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.

[lucasdasilva]

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.

[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.

[Observer]

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...

[raymondwest]

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

[alans]

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?

[ayushsinha]

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

[desirehannah]

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

[tqaquotes]

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

[UliSwag]

Awesome demonstration. Thank you for this.

[buzzmas]

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

[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.

[arielvillanueva]

precise. concise. to the point. thank you.

[snoopycharlie]

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

[RationalDiscourse]

I just wish he'd done the experiment with open-ended manometers instead of the linked ones. That way we could have estimated the actual pressures in the individual tubes.

[adityaraj]

Why is the liquid in third tube remains unaffected

[유지태-ls]

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.

[PiefacePete]

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

[anwenlune]

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

[russ]

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