PDA

View Full Version : Explanation wanted for a vibrating pole


OFSO
21st Feb 2015, 13:36
I parked and sat in the car in a small town today while the Mrs collected a roast chicken for lunch.

In front of me, set into the concrete pavement, was an extruded aluminum pole about 10' high, 3" across, slightly ridged sides for strength, obviously had at some time had some kind of sign attached to the top.

As I watched the top started vibrating from side to side, maybe two inches each way. Went on for maybe ten minutes died away, a period of perhaps five minutes, started vibrating again.

To the left was a tree, leaves hardly stirring in the breeze, and another pole with a 'no entry' roundel on top, didn't move at all.

So why was the aluminum pole vibrating ?

Traffic: no commercial vehicles, the occasional car, vibrations not related to the cars driving past. No workshops nearby, no trams, nearest railway about 15 miles away, no construction work of any sort in the vicinity.

In other words, I have no idea.......

Lon More
21st Feb 2015, 13:39
It was asked here some time ago IIRC Vibrating lampposts

airship
21st Feb 2015, 13:42
What did you have for breakfast OFSO?

toffeez
21st Feb 2015, 13:44
Was there a dancer attached?

Capot
21st Feb 2015, 13:51
There was this discussion (http://www.pprune.org/jet-blast/555612-swinging-lampposts.html) recently, but I can't find one on "vibrating lamp posts".

Some kind of vortex shedding, maybe, but the there was evidently little or no wind; wind was the main factor in "swinging lamp posts", as I understand it.

Fox3WheresMyBanana
21st Feb 2015, 13:51
I would guess the source of energy was likely your idling car engine.

OFSO
21st Feb 2015, 13:55
I would guess the source of energy was likely your idling car engine.

This is Europe. When parked the engine is turned off !

I admit, I was playing "Won't get Fooled Again" on the car system, but not THAT loudly.

What did you have for breakfast OFSO?

Cornflakes and sliced dates.....

airship
21st Feb 2015, 13:55
Maybe the roast chicken place has an underground kitchen, and the pole is a ventilation duct. Shaking everytime Mme. OFSO shouted "Get a move on down there. We're hungry!"

G-CPTN
21st Feb 2015, 13:59
33vc3U1X7co

Fox3WheresMyBanana
21st Feb 2015, 14:18
Just done a calculation of the natural frequency of such a (straight sided) pole. It would appear to be about 18 Hz.
Firstly, humans can't hear this, which is why you didn't hear the source.
ISTR a case involving this frequency where the source turned out to be a very large slow turning fan.
This would seem to indicate a light breeze might be the culprit, perhaps from vortex shedding. Other than that, some other form of vibration conducted through the concrete would be the culprit

Sir George Cayley
21st Feb 2015, 14:22
Fracking?

SGC

Fox3WheresMyBanana
21st Feb 2015, 14:28
For resonance at 18 Hz, the only formula I can find for vortex shedding indicates the windspeed required would be 7.2 m/s, or about 15 mph.
Is it possible the breeze was this strong?
There again, this would likely change for a ribbed pole, as would the natural frequency. My knowledge and available time do not extend to calculating this.

However, it would seem your pole could be vibrating due to a light breeze.

wings folded
21st Feb 2015, 14:35
Cornflakes and sliced dates.....

A combination which can cause flatulence at critical harmonics. Eat a helping of vindaloo and go back tomorrow, then report in.

OFSO
21st Feb 2015, 14:52
humans can't hear this, which is why you didn't hear the source.

No, I didn't hear the source because I was listening to this:

https://www.youtube.com/watch?v=g_SWP3qI7Rg

BTW after we got home and opened the packet of chicken, sausages and black pudding (cooked over a wood fire) consumed with Patatas Brava (wild potatoes) I actually TASTED the sauce.

To be serious (!) there was a very light breeze blowing so resonance might explain it.

ChrisVJ
21st Feb 2015, 20:10
Vibrating pole

Surely it is a tube not a pole.

I've got one of those, well it used to vibrate. Did you see any pretty females passing?

P6 Driver
21st Feb 2015, 20:14
Earthquake.

OFSO
21st Feb 2015, 21:12
Earthquake.

Indeed. We get a lot of small ones here. Just found there was a small earthquake at Llagostera, forty minutes drive south of here on the coast, this morning at about 3 a.m. local time. Possibly aftershocks ?

