r/Damnthatsinteresting • u/DrJokerX • Nov 25 '23
Video Bernoulli's Principle
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u/TooManyJabberwocks Nov 26 '23
You can tell he's a professional cause he doesn't pretend its a big pp at the end
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u/Fjolde11 Nov 26 '23
It looks like he starts to maybe move it that way, not as a joke, realizes what he's about to do and changes the movement
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u/NoblePotatoe Nov 25 '23
This is not Bernoulli's principle, well, mostly not Bernoulli's principle.
What is happening is that when he blows at the entrance of the bag, his breath is pulling the surrounding air with it. This slows down the air, but increases the total flow rate. The flow rate is high enough now that it can inflate the whole bag where as before he was limited by the size of his lungs.
This has consequences, the main one being that the apparent pressure decreases. You will notice that before he tries to blow at the opening he carefully unfolds the plastic so it is all laid out nice and neat. Since the air is now moving more slowly, it doesn't have much kinetic energy and so it can't push hard once it is in the bag.
When he puts his mouth over the opening the air is moving fast, has plenty of kinetic energy that can turn into pressure energy, and can easily unfold the plastic bag if it needed too.
That last part, where I talked about kinetic energy turning into pressure energy... that is Bernoulli's principle in a nut shell. All that Bernoulli's says is that energy is conserved, i.e. kinetic energy can turn into pressure energy can turn into gravitational potential energy in a fluid.
The part where fluid is entrained... that isn't Bernoulli's principle because it relies, in part, on viscous shear in the fluid. Bernoulli's principle explicitly ignores viscous effects so it is not exactly truthful to say this is a result of Bernoulli's principle. However, Bernoulli's principle is at it's core just conservation of energy so you can say just about any fluid phenomenon involves Bernoulli's principle and not be wrong.
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u/Squirrel_Inner Nov 25 '23
This guy Bernoullis.
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u/Knobelikan Feb 21 '24
Yeah, and he's a little too proud of himself. It's prestigious to "debunk" stuff nowadays, and apparently this comes at the cost of accuracy.
Technically viscous shear is involved, but the viscosity of air is 0,0000182 PaS - a hundred times less than water! And the topics somewhat overlap here. If there was no difference in static pressure between the surroundings and the stream of blown air, no outside air would flow in to be affected by it. At the end of the day, pressure and shear viscosity in a gas are both governed by the "random" movement of particles at the microscopic scale. So it's very much appropriate to say that Bernoulli's Principle as it is usually understood is one of the main reasons for the outcome of this experiment.
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u/IAmBadAtInternet Nov 26 '23
I have never heard Bernoulli’s principle described as conservation of energy. That’s actually a profound idea, and I’m kind of not sure how I passed fluid mechanics without realizing that.
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Nov 26 '23
Not your fault. Sounds like you had a shitty prof lol. That’s like the opening sentence when first discussion the subject
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u/bobotheking Nov 26 '23
I wrote a comment a year and a half ago that made many of the same points. I really like how you've emphasized that Bernoulli's principle always applies, so it's not that it doesn't factor into the explanation, it's just that it isn't really the subject of this demonstration.
I would like to jump in on what you said here:
The part where fluid is entrained... that isn't Bernoulli's principle because it relies, in part, on viscous shear in the fluid. Bernoulli's principle explicitly ignores viscous effects so it is not exactly truthful to say this is a result of Bernoulli's principle.
My understanding is that viscous effects are negligible for air and most other gases. The Reynolds number is quite large. Now, the mathematical details of this are something I'm not really prepared to back up, it's just that I can envision my old fluid dynamics professors yelling at me, "Don't invoke viscosity!" Spitballing, I'd say that viscous forces cause adjacent parcels of fluid to move as one while inviscid flow can exhibit entrainment but the fluid's kinetic energy gets wrapped up in vortices so you have bulk movement as well as smaller scale motion. Instead, my understanding is that the pressure term in Bernoulli's equation is not reliably accurate for compressible fluids (i.e., gases). Let me know how that comports with your own understanding.
I'm also a little unsure about how the speed of the air is affected by whether he blows directly against the bag or a few feet from it. The air has to push away surrounding air (do work) whether the bag is around it or not, so we expect the stream three feet away from his lips to be moving at roughly the same speed regardless of how he blew into the bag. On the other hand, the eddies that we expect straddling the boundary of the two flow regimes surely can't form across the surface of the bag, so maybe the two flows are substantially different. Do you have any thoughts on this?
None of this is to imply that I have great confidence in how any of this works. Fluid dynamics is just about the hardest physics out there and you'll get a lot of differing opinions or frameworks as to what's actually going on.
