Siphons

[zut]

*Originally posted by hussman *

Re: Zut’s experiment–you created a vacuum inside the tube, and it pulled the water up and over, the water being held together by tensile strength. Very clever idea though.

I’m going to have to disagree with you here. First, saying that the “vacuum…pulled the water up and over” is misleading at best. Vacuum doesn’t pull; pressure pushes. However, laying aside this bit of semantics, what you’re saying is: Pressure pushes the water, but tensile strength holds it together.

I don’t understand why (it appears) you think tensile strength is necessary. This experiment showed that air pressure (and gravity) is sufficient to power a siphon, in the absence of tensile strength. Granted tensile strength exists, but, if a siphon works without it, I find it hard to believe it can be a dominant effect.

*Originally posted by irishman *
Holy Flying Monkeys! I’m sitting here describing that analogy and why it applies, and suddenly I had a revelation. It’s NOT the air pressure pushing down that’s doing anything.

I’m going to have to disagree with you too. A siphon requires air pressure and gravity. The gravitational force is required to get the fluid moving from one tank to another, as you’ve said. If you just connect a hose from one tank to the other without raising the hose above the upper tank’s waterline, then gravity is all that’s required. BUT, if you raise the center of the hose above the upper tank’s waterline, creating a siphon, then you need air pressure* to drive the fluid over that hump.

[sub]*or tensile strength of the fluid, if it’s great enough, but let’s not get into that.[/sub]

[zut]

*Originally posted by Sleepy Weasel *

You [TheBrain] insist that gravity and air pressure are the contributing factors, but you’re missing something.

Missing what? No gravity means water won’t flow through the “down” side of the siphon. No air pressure means water won’t be pushed up the “up” side of the siphon. So BOTH gravity AND water are necessary (and sufficient).

*Originally posted by TheBrain *
The water is in tension on the falling side. In tension the tensile strength comes into effect. In this thread tension = pulling.

Well, no, the water is NOT in tension. Granted, the water is under lower than atmospheric pressure, but that does not equate to tension, which is less than zero absolute pressure. So while tensile strengths exists, I don’t see how it comes into play on the falling side any more than it would on the rising side.

[TheBrain]

Well, we are coming around to full circle. If you read my orginal post I also claim that the siphon would work in a near vacuum using only its cohesive properties.

If you do a force analysis you should include in it the tension in the water. If there is no movement in the tube and either side of the tube is the same length then a rope analogy is great to discribe the forces. On either side of the tube gravity is pulling down on the water just like a rope on a pully being pulled on both sides by you but it is not moving. You can pull until the rope breaks which in water would be cavitation. So before the water moves in the tubes we have a pre-stressed water under tension caused by gravity.

What happens to the tension after the water starts to flow?

Now we are getting to a very difficult part of the problem. What is causing the water to flow up. I have said it can be either or and on planet earth in atmosphere I claim it is mostly due to the atmospheric difference.

If you do the force analysis you will see that the atmospheric force is much larger than the tension force is. Much larger. The tension force is still there on both sides but on the rising side in the direction of flow the pressure force is larger so I attribute the rising to pressure. This is proven by the fact that bubbles can be in the siphon and it still works. Bubbles are like your buddy letting go of you while you are climbing. Once your buddy lets go he can not be pulling you up any more, right?

Now on the falling side. The water just gets pushed over the top of the tube and begins to fall. The water that is right behind it on the hump will be moving slightly, ever so slightly slower. Have you ever been walking a dog on a leash? If the dog is going the same speed as you the leash is loose. If the dog takes off running, viola, tension in the leash. Same with the water. The leash is it cohesiveness.

As in both cases gravity OR pressure can be exclusively driving the siphon. The conditions for either doing this alone are extreme but theoretically possible. On planet Earth atmospheric pressure pushes the fluid over the top with the water in compression, and then gravity and air pressure form a resultant vector that causes tension in the water as it falls down the other side.

Do a point force analysis of this if you don’t believe me and see for yourself.

[hussman]

*Originally posted by zut *
**
I’m going to have to disagree with you here. First, saying that the “vacuum…pulled the water up and over” is misleading at best. Vacuum doesn’t pull; pressure pushes. However, laying aside this bit of semantics, what you’re saying is: Pressure pushes the water, but tensile strength holds it together.

I don’t understand why (it appears) you think tensile strength is necessary. This experiment showed that air pressure (and gravity) is sufficient to power a siphon, in the absence of tensile strength. Granted tensile strength exists, but, if a siphon works without it, I find it hard to believe it can be a dominant effect.**

Okay, my last post on this. Seriously.

