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Old 8th August 2019, 07:47 AM   #81
Robin
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Incidentally, in the video at around 1:51 - 1:57 the actual attachment of the cone is obscured, but it looks to me as though the cone is inserted inside the already small outlet which would reduce the amount of air that can escape, or possible stop it altogether.

It doesn't help that we are watching a screen grab of the video rather than the actual video and have the low resolution and a pop up which obscures part of the process.
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Old 8th August 2019, 02:22 PM   #82
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Originally Posted by Red Baron Farms View Post
Wrong again.

Wow! Impressive argument and explanation.
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Old 8th August 2019, 03:12 PM   #83
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Originally Posted by Red Baron Farms View Post
not true. Don't confuse the man with the woo from the original thread
Correct. A lot of this stuff is counter intuitive. Common sense is not one's friend in this domain.

The cone itself actually creates lower air pressure. To understand why, you need the maths.

One of our protagonists up thread demanded that I provide that maths, so I did. It involved calculus. Not a peep since.

I wonder why?
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Old 8th August 2019, 03:37 PM   #84
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Originally Posted by Thor 2 View Post
The jet of air escaping the balloon is pushing against a static air mass which resists the flow and provides a reaction thus propelling the car.

When the cone is fitted the exit velocity of the air is greatly reduced so the resisting reaction much reduced.

The forces are very low so the rolling resistance of the vehicle is significant. The static friction would be higher than the effort provided with the cone fitted. The cone also adds mass and therefore increases rolling resistance.

There would be a slight rocket effect due to the acceleration of the air but this would be very slight because the accelerating force is low.
Mess about with the shape of the cone and one can get it to suck, moving the car backwards, were one so motivated.
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Old 8th August 2019, 07:11 PM   #85
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Originally Posted by DevilsAdvocate View Post
2. Now we add the cone. We have the same amount (mass) of air being moved out of the balloon during the same amount of time. We have the same force from that air, so we would expect the same force to be applied to the car and it would move at the same speed. But it doesn't.
You missed the point about the cone that Red Baron Farms made, which would give it a force in the opposite direction even before the car had started moving. Did you miss this?
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Old 8th August 2019, 07:59 PM   #86
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Originally Posted by Puppycow View Post
How is so much dumbth possible?

Trolls? Sincerely deluded fools?

Maybe it doesn't even matter.
Meh. I already provided the actual math involved.

Nobody cared, because math is too hard.
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Old 8th August 2019, 08:20 PM   #87
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Originally Posted by Red Baron Farms View Post
Wrong
It would help my understanding considerably if you explained why that is wrong rather than simply stating that it is wrong.

Originally Posted by Red Baron Farms View Post
The turbulence does have a minimal effect on the car as drag, but at such low speeds and small surface area, it really is so small as to be negligible.
That is what I am saying. To understand what is happening, I am looking at all the effects. In post #16 someone brought up the Krushnic Effect.

From what I have been able to learn, it seems like the Krushnic Effect wouldn’t really be in play. It seems to me that there would be some effect, especially in this rather crude experiment.

My guess is that the air from the balloon would come into the tube. It creates a low-pressure zone that keeps it mostly tube shaped. That air pushes against the atmosphere, which slows it down. When it slows down, the pressure differential decreases and it is no longer confined to that tube. It expands and hits the interior of the cone. The air is already moving back and the cone shape is directing the air back.

But there is probably a small amount that hits the cone at an angle where the air is forced back. That would create turbulence and backward force. But that would be a very tiny amount.

So I would expect that turbulence is a factor, but is essentially inconsequential in explaining why the car doesn’t move.

Originally Posted by Red Baron Farms View Post
No that is not right. Some sentences are sort of correct like "It creates enough resistance that there is not enough force on the car/balloon/cone overcome friction. So the car doesn't move." however your description is horribly convoluted and confused.
What isn’t right? What descriptions are horribly convoluted and confused?

Frankly, I feel horribly convoluted and confused.

Originally Posted by Red Baron Farms View Post
The pressure on the cone with the system at rest is 1 atmosphere. But it is the same inside the cone as outside.

