Inward Flow Increases the Rotational Speed

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• A tray containing water is fixed at the center of a tub that is placed on a turntable. There is a hole at the center of the tray, where a pencil is inserted as a plug.
• As you rotate the turntable, the water in the tray rotates with it. Then, we sprinkle coffee creamer powder on the water to make the water flow visible. The tray is painted black so that it is easier to see the powder.
• When you remove the pencil, water drains through the hole. The water inside the tray rotates, at the same time flowing slowly to the center (hole). Similar to the experiment “The Shorter the String, the Faster it will Turn,” the rotation speed of water increases as its distance from the center of rotation decreases. As a result, the rotation speed at the center is faster and inversely proportional to the radius. This phenomenon can be explained by the “law of conservation of angular momentum.”
• Typhoons, tornados, and the vortex formed as water drains from a bathtub are all similar types of flow. In these cases, the rotational speed (circumferential speed) is inversely proportional to the radius. Such vortices are called “free vortices.” In typhoons, the wind gets stronger as you near the center. (However, in the cases of typhoons and tornados, part of the region close to the center are an exception and become a forced vortex.)
• In the experiment shown in the video, there are four brick pieces at the side of the tub. This is to increase the angular momentum during rotation by placing masses at locations distant from the center. By doing this, the rotation lasts longer. The so-called “moment of inertia” of the rotating object is made larger. (Those of you who are interested are encouraged to conduct further research on the topic.)

[Keywords]	Law of conservation of angular momentum, Free vortex
[Related items]	The Shorter the String, the Faster it will Turn, Free Vortex and Forced Vortex, Which Direction does Typhoon Spin?
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs pp. 52–61.