Vortex Shedding in Water
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- เผยแพร่เมื่อ 27 ก.ย. 2024
- Wolfgang uses a pendulum partially immersed in a makeshift flow tank to show us the effect of vortex shedding on a small object.
When fluid flows around a cylindrical object, there is a range of flow velocities for which a von Karman vortex street is formed. The shedding of these vortices imparts a small, periodic force on the object. Here the object is a cylinder attached to a physical pendulum whose frequency of oscillation is adjustable. The end of the cylinder is submerged in flowing water. When the frequency of the pendulum is adjusted to match the frequency of vortex shedding, the cylinder swings transverse to the direction of flow with a peak-to-peak amplitude of a few centimeters.
For more details on our setup as well as references for further study see sciencedemonstr...
Can you please explain this to the engineer who designed the shaking lamp post outside my house?
that's funny
should weld spiral fins around the lamp post to make the eddies chaotic and not oscillate between one another
XD lool
Ahahaha! You are awesome 😂
@2:25 you should stop the pendulum completely. It seemed as though the pendulum once let go started the movement. Or see what happens when the pendulum is already existing prior to weather flow.
Yes I'm aware this is old.
nice water clock. 8~) This is great to show how a basic Vortex flow meter sensor reacts to flow and the St: Strouhal number.....thanks.
instablaster
أخي هل لديك معلومات أعمق حول هذا الموضوع وشكرا لك
I think the flow speeds upstream and downstream are the same, along the channel. The flux was constant and the cross-section was also a constant, i.e. this will tell us the flow speed can only be constant. Do you have a aquarium side view? if the water depth in the downstream section was a bit higher than the front section, then the fast-slow claim can be true. Anyway, this is a great video.
Yes the flow rate slows down near the end of the channel because of the transition to the deep reservoir.
Correct, the slower stream would result in a shedding of vortices at a lower frequency. If that frequency is different from the pendulum's, then the pendulum will not be resonantly excited. Furthermore, the velocity of the slower stream might be too slow for the alternate vortex shedding (von Karman vortex street) to occur.
Harvard Natural Sciences Lecture Demonstrations but what is causing the pendulum's frequency? is it the creation of the vortices? if it is then wouldnt it automatically match the frequency of the vortices causing it to be reasonantly excited?
@@changenoways9555 - I think it's more about the material it's made from. Different materials have different atomic structures and characteristics.
According to this paper - frequency is the velocity of the sound of this object (if I understood it properly). And you hear daily how different objects and materials sounds - density matters but this is atomic structure.
"The resonance frequency of an object is a function of its sound velocity - a material characteristic - as well as of its geometry. "
(PDF) Resonance frequency measurements of a few materials for temperature variations. Available from: www.researchgate.net/publication/281549045_Resonance_frequency_measurements_of_a_few_materials_for_temperature_variations [accessed Oct 27 2019].
in this design, the cylinder is fixed, and the fluid is flowing into it, but does anything change if the fluid is affixed and the cylinder is the moving part? like a soccer ball traveling through windless air.
So the slower stream, has a longer interaction with the obstacles surface area {the pendulum} And so has more mechanical affect whilst passing over the surface, very interesting if some what counter intuitive, makes me think, nice 1..thanks, interesting video guy.
Thom in Scotland.
That dim vertical line on the water, moving slightly. Edge of a monolayer? I've seen that effect on slow-flowing sunlit creek beds, caused by skin oils of human legs!
Amazing video,
Is it possible for can you to please share the parts that need to build this vortex shedding,
Some more details about our setup can be found here sciencedemonstrations.fas.harvard.edu/presentations/vortex-shedding
Pretty interesting effect.
Can this not be utilized to generate energy?
Why is the flow speed lower at the downstream location, is it because you have an inclination of the bottom plate (also, I had a hard time seeing any difference in the ping-pong ball test)? If not then I would rather guess that the upstream location is affected by entrance disturbances that destroy the regularity in the pattern.
It's due to friction. Where the water meets the bottom plate on a micro/nano level, you have to imagine that the surface of the bottom plate is not perfectly flat. Any surface you see around you right now is not perfectly flat. When zoomed in close enough, you will see a mountain like surface. Now where the water molecules meet these mountains at the surface of the bottom plate, you can imagine water molecules getting stuck in between these mountains (in the valleys). We also know that there are certain inter molecular forces between the water molecules. Here we want to focus on the attractive forces. As the molecule that is stuck in between the mountains is pulling on its neighboring molecules, it will slow them down (considering the moving fluid case of this experiment). These molecules will in turn slow down a higher fluid layer as there will be a velocity difference between the layers. So you can imagine that this concatenation of molecules pulling each other back will result in a slower moving fluid eventually.
