Bernoulli's Principle Demo: Venturi Tube

This is a demonstration of Bernoulli's principle using a Venturi tube.
It was created at Utah State University by Professor Boyd F. Edwards, assisted by James Coburn (demonstration specialist), David Evans (videography), and Rebecca Whitney (closed captions), with support from Jan Sojka, Physics Department Head, and Robert Wagner, Executive Vice Provost and Dean of Academic and Instructional Services.

[@lucasdasilva8697]

I don't like when teachers don't explain why the pressure is lower at that restriction point. The reason, why it's lower is because of the law of conservation of energy. The flowrate must remain the same, like he stated, and because of this, the velocity must increase to push the same amount of volume of fluid over time. But the increase of velocity has to come from somewhere, it cant just be created. This is the reason for the lower pressure. The pressure in this scenario is potential energy, and the velocity is kinetic energy. In order for velocity (kinetic energy) to increase pressure(potential energy) has to decrease. The potential energy (Pressure) turns into kinetic energy (velocity). Think of a full balloon, when it's full, it has lots of potential energy, and as you let air out, its losing that potential energy and turning it into kinetic energy. Hope this helps someone, I know it helped me finally understand this principle.

[@Ram-xy2vy]

You Helped Someone Thank You

[@lucasdasilva8697]

@@Ram-xy2vy yay!

[@Ram-xy2vy]

@@lucasdasilva8697WOOO!

[@nowrinzamanprova9420]

bro u literally answered all my doubts in just one paragraph!!! thanks a lot... respect!!!

[@PiefacePete46]

​@@Ram-xy2vy : Ditto! 😊

[@jameserayburn]

Outstanding. I really appreciated the way you show the pressure drop was recovered by the divergent end of the tube.

[@ezfzx]

I'm not sure the flow-rate description works as described because the air is compressible. Bernoulli's principle originally described situations where the flow was from incompressible liquids, which was essential to continuity. The demonstration is interesting, but involves more than Bernoulli.

[@ezfzx]

@@AuMechanic Perhaps, but THE "Bernoulli's Principle" equation, as taught in first year physics is most likely the one people will have in mind when they search for and find this video. Ironically, Bernoulli's equation was actually formalized by Leonhard Euler, about the time when Giovanni Venturi was only 6 years old. See, most non-physics majors taking physics will only ever see this one equation or hear about this one element related to Bernoulli, and still misunderstand THAT, when the explanations and special conditions regarding this principle and equation are not clarified. They're oblivious to extreme exceptions and corner cases. They will never know about Bernoulli's family life or other contributions. And they will continue to point to air-flow examples and squeal "Bernoulli!" inappropriately, without understand why, or why it's inaccurate. They will also never become familiar with all of the other contributors to fluid flow dynamics, the work of many of which is far more relevant to current applications in aerodynamics and hydrodynamics. The point is ... if they're only going to ever learn this one thing about this one guy, let us (as instructors) get it right.

[@piotrklepinowski6478]

@@ezfzx Thank you for bringing happiness to our lives.

[@Lalo-Salamanca.]

In Fluid Dynamics at Mach number < 0.3, airflow is considered incompressible. So we can use Bernoulli's Equation in such a case. Correct me if I am wrong sir.

[@ezfzx]

@@Lalo-Salamanca. Respectfully, my concern is this. There are far too many air-related demos that are described as Bernoulli, which either are simply not, or, at best "Bernoulli-adjacent". So demos one my colleagues did with open air flowing over the top of liquid-filled pipes from different directions had some very interesting and unexpected results which were inconsistent with Bernoulli's principle. Our department has a Venturi Tube demo like the one you're showing, and it's good to see the lower pressure, pressure differential, etc. (We also point out that Venturi came along about 50 years after Bernoulli.) My point is that any air-flow effect cannot be just attributed to Bernoulli without at least some kind of disclaimer. Bernoulli (the guy) was very clear about the conditions of his equation: incompressible, inviscid, laminar flow, closed system, no added energy. We can perhaps dismiss the slow air as approximately incompressible, but not the rest. Even slow moving air behaves differently moving into and out of a pinch, as we see in wind tunnel tests. Even the A/C repair people can tell you that. You can see where the dust piles up from the gentle turbulence. Academically, for students who don't continue to study fluid flow for a living (aircraft designers, for example), it becomes very easy for them to make this one association without appreciation for the nuanced exceptions, and should they go on to become teachers, simply recite it like gospel, perpetuating the misunderstanding. (I've had high school science teachers, who never worked in industry, try to argue with me about it, until I show them a few demos that contradict their understanding. That's the nice thing about physics ... I don't have to argue; I can just show it. And I'm looking for a video to show you too.) The demos are fun and exciting and amazing, so-much-so that it's tempting to perform them without the details, just to wow the crowd. However, for the scientist/engineer, there's something deeper and exciting going on, and it would be disappointing to miss that opportunity.

