back into the slot in the stern, and pushed out.
