Experimental proof of Venturi Effect

[boneh3ad]

Another option is rigging the system so that the fan is actually downstream of the funnel drawing air through it rather than pushing air through it. This is how wind tunnels operate.

 

[paradisePhysicist]

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.

Cool video with cool music:
https://ytprivate.com/watch?v=UNBWI6MV_lY

I never heard of Venturi effect until recently, and I didn't watch the whole video yet, so I am not sure. But my opinion of how it works is the pressure makes the gas move, the moving gas has less pressure because it is not moving vertically anymore but horizontally. This is how it has less pressure even though also in a tighter space.

 

[russ_watters]

My first attempt at this is shown in the photos below. It went much worse than I expected. Here are the results:

As you can see in the results, the velocity ratio is nowhere close to the area ratio. This is likely due to poorly developed (non-uniform) and swirling flow, caused by the "tube" being installed on the outlet of the fan instead of the inlet (the large central obstruction on the fan is probably part of that). In other words; the measurements are likely unreliable. I'll see if I can re-do the experiment with the fan reversed.

The poor pressure performance and corresponding loss of airflow with the addition of the obstruction was not a surprise. Even with a relatively modest area ratio and associated static pressure requirement, the fan airflow dropped by half and registered essentially no static pressure (0.02" or 5 Pa) in the big box.

Below is a video I uploaded to Youtube showing the performance change as the variable geometry outlet nozzle is actuated closed. Note, the fan shutoff pressure is 0.07" (17 Pa) and the the fan audibly slows due to the added resistance. By comparison, here's a random residential toilet exhaust fan that's a centrifugal blower, which can achieve around 0.6" shutoff pressure (the curve is cropped at the top though):
https://www.broan-nutone.com/getmed...9c93-b67cad266740/Spec-Sheet-695.pdf?ext=.pdf

 

[boneh3ad]

 

[russ_watters]

Early returns on the draw-through design are promising, with results within about 10% of predicted. Details tomorrow.

 

[cjl]

Draw through is almost certainly the way to go. It basically eliminates swirl and also gives much more consistent flow across the cross section, especially if you use a flow straightener on the inlet - a cheap and easy way to do this that I've seen used on homemade wind tunnels before is actually to just buy several large containers of drinking straws and cut them (the entire container, straws and all) to a length of a few inches and fill the inlet with these. You'll also get much better results if you have a diffuser on the exit for pressure recovery - this will greatly reduce the static pressure requirement on your fan, and allow it to operate much closer to its open air performance.

Something like this would be an excellent guide to follow, though I think that guide would get better results with a slightly shallower angle and smoother nozzle transition at the front end, and a bit larger exit on the diffuser. You can even further reduce the flow restriction and get higher velocities still if you put the flow straightener before the constriction, but if you do that, the constriction of the nozzle needs to be smooth enough to keep the flow nicely behaved and not to introduce a bunch of turbulence or separation off the walls. You can see those characteristics on this wind tunnel for example, which is a really good example of sort of what you want to shoot for.

 

[T C]

I have found a few videos (video 1, video 2, video 3) on youtube. In all these videos, the venturi is fitted after the blower i.e. downstream but performing pretty well and without any kind of flow straightener. IMO a cone shaped venturi can perform better than square shaped one created by russ_waters.
@cjl, if a fan/blower performs better in open air instead when attached to the throat i.e. draw through mode, what's the use of it?

 

[russ_watters]

if a fan/blower performs better in open air instead when attached to the throat i.e. draw through mode, what's the use of it?

What's the use of a duct? Lots of uses/reasons to keep air contained. Any time you want a coherent mass of air traveling from one place to another you need a duct. Or if you pressurize it or want to force it to go somewhere it doesn't want to go (like through a heating/cooling coil). You could just as easily ask why we put water in pipes!

 

[T C]

In case of a wind tunnel, IMO the main purpose is to create a flow at a specific speed. If a open air blower can generate more speed in comparison when fitted at the throat of a convergent nozzle shaped duct, why should one need that?

