Coandã effect
The Coandã effect is the tendency of stream of fluid to stay attached to a convex surface, rather than to follow a straight line down in its original direction. The Coandã effect is also known as "boundary layer attachment" and was named after the Romanian discoverer Henri Coandã, who was the first to understand the importance of this phenomenon for aircraft development.

What is known today as the Coandã effect, was described by the discoverer himself as the "Deviation of a plan jet of a fluid that penetrates another fluid in the vicinity of a convex wall". He made the discovery during experiments with his Coandã-1910 aircraft, which is the first aircraft ever to use a motorjet.
Causes
The Coandã effect can be explained by a spoon held in the instance of a stream of water and largely on the basis of surface tension or Van der Waals forces. In case of gas flow against a surface with ambient gas or liquid flow in ambient liquid, then the Coandã effect can be explained on the basis of momentum and entrainment of the fluid. In the event of gas flowing over an airfoil, the gas is drawn down to adhere to the airfoil by a combination of the greater pressure above the gas flow and the lower pressure below the flow caused by an evacuating effect of the flow itself, which as a result of shear, entrains the slow-moving fluid trapped between the flow and the down-stream end of the upper surface of the airfoil. The effect of spoon attracting a flow of water is basically caused by that same effect.


Aviation Applications
The Coandã effect has an important effect on aircraft performance and has several applications in various high-lift devices. Especially where air moving over the wing can be "bent down" towards the ground using flaps and a jet blowing over curved surface. For example, the flow from a high speed jet engine mounted in a pod over the wing produces enhanced lift through turbulent mixing that does not occur above a normal wing. This was first implemented in a practical sense during the United States Air Force's AMST project. Several aircraft have been built to take advantage of this specific effect, by mounting turbofans on the top of the wing in order to provide high-speed air over the wing, even at low airspeeds. At this moment only one aircraft has gone into production using this system to a major degree, Antonov An-72 "Coaler". Other aircraft like the McDonnell Douglas YC-15 and Boeing C-17 Globemaster III also employ this effect, though to a less substantial degree.
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