[@Lalo-Salamanca.]

@@ezfzx Thats fact. If we aren't involved in it for a living, we take things in approximation. I understand your point sir !

[@raymondwest5969]

Thank you for the easy to understand explanation an demonstration. And, I love the Utah State Tie!

[@Observ45er]

I'd like to see this same demo, start and end with the pump off, BUT with all three manometers referenced to atmospheric pressure. That left-hand level not changing much bothers me. That looks like a three pronged manometer, not three individual ones. That shows an interrelated relationship that isn't at first obvious. The pressures in the two wide sections should be closer. .. ALSO, that second narrow section on the far left is confusing things...

[@ayushsinha1813]

Thank you, here in India most teachers don't care about intuition at all, hence have no empathy when someone doesn't understand.

[@techguru6875]

They Only use 3 props,one is black board, other is duster and chalk.

[@Lalo-Salamanca.]

Schools are just meant to teach you values of life while the actual study is to be done by yourself! The more quickly your country understands it , the less problematic your school issue will become !

[@ayushsinha1813]

@@Lalo-Salamanca. If by values of life you mean ‘moral values’ then I don’t agree. Value systems should be developed by people themselves.If anything, children can be put into diverse environments to develop their own values. Same can be said about any other form of education, providing the best environment to learn something is what’s important.

[@arielvillanueva1127]

precise. concise. to the point. thank you.

[@a.k3771]

Thanks a lot....No one explained it so nicely as u did.

[@UliSwag]

Awesome demonstration. Thank you for this.

[@alans172]

I'm scratching my head to understand how the air flows from the second (narrow) chamber into the third (wide) chamber as it appears to be flowing from low pressure to (indicated by the length of the water column in the middle manometer) to a high pressure (indicated by a shorter water column in the third manometer). Doesn't a gas only flow from high to low pressure?

[@uccoskun]

Beatiful. You can improve this demo by using open ended venturi tubes, which sit in a open bucket of fluid to isolate one end of the tube to the other one.

[@desirehannah5494]

Makes it very clear as to what happens, Thank you, :)

[@wagagames2393]

He explains it much better than the french videos... Thanks

[@58russ]

Why were the water levels different on the two outside tubes?

[@tqaquotes9379]

0:57 can anyone tell logically (not mathematically) about why pressure is low?

[@Auxified]

Sure. If I shrink down the middle section until it is only big enough to fit 1 atom then the pressure is only equal to the kinetic energy of 1 atom right? And now because only 1 atom can fit in there he does not have to worry about bouncing into some other guy and losing energy and wasting time in the tube, he has the highest possible throughput. The wider the tube the more atoms go in there, which makes it harder for them to get through. They all keep bouncing around.

[@buzzmas8068]

This is a good demonstration, and i understand how the principle works, i just dont understand why. Why would the small part have a lower pressure when it is literally being squeezed tighter into a small place? Why do things at higher velocities have lower pressure? Isnt velocity relative? How would a fluid even “know” if it was moving fast or slow? Someone help, its 2:00 am. Idk maybe im just getting into some pretty complicated stuff that my poor brain could never comprehend, but im still gonna try

[@physicsdemos]

The easiest way to understand why the pressure is lower in the narrow part is that an element of fluid in the wide part must accelerate as it approaches the narrow part (because conservation of mass requires this). The only way for the fluid element to accelerate is if the pressure is greater behind it (in the wider part) than in front of it (in the narrower part), creating a net pressure force on the element that causes it to accelerate as it approaches the narrow part.