Wind now this evening gusting over 100 kph, so no longer a light breeze.

G-CPTN
21st Feb 2015, 21:20
Rural residents near an Army Training Range have been calling in about 'earthquakes' - said by the Army to be just their exercise activities.

Hexham Courant | News | Army exercise causes Tynedale ?quake? (http://www.hexhamcourant.co.uk/news/army-exercise-causes-tynedale-quake-1.1193851)

Flybiker7000
21st Feb 2015, 21:33
Does tram and railway include subways? Otherwise that might be the answer!

Loose rivets
21st Feb 2015, 22:54
It would appear to be about 18 Hz.
Firstly, humans can't hear this, which is why you didn't hear the source.


Erm . . . no . . . JSB, when he devised the evenly tempered scale, had his pipes fabricated to go down to 16 cps. Notoriously difficult to tune, but tune them they did. Pressure waves on one's nipples. Sublime. That Bach new a thing or two. But really, nothing compared to the fella in my link. Before going there, I'll just mention that I spent two years of my life trying to make a machine that would tune concert grand pianos in a noisy environment. It kind of worked, but it was not going to be a money-spinner, like most of the things I did in my life.

This is incredible. The bloke's a genius. So you thought tuning a Steinway was easy. Read on.

An Introduction to Historical Tunings (http://www.kylegann.com/histune.html)

Fox3WheresMyBanana
22nd Feb 2015, 00:33
Can sense, yes, but not with the ears.
There is some evidence that about 17 Hz causes feelings of unease, even fear in humans. Frequency generators at 17 Hz have been used in Haunted House attractions with some effect.

Flybiker7000
22nd Feb 2015, 02:00
If a known sub-frequency is played together with the double frequency, it wil be able to tune in according to the same interference that occours during tuning of equal frequences thus end up being the half frequency of the higher and hearable!

Hydromet
22nd Feb 2015, 02:01
Loose Rivets, after reading that, my brain Hertz.

Flybiker7000
22nd Feb 2015, 02:38
During motorbiking I've often detected another bike coming up from behind by an interferring with my own engine-noise - Before I would be able to actual hear the oncoming engine.
One should think that it depends of near equal rev's but slower reving engines with fewer cylinders (=impulses wich makes the frequency) might 'tune-in' in the same way.
Following: lower frequences can interfer with higher frequences!

Loose rivets
22nd Feb 2015, 11:13
Beat notes can be very pleasing and indeed produce (much lower power) harmonics of the harmonics. However, with a piano, because they're not 'flat tuned' i.e. just multiples, the Temperament is the only stretch of the keyboard that's standard between all pianos. After that, the guy tuning it must search out unpleasant harmonics and build slight adjustments on top of the normal keyboard 'Stretch'. So, an octave above A 440 isn't just 880, but slightly above, to the point where 1760 can be as much as 1764, even 5.

I've always wondered why this doesn't clash with other instruments. I supposed it does, but we've just learned to like it.

Back to the OP's post. That really would have piqued my interest. To buzz that pole like that would have taken a lot of energy, I wonder if there was an underground line there, intermittently carrying a lot of current.

Such lines can be buried and forgotten when earth is removed years later. A local builder here spent some time in the pub showing people the scars from an exploding elbow. Funny? No, scringe-making, it had been a heck of a mess. That's what 11,000 volts can do. Well, he said it was 11,000, but then I'd have said it was a million if I'd suffered that.

onetrack
22nd Feb 2015, 13:16
My money is on slight ground movement created by heavy traffic in the region.
How far away is the nearest Autovia, OFSO?
Could also be underground rail movements, as a previous poster suggested.

OFSO
22nd Feb 2015, 13:25
How far away is the nearest Autovia, OFSO?

About 20 miles at least.

Nearest railway (back line) about 12 miles (two trains an hour).

Nearest TGV (high speed railway) about 20 miles (one train every 90 minutes).

Heavy traffic in a small Spanish town ? You gotta be kidding. Corks popping from wine bottles more likely, heads falling forward on chests after a heavy lunch......

onetrack
22nd Feb 2015, 14:08
Hmmm, those distances tend to rule out the traffic and trains angle, then.
What makes it curious is that you say the pole was only 10' high.
I've seen and heard vibration resonance in a tall flagpole - say 30' or 35' high - from a light breeze - but I wouldn't have thought a 10' high pole would have enough height in it, to vibrate or resonate in the stated manner.

