Experimental proof of Venturi Effect

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Summary:
I want to know how much well experimented Venturi Effect is. If there are people here who experimented with Venturi Effect and can give references to experiments that can show the validity of Venturi Effect.
Venturi effect is known for centuries. And most probably that's why experimental proofs are rare because it's already accepted. But, I want to know how close real results are in case of experiments regarding Venturi Effect. I am especially interested in results of experiment regarding velocity of fluid through a convergent nozzle in case of subsonic flow. I want to know this just to see how other factors like viscosity can effect the outcome.
 

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  • #2
jack action
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I don't understand. Isn't the fact that the venturi meter is used to measure precisely the rate of flow is experimental proof enough? Or what about the fact that it was used in engine carburetors for decades?
 
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  • #3
boneh3ad
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It's an experiment performed in nearly every intro to fluid mechanics lab in most mechanical engineering departments worldwide. There are meters based on it. There are Venturi ejectors based on it. What additional proof do you need, exactly? It's been proven and reproven probably billions of times given the number of applications it has and constant usage of said applications.
 
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  • #4
russ_watters
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Agreed with the above. The Venturi effect is far beyond investigatory science, into engineering - but it's mundane even for engineering. You are of course free to devise your own tests of it, though they would probably be testing the experimental setup more than the effect.

Anyway, have you googled "Venturi effect experiment"?

In another thread you asked about how to measure airflow and we discussed pito-static tubes and manometer. Did you make one yet? Have you tried adapting it to this issue?
 
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  • #5
boneh3ad
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Why did this get moved to aerospace engineering? It really is more of a mechanical engineering topic.
 
  • #6
T C
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Actually, I want to see experimental proofs regarding increase in velocity inside a convergent nozzle in case of subsonic flow. As per Venturi effect, the velocity increases as the area decreases and that's why the maximum velocity can be found at the throat. I just want to see experimental proofs of that and also want to see that in real experiments, whether the increase in velocity is certainly in proportion to the decrease in area or not.
 
  • #7
russ_watters
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Why did this get moved to aerospace engineering? It really is more of a mechanical engineering topic.
Agreed. Though it may feel associated with wind tunnels (and might be for this thread), it is a concept with broad applicability to mechanical engineering.
 
  • #8
russ_watters
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Actually, I want to see experimental proofs regarding increase in velocity inside a convergent nozzle in case of subsonic flow. As per Venturi effect, the velocity increases as the area decreases and that's why the maximum velocity can be found at the throat. I just want to see experimental proofs of that and also want to see that in real experiments, whether the increase in velocity is certainly in proportion to the decrease in area or not.
What type of source would satisfy you? As said above, this is far too mundane to appear in current scientific literature. I suppose we could try to get ahold of some undergrads' lab reports, but it would probably be more instructive for you to do your own experiments. You can google for suggestions, or we could provide some as well.

Incidentally IMO, the velocity increase is the most obvious and least interesting aspect of the Venturi effect, and I would hope that by now you should understand why. Can you explain your understanding of it?
 
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  • #9
boneh3ad
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Agreed. Though it may feel associated with wind tunnels (and might be for this thread), it is a concept with broad applicability to mechanical engineering.

You can crack open a text on wind tunnel design and operation and I wouldn't be surprised if the word "Venturi" is never mentioned. There's just so much more going on, even if it's technically related.
 
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  • #10
T C
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What type of source would satisfy you?
Data from real experiments where the increase in velocity inside a convergent nozzle in case of subsonic flow, the velocity increased accordingly with decrease in cross-sectional area and the velocity at the exit is very close to the theoretical value as per Venturi Effect.
 
  • #11
russ_watters
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Data from real experiments where the increase in velocity inside a convergent nozzle in case of subsonic flow, the velocity increased accordingly with decrease in cross-sectional area and the velocity at the exit is very close to the theoretical value as per Venturi Effect.
Well as said you're not going to find such experiments in modern literature. I think your best bet may be to try to do your own. However, if it isn't clear by now, this is a very odd request. Perhaps if you explained more about an exact scenario you were looking at or ultimate goal, we could more directly address that.
 
