Forced Vortex

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• Make a cut on the cap of a plastic bottle, and thread through a string or piece of yarn.
  (The slot must be cut by an adult using a small saw. It is dangerous to use box cutters or other knives.)
• Fill water in the bottle. Then, twist the string and let go of the bottle. The bottle will start spinning.
• Once the rotation becomes stable, the water surface will form a paraboloid of revolution (a parabolic curve when seen from the side). This is a “forced vortex.” The peripheral speed (speed of rotation) is proportional to the radius, so the farther you go from the center, the higher the speed. This motion is similar to that of a spinning solid.

[Keywords]	Vortex, forced vortex
[Related items]	Free Vortex and Forced Vortex
[Reference]	"The Wonders of Flow" Japan Society of Mechanical Engineering, Kodansha Blue Backs, pp. 52-59