Vortex Shedding in Air
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- čas přidán 8. 06. 2010
- A thin wire, moving through the air, is made to vibrate in the audio range at the vortex shedding frequency. Power lines and other tension wires "sing" on windy days because of this same phenomenon. Here, instead of the air moving past the wire, we got the wire to move past the air!
For much more information on our setup as well as references for further study please see sciencedemonstrations.fas.har...
my right ear enjoyed this
@TheMick26 The vortex shredding frequencies are of the order of KHz , so it would not be possible to see these. The wire itself is whipping around at about 10 m/s too.
@minutewalt Yes. In fact, we introduce the demonstration with exactly what you suggest -- a fluid with an additive that makes the flow visible (it's called Rheoscopic Fluid).
It sounds like the original Enterprise at the beginning!
Great sounds! Stay the heck away from that thing when it's running, though: yikes!
lol
@Newtoon Yes, it is. In the Tacoma Narrows case, the wind over the roadway deck of the bridge produced vortices with a frequency of 1/5 Hz. The 5 second period of oscillation (of the bridge) was close to the period of the vortices and thus the oscillations built up in time (resonance phenomenon)>
No. That wasn't the cause of the Tacoma Narrows bridge collapse.
Wonder what would happen if you tuned the strings to the same note or different frequencies altogether. Great video
@bluentes yep!
Thanks for the explanation and the demonstration, sir.
Try adding a little smoke so that we can actually see the vortices being shredded.
Thanks,
Mick
BTW, I'm glad you didn't have safety guards over the belts and pulleys just to satisfy OSHA seeing how those aren't the most potentially dangerous parts of the aparatus. ;o) That would make one heck of a slicer/dicer.
Tyvm for this!
my right ear loved this video
left ear had no idea what was going on
Me too
Might this demonstration have been more effective with a nebulizer or fog machine and a well-placed light source?
12 year late party
My left ear is lonely.
Wow. That sounds like a long train passing by.
It sounds like the bridge of the original Enterprise!
@NatSciDemos Not even with the use of a high speed camera? As visceral as this is demonstration is, additional visual aid would be most welcome.
That being said, could a different demonstration be set up in a tank of transparent fluid (water being the most obvious choice) with dyes (possibly non-soluble with the tank's main liquid (ie: dyed oil in a water-filled tank)) being released by the vector(s) creating/shedding vortices? Would that slow things down enough for a more visual demonstration?
I think the German band Faust would love to have one of these things on their stage.
Both strings were almost tuned together but a notch off. This creates beats we hear when they resonate. If they were exactly tuned, we would probably hear a continuous harmonic. Another way would be to have one piano string on one side and a muted material on the other side.
@NatSciDemos Hé ! Very interesting indeed. I have an itching question : is it the illustration of what happened to the Takoma bridge. We know now that the accident was not pure resonant but seems better explained by a coupling between vortices shredding and the reaction of the bridge (cables + bridge itself) ????
Quick ! An answer ! It's itching my brain ! ;-)
@Newtoon Resonant oscillation was thought to be the sole culprit behind the Takoma bridge failure, until an explanation pops up, suggesting that the interaction of the bridge with the wind causing continuously varying 'angle of attack' led to the disaster. A point to note that it is very unlikely for naturally occurring wind to be regular enough to cause a regular vibration- especially when it comes to vibrating a bridge of that scale.
Where would we see this kind of thing in our every day lives? What were they working on when the vortex shedding was discovered? Very interesting video.
This particular demo is designed to replicate the phenomena of sound emanating from power lines and telegraph wires during windy conditions. Vortex-induced vibrations are important in that they can have a strong influence in situations ranging from tethered structures in the ocean, pipes bringing oil from the ocean floor to the surface, aeolian harps, tall buildings, and chimneys, to name but a few. For example, the tallest building in the world, the Burj Khalifa in Dubai, UAE, incorporates a variation in cross section with height to help ensure that vortices are not shed coherently along the entire height of the building.