 

[russ_watters]

In case of a wind tunnel, IMO the main purpose is to create a flow at a specific speed. If a open air blower can generate more speed in comparison when fitted at the throat of a convergent nozzle shaped duct, why should one need that?

Because a non-ducted fan doesn't generally generate higher velocity it only generates higher flow volume (if you use a proper fan). And it isn't coherent/uniform. It is also a huge waste of energy; most real/large wind tunnels are closed circuit.

 

[T C]

As per the post from cji, a non-ducted fan can generate more flow than a ducted fan especially when fitted at the throat of a convergent nozzle shaped duct. You are saying the opposite. Which one should I consider to be correct?

 

[russ_watters]

As per the post from cji, a non-ducted fan can generate more flow than a ducted fan especially when fitted at the throat of a convergent nozzle shaped duct. You are saying the opposite. Which one should I consider to be correct?

I don't see where he said that, but regardless, "flow" usually means volumetric flow rate, not velocity.

You have a bad habit of trying to connect unconnected things.

 

[T C]

and allow it to operate much closer to its open air performance.

That clearly means its ducted performance is worse than open air performance.

 

[russ_watters]

That clearly means its ducted performance is worse than open air performance.

"Performance". That word evidently doesn't mean what you think it does.

 

[T C]

Then kindly explain it.

 

[russ_watters]

Then kindly explain it.

I did!

 

[boneh3ad]

In case of a wind tunnel, IMO the main purpose is to create a flow at a specific speed. If a open air blower can generate more speed in comparison when fitted at the throat of a convergent nozzle shaped duct, why should one need that?

For one, a wind tunnel has a lot of other considerations other than just a specific speed. You have to worry about flow uniformity, angularity, and free-stream fluctuations.

 

[T C]

I just want to know whether the velocity will be same or less in case of flow through design or open air design.

 

[russ_watters]

I just want to know whether the velocity will be same or less in case of flow through design or open air design.

It depends on the fan. If the fan is capable of generating substantial static pressure the velocity through a duct can be higher than without.

But the volumetric flow rate will never be higher with a duct (for an axial fan, with the possible exception of a shroud/inlet cone).

 

[russ_watters]

Here's that second set of results in the draw-through configuration:

Notes:

• I tried to read the velocity on the short side of the large box and got zero (which is really just <50 fpm). The transition is too steep between the small and large box on the short side, and the flow separates from the side of the box, leaving a dead spot up against the side. Shape of the tunnel matters a lot.
• Shutoff pressure was 0.06", about the same as in the blow-through configuration. I didn't attempt to limit re-circulation at the tips. I suspect this wouldn't help much due to the noticeable rpm noise drop when closing the variable geometry inlet. In other words, while the fan shape is bad, the torque requirement for the asynchronous motor is a bigger problem. Most real-world HVAC fans or others with big motors are constant RPM and the torque increases as load increases (or the motor dies trying).
• Notice that in this test I was able to achieve a higher velocity in Box 2 than with the bare fan. But just barely. With a better fan, capable of generating static pressure when restricted, the velocity could be much higher in a small duct than at the fan outlet for a bare fan. But airflow (volumetric) will pretty much always be lower in a duct than on a bare fan.

 

[T C]

In this video, the proof of Venturi Effect is much clear and ratio is far better.

 

[russ_watters]

In this video, the proof of Venturi Effect is much clear and ratio is far better.

I don't see anything in that video about the Venturi Effect or the area/velocity ratio, with the possible exception of an out-of-focus/unreadable CFD snapshot. What results, exactly are you referring to? What ratio are you seeing?

Anyway, I wasn't trying to optimize accuracy, I was optimizing effort and cost. I'm quite certain you could produce better results than I did if you choose to put some effort into it, but your attempt was a far superior optimization of effort than mine was, so I doubt you will. I'm genuinely confused as to what you are after here (and I don't just mean this subject/thread, I mean your entire effort on PF).

