Free Vortex and Forced Vortex

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• Let us create two basic types of vortices.
• Connect two plastic bottles and poke a hole through the cap. Fill water in the lower bottle.
• When you turn the bottles upside down and rotate, a vortex is formed in the water. (If the water does not start flowing, give a quick squeeze to the top bottle to get it going.)
• The vortex formed in this case is called a “free vortex.” This is the vortex formed when draining a bathtub or a sink. The speed is faster as you get closer to the center; that is, the peripheral speed is inversely proportional to the radius.
• In contrast, the vortex seen in a water bottle fixed to the center of a turntable is called a “forced vortex.” The peripheral speed is faster as you move away from the center; it is proportional to the radius. The water moves together with the container and will not deform. The water behaves like a rotating solid.

[Keywords]	free vortex, forced vortex
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Koudansha Blue Backs, pp. 52-59