@@NatSciDemos That is cool! I came across this concept after hearing about it with knuckleball pitches in baseball.
@KafshakTashtak
oh and you think its pure coincidence that the sounds peak at piano string harmonics? sure there is noise out of the motor etc the acoustics of the apparatus could be better in that aspect, but listen to the high pitched sounds that peak and die out depending on the rotation speed and go higher and higher as the apparatus rotates faster. i bet that if you put that sound track through audio software and compared the peaks to the string frequency they would be harmonics of it
Clever job. Don't put your fingers in the vortex! lol
Could you get that with a high framerate high res camera? In a correctly lit environment so the wires contrasts well under the lens.
You could probably isolate out the motor noise by putting the device in a separate room with a hole in the wall.
I think the sounds are mostly due to your motor, belt, bearings and structure, finally some sound coming out of string.
Reminds me of that Gordon Lightfoot song, "The Wreck of the Edmund Fitzgerald":
"The wind in the wire made a tattletale sound..."
Hard to discern the engine noise from wire vibration (audible harmonic), interesting nonetheless; just wondering how we can be certain that this is from wire/air interaction and not from the apparatus holding the the wire out, as it has much more surface area? E.g. how do we know the sound is generated by the air/wire combo and not something to do with the arms holding wire & that is what’s interacting with air, or, if this can just be vibration from the engine/ pulley / whole assembly vibrating strings? To be clear, I do personally believe the wire / air can do this, just wondering for method used, how we can scientifically know this is the case and prove isolation from aforementioned…? Thanks for interesting, albeit brief, video!
Thanks for your question. The tension in the wires is set so that when they're plucked like a guitar string, their fundamental audible frequency is at about 120 Hz. When the wires are set in motion and moving through the air at about 4.75 m/s, the periodic shedding of vortices starts to happen, and the wires begin to audibly "sing" at about 845 Hz, which is very close to the 7th harmonic. Using a spectrum analyzer, we found higher audible harmonics are obtained by increasing the speed of the wires. More details on our setup can be found here sciencedemonstrations.fas.harvard.edu/presentations/vortex-shedding-air
it's gonna take off.....
A friend of mine is a piano tuner so I'm being picky.
The strings were not tuned to the same note. This causes an interference pattern in the sound both strings produce (hence the chopping sound).
Does this alter the way in which the system produces vortexes?
Could the strings be tunes to cancel the sound of the other and reduce the noise level of the system?
How would vortexes vary in a sound reduced system?
Your friend the piano tuner has a better ear than I. In any case, the slight beating of the two frequencies doesn't change the way vortexes are produced. I also don't think you can tune the two strings to cancel the effect.
If the frequency of the vortex shedding is primarily dependent on the cross sectional width of the string then I suspect they will both oscillate at the same frequency regardless of whether one is tuned different from the other. The tuning would affect the amplitude of oscillation. I hear Doppler shift though. The setup actually seems like it would be difficult to mis-tune, because the tensions and lengths of both strings are almost certain to be very similar.
Excellent. only the driving mechanism noise too annoying. Actually the intermediate frequency resonance is much more clear after the motor stopped, at 4:18.
I suggest using a strobe light, and maybe a "fog" made with dry ice in water.
What would happen if the RPMs where ALOT greater? Would the sound get to a frequency the human ears could hear?
They're already at frequencies the human ears can hear. The frequency would go up with increasing speed. In principle, they could go up into frequencies we cannot hear.
Oops! I should have said shed, not shredded. But with piano wire, they are probably being shredded as well. ;o)
Either way. thanks for the post.
Hell, I got great headphones and have to guess.
Should have included a sound FT visualization.
cool, now try to grab the rod in the middle
bye bye forearms
ear buds bad idea
looks and sounds like a part of a @fuck buttons song!
confused
Dangerous, Needs a cage/fence around it.
Or just... stand back and listen...
My cat did not like this one bit!
2:02 click to Skip the bla bla section