A jet of fluid ought to shoot straight out of its nozzle. Yet if a curved surface sits just beside it, the jet bends and clings to the curve. This is the Coanda effect — driven not by any pull, but by a drop in pressure beside the jet.
Hold a spoon under a running tap and the water curves to follow the spoon instead of falling straight. Push the back of your hand into a jet of air from a hairdryer and the stream bends towards your skin. In both cases a fast jet has attached itself to a nearby curved surface. This behaviour is named the Coanda effect, after the Romanian engineer Henri Coandă.
The nozzle on the left fires a jet to the right. The grey dashed line is where a free jet would travel. Instead, the jet bends down and hugs the Coanda surface. The arrows above show fluid being entrained — dragged along by the jet — which thins the fluid between jet and surface and drops the pressure there, marked as the low-pressure zone. The ambient pressure on the outside is left higher, and it is that imbalance that presses the jet onto the curve.
Engineers exploit the effect to steer a flow without moving parts. It shows up in blow-flaps and some vertical-take-off designs, in "air amplifier" nozzles that multiply a small compressed-air supply, in air knives that strip liquid off a surface, in Dyson-style fans that draw in extra room air, and in sprayers that let a fast gas stream break up a liquid.
A converging nozzle turns a slow, wide stream into a fast, narrow one, and the fluid leaves at high speed. Left alone, such a jet would travel in a straight line.
That straight path is drawn as the grey dashed line. Nothing in the jet itself knows about the surface beside it — the bending has to come from the pressure around it.
Wherever a fast jet moves, the fluid next to it is dragged along and swept away — this is entrainment. In particular, the fluid in the narrow gap between the jet and the curved surface gets pulled away and cannot easily be replaced.
Because that fluid is swept out, the pressure in the gap falls below the surrounding ambient pressure. The jet now has lower pressure on the surface side and higher pressure on the open side.
The higher ambient pressure pushes the jet towards the surface, so it attaches and clings to the curve. As the surface turns, the jet turns with it — bending further than a free jet ever would. This attached wall jet is the Coanda effect.
It is the tendency of a fast jet of fluid to follow a nearby curved surface instead of continuing in a straight line. The jet bends towards the surface and stays attached to it, even when the surface curves away from the original direction of travel.
Because the moving jet drags surrounding fluid along with it, a process called entrainment. Between the jet and the surface that fluid is not easily replaced, so the pressure there falls. The higher pressure on the other side of the jet then pushes it onto the surface.
No. Any fluid will do — water, air or a jet of gas. What matters is a fast-moving stream next to a curved surface; the same bending happens when a tap is held close to a curved object and the water clings to it.
It shows up in aircraft blown flaps and some VTOL designs, in air amplifiers and air knives, in Dyson-style fans that draw in extra air, in carburettors and sprayers, and even in how a teaspoon catches a stream of tap water.
They are closely related but not the same. Bernoulli links faster flow to lower pressure; the Coanda effect is the result of entrainment lowering pressure on one side of a jet, which then lets the surrounding higher pressure bend the jet onto a surface.