PHYS DEMOS |
GOOD STUFF |
NEW STUFF |
WHY DOES SMOKE 'RING?'William J. Beaty
If you throw a chunk of rock through the air, the rock flys along and the
air swirls around it. But what happens when you throw a chunk of AIR
through the air? Sometimes you get a "smoke ring." I always wondered how
smoke rings work. Below is one way to think about them.
In figure one, a chunk of air is being thrown from the right. The red
dots show where the "chunk" is located. The black dots show the
surrounding air. In the following animation, the air is
If the air is frictionless, a "chunk" of air can keep moving forever
without losing its shape. But real air has friction, and the chunk of
moving air will be stirred as in figure two below.
In the real
world, it turns into a moving donut-shape inside a spherical blob of air.
In other words, the red part in the animation becomes a ball, but it's a
ball with a vortex-ring inside. Did you ever play with
a "water weenie," one of those water filled balloon cucumbers? A chunk of
moving air behavies like a short, stubby "water weenie". Its outer
surface is dragged
backwards, and a central stream moves forwards.
Interesting note: in the above animation, NO AIR IS MOVING on average.
Think about it. If air was moving, then there would soon be less air at
the right side of the diagram, and more air on the left side. But every
time a dot moves forwards, some other dots are moving backwards. There is
an overall circular flow, but air in general is not moving from right to
left. If we put some smoke in the vortex, it appears that
something is moving from right to left. But the smoke misleads us, since
the smoke doesn't show the backwards flow which cancels out the forwards
Created and maintained by Bill Beaty. Mail me at: .
Best if viewed with ANY Browser.