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  • #12
T C
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Actually the problem started with my home made experiments. I have a small fan and I set a home made nozzle before it. But, every time I measure the velocity of flow from and fan and the velocity of the flow coming out of the throat, the velocity at the throat found to be less than the velocity at the entry. The anemometer can measure velocity upto 30 m/s and the velocity of flow coming from the fan is 1.9 m/s and the inlet to throat ratio of the nozzle is roughly 5:1. I want to know the reason.
 
  • #13
russ_watters
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Actually the problem started with my home made experiments. I have a small fan and I set a home made nozzle before it. But, every time I measure the velocity of flow from and fan and the velocity of the flow coming out of the throat, the velocity at the throat found to be less than the velocity at the entry. The anemometer can measure velocity upto 30 m/s and the velocity of flow coming from the fan is 1.9 m/s and the inlet to throat ratio of the nozzle is roughly 5:1. I want to know the reason.
Well, that's something we can help with. Please provide the details of the setup, including a dimensioned diagram and make and model of the fan, and a description of where and how you made the measurements.

Also, as far as I can tell, your issue here has nothing to do with the Venturi effect. You are asking about the area/velocity relationship, which is a mathematical proof/law, as a consequence of continuity for incompressible fluids. It has to be true. The Venturi effect has to do with the pressure change, which you didn't measure.
 
  • #14
T C
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1621268007856.png

It's the fan having 8 cm diameter blades.
1621268142777.png

It's the nozzle. 11 cm diameter at the inlet and 2.5 cm at the exit.
1621268268161.png
1621268297126.png
1621268319737.png

And you can see that when the nozzle is fitted, the reading at the anemometer has been shown to zero.
And, by the way, the pressure only decreases when the velocity increases. Does the pressure decrease when the velocity wouldn't increase accordingly?
 

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  • #15
russ_watters
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It's the nozzle. 11 cm diameter at the inlet and 2.5 cm at the exit.
That's a 19:1 area ratio, not 5:1.
And you can see that when the nozzle is fitted, the reading at the anemometer has been shown to zero.
That's not how this works. Continuity means that you are taking measurements on the same system at different places in the system (along a streamline). Here, you've measured two different systems:
  • System 1: A bare fan. (measuring its airflow)
  • System 2: A fan with a nozzle attached. (measuring its outlet airflow).
This is a common error people make when analyzing such systems.

And, by the way, the pressure only decreases when the velocity increases. Does the pressure decrease when the velocity wouldn't increase accordingly?
The pressure change is associated with the velocity change, yes -- that's what the Venturi effect is. If you don't have a velocity change, then you don't have a Venturi effect happening. In this case, what you said you wanted to check was the Venturi effect, but what you described wasn't. And what you actually measured wasn't the area/velocity relationship either.

[edit]
Also, to answer what the real question/issue is here: what you've learned is that your fan is too weak to push a measurable amount of airflow through your nozzle. It can't generate the necessary static pressure.
 
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  • #16
T C
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That's a 19:1 area ratio, not 5:1.
Actually, I have considered the diameter of the fan as the inlet, not the original inlet area. To calculate the ratio, you have to consider the fan diameter, not the entry of the nozzle.
Also, to answer what the real question/issue is here: what you've learned is that your fan is too weak to push a measurable amount of airflow through your nozzle. It can't generate the necessary static pressure.
The flow is subsonic, and it should accelerate inside the nozzle. Whatever may be the static pressure. That's the theory correct. And what minimum static pressure is needed to push the air through the nozzle?
 