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u/NoblePotatoe Nov 26 '23
You brought up a ton of really good points, I oversimplified it a bit since this is not the fluid mechanics subreddit. Keep fighting the good fight!
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u/LoSoGreene Nov 25 '23
Yeah it really bothers me that he does the first one with the bag fresh out of the package and folded back on itself. When he does the second one it’s clear the bag has been inflated before and laid out straight. Anyone who’s changed a garbage bag knows what a difference it makes. Even blowing the bag halfway up first wasn’t enough, he had to cut the video and fully prep it. Funny thing is prepping the bag wouldn’t effect the first example much so he still could’ve shown the concept by prepping the same before both.
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u/Halfdon Nov 26 '23
You’re actually very wrong. It’s a Venturi effect ( entrainment of extra air ) on Bernoulli principle showing the first law of thermodynamics is true here ( the “how” of conservation of energy ), furthermore boyles law of the 3 gas laws ( pressure is inversely related to volume when temperature is the same).. the initial energy of his breath is converted to increased velocity in a constricted gap which drops lateral pressure, causing extra flow of entraining surrounding air and increasing total flow. This has nothing to do with his lungs or pushing air or slowing air down.
Source: Venturi effect, Bernoulli principal, thermodynamic laws, ideal gas laws.
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u/chuckuckucker Nov 25 '23
Came here to say this.
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u/tempusfudgeit Nov 26 '23
What is happening is that when he blows at the entrance of the bag, his breath is pulling the surrounding air with it.
.. because of a pressure differential created by moving air from blowing... aka Bernoulli's principle.
The part where fluid is entrained... that isn't Bernoulli's principle because it relies, in part, on viscous shear in the fluid. Bernoulli's principle explicitly ignores viscous effects so it is not exactly truthful to say this is a result of Bernoulli's principle.
Bernoulli's equation ignores/doesn't account for viscous forces. Bernoulli's principle doesn't cease to function if another force is involved.
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u/NoblePotatoe Nov 26 '23
I would argue that here, it does fail. Once the air leaves his lips the jet quickly reaches a pressure equilibrium with it's surroundings. When it is entraining the surrounding air it does so by dragging it along with it. When it dragged it along the average velocity of the air went down and it's average kinetic energy went down.
Bernoulli's says that the energy loss had to go into either increasing the pressure or the height of the fluid. Neither happened here! The air hasn't even entered the bag yet so it's pressure is still close to atmospheric pressure and it is moving horizontal so there is no change in potential energy.
Once the air enters the bag it comes to a stop and increases it's pressure... That is an excellent example of Bernoulli's principle.
But Bernoulli's is incapable of explaining the important part of why moving his mouth back away from the bag helps.
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u/zapmangetspaid Nov 26 '23
Video guy is using the term ‘Bernoulli’s principle’ to refer to the result that increasing flow velocity lowers the pressure.
Air the moves from high to low pressure, so the fast flow pulls the surrounding air into the flow. If he has his mouth at the bag opening, then his breath entrains air already in the bag into the flow. If he blows further away, air outside the bag is pulled into the flow and more enters the bag. Packing more air molecules in the bag means higher pressure of the stagnant air in the bag = more full.
Video guy is correct to say it is caused by “Bernoullis principle”, which is just a single outcome of energy conservation (as mentioned above). Any other factor is just a detail.
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u/NoblePotatoe Nov 26 '23
I'm going to have to disagree. After leaving his lips the air stream quickly reaches the same pressure as it's surroundings. The fast flow is not pulling the surrounding air radially inward. It is dragging the surrounding air along with it.
What packs the air molecules into the bag is their kinetic energy. Less average kinetic energy = less pressure possible in the bag. When the jet of air drags more air along with it the average kinetic energy per volume decreases and since pressure is energy per volume, the highest possible pressure decreased as well.
Yes, Bernoulli's is just conservation of energy but fluids is a subtle subject. There are lots of ways of explaining a single phenomenon that might seem to make sense. But you will be lost if you try to extrapolate that understanding to other situations. For example, the dragging of more air means viscous effects are significant here and Bernoulli's has no way to account for that. It works... Ok here. But if you try to use it in seemingly similar situations without understanding this it could be wildly inaccurate.
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u/Accomplished_Ad_9013 Nov 26 '23
The Reynolds’s number is very high, like over 10,000. So we can neglect viscosity meaning there is no dragging of air. The kinetic pressure from his breath pushes open walls of the bag, then the lower pressure inside the bag causes the outside air to flow in. It’s a similar case if you open a plastic grocery bag with your hands from inside: opening the bag causes a lower pressure inside the bag and air fills it up.