When the balloon is allowed to vent air through the cone as a simulated rocket engine, the air inside the cone drops in pressure. The air outside the cone is still 1 atmosphere. So the force outside the cone presses inwards against the cone. But the cone has an angle. So a % of the force is actually pushing backwards against the car. The simulated rocket engine pushes forward, the cone pushes backwards.
I’m not sure how that is different from what I said.

Except it seems to that the pressure isn’t actually pushing back on the cone, but rather just preventing the cone from moving forward.
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Old 8th August 2019, 09:50 PM   #88
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Originally Posted by Ziggurat View Post
More specifically, the acceleration of this mass of air has force.
Right. That’s what I was thinking, but I said it wrong. It would be acceleration because the mass of air is changing from the velocity of zero to a velocity greater than zero over time. And it is going in a specific direction. So, we have a vector. That is acceleration. And the acceleration times the mass is the force.

I think I got that all right, just using the wrong words.

Originally Posted by Ziggurat View Post
Bernoulli's principle is easy to get tripped up on (it's based upon some assumptions which don't always hold), but to first order, the pressure drop you're examining with Bernoulli's principle is NOT between the escaping air and the surrounding atmosphere, it's between the air inside the balloon and the escaping air. And the air inside the balloon is at higher pressure than the surrounding air. The air as it escapes is at atmospheric pressure, because if it were lower pressure, it couldn't escape, it would get pushed back in.

]That's true, but it doesn't constrict the flow of escaping air, because the air escapes basically at atmospheric pressure. It has dropped in pressure from what's inside the balloon.
I read about this on the Wikipedia page for “Misapplications of Bernoulli's principle in common classroom demonstrations”. It talks about how holding a piece of paper and blowing along the top and the paper rising up isn’t actually due to Bernoulli's principle. That has me really confused.

Am I wrong when I am saying that the car doesn’t move because there is pressure on the outside of the cone preventing the cone (and therefore the car) from moving forward? It sounds like that is wrong. Because that wouldn’t explain why the air escaping the balloon slows down.

Now I’m back to post #22:

Originally Posted by Ziggurat View Post
In order for the air to enter the cone at high velocity but leave the cone at low velocity, it has to slow down. So a net force has to act on the air. The cone has to, in effect, "suck" at the air. In more detail, the air is at lower pressure inside the cone than outside the cone, with a pressure gradient that slows down the air (Bernoulli's principle). Because of the cone's shape, lower pressure inside than outside means there's a net force pushing backwards on the cone, so that the high force at the entrance to the cone is largely cancelled by the forces acting on the cone itself.
I’m confused because the first few sentences say that the effect is that the velocity of the escaping air slows down. But the last two sentences seem to say that effect is pressure on the outside of the cone pushing the cone backward.

Those seem like different things. How do I put those two together?

It might help if I understand how the air inside the cone slows down. What is the “pressure gradient” that slows down the air? How does that work?
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Old 8th August 2019, 09:59 PM   #89
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Originally Posted by abaddon View Post
Mess about with the shape of the cone and one can get it to suck, moving the car backwards, were one so motivated.
That's interesting. What would the shape of such a cone look like?
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Old 8th August 2019, 10:04 PM   #90
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Originally Posted by Robin View Post
You reached the conclusion that adding the cone should not prevent the car from moving, so you must have made at least a rough estimate of this.
The statement you quoted from me was not a conclusion. I was stating an intuitive assumption; what I might intuitively expect. I then when on try to explain, in my own words, why that intuitive assumption is incorrect.
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Old 8th August 2019, 10:12 PM   #91
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Originally Posted by Puppycow View Post
How is so much dumbth possible?

Trolls? Sincerely deluded fools?

Maybe it doesn't even matter.
If this was directed at me, I certainly do not appreciate the insult.

I'm trying to learn something. I'm trying to understand some physics that I don't understand. I would expect a skeptic would be appreciative of such an endeavor.
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Old 8th August 2019, 11:05 PM   #92
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Originally Posted by DevilsAdvocate View Post
The statement you quoted from me was not a conclusion. I was stating an intuitive assumption; what I might intuitively expect. I then when on try to explain, in my own words, why that intuitive assumption is incorrect.
I am also trying to understand. See if we have the same idea. I created an animation to try and grok it:

https://photos.app.goo.gl/BKmL4iqZcdM31iii7

Basically it is a cross section of the cone with air all around and the pressure on the inside is the same as the pressure on the outside.