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  • #17
russ_watters
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Actually, I have considered the diameter of the fan as the inlet, not the original inlet area. To calculate the ratio, you have to consider the fan diameter, not the entry of the nozzle.
Then that would be 10:1, but the mismatch in fan and nozzle inlet diameters can do nothing but cause problems. It leaves room for air to spill out around the blade tips. At that point, the true inlet diameter becomes very poorly defined. This will no doubt add error to an area/velocity ratio measurement.
The flow is subsonic, and it should accelerate inside the nozzle. Whatever may be the static pressure. That's the theory correct.
Yes, and surely it does. But you didn't measure the velocity change - and your anemometer apparently isn't capable of measuring such low velocities anyway.
And what minimum static pressure is needed to push the air through the nozzle?
Bernoulli's Principle says the total pressure must be constant along the streamline. Since we lack a defined scenario I can't answer that question; you will need to pick your inputs and then you can plug them into Bernoulli's equation to get the answer to your question*.

*And probably should assume the nozzle isn't 100% efficient and add a factor for that. Maybe 75%?
 
  • #18
T C
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Yes, and surely it does. But you didn't measure the velocity change - and your anemometer apparently isn't capable of measuring such low velocities anyway.
What's the reason that the anemometer can measure velocity without nozzle, but not the velocity with nozzle.
 
  • #19
russ_watters
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What's the reason that the anemometer can measure velocity without nozzle, but not the velocity with nozzle.
I answered that already:
Me said:
Also, to answer what the real question/issue is here: what you've learned is that your fan is too weak to push a measurable amount of airflow through your nozzle. It can't generate the necessary static pressure.
 
  • #20
T C
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Can you give me some idea about how much static pressure is necessary to get the desired results?
 
  • #21
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There is something important: the [edit]Venturi effect statement the velocity area relation [/edit] is not about a local velocity, it is about the average velocity. So, the average velocity (averaged over the entire inlet) at the inlet is related to the average velocity at the outlet by their areas.

So, the relation ##\bar{V}_{in} A_{in} = \bar{V}_{out} A_{out}## is actually about volume flow (cubic meters per second). The volume flow of air into the nozzle has to equal the volume flow of air out of the nozzle. This, on its turn, is equal to stating that the amount of mass into the system has to equal the amount of mass out of the system. This statement about mass is true because we assumed incompressibility (actually, that is not necessary, you can also regard a compressible flow not changing in time (i.e. steady state), then the amount of mass into and out of the system also has to be equal as well).

That mass is conserved is explicitly stated in the continuity equation (which is usually regarded as part of the Navier-Stokes equations), this equation is validated in literally every fluid flow measurement that has ever been performed. If you 'disprove' it, you need to explain where the mass went... It is probably easier to find flaws in the setup.

Your setup is very flimsy, the velocity at the inflow of the nozzle is blocked because the air would rather spill along the sides of the nozzle, than accelerate through it (which requires static pressure as @russ_watters already pointed out). The smaller the outlet area, the more air is spilled along the sides. You assume that the inflow velocity into the nozzle is equal to the free stream velocity of the fan, which is far from true.
 
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  • #22
T C
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Thanks for your reply. Can you tell me how to prevent the spilling of air?
 
  • #23
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You need an enclosed system with a proper fan in front, you'll need to build static pressure. The fan you use is not made for building static pressure. The fan needs to be enclosed in the system at the tips as well, and the blade need a different shape (flat at the tips) such that you have no flow leakage.

You also cannot change the setup as you do measurements. So you need to find a non-intrusive way to measure the flow velocity into and out of the nozzle. The flow velocity you measure needs to be related to the average velocity of the flow at that position, and you need to calibrate for that.

In fact, you need something like a wind tunnel. Doing real accurate measurements is hard...
 
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  • #24
russ_watters
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Can you give me some idea about how much static pressure is necessary to get the desired results?
Maybe a centimeter or two of water column, but again, that depends on what "the desired results" are, which you haven't told us.
Can you tell me how to prevent the spilling of air?
Make the fan diameter be just slightly smaller than the inlet.

Surely you can put together a better setup than this? It's sooooo crude. I'd expect better from a high school student.
 
  • #25
T C
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@Arjan82: Can you give me some references about such kind of experiments?
 

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