It is quite a strange phenomenon of the mechanics of the fluids, discover by chanche, thanks to a mishap during an experience of aerodynamics by the Rumanian engineer Henri Coanda,in 1911.

Henri Coanda is now known as being the father of thejets principle, his works were soon recovered during the second worl war to give birth to the first rockets
It is this Coanda effect, or effect of bearing pressure, which allows planes and bird to fly

The Coanda effect is a physical phenomenon characterized by the adhesion of a liquide or a gas to the support on wich it runs

The experience shown above illustrate very well the phenomenon. Look how the thin trickle of water sticks to and follow the side. Once the fluid emerges, it tends to fit the outline of the part. Under certain conditions, the fluid could even make a"hairpin blend".

Now try to imagine the same experience at the exhaust of a circular form, that is to say a pipe.
You may understood that the result is the principle of the Coanda durit effect.