Then air starts to come out, so the air is travelling more quickly inside the cone so that there is less pressure on the inside surfaces of the cone than the outside. Thus the sum of the forces on the surface of the cone will be pointing towards the back of the cone, in the opposite direction to the way the car should be travelling.

Is that your understanding too?
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Old 8th August 2019, 11:29 PM   #93
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Originally Posted by Robin View Post
Is that your understanding too?
I don't think so. That was what I was thinking, but Ziggurat has me rethinking that.

I confused about why it is sometimes said that faster moving air creates lower pressure, but also saying that it actually doesn't. There is also that "pressure gradient" that I don't understand.

If it is just pressure on the outside of the cone, that wouldn't explain why the air inside the cone loses velocity. And I think it does.

My current (pun intended) thinking is that there is something going on inside the cone. The fast moving air creates low pressure. So the higher pressure air inside the cone (and from the back of the cone) would want to replace that lower pressure air. It would push back against the escaping air. That would slow down the velocity of the escaping air. With lower velocity, there is less opposite force to push the car forward.
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Old 9th August 2019, 12:05 AM   #94
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Two observations.

In experiment 1, the car drives very slowly.
In experiment 2, the balloon takes at least double the time from experiment 1 to empty.

1. Tells us the amount of thrust is just enough to overcome friction.
2. Tells us, the air is escaping more slowly and thus has a slower speed and lower thrust.
3. which brings us back to 1. The thrust was low to begin with, so aything that lowers it, will bring it under the friction threshhold very quickly.

Why does the air move more slowly? I think it is simply because of friction within the cone itself. The cone is very narrow, almost tubelike. It is made of paper and thus most likely has a lot of friction against the air coming past.
I think you will see the same effect if you enlarge the original 'nozzle' from 1. with a tube or anything at all.
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Old 9th August 2019, 12:55 AM   #95
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Originally Posted by erwinl View Post
In experiment 2, the balloon takes at least double the time from experiment 1 to empty.
We can't compare the rates. In the first experiment, we don't see the balloon fully deflate. In the second experiment, the balloon is blown up much larger.

I find it frustrating when people do these experiments and for the second test they do something like blow up the balloon larger to try to show more spectacularly what they are trying to prove. That just throws in another questionable variable. Do the experiments the same each time, and do them fully, so that they can be compared without adding in other variables.

Originally Posted by erwinl View Post
Why does the air move more slowly? I think it is simply because of friction within the cone itself. The cone is very narrow, almost tubelike. It is made of paper and thus most likely has a lot of friction against the air coming past.
I think you will see the same effect if you enlarge the original 'nozzle' from 1. with a tube or anything at all.
I don't think so. If it is friction, then putting a paper tube should cause the same effect where the car doesn't move. My guess is that with a paper tube the car will still move. It would be a good experiment, though.

I'm hoping to get together some materials and do this experiment myself. I'm not sure when I will have the time. But I would like to try some things. This is actually a really neat experiment.
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Old 9th August 2019, 01:48 AM   #96
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So here's my attempt at an intuitive understanding.

A gas consists of molecules bouncing around. They bounce into and off each other in a way comparable to golf balls or ping pong balls.

They will also bounce against the walls of any vessel they are in. The constant drumming of a gazillion air particles is what we feel as air pressure.
What happens when we compress those air molecules/bouncing balls? We have the same number of "balls" in a smaller volume. A "ball" that bounces against the wall and back into the vessel travels a shorter distance before bouncing into another "ball" and back against the wall of the vessel.

A balloon compresses the air inside slightly. The air pushes against the skin of the balloon. It exerts a constant pressure.
Now when you make a hole in the balloon the air will escape, instead of pushing against the skin. On the opposite side the air is still hammering against the skin. The pressure is no longer balanced and that's what drives the balloon forward.