[@buzzmas8068]

Physics Demos makes sense that there would need to be a pressure difference, so does that mean that the larger tube gains pressure, or does the smaller tube lose pressure, if you know what i mean? I would instinctively think the large one gains pressure cause all the particles hitting the bottleneck would sort of pile up on each other, and force the air through the small hole, but once it gets going through the hole, the air is able to move freely again and return to normal pressure since nothing is blocking it anymore. So if you added another tube comparing the small section with the atmosphere, would the small tube be the same, if not slightly higher pressure? Also thanks for replying

[@physicsdemos]

@@buzzmas8068 The pressure buildup from the particles hitting the bottleneck is a good way of thinking about why the pressure is larger in the wider region, relative to the narrow region. You can think of it either way - larger tube gains pressure or the smaller tube loses pressure. None of the pressures is "normal."

[@Observ45er]

@@physicsdemos This is one of the better demos because it shows the static pressure rise in the first wide section when flow occurs. .. What I don't understand is why no one addresses this increase as caused by the restriction effect of the narrowing of the pipe. The fact is, that the restriction is attempting to reduce the flow and, therefore, *causes* the increased static pressure in the right hand wide section. The fluid is not being squeezed in the narrow section, it is the outlet that allows the fluid to "escape" the higher pressure section. .. Think of the wide inlet section as a pressurized tank with an outlet that is relatively small. The pump is forcing high pressure fluid in from the right. This is a characteristic of a constant flow system, or pump. Everyone just says "Conservation of energy" and that the energies in effect exchange places as cross section changes. I find that to be disingenuous, or at least incomplete. If no energy is added or removed, and the fundamental setup prevents and energy change, then this is a given, not a cause, . .. Air has mass and we know, ever since Newton, that a force is required to accelerate a mass. Therefore, there must be some force/pressure causing the acceleration and it is our responsibility to explain that pressure. .

[@Observ45er]

+Buzzmas , Here's WHY .. All pressures mentioned here are static pressures. .. The narrow section is not squeezing the air because it's free to exit on the left. .. IMPORTANT: NOTE that the pressure in the right wide section INCREASES when the flow starts. This is very significant. It tells us that something is restricting/resisting the flow above and beyond the atmospheric pressure at the left. THAT resistance is caused by is the NARROWING entering the narrow center section. The narrowing increases the pressure UP-STREAM. . With a higher pressure there, the narrow section is now an outlet which allows the air to escape. Think of the right-hand wide section as a pressurized tank. The narrow section is now a hole in the tank letting air escape. .. If you started with a *completely wide* pipe exiting into the atmosphere, it would be atmospheric pressure at the left and pretty much atmospheric pressure all along the length of an all-wide tube. ... If you could vary the cross section of an all-wide tube, you would see the pressure on the right go up when it was narrow in the center or anywhere down stream.. .. Help?

[@snoopycharlie8718]

I understand the concept and the explanation is good. However, why does it feel like there is more pressure coming from a hose pipe when you use a finger to restrict the hose opening (when watering flowers for example)? The water increases in velocity due to the restriction, as the video says, and so travels further from the hose, but it also feels as if the pressure has increased when you pass your hand through the resultant spray of the mostly blocked hose. Compared to the low pressure you'd feel on your hand with the hose opening completely uncovered, it seems increased when partially blocked. Does anyone have any insight? Thanks

[@anjanathilakumara1937]

According to the continuity equation (A1*V1=A2*V2) when you cover the opening of the hose the velocity of water increases. When a high velocity object hits a barrier the force acting on the barrier is high compared to a low velocity object. Pressure= Force / Area. As the area is same in both cases you *feel like* the pressure is high when the water hits your hand.

[@user-pr3ry9wf4e]

Thanks

[@adityaraj8159]

Why is the liquid in third tube remains unaffected

[@anwenlune3478]

This helped me a lot with my homework, thank you very much :) !!

[@tqaquotes9379]

The constriction reduced pressure in the tube connected to it BUT increased pressure in the first tube. ??????

[@muhammednadeempaf7221]

EXCELLENT DEMONSTRATION OF VENTURI TUBE

[@snehasishpadhi1490]

absolutely fantastic demonstration sir . I agree on this but unable to figure out despite of pressure energy would be higher in the narrow cross-section why it is eventually low ?

