Coanda Effect
Coanda Effect is the effect used to describe the characteristic of a fluid jet to stay attached to a sloped surface. It is named after the Romanian aerodynamicist, Henri Coanda, incidentally, was the first to recognise this and apply it for aircrafts.
To explain this effect, consider a fluid exiting a pipe at some velocity. Let us assume this fluid as a jet of air. Now, the fluid exiting the pipe will come under the influence of atmospheric air pressure soon after exiting the orifice as shown in the below figure.
As seen in the figure, as the jet exits the orifice, it meets air below and above it. This air, around jet, is called ambient air and will exert its force on the jet from both the sides, called ambient pressure and thus the jet continues to move in a linear path corresponding to the way it emptied out from the pipe.
Now consider the situation where this jet is made to exit above and adjacent to some smooth surface. Let this surface gently curve away from the jet of air as shown below.
It is clear that the jet doesn’t flow linearly as in the previous case, instead it closely follows the shape of the surface adjacent to it. This behaviour is called as the Coanda Effect.
The jet behaves this way in the second case as, after exiting the orifice, there is no air under the jet, consequentially, no ambient pressure from below. It is replaced by a smooth and hard surface which doesn’t exert any force on the jet, and because the only influencing factor here is the air above, it is pushed downwards, thereby forcing the air to closely follow the surface under it. This is very clear from the adjoining figure.
Essentially, this is what occures when an air stream flows above the surface of an aerofoil, which can be viewed in the figure below. It happens to follow the shape of the aerofoil because of Coanda Effect and the explanation remains identical to what was presented above.
However, Coanda effect is sometimes confusingly used. One of the most common example where it is wrongly identified is when a liquid moves along the surface of an object, like water slowly overflowing from a cup. Though Coanda Effect is present and the ambient pressure exists, it isn’t strong enough to cause the heavier liquid to stick to the object’s surface. Rather here, the more suitable explaination is surface tension.
