Circling Around a Pipe

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Watch the video on YouTube.

What type of experiment is this?

Experimental procedure and explanation:

• Squirt water from a squirt gun onto a transparent pipe, and watch the water circle around the pipe. The water is directed further to the inside of the position where it would contact the pipe on the side. In other words, the water is sprayed directly onto the front side of the pipe. Due to the Coanda effect, the flow of water wraps around to the back of the pipe. Moreover, if the experiment is performed skillfully, the water will circle completely around the pipe once or more than once.
• The trick is to hit the front side of the pipe so that the water begins to curve along the pipe at the front half of the pipe. As shown in the lower left explanatory diagram, if the water is, instead, directed so that it contacts the side of the pipe, a pressure difference in a direction orthogonal to the flow of water cannot be generated. Therefore, the Coanda effect does not easily occur. Moreover, if air bubbles are mixed into the stream of water that is squirted from the squirt gun and the water appears cloudy and turbid, this experiment will not succeed. This is because the flow itself is pulled away from the location where the air bubbles in the stream of water contact the outside air and is then separated from the pipe. Another trick to this experiment is to strengthen the force of the water flow.

[Keywords]	Coanda effect
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[Reference]	Isao Imai, “Fluid Dynamics, Vol. I,” Shokabo, pp. 66–67. The Japan Society of Mechanical Engineers, “The Wonders of Flow (Nagare no Fushigi],” Kodansha Blue Backs, pp. 128–133. Ryozo Ishiwata, “Illustrated Fluid Dynamics Trivia (Zukai Zatsugaku Ryutai Rikigaku),” Natsume Publishing, pp. 214–215 and pp. 206–209.