[@syazwansofian5186]

Also having the same question here

[@alans172]

At 0:35 he INCORRECTLY claims that the volumetric flow rate, the number of cubic centimetres per second, passing through the two spots has to be the same. That would be true if the fluid were a liquid, in which the density is constant. But it's air, a gas. Its density is dependent on the pressure and temperature. And we know, from the heights of the water in the tubes, that the pressure are different in the two spots. What is constant through the system is not the volumetric flow rate, but the MASS FLOW RATE, in gm/sec. Bernoulli's equation (excluding gravity: P1 + 1/2 ρV1^2 = P2 + 1/2 ρV2^2) depends on constant density, ρ. Gases don't have constant density. Bernoulli's equation doesn't apply to gases. The pressure differences are due to the gas laws: PV = nRT, and W = (p2V2- p1V1)/ (γ-1). I haven't yet solved this experiment for the mass flow rate through the system, but I'm sure someone can.

[@engineeringart982]

Thanks for the practical demonstration.

[@kushalkollu8628]

Why is the liquid in third tube remains unaffected . Someone please explain 🥺

[@avyayjamadagni1161]

If you look closely, the liquid in the third tube rises a little. This could be because the velocity of air in the second wider section is higher than the first wider section as the air is coming from the thin section (where the air is travelling at the highest velocity). This causes the liquid in the third tube to rise a little bit, but not as much as in the second tube.

[@stevenbellz]

My assumptions is that it is because the third tube is near the exit of the duct, and is therefore closer to the atmospheric pressure in the room, which is the pressure inside the tubes when the flow is turned off. This might explain why the height of the water columns doesn't change as significantly.

[@Dr_Asma_physio]

Very nice........I really like this demo

[@TheLetterB0X]

A rather splendid explanation indeed! I enjoyed and found use of your precise and accurate explanation, I do not know exactly how to present my appreciation for this wonderful piece of art that some imbeciles call just a "video", this is truly a masterpiece, a 1 of a kind... Thank you very much my good sir, Your sincerely, TheLetterB0X

[@Aramalisadventures]

I tried to search over the net for a similar venturi tube with 3 tubes connected to each other and i couldn’t: could u show me the source of this tube? Where can i find it and buy it?

[@mustapha2]

Thank you sir

[@Rinmawia_ralte]

Very clear..

[@user-ex4si2md6r]

Did the ancient Greeks us that similar way in the in the past and the oculus was like the same thing

[@Brickleton]

Where did you buy the Venturi Tube? I am very interested in this demo for school. I would love for someone to reach out to me for this tube. Thanks! Also, the video is great!!!

[@avyayjamadagni1161]

Thank you for the explanation. I have a question. You stated that the same amount of air has to pass through the wider region in a given period of time as does the air passing through the thinner region. However, won't the compression of air prohibit that from happening?

[@dragonfly9786]

Check out the concept called continuity in fluids

[@Troeltsch7873]

Yes the amount of air per unit time sounds like the volumetric flow rate. As far as I can tell it's the same thing. It must be that sometimes a volumetric unit of a gas means the amount of it that occupies the unit volume when at standard pressure and temperature.

[@jeffkey5335]

Make a windmill like carburetor choke and store pumped water energy battery or dam face lift and or ram pump

[@PiefacePete46]

For the last sixty years, I thought I understood Bernoulli's principle... now I am not so sure.

[@larsmees4952]

Does the narrow part of the Venturi tube also get colder than the wide part?

[@physicsdemos]

I don't know.

[@nirjharbhatnaagar1982]

may be low pressure, low density, thus molecules are not that active...like in the upper atmosphere of the earth low pressure thus temperature goes in "-Celsius"...may be i don't know.

[@FLYWITHDAVINCI]

WATCHED.✔

[@user-tm9eo9rv2r]

The meaning of pressure in Bernoulli's law is unclear, leading to misunderstandings. Bernoulli's pressure is the pressure exerted on the wall. If you measure pressure in the direction of flowing fluid, the faster the flow rate, the stronger the pressure will be. If you don't agree, I'll shoot an air compressor in your face. Bernoulli's law should be understood in terms of cohesion, not pressure. Fluids attract each other. The faster the flow rate, the more molecules are attracted. Do not think that the faster the flow rate, the lower the pressure. It is more intuitive to think that the faster the flow rate of the fluid in contact, the stronger the pull.

[@yasharaltay2766]

how does the increase the venturi effect?

[@philindeblanc]

geez, that was brief and left a lot out. Even the video and airflow was cut short

[@user-zf6fb6pv1n]

😘😘😘

[@jonathananeng]

Practical

[@gharne3436]

1:13 swallows phlegm lolol

[@hi92446]

you are so cute!

[@billshiff2060]

Demonstration explains nothing.

[@givingittoyouraw4101]

Thank you for the great demonstration!