5.7 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Coandă effect | 1/5 | https://en.wikipedia.org/wiki/Coandă_effect | reference | science, encyclopedia | 2026-05-05T10:54:48.580764+00:00 | kb-cron |
The Coandă effect ( or ) is the tendency of a fluid jet to stay attached to a surface of any form. 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.
== History == A description of this phenomenon was provided by Thomas Young in a lecture to The Royal Society in 1800:
The lateral pressure which urges the flame of a candle towards the stream of air from a blowpipe is probably exactly similar to that pressure which eases the inflection of a current of air near an obstacle. Mark the dimple which a slender stream of air makes on the surface of water. Bring a convex body into contact with the side of the stream and the place of the dimple will immediately show the current is deflected towards the body; and if the body be at liberty to move in every direction it will be urged towards the current... More than a hundred years later, Henri Coandă identified an application of the effect during experiments with his Coandă-1910 aircraft, which mounted an unusual engine he designed. The motor-driven turbine pushed hot air rearward, and Coandă noticed that the airflow was attracted to nearby surfaces. In 1934, Coandă obtained a patent in France for a "method and apparatus for deviation of a fluid into another fluid". The effect was described as the "deviation of a plain jet of a fluid that penetrates another fluid in the vicinity of a convex wall". The first official documents that explicitly mention the effect were his two 1936 patents. This name was accepted by aerodynamicist Theodore von Kármán, who had a long scientific relationship with Coandă on aerodynamics problems.
== Mechanism ==
A free jet of air entrains molecules of air from its immediate surroundings causing an axisymmetrical "tube" or "sleeve" of low pressure around the jet (see Diagram 1). The resultant forces from this low pressure tube end up balancing any perpendicular flow instability, which stabilises the jet in a straight line. However, if a solid surface is placed close, and approximately parallel to the jet (Diagram 2), then the entrainment (and therefore removal) of air from between the solid surface and the jet causes a reduction in air pressure on that side of the jet that cannot be balanced as rapidly as the low pressure region on the "open" side of the jet. The pressure difference across the jet causes the jet to deviate towards the nearby surface, and then to adhere to it (Diagram 3). The jet adheres even better to curved surfaces (Diagram 4), because each (infinitesimally small) incremental change in direction of the surface brings renews the effect of the initial bending of the jet. If the curvature is not too sharp, the jet can adhere to the surface even after flowing 180° around a cylindrically curved surface, and thus travel in the opposite direction. The forces that cause these changes cause an equal and opposite force on the surface along which the jet flows. These forces can be harnessed to cause lift and other forms of motion, depending on the orientation of the jet and the surface to which the jet adheres. 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. The Coandă effect can be induced in any fluid, and is therefore equally effective in water and air. A heated airfoil significantly reduces drag.
== Existence conditions == 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.
=== Wall jet === To compare experiment with a theoretical model, a two-dimensional plane wall jet of width (h) along a circular wall of radius (r) is referred to. A wall jet follows a flat horizontal wall, say of infinite radius, or rather whose radius is the radius of the Earth without separation because the surface pressure as well as the external pressure in the mixing zone is everywhere equal to the atmospheric pressure and the boundary layer does not separate from the wall.