 

[T C]

I don't see anything in that video about the Venturi Effect or the area/velocity ratio, with the possible exception of an out-of-focus/unreadable CFD snapshot. What results, exactly are you referring to? What ratio are you seeing?

Just look at the RPM with and without the flaps, that will be good example of velocity increase because RPM will increase in direct proportion to velocity. IMO, the best way get a proper venturi effect demonstration is using plastic or metal cones.

 

[russ_watters]

Just look at the RPM with and without the flaps, that will be good example of velocity increase because RPM will increase in direct proportion to velocity. IMO, the best way get a proper venturi effect demonstration is using plastic or metal cones.

It's just so disappointing that after all this effort trying to explain it to you that you think that has anything directly to do with the Venturi Effect. I feel like either you aren't trying at all or you are messing with us here.

 

[T C]

My main motto was to understand why my homemade experiment is unsuccessful and what is necessary to make it successful.

 

[russ_watters]

My main motto was to understand why my homemade experiment is unsuccessful and what is necessary to make it successful.

In that case you should re-read this thread from the start and put some real effort into learning the concepts. And for experimenting yourself, I showed you what could be done with a bigger fan, two boxes, a roll of duct tape and virtually no money or effort. If you put in even a small amount of effort and money you could surely do vastly better.

Or maybe better, you should probably enroll in an introductory fluid dynamics class, because you seem completely incapable of self-directed or forum-assisted learning. You may need an absolutely rigid structure that forces you to focus and doesn't allow you to wander off track. Here we just close threads and issue infractions. Maybe failing a test would help you focus.

 

[cjl]

In case of a wind tunnel, IMO the main purpose is to create a flow at a specific speed. If a open air blower can generate more speed in comparison when fitted at the throat of a convergent nozzle shaped duct, why should one need that?

I know I'm a little late here, but to clear up the misconception:

The volumetric flow rate, as Russ points out, will nearly always be highest when the fan is out in the open. There might be some way to slightly exceed that with careful design, but really, open air is about as good as you'll get. However, flow velocity will be higher inside a well designed wind tunnel, with a converging nozzle, a test section, and then a diffuser with the fan at the exit of the diffuser. In addition, as boneh3ad correctly states, velocity is not the only goal of a wind tunnel - flow uniformity and straightness, and having an extremely controlled environment are also very important.

 

[T C]

Is there any reason behind it? If so, what's that?

 

[russ_watters]

Is there any reason behind it? If so, what's that?

Conservation of energy, f=ma, etc.

 

[T C]

Conservation of energy, f=ma, etc.

Can you explain how the position of the fan (inlet or exhaust) can affect such laws?

 

[russ_watters]

Can you explain how the position of the fan (inlet or exhaust) can affect such laws?

It doesn't. The laws govern the flow and the addition of certain elements to the system affects the flow in keeping with the laws. Specifics on particular elements/fittings can be complicated, but much of that has already been discussed.

 

[T C]

So far, there is nothing in this thread that can clearly explain why a wind tunnel performs better when the blower is fitted and the exit instead of the the inlet.

 

[cjl]

It doesn't. At least not necessarily. That's more related to keeping minimum turbulence and swirl in the test section, but from an energy and flow velocity standpoint, you could put the fan on either side (but you'd still always want a nozzle and diffuser to maximize performance, not just a fan in a tube).

 

[T C]

Nozzle is a must to get higher velocity at lesser power consumption. I can understand that.

 

[cjl]

Both a nozzle and a diffuser. Without the diffuser, the pressure requirement to drive the nozzle is much higher, dramatically reducing the flow.

 

[T C]

Both a nozzle and a diffuser. Without the diffuser, the pressure requirement to drive the nozzle is much higher, dramatically reducing the flow.

Why?

 

[russ_watters]

Why?

Air doesn't like abrupt direction changes/transitions.

 

[cjl]

Why?

While Russ's statement is true, I don't think it really explains this very well. This really comes from 3 factors. First of all, you have to understand what a nozzle and diffuser do. A nozzle exchanges pressure for velocity, a diffuser does the opposite. So, at the entry of a nozzle, the pressure will be high and velocity low, while at the exit, the pressure will be lower and the velocity higher. On a diffuser, the entry will be at relatively high velocity and low pressure, and then as it slows the flow down, the pressure increases such that the exit conditions are a higher pressure and lower velocity.

