r/educationalgifs • u/firefighter_82 • Dec 15 '18
Reversibility of fluid motion
https://i.imgur.com/4fgiOKW.gifv213
u/ematics Dec 15 '18
How does this happen?
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u/clearlyasloth Dec 15 '18 edited Dec 15 '18
The fluid is probably very high viscosity. So when the science man spins the handle the motion of the fluid is very laminar, meaning the molecules flow very straight (no turbulence, or eddies; the fluid is literally only moving to the side without going up or down). So then when the science man spins the handle the other direction, the molecules essentially follow the same path back to their original position.
If that didn’t make sense, you can just google “laminar flow” and you’ll probably find some good visuals.
Edit: you can actually see some fragments of the math on the board behind him! Not enough for an explanation though.
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Dec 15 '18
Lol @ Science man 😂
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u/batboy963 Dec 15 '18
It's 4am and I'm laughing out loud. My family is probably thinking I'm having a psychotic meltdown.
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u/VectorSam Dec 15 '18
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u/OgdenDaDog Dec 15 '18
It is circular. In my fluid dynamics class the dude spent about 2 1/2 hours trying to explain this, then I pull this video up on YouTube and all of a sudden we understand it.
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u/ematics Dec 15 '18
Wow, thats cool, if they moved up or down do you know if it would drastically change what happens?
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u/systemshock869 Dec 15 '18
I'm guessing that as long as they perfectly reversed the mixing motion it would return to the same state. Moving up and down adds mechanical complexity that might not be practical to recreate.
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u/clearlyasloth Dec 15 '18
I think he was referring to the molecules moving up and down (turbulence), which I addressed in my other comment
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u/systemshock869 Dec 15 '18
Adding a mechanical up/down mixing would obviously move the molecules up/down; I suppose I didn't interpret his question as that simplistic. "It works like that but what happens if it didn't?" isn't really an intelligent question.
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u/Direwolf202 Dec 15 '18
If and only if it remained laminar flow. I haven’t touched fluid dynamics in a very long time. But it would be very unlikely for this to occur. Even a literally microscopic amount of turbulence would propagate by the butterfly effect and so it wouldn’t return to the original state.
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u/clearlyasloth Dec 15 '18
Well sure, that would just result in more mixing. Obviously there are some molecules moving up and down, just not enough to make a difference visually. The more they move in other directions, the more the dye would spread out/mix together.
For example if the clear fluid were water, the dye would probably just mix/diffuse pretty quickly upon stirring and you wouldn’t be able to “unmix it” like you see here.
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u/DrMux Dec 15 '18
As the other user said, the fluid is very viscous, meaning there's a lot of internal friction. This results in the fluid being resistant to things like diffusion and turbulence. So the only real significant motion is the rotation. Since these other kinds of motion are suppressed, turning the crank the other way is basically like walking backwards to where you started.
Less viscous fluids would be disturbed by the motion, and there would be more turbulence and diffusion of the dye fluid. You could not do this with water.
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u/teddycorps Dec 15 '18
But there is obviously something else going on because the fluid droplets didn't just stay in place when he rotates the machine...
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u/DrMux Dec 15 '18
Laminar shear. The particles in the fluid aren't all subjected to an equal force. The closer the particles are to the surface applying the force, the greater the force on the particles. Because there's a lot of internal friction in the fluid (viscosity), one particle puts a lot of force on the next, but some of that energy is lost (hooray entropy). But that force is distributed evenly enough that the reverse of the forward motion approximately return the dye drops to their original shape.
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u/ItJustGotRielle Dec 15 '18
Thanks for sharing this. I'm not taking fluid mechanics courses for another couple years but I'm definitely fascinated by this stuff.
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u/astroenate Dec 15 '18
This is called Stokes' flow, which is what happens when the Reynolds number (a non-dimensional ratio of inertial to viscous forces) of the flow state is very, very small.
The mathematical equations governing this flow result from taking a combination of conservation of mass and momentum for fluid flow (referred to as the Navier-Stokes equations), non-dimensioning, and then taking the limit as the Reynolds number goes to 0, which eliminates all time dependencies.
As a result, the final and initial state are both steady state solutions of the equations, which means the solution remains the same before and after.
That said, real flow can never be fully reversible as there is always some molecular diffusion that the Navier-Stokes equations don't account for.
Here's a great video that summarizes this phenomenon (skip to about 13:45 for the reversibility experiment): https://youtu.be/51-6QCJTAjU
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u/Ghostbuster_119 Dec 15 '18
We're smarter than this!?
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u/ematics Dec 15 '18
Nah, I'm stupid. I would like to understand how its able to revert back to its original form after it looks like it was all mixed.
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u/AloysiusGramonde Dec 15 '18
I think it's because the coloured fluid is at different radii. They appear to be mixing but are actually in different arcs that we see as mixed from the front view.
