Experimental proof of Venturi Effect

[T C]

TL;DR 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.

 

[jack action]

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?

 

[boneh3ad]

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.

 

[russ_watters]

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?

 

[boneh3ad]

Why did this get moved to aerospace engineering? It really is more of a mechanical engineering topic.

 

[T C]

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.

 

[russ_watters]

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.

 

[russ_watters]

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?

 

[boneh3ad]

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.

 

[T C]

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.

 

[russ_watters]

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.

 

[T C]

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.

 

[russ_watters]

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.

 

[T C]

It's the fan having 8 cm diameter blades.

It's the nozzle. 11 cm diameter at the inlet and 2.5 cm at the exit.

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?

 

[russ_watters]

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.

 

[T C]

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?

 

[russ_watters]

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%?

 

[T C]

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.

 

[russ_watters]

What's the reason that the anemometer can measure velocity without nozzle, but not the velocity with nozzle.

I answered that already:

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.

 

[T C]

Can you give me some idea about how much static pressure is necessary to get the desired results?

 

[Arjan82]

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.

 

[T C]

Thanks for your reply. Can you tell me how to prevent the spilling of air?

 

[Arjan82]

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...

 

[russ_watters]

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.

 

[T C]

@Arjan82: Can you give me some references about such kind of experiments?

 

[Arjan82]

Lavoisier discovered the law of conservation of mass in the late 18th century. You'll have to look up his work somewhere.

[edit] ah, here it is, good luck![/edit]

 

[T C]

Lavoisier discovered the law of conservation of mass in the late 18th century. You'll have to look up his work somewhere.

I am not looking for the proof of Conservation of mass. But rather the way to stop the spill over.

 

[boneh3ad]

I am not looking for the proof of Conservation of mass. But rather the way to stop the spill over.

That's already been answered. Several ways.

 

[T C]

That's already been answered. Several ways.

I just want to see how the explained set up (including the fan with all its details) will look like in reality.

 

[russ_watters]

I just want to see how the explained set up (including the fan with all its details) will look like in reality.

C'mon, do you really need one of us to get a cardboard box and a roll of duct tape and literally demonstrate how someone in junior high could engineer this?*

Step 1: Get a better fan (at least 30cm dia), a cardboard box of similar dimensions to the fan and a roll of duct tape. Let us know when you have acquired these materials.

Or are you asking about better experiments? Have you watched a bunch of YOUTUBE VIDEOS showing such demonstrations yet?

*Note: my table fan broke last year, so I just ordered a new one on Amazon. If I'm bored this weekend, I may use the box it comes into set up a test.

 

[T C]

One question suddenly struct my mind. In a wind tunnel, the flow will be created by a blower and in my experiment too, the flow has been created by a blower. Why there will be no spill over in wind tunnels?

 

[T C]

*Note: my table fan broke last year, so I just ordered a new one on Amazon. If I'm bored this weekend, I may use the box it comes into set up a test

i will wait to see the outcome.

 

[russ_watters]

One question suddenly struct my mind. In a wind tunnel, the flow will be created by a blower and in my experiment too, the flow has been created by a blower. Why there will be no spill over in wind tunnels?

Quite simply, they are designed and constructed better. But again, you've already been told several of the reasons: Even a large fan in a real wind tunnel will have a smaller gap between the blades and the tunnel than your 11cm fan. And your blades are the wrong shape. Also your area ratio is pretty high.

 

[T C]

Also your area ratio is pretty high.

It's still subsonic. I can decrease the outlet area as long as the flow velocity at the exit velocity reaches sonic. Right?

 

[russ_watters]

It's still subsonic. I can decrease the outlet area as long as the flow velocity at the exit velocity reaches sonic. Right?

No. The larger the ratio the higher the required pressure, and the bigger the problem if your fan is not designed to generate significant pressure/has flaws such as listed above. E.G., the higher the pressure the more spillage you will get. You can't just use any random fan for any random subsonic application.

 

[T C]

You can't just use any random fan for any random subsonic application.

Which kind of fan is most suitable?

The larger the ratio the higher the required pressure

Is there any way to calculate the necessary pressure?

 

[russ_watters]

Which kind of fan is most suitable?

It's more complicated than just saying "kind of fan". It's still an axial fan, but you can google for photos of them and see how they differ from yours. More, narrower blades, smaller gap, different pitch and rpm, etc. Optimizing the exact fan for the exact application is a fairly deep engineering issue. Your fan is explicitly/purposely designed to *not* generate static pressure.

Any quality fan that is meant to be ducted (or otherwise obstructed) and provide static pressure will also be provided with a fan curve by the manufacturer that tells you how its performance varies with back-pressure.

Is there any way to calculate the necessary pressure?