At the start of the ping-pong test, the ball needs to be accelerated first. When the ball is thrown in the water, its initial velocity is zero. The water molecules around the ball are moving fast, as the demonstrator states. The ball also does not have a perfect flat surface. Again the water molecules get stuck in between the mountains of the balls surface. The same pulling motion now causes the water to pull the ball along, as it is not a fixed object (unlike the bottom plate). We know from newtons second law that F=ma, and therefore the pulling force of the water will cause the ball to start moving as the force causes an acceleration a which increases the balls velocity v. After some initial acceleration, the ball will have the same velocity as the fluid. At this point, there will be no resulting pulling force on the ball anymore. So the reason why you can't see the difference in the fluid velocity is because the ball has to accelerate first. Once its velocity is roughly equal to the fluid velocity, the fluid velocity has already decreased due to the bottom plate friction.
Hope it helps:)
I wonder if the vortex frequency is similar to the pendulum swing frequency when the swing amplitude is large? Does the pendulum swing frequency be related to the environment in which it is located, in the water and in the air, or in the lower part of the water, as in the video?
Yes when the vortices are shed at a frequency that corresponds to the natural frequency of the physical pendulum, the amplitude becomes large. The situation is analogous to pushing someone on a swing: when you push at the right intervals, the person on the swing will travel the maximum distance between pushes.
The natural frequency of the physical pendulum is set by its length, relative to the pivot (e.g. a longer length gives a slower frequency, a shorter length gives a faster one). When the end of the pendulum is in the water, the natural frequency will slow down a little bit, due to the drag force of the water.
Very interesting. Thanks.
nicely done. consider trace lines.
The vortex structures are mechanistic phenomena that shouldnt change the material properties of the fluid. Why is there a change in the optical behavior due to these structures ? Is it because of underlying impurities in the fluid that get concentrated by the vortices ?
The vortices cause the surface of the water to no longer be perfectly flat, and so refraction at the air-water interface can bend light away and cause shadows at the bottom of the tank.
@@NatSciDemos Thanks !
And that is how galaxies are formed
Imagine if you use that motion and create electricity in the form of hydropower
is there a paper about this experiment
+ian prakoso You can find references for further study cited in our writeup sciencedemonstrations.fas.harvard.edu/presentations/vortex-shedding
Cool. Thanks
🐺WOLF GANG🐺
Thanks teach
Please let the real fish swim in the tank in next experiment and we can see how the real vortex made in nature. Change of water temperature and vortex are really related?
Now put some trip lines on it...
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His head is in the shape of a spade....
MY SOCKS !!
I vale from that accident wich 9 people died un the mountains
Hi. “Game Ready.” Geneveieve as well as England NSA Acknowledged 9/6/2020
“The shedding of the Internet will be check mate.” Helmholz resonance oscillating, I hear it. 🧠☎️ What do your socks mean at the end?
The socks diffuse the water flow to minimize a strong directed stream.
Thank you for sharing 🙏🏻✊🏻❤️
Que
So what?
I love these videos, many thanks to those involved
I'm pretty excited that I soon will start a PhD about this phenomenon. :)
goo.gl/MMi1XL
badass dude
how are u doing?
this is really creative ! I got why the skyscrapers and long chimney are tested for karman street
3:42 - I name you SOPENTURAN - Sock Powered Enterprise Turbulence Annihilator.
How to reconcile the results. At 1st the pendulum swings faster downstream with 'slower' flow rate. At reduced flow rate it swings more slowly ?
Thats interesting but why pendulum fluctuate more at the end not at the beginning?
Is it possible to construct this entire set up for a group project on vortex shedding?
Yes.
Are those socks drying at the end?
Is this the same phenomenon that causes a baseball thrown with no spin on it to "dance" around?
Kenneth Boyd Interesting question. The knuckleball definitely deviates from a normal trajectory due to variations in pressure. There can be several factors that lead to these variations (local turbulence, temperature variations, etc). But I would imagine that vortex shedding could plausibly play a role.
Ingenious and simple setup.
thank you
@@wolfgangrueckner7151 Hallo Herr Wolfgang! Could you please share the design details of the flow tank?
But is there something more than that?
hi, can i know what is the function of the socks?
did u just tied the end of the socks under the platform?
el tigre.
Like saying half a sentence and.................................................................... .........end.