The second important fact is that at any point where a wind tunnel is open to the atmosphere, the pressure must be equal to ambient pressure. This is because it is open to the atmosphere (admittedly, there are some minor caveats here, but we can largely ignore them for the moment).

Third, as was already discussed previously, fans flow more air (volumetrically) when the pressure change across the fan disk is as small as possible.

Now, taking those three things, let's think about a wind tunnel with only a nozzle. As I already said, the entrance of a nozzle must have a higher pressure than the exit. However, if the exit is open to the atmosphere after the test section, this means that the pressure just after the fan (just before the nozzle) must be higher than ambient, in order to drive the flow through the nozzle. This means you have to have a significant pressure gradient across the fan, reducing its flow. Having the fan on the exit doesn't help you that much either, since now the exact same logic still applies, except that the pressure just before the fan must be below ambient (since the entry to the nozzle is at ambient), again requiring a significant pressure jump across the fan.

However, now imagine a system with a nozzle and a diffuser. The air enters the nozzle, then loses pressure as it gains velocity. It then traverses the test section, and then in the diffuser, it loses velocity and the pressure rises again, and it is then exhausted to ambient. This means that the exit of the nozzle now doesn't have to be at ambient pressure - it can be below ambient pressure, since the flow will gain pressure in the diffuser. If we're still imagining a setup with a fan on the front, and you have the same pressure ratio across the nozzle, this means that less pressure is needed at the entrance to the nozzle than would be needed without the diffuser, since now the nozzle exit is actually below ambient pressure. Because of this, the pressure ratio across the fan can be much smaller, allowing it to flow more air (or, alternatively, to flow the same amount of air with less power).

This also still applies with an exhaust fan setup - basically, allowing the test section to operate below ambient pressure allows for a much smaller pressure change across the fan disk, improving operation substantially.

 

[paradisePhysicist]

However, now imagine a system with a nozzle and a diffuser. The air enters the nozzle, then loses pressure as it gains velocity.

Seems like it would gain pressure in the forward direction, but maybe lose perpendicular pressure.

 

[boneh3ad]

Seems like it would gain pressure in the forward direction, but maybe lose perpendicular pressure.

Define "perpendicular pressure." Also define "pressure in the forward direction." Given that pressure is a scalar quantity, I have no idea what you mean here.

 

[paradisePhysicist]

Define "perpendicular pressure." Also define "pressure in the forward direction." Given that pressure is a scalar quantity, I have no idea what you mean here.

Pressure equation is 2d (p=f/a) therefore it can have forward or perpendicular.

In the post I quoted there is a cone, nozzle where the water is flowing through, forward pressure I am referring to is the water streaming out the front of the cone, perpendicular is the pressure on the side walls of the cone.

 

[cjl]

Pressure is isotropic though - that is to say, it is the same value in all directions. Yes, the standard definition of pressure is f/a, and that area needs to be in a particular direction, but one of the fun things about (static) pressure is that it acts equally in every direction.

 

[paradisePhysicist]

Pressure is isotropic though - that is to say, it is the same value in all directions. Yes, the standard definition of pressure is f/a, and that area needs to be in a particular direction, but one of the fun things about (static) pressure is that it acts equally in every direction.

Hmm, NASA says that dynamic pressure is of a moving gas, or in this case I'd assume liquid such as water.

"we can define a pressure force to be equal to the pressure (force/area) times the surface area in a direction perpendicular to the surface. If a gas is static and not flowing, the measured pressure is the same in all directions. But if the gas is moving, the measured pressure depends on the direction of motion. This leads to the definition of the dynamic pressure. "
https://www.grc.nasa.gov/WWW/k-12/rocket/dynpress.html

I of course am not a seasoned expert and new to all this but this is just my take.

