Sunday, 30 January 2011
Can you shoot air?
Aim the barrel at the curtains. Then sharply hit the rubber drumhead of the barrel.
A sharp hit on the rubber drumhead of the barrel forces the air inside the barrel to move. A portion of the air exits the opening at the other end of the barrel and, within a few seconds, hits the curtains hanging from the ceiling.
The air that leaves the barrel does not exit the hole as a flow of air, but rather as a ring of air. The phenomenon concerns a ring-like air vortex, in which the air is moving forward as well as circling around inside the ring. The vortex is essential to maintain the shape of the ring. The pressure of the air flow is lower than the pressure of air that is still. The progressing ring of air is held together by the higher pressure of the surrounding air that is still.
The shape of this air cannon’s ammunition may be difficult to visualise, since it cannot be seen in the same way as, for example, smoke rings. In water, however, rings of air are clearly visible. Dolphins use this phenomenon to their advantage. They blow rings of air around schools of fish. The fish are trapped inside this “web” formed by the rings of air, and the dolphins can easily catch even the entire school.