- a candle,
- a bottle,
- match box, or any other small rectangular box.
- Light the candle;
- Place the bottle between you and the candle, close as possible but without touching each other
- Place yourself near the on the opposite side and blow to the bottle, like you wish to extinguish the candle, what happens?;
- Light the candle again;
- Replace the bottle for the box;
- Now blow, try to blow with same intensity.
When you e a bottle the candle extinguish at first blow. When you use the box, the candle resist, even if you blow harder.
Eventually it will extinguish if we blow very hard, that’s because a vortex will be formed and the air circulation will extinguish the candle at the end.
This is a simple Coanda effect demonstration.
Coanda effect was discovered in 1910 by the Romanian engineer Henri Coanda (1885-1972), Coanda began to study this phenomenon after having he crashed a prototype aircraft (Coandă-1910). (1)
To the capacity of a fluid, to follow on a curve surface, like it's glued to it, we call Coanda effect- of course this is a very simple way to explain this, in fact is far more complex.
Bottle surface is smooth and curved, and the air we blow travels like its glued to the glass surface, the air goes around the bottle circumference, this allow us to blow the candle with just a little breath. On the other hand, the match box is a rectangular and plain surface, in this case the air flows to the corner of the box, this corner is a 90º angle, and therefore the air can't travel glued to the surface, like in first example. In fact the air is expelled away from the candle; this will form a vortex, which will eventually extinguish the candle.
This effect helps to explain why the plains stay in the air.
Simple and fun!