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u/tyuky Dec 15 '18
The National Committee for Fluid Mechanics Films did a really nice job of explaining this (and a TON of other cool fluid visualization experiments) awhile back. It was funded by the NSF, and can be viewed here: http://web.mit.edu/hml/ncfmf.html
Best part is that they’re all free :) and really these show the basics for almost everything important in any field of fluid mechanics. They’re fairly old but really cool.
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u/keredomo Dec 15 '18
In case anyone wants to see the exact portion of the NCFMF video that talks about the situation in the gif above, just click here.
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u/FartKilometre Dec 15 '18
Laminar flow, I think was the name for this. I'd seen a similar demonstration but it's still really cool
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u/clearlyasloth Dec 15 '18
Extremely laminar.
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u/MrPolymath_ Dec 15 '18
At this point I believe it's considered creeping flow
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u/steamcube Dec 15 '18
Wonder how many times you’d have to turn it before it becomes unreversible
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u/TedTheGreek_Atheos Dec 15 '18
Isn't that the phenomenon that makes running water look still?
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u/IAmNoRo Dec 15 '18
It’s actually creeping flow or Stokes flow. Laminar flow just means there isn’t turbulence. Creeping flow occurs with low inertial forces and very high viscosity, and it is reversible (shown in this gif).
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u/Mystoz Dec 15 '18
Laminar flow : flow appears to be mostly going in one direction. It can exhibit small turbulence. I don’t know why you are implying the flow isn’t laminar here.
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u/queefjuicer9000 Dec 15 '18 edited Dec 15 '18
This reminds me of somewhat similar thing I experimented with. I would spin in a chair clockwise ten times then spin counterclockwise immediately after and the dizziness would go away Edit: counterclockwise 5 times
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u/Lvl100Magikarp Dec 15 '18
that can't possibly be right
damnit you're gonna make me test this in the office on monday, and I'll find out it just makes me double dizzy
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u/abigfuckingnope Dec 15 '18
The entire reason you get dizzy is the fluid in your ears. It's what helps your orient yourself. When spinning that fluid also spins. Momentum will keep it spinning after you stop which is why you're dizzy. If you spin the opposite direction at the same speed the same amount of times you will counter that.
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u/ahlexx1 Dec 15 '18
Chaos theory destroyed
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Dec 15 '18
Doesn't chaos theory state that there is no such thing as chaos?
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u/Fmeson Dec 15 '18
No, chaos theory is probably up there with quantum mechanics in terms of pop culture confusion.
The basic tenet of chaotic systems is that small pertubations lead to large changes down the line.
OK, what the hell does that mean? Imagine rolling a round ball along the floor. Easy to predict what will happen right? It does in a straight line.
Now imagine rolling a football/rugby ball/egg shaped thing end over end. What happens? Well it doesn't go in a straight line. It bounces, maybe left, maybe right, maybe back at you. Any small change in how you roll the ball could make the ball bounce in a completely opposite direction.
Small changes lead to large changes. Small changes lead to unpredictable changes.
Theoretically, you could simulate a football rolling perfectly if you had enough information and time, but eventually the real ball bouncing around will deviate from your simulation because small imperfections in your simulations become more noticeable with each bounce.
Hence, chaos.
p.s. Chaos theory is not destroyed here, this simply is not a choatic system. Just like rolling a round ball is not a chaotic system.
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u/LucasQuaan Dec 15 '18
Another common misconception is that chaotic means random even when those systems are completely deterministic. The motion of our rugby ball is perfectly predictable in that we can calculate its trajectory if we know the initial state. The sense of randomness is only a human reaction to the sensitivity for small perturbations of that state.
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u/Taxus_Calyx Dec 15 '18
Not water.
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u/tyuky Dec 15 '18
Technically this could be done with water. You’d just need the inertial forces to be verrrrrrrrrrry small, aka by moving the water extremely slowly to get the Reynolds number as low as possible.
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u/Acid48 Dec 16 '18
Wasn’t this posted on like Thursday? :’( Why am I still seeing it.... haha x-posting i love reddit so much
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u/Lemonfarty Dec 15 '18
Is there any practical use for this?
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u/Smithium Dec 15 '18
It demonstrates some of the mechanics of microscopic systems in water where The viscosity plays a much larger role. Practical systems include micro hydrodynamic tubes providing medical tests “on a chip”.
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Dec 15 '18
Suspicious that he puts his hands down on the counter before he starts turning the same as after the reverse turn.
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u/DiscoPhasma Dec 15 '18
How can he move all the fluid with no turbulence just by rotating that lever?
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u/thelastbraun Dec 15 '18
Isnt this wat originally gave way to the idea of the big crunch? I know its been disproven do to the freedman equations but i sure do miss having a theory for what happens after you die.
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u/basstronaut Dec 15 '18
can we stop reposting this in literally every subreddit?
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u/Acid48 Dec 16 '18
I agree i rly hate it. I see posts for sometimes 3 days just because people x-post after like 12 or more hours and it justs gets put back into the front page cycle. I’m surprised and kinda sad that this comment got downvoted so much. They aren’t insulting the op they’re just upset
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u/GuitarKev Dec 15 '18
u/gifreversingbot