OFSO
22nd Feb 2015, 15:36
I did wonder whether the fact that the sea is less than half-a-mile from the pole might be doing it; although yesterday (unlike today) there were no waves to speak of. And yesterday, unlike today, we didn't have a force nine blowing.

Loose rivets
22nd Feb 2015, 16:54
Despite being aluminium, oscillating magnetic forces could still effect movement of the pole - as in the old speedo disks.

Do you have any idea of the frequency? i.e. a quick guess, did it look like a natural resonance for such a tube? This would be in the audio range. No sound heard?

Was the oscillation just one length of the pole or did it whip in the middle to give a wave formation within the 10' length?

MarcK
22nd Feb 2015, 17:13
I've always wondered why this doesn't clash with other instruments. I supposed it does, but we've just learned to like it.
That's why concert pianos are 9 feet long. Less stretch required for the piano.

603DX
22nd Feb 2015, 18:28
Lots of assumptions necessary to have a stab at an Aeolian wind-induced vibration "guesstimation". For a tubular section 3" diameter aluminium pole with a wall thickness of say 0.25" and height of 10'-0", I estimate the fundamental frequency at about 10 Hz. Vortex shedding excitation often establishes within a steady wind speed range of 5 - 15 metres/sec, so taking the lower limit as 5 m/sec (11 mph) to suit the "light wind" description, the Reynolds number would be definitely subcritical. This would correspond with a Strouhal number of 0.18, which gives an excitation frequency of about 12 Hz.

Given the inaccuracies involved, this seems to at least be within the same ball park as the tube frequency, so resonance might be a feasible answer.

OFSO
22nd Feb 2015, 18:32
Did it look like a natural resonance for such a tube? This would be in the audio range. No sound heard?

Yes and no. no sound heard.

Was the oscillation just one length of the pole or did it whip in the middle to give a wave formation within the 10' length?

It was the top oscillating, and only the top.

RJM
22nd Feb 2015, 21:57
Ofso, is it correct that the tube was open at the top? How. Thick do you think the tube walls were?

Loose rivets
23rd Feb 2015, 00:55
The thing is, the tube was fluted in a way that would increase its rigidity. That rather messes up the fine guesstimations above.

gemma10
23rd Feb 2015, 03:29
If it has a split near the bottom [hence why its no longer in use] then its probably behaving like an open diapason. Think organ pipes.

OFSO
23rd Feb 2015, 11:38
Ofso, is it correct that the tube was open at the top?

If you think I'm going to shin up a ten foot pole to see if it's open....

hence why its no longer in use

It is no longer in use because someone came along and put up another pole next to it and put a "no entry" sign on that. It would not occur to local council workmen told to erect a pole to use one that was already there.

If it has a split near the bottom

None that I could see

Think organ pipes.

I do, I do. I'm feeling rather fugue-ey

603DX
23rd Feb 2015, 12:04
The thing is, the tube was fluted in a way that would increase its rigidity. That rather messes up the fine guesstimations above.


OFSO, your OP actually said "slightly ridged for strength" rather than "fluted", which suggests to me that it may be a polygonal cross section with multiple small flats around the perimeter. That is often done to increase the local wall buckling strength of thin-wall closed sections used for "street furniture" poles. Compared with the potentially large error introduced by the need to guess the wall thickness as 0.25", a polygonal section like that would have little effect on the Strouhal number and vortex-shedding characteristics of the smooth tube which I assumed.

Is it possible to give a bit more detail of what you meant?

OFSO
23rd Feb 2015, 13:16
Is it possible to give a bit more detail of what you meant?

Next time I'm there I'll take a photograph. Until then curb yourself and just count the photons.:D

gemma10
23rd Feb 2015, 17:39
Why didn`t you take a photo last time, might have saved us all a lot of brain ache :E

OFSO
23rd Feb 2015, 18:18
Right then.

Just been a 5.3 Richter Scale earthquake down t'coast from us. Maybe it was winding up for that. Or the (officially, now cancelled) plan to store zillions of cubic metres of gas off-shore in underground cavities.