Ring of Air

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• A type of dolphin can make a ring of air. Let’s make a similar ring of air.
• Using an air pump, make tiny air bubbles inside a water tank.
• Open a round hole on the side of a milk or juice paper container and make the same configuration as an air cannon.
• Submerge this container in the water, and push on both sides of the container, in a manner similar to what you do to an Air Cannon. The shorter the duration of the push, the better it works.
• Like an air cannon, water squirting from the hole forms a core, and a vortex ring forms around that.
• When this vortex ring passes air bubbles, the bubbles will connect in a ring-like manner, forming an air ring. There is no need to blow air as a ring from the beginning.
• Due to the rotation of the vortex, a higher pressure forms on the outside, while the pressure is lower near the center. Air is lighter (less dense) than water, and thus it will concentrate toward the center of the vortex where the pressure is lower. This is similar in principle but opposite to what we saw in the centrifuge, in which a substance with a larger density collects at the outside of the revolving fluid.
• The trick to making a good air ring is to rapidly (in a short duration) push the paper container, and to make the air bubbles as fine as possible.
• A bubble bath might work as well, though different from what is shown in the video. Theoretically, if there is enough air in the water inside the paper container, it should work as well (though I have not yet succeeded in doing so).

[Keywords]	Vortex, Vortex ring
[Related items]	Air Cannon, Free Vortex and Forced Vortex, Centrifuge
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs pp. 52–61.