The Coanda effect is a very strange phenomenon in fluid mechanics discovered accidentally following a mishap during an aeronautical experiment by the Rumanian engineer Henri Coanda (1886-1972) who gave it his name.
Henri Coanda is recognised today as the father of the principle of jet planes and his work was taken up early on by the Nazis (and led to the V1 and V2 rockets).
The Coanda effect proper takes the following form: when a fluid (gas or liquid) emerges from a recipient through an orifice or tube, part of the emerging fluid has a tendency to closely follow the contour of the exterior of the recipient, even if, to do this, it has to execute a hairpin bend. The commonest example of the Coanda effect is the annoying way in which tea comes out of a teapot when the spout is not tipped enough: the tea does come out, but the stream adheres to the outer wall, dripping elsewhere than in the cup where it was meant to go; the Coanda effect is thus at work without you realising it.
The existence of this phenomenon closely depends on several crucial parameters amongst which are the speed of the jet flow, the flow rate and the exact profile of the outlet. So if the Coanda effect occurs when pouring tea, it is enough to pour the tea faster to make the phenomenon disappear.
The Coanda effect, which can actually have beneficial consequences in certain circumstances also occurs, and even more so, in gas flows, in particular in aerodynamics where it can give rise to very significant effects through the drag exerted on the surrounding air. This drag can involve sufficient quantities of air to lead to practical applications. Thus, in a certain type of air cushion vehicle, a current of air is ejected upwards (and not downwards as in conventional air cushion vehicles such as the hovercraft) through an annular slit at the top of the vehicle. This air jet, after emerging upwards, undergoes the Coanda effect and flows down to the ground along the sides of the vehicle. In doing so, it drags part of the air from above the vehicle with it. Thus, above the vehicle a depression forms, which, in conjunction with the over-pressure exerted under the vehicle, produces a big enough ground effect to lift the vehicle. This phenomenon is used by the Russian air force for aircraft flying just above wave level and having a low wing surface (ground effect aircraft).
In a completely different field the Coanda effect is applied in logic circuits using a fluid, also called fluidic circuits. In these circuits where compressed air circulates (or eventually liquids under pressure), the same "all or nothing" phenomena occur as those that govern the circulation of electronic pulses in computer circuits. They can therefore be used in the same practical applications, i.e. elementary calculations or logical decisions. Fluidic circuits are obviously much slower than electronic circuits, but they have the advantage of being totally insensitive to thermal, electromagnetic and mechanical disturbances (vibrations).
Thanks to 1100F for these excellent explanations!