Yes, like I said, by applying Bernoulli's equation to the Venturi effect. By now you really should be able to calculate the component pressures at various points in the system using the Bernoulli equation in addition to using the Venturi short-cut for static pressure only.
https://en.wikipedia.org/wiki/Venturi_effect

[oops, simultaneously deleted duplicate posts. Fixed now.]

 

[T C]

The link you provided has no reference to how to stop spilling over and how much static pressure is necessary to push a specific amount of flow to pass through a convergent nozzle as long as the flow remains subsonic.
Maybe I am not that adept and efficient to calculate it, but I am sure there are people here in this forum who can do that.

 

[russ_watters]

I'm calculating that with a 50% loss coefficient through the nozzle and your specified 1.9 m/s inlet and 19:1 area ratio, you would need 1200 Pa of static pressure at the fan. That's a pretty substantial fan, comparable to a heating/air conditioning unit pushing air through heating and cooling coils, HEPA filters and a large ductwork system. It's probably 10x what your fan is capable of.

 

[russ_watters]

The link you provided has no reference to how to stop spilling over..

The gap. The gap is too big. Reduce the gap. You have to reduce the gap.

and how much static pressure is necessary to push a specific amount of flow to pass through a convergent nozzle as long as the flow remains subsonic.

Obviously, you have to calculate that given your chosen condition. It sounds like you need a walk-through of this. Let's start here:
The Venturi effect page has a basic equation for calculating pressure difference. Do you see it?

 

[T C]

With minimal gap, if the flow is first released into a short tunnel before entering the nozzle, do we get better result then?

 

[jack action]

One question suddenly struct my mind. In a wind tunnel, the flow will be created by a blower and in my experiment too, the flow has been created by a blower. Why there will be no spill over in wind tunnels?

You don't measure the inlet velocity of the convergent nozzle at the outlet of the wind tunnel fan. They are not necessarily the same.

You've been told to take care of spillage in your apparatus. But this is not really your problem. You have to measure the inlet and outlet velocities without changing your setup. Right now, you are measuring the outlet velocity of a fan without a nozzle in front of it (apparatus #1) and the outlet velocity of a fan with a convergent nozzle in front of it (apparatus #2). You cannot assume everything is the same in both apparatuses.

You must have two anemometers: one in front of your fan - with the convergent nozzle over it - and another one at the outlet of your convergent nozzle. Of course, the anemometer you have right now is too big compared to your nozzle and will obstruct the flow to the point that it will affect greatly the measurements. (i.e. they will not represents the values without the anemometers present.)

 

[T C]

In short, the problem is in the anemometer size and shape, right?

 

[jack action]

The problem is that you change your apparatus in between your measurements. And you do it because your anemometer is too big.

 

[T C]

At present, my biggest problem is to reduce the spilling over of the flow to the minimal and measure the inlet and exit flow after that. Just that and nothing else.

 

[russ_watters]

At present, my biggest problem is to reduce the spilling over of the flow to the minimal and measure the inlet and exit flow after that. Just that and nothing else.

Not really, no. The inappropriate fan and low minimum velocity of the anemometer are big issues too. Other than simply measuring the area/velocity relationship you haven't specified a goal, and any/all of these issues will prevent you from measuring the area/velocity relationship.

 

[russ_watters]

You must have two anemometers: one in front of your fan - with the convergent nozzle over it - and another one at the outlet of your convergent nozzle.

You can use the same anemometer in two different places as long as you don't change the system in between. I was going to suggest on the inlet of the fan. I know that's not as reliable as the outlet, but it would be something. But that doesn't even matter since the fan is stalled and the velocity is lower than the anemometer is capable of measuring. As far as I can tell, the current setup can't be cleaned-up to make it work. He needs to start over from scratch with a bigger fan at least.

 

[boneh3ad]

At present, my biggest problem is to reduce the spilling over of the flow to the minimal and measure the inlet and exit flow after that. Just that and nothing else.

 

[boneh3ad]

You can use the same anemometer in two different places as long as you don't change the system in between. I was going to suggest on the inlet of the fan. I know that's not as reliable as the outlet, but it would be something. But that doesn't even matter since the fan is stalled and the velocity is lower than the anemometer is capable of measuring. As far as I can tell, the current setup can't be cleaned-up to make it work. He needs to start over from scratch with a bigger fan at least.

That anemometer is large enough (as a fraction of the duct size) that it almost certainly appreciably changes the flow.

 

[russ_watters]

That anemometer is large enough (as a fraction of the duct size) that it almost certainly appreciably changes the flow.

Agreed. And it could be increasing or decreasing the velocity depending where the fan is on the curve. One of the reasons I suggested a bigger fan is both the anemometer size issue and the gap/re-circulation issue are decreased just by getting a physically larger fan/system (assuming same gap size in cm).