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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 FRICTIONLESS.

[chunk moving left through dots,
layer of dots flows
back over chunk's surface]
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 the 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.

[same as above, but chunk isn't
solid, it swirls like
two adjacent tornadoes]
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 inside.

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 flow.

 




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