It puzzled me for days — what caused an old aerial pole on a neighbour’s house to oscillate wildly on a seemingly windless day? The answer, it turned out, wasn’t UFOs, but an equally fascinating phenomenon with its explanation rooted firmly in the science of fluid mechanics.

I was genuinely baffled by the mystery of the vibrating aerial pole on my neighbour’s house, which I wrote up in a post a few days ago. The coincidence of there being a UFO sighting in the area on the same day seemed to suggest a connection, and I speculated about whether electro-magnetic field disturbances might be responsible. I posted links to my item in the Unknown Country Forum and on the Fortean Times Bulletin Board. It was in the discussion thread at the FT board that what I now believe to be the answer was suggested (by the appropriately named contributor “misterwibble”): a phenomenon known as Vortex Shedding.

Wikipedia has an entry on vortex shedding, together with an animation that shows what happens when a fluid in motion — such as water or air — meets a solid blunt object (including, as in our case, a cylindrical object):

Von Kármán vortex street animation, courtesy Cesareo de La Rosa Siqueira

The Wikipedia entry explains the effect in detail, including equations for working out how fluids of different velocities passing cylinders of different diameters will produce vortices possessing different qualities, but for our purposes, the animation is enough to make it obvious what is happening — as each vortex is spun off the leeward side of the cylinder, alternating on the left and right sides, pressure-drops are caused on the opposing side which pulls the cylinder first one way, then the other. If the frequency of the production of the vortices corresponds with the resonating frequency of the cylinder, the motion (oscillation) of the cylinder back and forth will be accentuated until, in some cases, the structure will fail.

Vortex shedding is something I’d never heard of before, but now I know about it, it’s the obvious answer. In the case of the aerial pole, only a very light breeze is necessary to make it oscillate because its (presumably) aluminium construction is so thin, as can be seen in this photo, looking up the pole from underneath. When the wind is blowing from east to west, as it has been when I’ve seen the pole oscillating, it’s blowing from directly behind the pole from my viewpoint and towards me, so the movement I see is left to right. At other times — say, when the wind is blowing from south to north — any pole oscillations would be back and forth from my perspective and therefore much more difficult to observe from a distance.

I was confused as to why the pole vibrates in light breezes but not when the wind force is higher. Vortex shedding provides an answer: the pole doesn’t oscillate in higher winds because the frequency with which the vortices are spun off the leeward side of the pole is higher than the pole’s natural resonating frequency, and so it doesn’t react in ‘sympathy’.

Finally, a few videos at YouTube put the icing on the cake, as it were: here’s a traffic light pole being affected by vortex shedding:

This is a video explaining the phenomenon in the laboratory:

Lastly, a more light-hearted explanation from some science students, which still gets the science message across:

It’s a pity that UFOs weren’t responsible for the wobbly pole. But I’m always happy to admit when I’m wrong — and so, with thanks to all the contributors who helped solve the problem, it’s QED!

Tags: Banwell, Oddities, Science, Somerset, Video

This entry was posted on Thursday, November 13th, 2008 at 11:10 am and is filed under Oddities. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.