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The Navier-Stokes fluid dynamics equations, said that, as Sir William Thomson (or Lord Kelvin) predicted:

  1. When two smoke-rings are moving in the same direction, with the same speed, one behind the other, the 'leading' ring will slow down and enlarge, while the 'following' one will get smaller and speed up, passing through the ring in front, and this will keep on happening until they fade away. It was experimentally proven by Kelvin.

  2. When two smoke-rings move towards each other, rather than colliding and annihilating in a smokey mess, they actually slow down and enlarge, never meeting, just getting larger until they fade away. It was experimentally proven by Kelvin.

My questions:

  1. How does phenomena number 1 happen?
  2. How does phenomena number 2 happen?
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Smoke rings are very resilient things, because they are topologically protected from disappearing: small disturbances don't usually break them because they'd have to fundamentally alter the flow's topology. I suspect this is behind, at least, the second phenomenon you ask about. – Emilio Pisanty Dec 10 '12 at 23:55
up vote 4 down vote accepted

I currently don't have access to the paper, but this should meet your needs:

Vortex Ring Interactions (by Wakelin-Riley)

Here is my interpretation: Consider two rings moving co-axially in the positive x-direction with their rings in the y-planes (ring A in front of ring B). The particles in each circle of the (fattened) ring circulate and force spreading of the air in front of it (see Then
1) A's circulations will force B to spread and enlarge (visually look at the vectors in the vicinity of A). Then A moves through and this repeats, because B's circulation will now spread A.
2) Here ring A is moving in positive x-direction and B in negative x-direction. Their respective circulations will stretch each other and slow them down until they meet at rest and fade.

Here are more direct papers:
A Note on the Leapfroggeing Between Coaxial Vortex Rings at Low Reynolds Numbers (by Lim)
Interaction of Two Vortex Rings Moving along a Common Axis of Symmetry (by Oshima)

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The first phenomenon is called vortex pairing. It occurs in both 2D and 3D flows. When two nearby vortices rotate in the same direction, their circulation will cause them to revolve around each other until they merge. What probably happens with the smoke rings is that the second ring is stronger than the first one, its flow will cause the first ring to increase in size as it approaches. Depending on their positions and strengths, they will either merge or keep on interacting with each other for a while...

There is an analog phenomenon in atmospheric circulation called the Fujiwhara effect.

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I will look into the second point more and maybe do some diagrams if I find the time... – FrenchKheldar Dec 11 '12 at 0:03

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