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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Coandă effect | 5/5 | https://en.wikipedia.org/wiki/Coandă_effect | reference | science, encyclopedia | 2026-05-05T10:54:48.580764+00:00 | kb-cron |
== Demonstration == The Coandă effect can be demonstrated by directing a small jet of air upwards at an angle over a ping pong ball. The jet is drawn to and follows the upper surface of the ball curving around it, due to the (radial) acceleration (slowing and turning) of the air around the ball. With enough airflow, this change in momentum is balanced by the equal and opposite force on the ball supporting its weight. This demonstration can be performed using a hairdryer on the lowest setting or a vacuum cleaner if the outlet can be attached to the pipe and aimed upwards at an angle. A common misconception is that the Coandă effect is demonstrated when a stream of tap water flows over the back of a spoon held lightly in the stream and the spoon is pulled into the stream (for example, Massey 1979, Fig 3.12 uses the Coandă effect to explain the deflection of water around a cylinder). While the flow looks very similar to the air flow over the ping pong ball above (if one could see the air flow), the cause is not really the Coandă effect. Here, because it is a flow of water into air, there is little entrainment of the surrounding fluid (the air) into the jet (the stream of water). This particular demonstration is dominated by surface tension. (McLean 2012, Figure 7.3.6 states that the water deflection "actually demonstrates molecular attraction and surface tension.") Another demonstration is to direct the air flow from, e.g., a vacuum cleaner operating in reverse, tangentially past a round cylinder. A waste basket works well. The air flow seems to "wrap around" the cylinder and can be detected at more than 180° from the incoming flow. Under the right conditions, flow rate, weight of the cylinder, smoothness of the surface it sits on, the cylinder actually moves. Note that the cylinder does not move directly into the flow as a misapplication of the Bernoulli effect would predict, but at a diagonal. The Coandă effect can also be demonstrated by placing a can in front of a lit candle, such that when one's line of sight is along the top of the can, the candle flame is completely hidden from view behind it. If one then blows directly at the can, the candle will be extinguished despite the can being "in the way". This is because the airflow directed at the can bends around it and still reaches the candle to extinguish it, in accordance with the Coandă effect.
== Problems caused == Aside from the numerous possible advantages of the Coandă effect being exploited in engineering, its use can engender disadvantages as well. In marine propulsion, the efficiency of a propeller or thruster can be severely curtailed by the Coandă effect. The force on the vessel generated by a propeller is a function of the speed, volume and direction of the water jet leaving the propeller. Under certain conditions (e.g., when a ship moves through water) the Coandă effect changes the direction of a propeller jet, causing it to follow the shape of the ship's hull. The side force from a tunnel thruster at the bow of a ship decreases rapidly with forward speed. The side thrust may completely disappear at speeds above about 3 knots. If the Coandă effect is applied to symmetrically shaped nozzles, it presents resonance problems.
== See also ==
== References ==
=== Notes ===
=== Citations ===
=== Sources ===
== External links == Flight 1945 Coandă effect video (1) Coandă effect video (2) Information on the patents of Coandă New UK based UAV project utilising the Coandă effect Report on the Coandă Effect and lift How to see the Coandă effect at home (www.physics.org comic)