Think of a miniature sailing boat. And imagine that you stand on the shore and throw golf balls against the sail. Every time a ball hits the sail, the boat will be driven forward. It doesn't matter where the ball goes or where it came from (as long as it wasn't shot from the same boat, of course. Mythbusters did a segment on blowing your own sail.).
A faster golf ball will give a bigger push than a slower one. A bowling ball will give a bigger push than a golf ball at the same speed.

In a fundamental sense that's just the same thing driving as what's driving the balloon. Each collision between air molecules, or between air and the skin of the balloon, satisfies the laws of mechanics. That means that we can look at the air escaping and conclude that an equal mass of air at the same speed has been pushing the balloon forward.

Let's consider what the atmosphere does to the balloon. The air inside the balloon is pressurized by the rubber squeezing it together but also by the atmospheric pressure.
The air molecules outside the balloon constantly hammer against the skin and balance part of the hammering of the inside molecules. If you took the balloon into a vacuum it would expand.
When you open up the balloon, the higher pressure air escapes. The elastic skin of the balloon maintains the higher pressure by contracting.
As the air molecules inside escape they run into the constant pounding of the air molecules ouside. Having to push against the atmosphere actually negates part of the thrust that might be there.

About the cone. A cone has a narrow end and a wide end. Now suppose you hold the narrow end closed and hold it like a cup. You start to fill it with a steady stream of water. Let's say that the water level reaches 1 cm (or 1 inch, whatever) after 1 second. In the next second the same volume of water flows into the cone-cup. But because the cone-cup widens this is not enough to raise the level by another cm (or inch). The rate (volume per time) at which the cone-cup is being filled is constant but the speed at which the water level rises slows steadily.

The cone is already filled with air. When the stream from the balloon enters at the narrow end, it will try to push that air back out. Of course the stream that leaves at the wide end will have a larger diameter than the stream that enters at the narrow end. That means that the volume trying to leave at the wide end is larger than the volume entering. That would leave the cone empty pretty soon and that doesn't happen, of course. Instead the air slows down.
That means, by definition, you have a force acting on the air, slowing it down. In terms of bouncing balls, it means that fast travelling balls from behind bounce into slower balls in front and get deflected back. It's like the cone is getting jammed.

That means that the air exiting is slower due to the cone. That means that the air pushing the balloon ahead on the other end is also slower: Same mass but slower.

ETA: to add more on how the cone makes a difference. Pressure is force per area. The larger an area is, the more gas molecules will be pounding against it.
If the cone wasn't there, the air would still get slowed down. But because of the cone, the "push-back" from a large area is directed straight at the small area through which air escapes from the balloon.
Without the cone, fewer air molecules would be "pounded back" towards the narrow part of the nozzle. And those molecules that are not bounced back at the escaping air won't impede its flow.
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Old 9th August 2019, 02:14 AM   #97
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Originally Posted by DevilsAdvocate View Post
It might help if I understand how the air inside the cone slows down. What is the “pressure gradient” that slows down the air? How does that work?
First off, this may be obvious already, but the fact that air enters the narrow end at high velocity and leaves the fat end at low velocity isn't a function of pressure, it's just a function of mass conservation. But it has consequences for pressure.

Let's imagine a small volume of air that we can watch as it travels down the cone. It starts out going fast, and then slows down. That means a force must have acted on it. But if that bit of air isn't touching the walls of the cone, then what force could possibly have acted on it? Well, air pressure from the air around it. In order for there to be a net force on our bit of air, there must be less force on one side than the other. Since it slows down as it travels along the cone, then the force from the air on the wide end of the cone must be larger than the force from the air on the narrow end of the cone. And this means that the air pressure must be higher at the wide end than at the narrow end. It varies smoothly (ie, it's a pressure gradient) from the wide end to the narrow end. That's the only way we can get the change in velocity we need in order to conserve mass flow.
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Old 9th August 2019, 03:36 AM   #98
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Originally Posted by DevilsAdvocate View Post
That's interesting. What would the shape of such a cone look like?
A cone.

