WHY DOES SMOKE 'RING?'
|fig. 1    Throw a chunk of air forwards|
When any object moves through the air, it must push the air out of the
way. As the air in front of the object makes way for it, the object moves
forward, and an empty space is left behind. If the air is frictionless,
then figure 1 shows what happens. The "chunk" of air moves foward, and a
very thin layer of air moves backwards over it. Air is pushed away from
front of the chunk, and it fills the space behind.
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.
|fig. 2     Same chunk, but with friction|
In fig. 2, as the air ahead of the red chunk is spread apart, it makes the
air inside the red chunk spread apart too. As the red chunk moves
forwards, its outer layer is dragged backwards. A central stream of air
starts moving forward through the red chunk, and the chunk swirls
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