Coanda effect is the phenomena in which a jet flow attaches itself to a nearby surface and remains attached even when the surface curves away from the original jet path.
The phenomena derive its name from a Romanian born aeronautical engineer – Henri Coanda.
In 1910, Coanda designed and built what was arguably the world’s first jet-propelled aircraft. He performed the first reactive flight on 16 December 1910 – which unfortunately ended in an accident. After the plane took off, Coanda observed that the flames and burned gases exhausted from the engine “hugged” the fuselage and the aircraft side rapidly caught fire. This phenomenon was later named the Coanda effect.
A natural question is “how the does the wing divert the air down?”
To see how fluid stays attached to a convex surface (Coanda effect) in full force in real life, grab a spoon and find a wash basin. No need for state of the art or high-end laboratories.
Dangle the convex side of the spoon next to the water stream running down the water tap. Hold the spoon loose enough so it can swing back and forth a bit. It helps if you attach a piece of tape at the handle end to act as a hinge. Move the spoon up to the periphery of the stream, so that the spoon barely grazes it. You will see the water following the convex profile of the spoon, and upon reaching the trailing edge rather than water stream going vertically downwards, it continues its trajectory along with the profile of the spoon.
In free surrounding, a jet of fluid entrains and mixes with its surroundings as it flows away from a nozzle.
When a surface is brought close to the jet, this restricts the entrainment in that region. As the flow tries to balance the momentum transfer, a pressure difference is created across the jet. Therefore, the stream is deflected closer to the surface – eventually attaching to it.
Even if the surface curves away from the initial direction, the stream tends to stay attached. This effect can be used to change the jet direction.