Completely non linux question for your perusal...

This morning while out walking dog on a local playing field which has a
football pitch ( UK, soccer ) I noticed a peculiar phenomenon.

The weather here in North Wales was bright and fresh, slight hoare frost and
probably just above freezing, I looked at one goalpost and saw water
dripping from it as the sun came up over the mountain and hit the crossbar.

On closer inspection ( I threw the ball for the dog ) :slight_smile: I saw that the
crossbar was moving up and down in a sine wave, and did not vary in
frequency after about 5 mins of observing.

These bars are made of metal pipe ( not sure what ) and it is something I
have never seen happen before…

Anyone got a clue please?

I have tried gurgling but all I seem to find is about harmonics in
electrical systems which I understand but not in a stand alone metal pipe…

I would surmise it is probably due to thermal expansion/contraction but as I
am no engineer I thought somebody here might be able to cast a light on it
for me :slight_smile:

Your task today Jim is to find some explanation for me :slight_smile:

This post will self destruct in 24 hours.

Cheers.


Mark
Caveat emptor
Nullus en verba
Nil Illigitimi Carborundum

On Wed, 03 Dec 2014 10:25:48 GMT
baskitcaise <baskitcaise@gmail.com> wrote:

> This morning while out walking dog on a local playing field which has
> a football pitch ( UK, soccer ) I noticed a peculiar phenomenon.
>
> The weather here in North Wales was bright and fresh, slight hoare
> frost and probably just above freezing, I looked at one goalpost and
> saw water dripping from it as the sun came up over the mountain and
> hit the crossbar.
>
> On closer inspection ( I threw the ball for the dog ) :slight_smile: I saw that
> the crossbar was moving up and down in a sine wave, and did not vary
> in frequency after about 5 mins of observing.
>
> These bars are made of metal pipe ( not sure what ) and it is
> something I have never seen happen before…
>
> Anyone got a clue please?
>
> I have tried gurgling but all I seem to find is about harmonics in
> electrical systems which I understand but not in a stand alone metal
> pipe…
>
> I would surmise it is probably due to thermal expansion/contraction
> but as I am no engineer I thought somebody here might be able to cast
> a light on it for me :slight_smile:
>
> Your task today Jim is to find some explanation for me :slight_smile:
>
> This post will self destruct in 24 hours.
>

I’m reminded of an article I read in the Novosti Press back in the late
1960s. Unfortunately, it has almost nothing to do with your goalposts
but I think it’s quite interesting.

In the depths of the Siberian winter, there had been a rash of
breakages in power lines. First thoughts were that they must have been
caused by ice on the lines or high winds or a combination of the two.
When the weather reports were checked, it was found that all the
breakages had occurred in calm weather, so ruling out the wind as a
cause. Inspections also showed that there had been no undue build-up of
ice so that was also factored out. The next step was to just watch the
lines and see what was causing the breakages.

Eventually, the cause was found. Watchers reported that in particularly
cold and calm weather, the cables started slowly dancing up and down.
This upward and downward motion gradually increased until the lines
snapped. Having found out what was happening, they now had to find the
cause.

What happens in cold, clear nights is that the temperature falls
rapidly at ground level. If there’s no wind, there is no turbulence to
carry this colder air upwards and you get a strong temperature
inversion. Even in the UK, I’ve seen a surface air temperature of -10C
with a value of +10C only about a hundred feet higher up. In Siberia,
the lowest part of the cable’s catenary would be in really cold air and
so would contract. As it did so, it was raised into warmer air and
started to expand again. This wouldn’t have mattered except that the
distance between the pylons enabled simple harmonic motion to amplify
the movement of the cable.

When the cause was discovered, the length between pylons was changed
and the breakages stopped.

As I say, I can’t see how this explains the motion of your goalpost as
there would be little temperature change over such a short vertical
distance though perhaps the bar could have been moving in and out of
sunlight?


Graham Davis [Retired Fortran programmer - now a mere computer user]
openSUSE Tumbleweed (64-bit); KDE 4.14.3; AMD Phenom II X2 550
Processor; Kernel: 3.17.2; Video: nVidia GeForce 210 (using nVidia
driver); Sound: ATI SBx00 Azalia (Intel HDA)

Was there a breeze at the time? I’m thinking about halyards on yachts which don’t need much air movement to start them slapping against the
masts. I don’t think temperature changes would happen fast enough to make the crossbar flap. What sort of rate was it flapping at and was it up and down or side to side?

Fourier trasnform your crossbar displacment and then come back to us and tell us it was moving in a sine wave. If you
want to account for phase displacement information, use the FFT to calculate the power spectrum. I suspect however that
your solution isn’t non-trivial because `einmal ist keinmal’…

The temperature difference between the inner and outer surfaces of your eyeballs caused an illusion, or maybe the dog has special powers. lol!