Қоңда эффектісі: сұйық ағынының дөңгелек бетке жабысу тенденциясы

Tendency of a fluid jet to stay attached to a convex surface

The Coandă effect ('/k//w//ɑː//n//d//ə/ or '/k//w//æ// /) is the tendency of a fluid jet to stay attached to a convex surface. Merriam Webster describes it as "the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to entrain fluid from the surroundings so that a region of lower pressure develops." 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. It was first documented explicitly in two patents issued in 1936.

[[File:Coanda effect 6. jpg|thumb|300px|right|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. The jet adheres even better to curved surfaces (Diagram 4), because each (infinitesimally small) incremental change in direction of the surface brings about the effects described for the initial bending of the jet towards the surface. If the surface is not too sharply curved, the jet can, under the right circumstances, adhere to the surface even after flowing 180° around a cylindrically curved surface, and thus travel in a direction opposite to its initial direction. The forces that cause these changes in the direction of flow of the jet cause an equal and opposite force on the surface along which the jet flows. A small surface "lip" at the point where the jet starts to flow over that surface (Diagram 5) increases the initial deviation of the jet flow direction. This results from the fact that a low pressure vortex forms behind the lip, promoting the dip towards the surface.

Early sources provide theoretical and experimental information needed to derive a detailed explanation of the effect. The Coandă effect may occur along a curved wall either in a free or wall jet. 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. On the right image, the effect occurs along the curved wall as a wall jet. The image here on the right represents a two dimensional wall jet between two parallel plane walls, where the "obstacle" is a quarter cylindrical portion following the flat horizontal rectangular orifice, so that no fluid at all is entrained from the surroundings along the wall, but only on the opposite side in turbulent mixing with ambient air.

L. C. Woods also made the calculation of the inviscid two dimensional flow of a free jet of width h, deflected round a circularly cylindrical surface of radius r, between a first contact A and separation at B, including a deflection angle θ. Again a solution exists for any value of the relative curvature and angle θ. 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. The surface pressure distribution is then calculated using Bernoulli equation. Let us note the pressure (pa) and the velocity (va) along the free streamline at the ambient pressure, and γ the angle along the wall which is zero in A and θ in B. Then the velocity (v) is found to be:

An image of the surface pressure distribution of the jet round the cylindrical surface using the same values of the relative curvature , and the same angle θ as those found for the wall jet reported in the image on the right side here has been established: it may be found in reference (15) p. 104 and both images are quite similar: the Coandă effect of a free jet is inertial, the same as Coandă effect of a wall jet. However, an experimental measurement of the corresponding surface pressure distribution is not known. Experiments in 1959 by Bourque and Newmann concerning the reattachment of a two dimensional turbulent jet to an offset parallel plate after enclosing a separation bubble where a low pressure vortex is confined (as in the image 5 in the preceding section) and also for a two dimensional jet followed by a single flat plate inclined at an angle instead of the circularly curved wall in the diagram on the right here describing the experience of a wall jet: the jet separates from the plate, then curves towards the plate when the surrounding fluid is entrained and pressure lowered, and eventually reattaches to it, enclosing a separation bubble. The jet remains free if the angle is greater than 62°. 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. The surface pressure distribution as well as the reattachment distance have been duly measured in both cases, and two approximate theories have been developed for the mean pressure within the separation bubble, the position of reattachment and the increase in volume flow from the orifice: the agreement with experiment was satisfactory.

In air conditioning, the Coandă effect is exploited to increase the throw of a ceiling mounted diffuser. Because the Coandă effect causes air discharged from the diffuser to "stick" to the ceiling, it travels farther before dropping for the same discharge velocity than it would if the diffuser were mounted in free air, without the neighbouring ceiling. Lower discharge velocity means lower noise levels and, in the case of variable air volume (VAV) air conditioning systems, permits greater turndown ratios. Linear diffusers and slot diffusers that present a greater length of contact with the ceiling exhibit a greater Coandă effect.

In cardiovascular medicine, the Coandă effect accounts for the separate streams of blood in the fetal right atrium. It also explains why eccentric mitral regurgitation jets are attracted and dispersed along adjacent left atrial wall surfaces (so called "wall hugging jets" as seen on echocardiographic color doppler interrogation). This is clinically relevant because the visual area (and thus severity) of these eccentric wall hugging jets is often underestimated compared to the more readily apparent central jets. In these cases, volumetric methods such as the proximal isovelocity surface area (PISA) method are preferred to quantify the severity of mitral regurgitation. In medicine, the Coandă effect is used in ventilators.

In meteorology, the Coandă effect theory has also been applied to some air streams flowing out of mountain ranges such as the Carpathian Mountains and Transylvanian Alps, where effects on agriculture and vegetation have been noted. It also appears to be an effect in the Rhone Valley in France and near Big Delta in Alaska.

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.

The Coandă effect can be demonstrated by directing a small jet of air upwards at an angle over a ping pong ball. The jet is drawn to and follows the upper surface of the ball curving around it, due to the (radial) acceleration (slowing and turning) of the air around the ball. With enough airflow, this change in momentum is balanced by the equal and opposite force on the ball supporting its weight. 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. A common misconception is that the Coandă effect is demonstrated when a stream of tap water flows over the back of a spoon held lightly in the stream and the spoon is pulled into the stream (for example, uses the Coandă effect to explain the deflection of water around a cylinder). While the flow looks very similar to the air flow over the ping pong ball above (if one could see the air flow), the cause is not really the Coandă effect. Here, because it is a flow of water into air, there is little entrainment of the surrounding fluid (the air) into the jet (the stream of water). This particular demonstration is dominated by surface tension. ( states that the water deflection "actually demonstrates molecular attraction and surface tension.") Another demonstration is to direct the air flow from, e. g., a vacuum cleaner operating in reverse, tangentially past a round cylinder. A waste basket works well. The air flow seems to "wrap around" the cylinder and can be detected at more than 180° from the incoming flow. Under the right conditions, flow rate, weight of the cylinder, smoothness of the surface it sits on, the cylinder actually moves. Note that the cylinder does not move directly into the flow as a misapplication of the Bernoulli effect would predict, but at a diagonal. The Coandă effect can also be demonstrated by placing a can in front of a lit candle, such that when one's line of sight is along the top of the can, the candle flame is completely hidden from view behind it. If one then blows directly at the can, the candle will be extinguished despite the can being "in the way". This is because the airflow directed at the can bends around it and still reaches the candle to extinguish it, in accordance with the Coandă effect.

The engineering use of Coandă effect has disadvantages as well as advantages. In marine propulsion, the efficiency of a propeller or thruster can be severely curtailed by the Coandă effect. The force on the vessel generated by a propeller is a function of the speed, volume and direction of the water jet leaving the propeller. Under certain conditions (e. g., when a ship moves through water) the Coandă effect changes the direction of a propeller jet, causing it to follow the shape of the ship's hull. The side force from a tunnel thruster at the bow of a ship decreases rapidly with forward speed. The side thrust may completely disappear at speeds above about 3 knots. If the Coandă effect is applied to symmetrically shaped nozzles, it presents resonance problems.