Coandă effect – Wikipedia

L’ Effetto Coandă (VIOLENCE: [In Ko̯an] ) is the trend of a jet of fluid to follow the outline of a nearby surface. ^[first] The phenomenon owes its name to the pioneer of the Romanian aerodynamics Henri Coandurn, who patented in 1936 first in France ^[2] And then in the United States ^[3] Some tools that exploited the property of deviating a jet.

The fluid, moving along the surface, is subjected to both a friction force, which slows down the fluid particles immediately in contact with the surface, and to an adhesion force, which attracts them to the surface, thus deviating the layer of fluid a contact with the surface. External fluid particles, due to attractive molecular interactions, tend to follow the internal ones. The overall effect is that the layers of fluid closest to the surface tend to join the surface itself, deviating their path.
The Coda effect in the definition provided by Henri Coanda itself consists of three concomitant effects:

• the phenomenon of acceleration of the fluid – and consequent reduction of pressure – on the convex surface,
• the consequent adhesion of the fluid flow to the convex wall;
• The attractive phenomenon that the lower pressure compared to the external environment generates on the surrounding fluid.

When, due to the viscous phenomena that are generated in Limit layer , the pressure increases in the limit layer until you reach the atmospheric one, then you have the detachment and the adhesion phenomenon ceases. ^[4]

The effect can be demonstrated mathematically starting from the integration of Euler’s equations in the normal direction to a curved flow line.

By neglecting the gravitational forces, in fact, a pressure gradient is obtained:

${displaystyle {frac {partial p}{partial r}}=rho {frac {v^{2}}{r}}}$

in which r It is the density of the fluid, in speed and r It is the radius of curvature of the flow line. The same formula is reserved considering the radial balance of an infiniteism element such as the one in the figure: the pressure difference must balance the centrifugal force.

This term, being increasing than zero, denotes the existence of pressure gradients in a normal direction to the curved and directed curved flow lines. Concretely this means that the pressure increases by moving away from the center of curvature, thus pushing the fluid towards it.

More correctly, since we are talking about differential equations, the pressure condition relating to the pressure comes from the outside of the fluid (the pressure outside this viscous layer is bound while the one at the point of contact with the surface can be any) For this reason, it is more correct to describe the change of pressure as a decrease in the same approaching the surface if this is convex, vice versa as an increase in pressure if you approach a concave surface.

This decrease in pressure when approaching the surface therefore justifies the phenomenon of attachment of the flow around a wall known precisely as a coandurn effect, until the viscosity causes separation phenomena and therefore the variation of curvature of the current lines.

The airplanes fly in conditions of high incidence in part thanks also to this effect, which tends to remain the aerodynamic flow adherent to the back of the wing, delaying the stall.

The same considerations can be traced back in the case of rotating wings such as aeroplani propellers and helicopter rotors. However, it is incorrect to say that the flight of a plane takes place thanks to the existence of the Coanda effect. In fact, it should be remembered that in normal flight conditions the COANA effect affects the general aerodynamics of the aircraft in a rather negative way since the cause of the almost all of the entire friction resistance generated in the subsonic regime.

For years, in fact, primarily NASA, he tried through suitable studies to solve the problem of the limit layer, the air layer adhering to the surface bathed by the flow in which the air molecules slowed down to the external flow and flow on those adhering to the surface and therefore at speed nothing with considerable creation of aerodynamic resistance.

For the purpose, suction systems were studied through widespread microphors along the entire wing attack on the purpose of continuously and instantaneously eliminating the infinitesimal layer of stopped air adherent to the surface of the attack edge, in fact the generating area of ​​the most resistance intense.

Starting from the original Newman demonstration ^{[5] [6]} e Bradshaw ^[7] Several models have been produced. A milestone in the direction of a better understanding of the phenomena that govern the COANA effect is certainly the Acheon project ^[8] This project made it possible to model the coanda effect produced by a jet on a convex surface ^[9] and the adhesion produced by two jets at different speeds. ^[ten] This model has been largely verified and validated by various authors. ^{[11] [twelfth] [13]} Starting from this theoretical work they have been possible models of dried planes with deflection systems of the jet of this type. ^{[14] [15] [16] [17] [18]} This project also allowed to study the influence of thermal phenomena on the Coanda effect. ^[19] The final synthesis of this activity is the recent mathematical model proposed by Trancossi. ^[20]

By bringingly a kitchen spoon to a jet of water, the jet will be diverted to the surface of the spoon: in fact, if a current of water flows along a solid surface that is slightly curved (convex), the water tends to follow such surface. Keeping the spoon in such a way that it can oscillate, it can be felt clearly attracted to the jet. ^[21]

Some studies aim to use the Coandurn effect to develop aircraft with particular profiles: some vaguely resemble the flying disks of science fiction films. The advantage would be their maneuverability and their ability to tweet in the air. ^[22] A practical example of this application is found in the Antonov An-72 aircraft, in which the particular positioning of the engines has been designed specifically to exploit the Coandurn effect.

It was used for a few years in Formula 1 until 2013, through particular forms of bodywork capable of directing the flow of hot air from the exhausts on the sides of the rear extractor profile, creating a sort of aerodynamic seal useful to increase the efficiency of the ‘extractor himself and therefore the aerodynamic load. ^{[ without source ] [23]}

Typical applications to encourage the adhesion of a fluid to an aerodynamic surface, accentuating the coandurn effect, are the defurbulators, the shark skin type devices.