The relevant term there is A/dA as earlier provided.
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Old 9th August 2019, 03:45 AM   #99
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Originally Posted by Red Baron Farms View Post
When the balloon is allowed to vent air through the cone as a simulated rocket engine, the air inside the cone drops in pressure. The air outside the cone is still 1 atmosphere. So the force outside the cone presses inwards against the cone.
This is wrong, as Zig pointed out, but I think it bears highlighting.
If the air inside the cone had less pressure than the surrounding atmosphere, the air from the atmosphere would enter the cone.
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Old 9th August 2019, 03:58 AM   #100
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Originally Posted by DevilsAdvocate View Post

I find it frustrating when people do these experiments and for the second test they do something like blow up the balloon larger to try to show more spectacularly what they are trying to prove.

.
Also the little issue that the entire video is howling gibbering lunacy on a grand scale from beginning to end, which is almost as frustrating as inconsistently inflated balloons.
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Old 9th August 2019, 04:03 AM   #101
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Originally Posted by GnaGnaMan View Post
This is wrong, as Zig pointed out, but I think it bears highlighting.
If the air inside the cone had less pressure than the surrounding atmosphere, the air from the atmosphere would enter the cone.
Not necessarily; if there's air already moving inside the cone, the pressure differential decelerates the moving air, rather than accelerating stationary air into the cone. As the cone widens, the air slows and its pressure increases, to the point where it exceeds atmospheric pressure at the end of the cone, and therefore is expelled, but its pressure doesn't need to exceed atmospheric at every point in the cone in order for this to be the case.

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Old 9th August 2019, 04:10 AM   #102
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Originally Posted by Dave Rogers View Post
Not necessarily; if there's air already moving inside the cone, the pressure differential decelerates the moving air, rather than accelerating stationary air into the cone. As the cone widens, the air slows and its pressure increases, to the point where it exceeds atmospheric pressure at the end of the cone, and therefore is expelled, but its pressure doesn't need to exceed atmospheric at every point in the cone in order for this to be the case.

Dave
Yes, that seems right. It would be a coincidence, though, and not relevant to the point.
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Old 9th August 2019, 06:53 AM   #103
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Originally Posted by GnaGnaMan View Post
This is wrong, as Zig pointed out, but I think it bears highlighting.
If the air inside the cone had less pressure than the surrounding atmosphere, the air from the atmosphere would enter the cone.
I just tried right now. I made a cone from a piece of A4 paper and then used a tube to blow into it.

The cone flattens longitudinally.

This suggests to me that Red Baron Farms is correct and that the pressure on the inside surface of the cone decreases as the speed of the air going through it increases so that the pressure on the inside surfaces of the cone is much less than the pressure on the outside.

If the cone had been too rigid to flatten then that force would have been exerted towards the large end of the cone.

It is pretty easy to try yourself.
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Last edited by Robin; 9th August 2019 at 07:16 AM.
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Old 9th August 2019, 07:47 AM   #104
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Originally Posted by Dave Rogers View Post
Not necessarily; if there's air already moving inside the cone, the pressure differential decelerates the moving air, rather than accelerating stationary air into the cone. As the cone widens, the air slows and its pressure increases, to the point where it exceeds atmospheric pressure at the end of the cone, and therefore is expelled, but its pressure doesn't need to exceed atmospheric at every point in the cone in order for this to be the case.

Dave
It doesn't exceed atmospheric pressure at every point. It's only at the exit point of the cone where it needs to match atmospheric pressure. Elsewhere it will be lower.
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Old 9th August 2019, 08:23 AM   #105
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Originally Posted by Robin View Post
I just tried right now. I made a cone from a piece of A4 paper and then used a tube to blow into it.

The cone flattens longitudinally.

This suggests to me that Red Baron Farms is correct and that the pressure on the inside surface of the cone decreases as the speed of the air going through it increases so that the pressure on the inside surfaces of the cone is much less than the pressure on the outside.

If the cone had been too rigid to flatten then that force would have been exerted towards the large end of the cone.

