The Coandă effect 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."
A ping pong ball is held in a diagonal stream of air. This is a demonstration of the Coandă effect. The ball "sticks" to the lower side of the air stream, which stops the ball from falling down. The jet as a whole keeps the ball some distance from the jet exhaust, and gravity prevents it from being blown away.
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.
Pressure distribution along the circular wall of a wall jet
The first Avrocar being readied at the Avro Canada factory in 1958
Henri Marie Coandă was a Romanian inventor, aerodynamics pioneer, and builder of an experimental aircraft, the Coandă-1910, which never flew. He invented a great number of devices, designed a "flying saucer" and discovered the Coandă effect of fluid dynamics.
Coandă at a meeting with Nicolae Ceaușescu in 1967
Coandă-1910 airplane with the turbo-propulseur on separate display
1912 Bristol-Coanda monoplane
A 1982 stamp and postcard of Romania dedicated to Coandă
