Coanda effect

This post offers a simple description of "The Coanda Effect". My obsession with "effects" continues. It started with the Magnus Effect, then the Domino effect and now the Coanda effect. While the magnus and domino have a strong theory behind them, the Coanda effect is more of a physical phenomenon. I'll briefly write my understanding of this effect.

The Coanda effect is the tendency of a fluid jet to get attracted towards a surface that is close to it. The picture below (Source: Wikipedia) illustrates this phenomenon

The reason for this effect is not all that complicated according to my perception of it. When there is a fluid jet, the nearby fluid gets "entrained" around the jet (Courtesy: wikipedia). This can be best visualized when you blow air into an empty and open polythene bag. The bag gets filled with air immediately. This is because when air is blown into it from the outside, the surrounding air also joins the stream and enters the bag to fill it. This is what I referred to as "entrainment" previously. If a surface is preventing the ambient fluid to get entrained, the jet moves towards the surface and this is the Coanda effect. 

It is to be noted that the this effect is possible only when the jet fluid and the ambient fluid are the same. For the flow to get attracted and stick to the surface, the surface must be smooth and curved. This is why aero foils have a smooth curvature. Even when the angle of attack is increased, the coanda effect directs the fluid to flow along the surface of the foil thus ensuring lift. When the angle of attack is too high, the coanda effect will no longer be able to keep the flow sticking to the surface. The flow will separate and it stalls. (Image courtesy: discoverhover.org)

There was once an aircraft built to use only the coanda effect for producing lift. That effort did not succeed and aircraft never took off the ground. But this effect has been utilised in many modern day aircrafts to augment its aerodynamic abilities.