 

[boneh3ad]

Pressure equation is 2d (p=f/a) therefore it can have forward or perpendicular.

In the post I quoted there is a cone, nozzle where the water is flowing through, forward pressure I am referring to is the water streaming out the front of the cone, perpendicular is the pressure on the side walls of the cone.

Force has direction, but it does not inherit that direction from the pressure, but from the surface being acted on by pressure. In equation form,
\vec{F} = p\vec{A} = pA\hat{n}.
Pressure is a scalar. Force is a vector and, when related to pressure, comes from a scalar pressure acting on a surface that has direction.

Hmm, NASA says that dynamic pressure is of a moving gas, or in this case I'd assume liquid such as water.

"we can define a pressure force to be equal to the pressure (force/area) times the surface area in a direction perpendicular to the surface. If a gas is static and not flowing, the measured pressure is the same in all directions. But if the gas is moving, the measured pressure depends on the direction of motion. This leads to the definition of the dynamic pressure. "
https://www.grc.nasa.gov/WWW/k-12/rocket/dynpress.html

I of course am not a seasoned expert and new to all this but this is just my take.

Dynamic pressure ($q$) is effectively the difference between static (thermodynamic) pressure ($p$) and stagnation pressure ($p_0$). If a fluid is at rest, they are the same. If a fluid is moving, then it must be slowed down in order to reach stagnation, so $p$ rises and $p_0>p$. The difference between them in an incompressible flow is $q$ and is the kinetic energy per unit volume in the fluid, or
q = \frac{1}{2}\rho v^2.
This is Bernoulli's principle, $p_0 = p + q$.

 

[paradisePhysicist]

Force has direction, but it does not inherit that direction from the pressure, but from the surface being acted on by pressure. In equation form,
\vec{F} = p\vec{A} = pA\hat{n}.
Pressure is a scalar. Force is a vector and, when related to pressure, comes from a scalar pressure acting on a surface that has direction.Dynamic pressure ($q$) is effectively the difference between static (thermodynamic) pressure ($p$) and stagnation pressure ($p_0$). If a fluid is at rest, they are the same. If a fluid is moving, then it must be slowed down in order to reach stagnation, so $p$ rises and $p_0>p$. The difference between them in an incompressible flow is $q$ and is the kinetic energy per unit volume in the fluid, or
q = \frac{1}{2}\rho v^2.
This is Bernoulli's principle, $p_0 = p + q$.

Basically I was saying that, if there is a tube of flowing water, the front is going to have more force and thus more pressure. An example would be a flowing river, if you are standing at the front of the river you are going to have much more pressure to deal with, but if you are sitting on the river bank and put your foot in the river the water pressure (the perpendicular pressure) will be much less.

 

[boneh3ad]

Basically I was saying that, if there is a tube of flowing water, the front is going to have more force and thus more pressure. An example would be a flowing river, if you are standing at the front of the river you are going to have much more pressure to deal with, but if you are sitting on the river bank and put your foot in the river the water pressure (the perpendicular pressure) will be much less.

What you are describing is the concept of stagnation pressure. If a fluid (say, water) is flowing at a constant speed throughout, it has both static pressure and dynamic pressure. Static pressure is still a scalar and acts equally in all directions. If you were to non-intrusively measure the pressure in the flow, you would measure static pressure. It's the pressure felt by an object in the flow that is moving along with it and is how you would calculate the force on an immersed body.

Dynamic pressure is also a scalar quantity (it has the magnitude of velocity squared in it). This is the "extra" pressure you get when a fluid is moving, but the key here is that you never "feel" dynamic pressure.

What happens if you put an obstacle in the flow (such as yourself) is that the velocity of the fluid slows to 0 as it encounters you, which converts all of that dynamic pressure back into static pressure. At the very front end of you at the stagnation point, all dynamic pressure has been converted to static pressure. The static pressure at that point is therefore equal to the stagnation (or total) pressure, which is the constant in Bernoulli's principle. In essence, you still feel static pressure, but the static pressure has risen due to the change in flow conditions near your body.