It is pretty easy to try yourself.
I have and I can give you a couple ways in which the cone could appear to flatten by a different mechanism.
If the assertion were true, then one would expect pipes to collapse occasionally when gas travels through. This should be an issue wherever gas is piped out of pressure vessels, eg welding. The more pressure in the vessel, meaning gas escaping faster, the more easily that should happen.
Is that a thing?
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Old 9th August 2019, 08:37 AM   #106
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This is all well-known fluid dynamics. I suggest those who want their minds expanded look at a classic example of conical nozzles in action, the triple cone injector. Steam from a boiler is accelerated in a narrowing cone until its pressure is below atmospheric pressure, allowing it to suck water from a supply pipe; the steam condenses into water in a second narrowing cone; and the water is then decelerated in a third, widening cone, until it's at higher pressure than the steam pressure in the boiler, and can therefore be forced into the boiler. In the second and third cone, the pressure goes from below atmospheric to several atmospheres - it has to, for the device to work, and they've been in use for over a century and a half.

Dave
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Old 9th August 2019, 09:01 AM   #107
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Originally Posted by Dave Rogers View Post
This is all well-known fluid dynamics. I suggest those who want their minds expanded look at a classic example of conical nozzles in action, the triple cone injector. Steam from a boiler is accelerated in a narrowing cone until its pressure is below atmospheric pressure, allowing it to suck water from a supply pipe; the steam condenses into water in a second narrowing cone; and the water is then decelerated in a third, widening cone, until it's at higher pressure than the steam pressure in the boiler, and can therefore be forced into the boiler. In the second and third cone, the pressure goes from below atmospheric to several atmospheres - it has to, for the device to work, and they've been in use for over a century and a half.

Dave
I don't see how that's related. Please explain.
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Old 9th August 2019, 09:03 AM   #108
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Originally Posted by GnaGnaMan View Post
This is wrong, as Zig pointed out, but I think it bears highlighting.
If the air inside the cone had less pressure than the surrounding atmosphere, the air from the atmosphere would enter the cone.
Me and Zig are in agreement, so you are missing something. Not sure what exactly.
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Old 9th August 2019, 09:16 AM   #109
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Originally Posted by Thor 2 View Post
Wow! Impressive argument and explanation.
Sorry, but the explanation you gave involved pushing on the outside air. It is wrong. It's the woo we are addressing if what you said were true, then rockets might not work in vaccuum!

It shows me you are thinking about it the wrong way.

Air speeds up through the throat of a carb...pressure drops enough to pull fuel from the float bowl.

Air speeds up over the top of a wing... pressure drops enough to lift a whole airplane.

Air speeds up through this cone and pressure drops enough to cancel the thrust from the balloon.

Don't over complicate this.
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Old 9th August 2019, 09:18 AM   #110
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Originally Posted by Red Baron Farms View Post
Me and Zig are in agreement, so you are missing something. Not sure what exactly.
I'll re-read what you wrote and get back to you.
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Old 9th August 2019, 09:19 AM   #111
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Originally Posted by GnaGnaMan View Post
I don't see how that's related. Please explain.
It's an example of a well-known device in which a jet of fluid enters one end of an expanding cone at below atmospheric pressure and exits it at well above atmospheric pressure, and therefore a demonstration of the well-known principle that the pressure of a moving fluid in a sealed, but varying in area, pipe decreases as its velocity increases (and of course increases as its velocity decreases). The third cone in an injector experiences an inward force from outside at its narrow end - it must, otherwise the injector could not pick up water - but an outward force at its wider end - which also it must, or it could not inject that water into the boiler. And it's a thoroughly well demonstrated example, because it's been an essential component of every steam locomotive built since the 1860's. Therefore, it's a practical example of the phenomenon you were calling into question - that a jet through a cone can be at lower pressure than ambient at the narrow end, and at higher than ambient at the wide end.

Dave
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Old 9th August 2019, 09:39 AM   #112
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Originally Posted by Dave Rogers View Post
It's an example of a well-known device [...]
Maybe you could expand on this. There are literally no google hits for "triple cone injector".

Quote:
Therefore, it's a practical example of the phenomenon you were calling into question - that a jet through a cone can be at lower pressure than ambient at the narrow end, and at higher than ambient at the wide end.
I did no such thing.
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Old 9th August 2019, 10:24 AM   #113
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Originally Posted by Red Baron Farms View Post
Me and Zig are in agreement, so you are missing something. Not sure what exactly.
I thought I wouldn't get back to this till tomorrow but I was wrong.