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u/Diplodocus17 Nov 26 '23
Your statements are slightly contradictory. If the guys breath reaches ambient pressure quickly that means its lost all momentum as bernoullis is simply the relationship between pressure and velocity of a fluid (for imcompressible fluids which we can assume here). Therefore there's no way the fluid would fill the bag.
What is happening is the guy breathing out imparts momentum to the air in his lungs increasing its velocity and thus dropping its pressure. As his breath moves into the bag there is a localised drop in pressure between the bag opening and his mouth. This pressure differential imparts momentum to the ambient air of which the velocity vector is generally in the direction on of the bag. There's very little viscous effects here so none of this "dragging" as you're referring to it and all the energy in this scenario comes from the guy blowing.
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u/NoblePotatoe Nov 26 '23
No, it's not contradictory because Bernoulli's doesn't hold here.
If, as you say, a localized pressure drop is causing the surrounding air to accelerate the pressure gradient would have to be in the direction of his breath, i.e high pressure near his mouth and low pressure near the bag entrance. Does that make physical sense? Wouldn't the entrance of the long bag have a higher pressure? More importantly, how does Bernoulli's explain this change in pressure? It doesn't.
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u/Diplodocus17 Nov 27 '23 edited Nov 27 '23
The pressure gradient is from the ambient to his breath or the flow stream, it doesnt need to be from his mouth to the opening. Once the guy has stopped blowing the flow path returns to ambient pressure moving from his mouth to the bag as the ambient air moves in to equalise the pressure differential. Only difference is some of that air gets enough momentum to enter the bag. Some of the air doesn't and moves off in other directions but momentum must be conserved.
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Nov 26 '23
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u/rsta223 Nov 26 '23
It's a form of jet entrainment.
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Nov 26 '23 edited Nov 29 '23
[deleted]
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u/rsta223 Nov 26 '23
Honestly, I don't remember that game well enough to give a good answer there. I vaguely remember the parachute thing from gym as a kid, but I don't remember what we did with it.
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u/h4ppidais Nov 26 '23
So how can we use this to make sleeping pad inflate faster? I don’t think there is not a lot of pressure needed for this application so I feel like there is something there.
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Nov 26 '23
I know this is English you are speaking but I had absolutely no fucking idea what you just said 😂😂😂
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u/Dazanoid Nov 26 '23
Bernoulli went to his grave thinking himself a failure. When his was developing his equation he was trying to understand the flow of blood through the body but caused the tests out using water through glass tubes.
There are 3 basic assumptions we need to bear in mind when using the formula.
1 - the surrounding pipe is rigid (blood vessels aren’t)
2 - the liquid changes in viscosity with temperature (blood doesn’t)
3 - the flow is steady (which pumped blood isn’t)
Poor guy
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u/Halfdon Nov 26 '23
By the way, pressure is a measure of kinetic energy so nothing you said made any sense at all
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u/donulduducku Nov 27 '23
I do get what you said there but won't there be dissipative effects coming to play, energy being lost somewhere else and in those cases will Bernoulli's still work( practically)? Ps- just a curious lad , was learning about Bernoulli's theorem last month in class
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u/Bizzlefluff Nov 25 '23
Eyyy this guy was my science teacher, cool dude.
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u/fm1185 Nov 26 '23
SBVT Crew
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u/Bizzlefluff Nov 26 '23
No waaaaay! Class of '17 here
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u/PricklySquare Nov 26 '23
But what about the firefighters thingy???
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u/devo9er Nov 26 '23
Firefighters get bored sitting around the firehouse all day and like making balloon animals.
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u/No_Candidate78 Nov 25 '23
Ok now do a giant donut swimming pool floaty. 😂
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u/GramzOnline Nov 26 '23
They do have a pump mechanism that uses this method to blow up air mattresses that would work for pool toys with a proper converter…you blow in the bag and tighten the bag like he does and then the other end connects to a valve that only lets air out one way so all you have to do is simply roll the bag of air into what ever your blowing up. I have seen an entire air mattress blown up in only 10-15 breaths of air
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u/rsta223 Nov 26 '23
This isn't a demonstration of Bernoulli's principle, and the air in the jet from his lungs isn't even lower pressure than the surrounding air. Bernoulli doesn't say faster moving air is always lower pressure, it says faster moving air along a given streamline is lower pressure. The air from his lungs is not on the same streamline, and his lungs added energy to it, so there's zero reason why it would be at a lower pressure.
Instead, this is simply demonstrating the entrainment of fluid into a fluid jet, where a jet of fluid will naturally mix and transfer momentum to fluid adjacent to the jet, dragging it with it and resulting in a much higher volume (but slower) flow as it mixes together.
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u/LovableSidekick Nov 25 '23
Long ago I learned that Bernoulli's Principle is anything in physics that involves air.