TheBrian–You are right, I am not reading your posts because you have been very antagonistic, rude, and unaccepting of other views. Everyone else is trying to be respectful, except for you. Try to present posts in a friendly format, and others will respond in kind. You’ll note in this post, I did some homework.

Zut–You are making good points, and I may not be expressing mine clearly. Our differences may be semantic.

First, tensile strength of fluids is not surface tension. The tensile strength of a fluid can be visualized by having a sealed, inverted piston/cylinder arrangement completely filled with a fluid (let’s say mercury) with a weight hanging on the end. The tensile strength is the stress the fluid can hold before cavitation occurs. Refer to the abstract I borrowed from the Journal of Fluid Mechanics, the results are not presented, but the definition given.

Okay, back to the bladder experiment I mentioned before. Let’s say the hose of vessel A is at the base, and lies horizontally as vessel B drops. The gravity causes the fluid to move from the fluid level to a lower level, and the pressure difference is determined by the hydrostatic equation.

Now, let’s let the hose go up a meter then down, the pressure difference is the same for similar drops in elevation (minus friction losses in the tube). Why? Because the overall energy change is the same at the endpoints, and if the fluid is to rise in the pipe while conserving energy, it must be ‘pulled’ up the tube without a loss of energy.

But you know guys, to sink this ship we should form a very polite letter and ask a real expert, Dr. Daniel Joseph at the fine University of Minnesota.

Again, very nice comments by all.–dfh

[sub]Cavitation and the state of stress in a flowing liquid

Author(s): Joseph, Daniel D.
Author Affiliation: Univ of Minnesota
Source: Journal of Fluid Mechanics v 366 Jul 10 1998 Cambridge Univ Press New York NY USA p 367-378 0022-1120 JFLSA7
Abstract: The problem of the inception of cavitation is formulated in terms of a comparison of the breaking strength or cavitation threshold at each point in a liquid sample with the principal stresses there. A criterion of maximum tension is proposed which unifies the theory of cavitation, the theory of maximum tensile strength of liquid filaments and the theory of fracture of amorphous solids. Liquids at atmospheric pressure which cannot withstand tension will cavitate when and where tensile stresses due to motion exceed one atmosphere. A cavity will open in the direction of the maximum tensile stress which is 45° from the plane of shearing in pure shear of a Newtonian fluid. Experiments which support these ideas are discussed and some new experiments are proposed. English (Author abstract) 18 Refs. [/sub]

[TheBrain]

I will accept there is a God when he is standing in front of me so I can kick his ass. Not because people THINK there is a god.

I will accept your views when you are right. I apologised for being rude. Again, sorry. However, that does not mean you are any closer to being right. In reality I think you know what is going on; you just can’t relate it with any clarity to these people. Which is why I feel so bad about you being a teacher. I am sure you can see my point.

And before you you go off on me. Read where I already stated I am probably wrong about some of this. But, I am very much closer to being right than you are. So take that. nya nya nya.

Either way the siphon still keeps siphoning. Are you getting it yet hussman?

[NanoByte]

I don’t understand how zut concludes, apparently by definition, that tension only occurs where there is “zero absolute pressure”. Tension in a material surely occurs at any time there are components of force/pressure that oppose each other, in an away-from sense, while applied at different points of the material (here, on the downward side, gravity opposed to cohesive molecular forces). How does absolute pressure get involved in an issue involving only difference of forces/pressures?

Ray (Is he pulling our legs, pushing them, or does it depend on whether one see the source tub is half full or half empty? When push comes to shove, you can’t pull the wool over my eyes, though the forces may stretch my imagination.)

[jdnewmil]

TheBrain states that the “pressure difference … is being caused by the difference in fluid velocities in the tube.” Seeing this, and freightliner’s comments about dynamic pressure, forced me to go back and look at the books again, and sure enough, I neglected this effect. However, it only modifies the situation in two ways.

It turns out that if you insert manometer tubes (open to the air) into the water, the height to which the water will rise in them depends on how the opening in the tube is oriented with the flow rate. If the opening is “facing” the direction of flow, the water level in the manometer tube will rise height D, less some amount due to friction losses between D and the measurement point. If the opening is perpendicular to the flow, it will rise to a lowered level determined by the velocity of the flow. This pressure would determine the height at which cavitation occurs.

(Since manometer tubes measure relative to air pressure, it would be more practical to simply orient absolute pressure sensors in the flow rather than manometer tube inlets for when the pressure drops below atmospheric pressure.)

Note that if the diameter of the siphon tube is uniform along its length, the dynamic pressure is a constant offset through the whole tube. Therefore, it is not a driving force for the siphon effect, though it will reduce the height over which the siphon can work. This is evident as well because a siphon will work even when the flow velocity approaches zero.