So here's the thing:

The air at the narrow end of the cone is faster than in the balloon, so by Bernoulli it is at lower pressure. It is also faster than at the wide end of the cone, so again lower pressure.
We know that the pressure at the wide end is higher than atmosphere because air exits.
That means we know that the air at the narrow end of the cone is lower than above atmosphere. So how do we get from below above atmosphere to simply below atmosphere?
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Old 9th August 2019, 04:36 PM   #114
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Originally Posted by GnaGnaMan View Post
I thought I wouldn't get back to this till tomorrow but I was wrong.



So here's the thing:



The air at the narrow end of the cone is faster than in the balloon, so by Bernoulli it is at lower pressure. It is also faster than at the wide end of the cone, so again lower pressure.

We know that the pressure at the wide end is higher than atmosphere because air exits.

That means we know that the air at the narrow end of the cone is lower than above atmosphere. So how do we get from below above atmosphere to simply below atmosphere?
Lower than atmospheric pressure on the inside surface of the cone higher than atmospheric pressure on the air at the end of the cone doesn't involve a problem since the air is moving towards the wide exit.
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Old 9th August 2019, 04:41 PM   #115
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Here is my understanding.

There is low pressure exerted on the sides so the air pressure on the outside of the surface is greater than the pressure on the inside of the surface, even at the widest part of the cone.

But the air pressure against the air outside is higher than the air pressure exerted from the outside towards the opening, so air still exits.

https://photos.app.goo.gl/BKmL4iqZcdM31iii7
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Old 9th August 2019, 05:14 PM   #116
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Originally Posted by Robin View Post
Here is my understanding.

There is low pressure exerted on the sides so the air pressure on the outside of the surface is greater than the pressure on the inside of the surface, even at the widest part of the cone.

But the air pressure against the air outside is higher than the air pressure exerted from the outside towards the opening, so air still exits.

https://photos.app.goo.gl/BKmL4iqZcdM31iii7
We can see the same thing with the paper cone I described above. Blow through a tube attached to the thin and and the paper cone will flatten - even at the ends, but the air still comes out the end (although there is a bit of flapping as the end of the cone closes and opens).
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Old 10th August 2019, 01:40 AM   #117
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Originally Posted by Robin View Post
Lower than atmospheric pressure on the inside surface of the cone higher than atmospheric pressure on the air at the end of the cone doesn't involve a problem since the air is moving towards the wide exit.
How do you know that it is lower than atmosphere?
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Old 10th August 2019, 02:04 AM   #118
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Originally Posted by Robin View Post
Here is my understanding.

There is low pressure exerted on the sides so the air pressure on the outside of the surface is greater than the pressure on the inside of the surface, even at the widest part of the cone.

But the air pressure against the air outside is higher than the air pressure exerted from the outside towards the opening, so air still exits.
I took a bit of saran wrap and wrapped it around a drinking straw. Then I pushed the saran tube over the end of the drinking straw so that I had a tube of saran wrap that closed tightly with the straw. It was not a sturdy tube.
I was unable to collapse it by blowing through.
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Old 10th August 2019, 03:01 AM   #119
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Originally Posted by GnaGnaMan View Post
I took a bit of saran wrap and wrapped it around a drinking straw. Then I pushed the saran tube over the end of the drinking straw so that I had a tube of saran wrap that closed tightly with the straw. It was not a sturdy tube.
I was unable to collapse it by blowing through.
I am sorry, in what way is that analogous to what I did or the matter under discussion? I am not even sure what you are describing.

Are you doubting that i could collapse the paper cone by blowing through it? Do I really have to post a video?
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Old 10th August 2019, 03:06 AM   #120
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Originally Posted by Robin View Post
I am sorry, in what way is that analogous to what I did or the matter under discussion? I am not even sure what you are describing
Here's a quote from RBF:
Originally Posted by Red Baron Farms View Post
It's all based on Bernoulli's laws that a faster moving fluid has lower pressure. So the inside of that long cone has less air pressure than the outside of that long cone.
According to him, it does not matter if we are dealing with a cone.
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