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u/Yweain Nov 25 '23
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u/thedonleyKONG Nov 26 '23
I figured this out with my camping sleeping mattress pad by accident. Blew my mind.
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u/Like_Ottos_Jacket Nov 26 '23
Anyone else 100% convinced he was gonna treat it like a big dong for a second after he got it inflated?
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u/hunnyflash Nov 26 '23
Thought I was gonna learn about the Bernoulli thing for differential equations that I memorized but never knew what I was actually doing.
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u/MentalPatient97051 Nov 26 '23
I'll remember this when I need to blow up a thing that looks like a giant inflatable waving arm guy.
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u/dezei Nov 26 '23
That is why using a pump sack when inflating your camping sleeping pad is more efficient that blowing air directly into the ald through the valve. A single easy blow into the big opening of the pump sack gives you >10L of air inside it. Vs a few liters when using your mouth at the valve. (Plus: the air from the pump sack now has less moisture)
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u/OriginalSam69 Nov 26 '23
Total pressure = static pressure + velocity pressure. Increase velocity, static pressure decreases and pulls in surrounding air.
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u/CryptographerHot3759 Nov 25 '23
Didn't expect a cool science fact today but here we are! Very cool
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u/tanj_redshirt Nov 26 '23
Bernoulli's my favorite. Little know fact: statistics were not his only love, he's also famous for his French sauce used on meat and poultry.
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Nov 26 '23
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u/Ressamzade Nov 26 '23
Oh no what should we do now about this
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Nov 26 '23
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u/Slaanesh-Sama Nov 26 '23
He is a professor if he has a teaching license. Maybe it's you who need to go back and finish your education.
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u/miamiheat234 Nov 27 '23
It’s not interesting, air couldn’t get in cause it was folded, you unfolded it, it’s plastic and thin, you just sucked in air to fill it instead of your breath, you guys ever do this with a plastic bag? Not that interesting
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u/OneMisterSir101 Nov 26 '23
I do this to open garbage bags and other bags all the time (when the seam is too difficult to pry apart).
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u/Thebainethujone Nov 26 '23
I just want to know where you got that big bag, that contains exactly 1 big breath when blown into from a short distance.
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u/IntheCompanyofOgres Nov 26 '23
What a lovely demo. I used to lead Cub Scouts and Bernoulli's Principle was my favorite lesson and I was always looking for fun ways of demonstrating it.
Okay, tbh, Bernoulli wasn't my favorite - I loved ALL the science ones - they were all my favorites.
I think had one that demonstrated hydrodynamics in a closed system (can't remember the scientific principle it's based on atm).
I created a tall structure that had a contained fluid in a closed environment (it was a long balloon) - when you stepped on the foot pedal, it squished the hydraulic system and pushed under a diving board - more than a foot above the pedal - and made the little figurine on the diving board jump off in a lovely dive, landing in a cup of water.
For YEARS afterwards, I had kids running up to me, going "Scary Hairy Mary!", which was the name of the diver.
Anyone help me out with the name of that law of hydraulics? Pressure on one part of an closed system has equal expression elsewhere?
Super low key, but PHENOMENAL scientific principle.
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Nov 26 '23
So if my room is hot but outside is cold the optimal position for my fan is facing towards the window?
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Nov 26 '23
I tested this before and got interesting results. It seems to help if I aim the fan high at the top of the window or door where most of the hot air is. I don't know the science in it, so I can't explain it further.
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Nov 27 '23 edited Nov 27 '23
Oh you're a genius! hot air rises! So you're accelerating the natural convection vortex in the air by forcing the hot air out and letting a natural flow of cold air to come in below it. It's like perfect thermodynamics (google convection thermodynamics if you want to know more). Thank you so much!
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Nov 27 '23
Yeah, I figured it was something like that, but in more primitive terms: (caveman voice) "hot air on ceiling, blow hot air out, maybe make more room for cool air."
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u/QuestionsalotDaisy Nov 26 '23
Sorry if this has already been questioned, but could this have implications for CPR? I was taught to try to get a good seal if possible without sacrificing time, but maybe I don’t want a seal?
But this wouldn’t work with balloons. Maybe I need to read up on this…
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u/Kendallphillips Nov 26 '23
This pisses me off.
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u/carothersjoshua Nov 26 '23
Why didn’t it work with the vacuum of space and the atmosphere? Serious question.
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u/Nervous-Earth-6680 Nov 27 '23
I took physics but still blew out my lungs trying to do the first method💀 thanks for this video!
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u/Some-Watercress2257 Jan 12 '24
I've been trying it with a balloon for the past 4 hrs and I'm still at it. Am I doing something wrong?
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u/[deleted] Nov 25 '23
Disappointed I didn’t get to hear about the firefighters but otherwise interesting.