Air Cannon

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• This is the well-known air cannon. Cut a round hole in a cardboard box. When you tap on the box, a pop of air comes out. Flow coming from a hole or hose like this is called a “jet.”
• The round, cylindrical section of air leaving the hole is called the “core.” Because this blowout is instantaneous, the core will slow down rapidly over a short distance due to viscous friction between the core and the surrounding stationary air.
• When the core pops out, viscous friction between the rapid flow of the core and the surrounding stationary air generates a force that tries to rotate the surrounding air, thereby creating a vortex.
• This vortex forms a continuous ring around the core and is called a “vortex ring.”
• Immediately after the instantaneous blowout, surrounding air flows to fill in behind the moving core; this also helps create the vortex ring.
• The vortex ring travels at about half the speed of the core. Even after the core has lost its speed, the vortex ring can travel by its own rotation (this velocity is called the self-induced velocity) and can travel for a fairly long distance.
• The trick with activating the air cannon is to tap the box vigorously (with a short duration). The instantaneous flow of the core makes it easier for the vortex ring to form.

[Keywords]	Vortex, Vortex ring, Jet, Viscous friction
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs pp. 16–21.