I don’t think the “marbles” model can be extended to explain dynamic pressure, unfortunately.

The second alteration to my arguments is that now I think that friction losses in the tube are likely much smaller than the turbulent losses in the receiving container, so my arguments for reconciling the hydrostatic equation don’t hold. Bernoulli’s equation probably gets pretty close when applied between D and B, and the large discrepancy in a similar calculation between C’ and B is due to the pressure gradients involved in slowing down the fluid. In the “marbles” model, this would correspond to having them slide through a long, convoluted “skid area” to slow them down at the outlet of the tube.

Chesnakas suggests the use of mercury in these thought experiments, for its low vapor pressure (nonexistent at room temperature). Another key point is that it also doesn’t adhere (meniscus) to most materials we would make containers and tubes with. A fluid at a temperature such that it has no vapor pressure contained in a material to which it will adhere might be able to support siphoning in a vacuum or above the point at which the static pressure is zero. Maybe mercury in uranium?

TheBrain says in a recent message: “If you do a force analysis you should include in it the tension in the water. If there is no movement in the tube and either side of the tube is the same length then a rope analogy is great to discribe the forces. On either side of the tube gravity is pulling down on the water just like a rope on a pully being pulled on both sides by you but it is not moving. You can pull until the rope breaks which in water would be cavitation. So before the water moves in the tubes we have a pre-stressed water under tension caused by gravity.”

This is incorrect. The atmospheric pressure on both ends of the siphon is forcing the water up into the siphon tube, and the entire volume is under pressure, not tension. The pressure varies according to hydrostatic or hydrodynamic principles (depending on the velocities) but nevertheless it is not in tension. Imagine a silly putty rope hung over a stick… pull on it, and its cross-section will shrink. If you attempt to pull on the mercury in a glass tube with a piston you will find that it will simply separate from the glass walls leaving a vacuum… and the mercury will not be under any “tension,” just zero pressure.

[ZenBeam]

NanoByte writes

I don’t understand how zut concludes, apparently by definition, that tension only occurs where there is “zero absolute pressure”.

Oh, come on. He wrote “less than zero absolute pressure.” (bolding mine, leg pulling NanoByte’s).

hussman writes

Re: Zen Beam’s–If there are bubbles, assuming the whole cross-sectional area of the tube isn’t vapor, then there should be sufficient force to pull it over. I don’t know how high the siphon would work, less than 34 feet I would presume. We also can have pressure in a closed system without atmospheric effects

I redid the experiment using water. I used the hose I had on hand, so the siphon only rises about three feet above the water line. The hose is clear, so I can easily see that the water does not coat in any way the inside surface of the tube along the length of the bubble (three to four inches long) with no water flowing (the outlet end was stopped up). Upon unstopping the outlet end, the water immediately began flowing. So, no tensile strength is necessary for the siphon to work.

TheBrain writes

The water is in tension on the falling side. In tension the tensile strength comes into effect. In this thread tension = pulling. You people kind of get this but you don’t take into account the additional force due to the pressure difference.

zut replies

Well, no, the water is NOT in tension. Granted, the water is under lower than atmospheric pressure, but that does not equate to tension, which is less than zero absolute pressure.

On the outlet side, the water may or may not be in tension. This will depend on how quickly the water is flowing, how tall the siphon is, and on how long the outlet tube is. If the outlet tube empties into a second tank, whose water level is only a little below the inlet tank water level, the siphon rises less than 34 feet, and the water is flowing slowly, the water will always be under positive pressure, never under tension. Lengthen the outlet side of the hose and drop the outlet-side tank far enough (or do away with it completely), and the water near the top of the outlet side can be under tension. Raise the siphon height above 34 feet, and if the siphon is still working, the water is obviously under tension.

hussman writes

The tensile strength of a fluid can be visualized by having a sealed, inverted piston/cylinder arrangement completely filled with a fluid (let’s say mercury) with a weight hanging on the end. The tensile strength is the stress the fluid can hold before cavitation occurs.

In this experiment, either the device must be placed in a vacuum, or the atmospheric pressure must be accounted for.

if the fluid is to rise in the pipe while conserving energy, it must be ‘pulled’ up the tube without a loss of energy.

If the fluid in the bladder is under pressure, it will be able to rise in the tube. Surely, hussman, in a barometer, you understand there is nothing ‘pulling’ the mercury up the tube, only atmospheric pressure pushing from below? Please say yes.

Finally, the abstract you quoted reads (italics mine),

Liquids at atmospheric pressure which cannot withstand tension will cavitate when and where tensile stresses due to motion exceed one atmosphere.

which supports the contention that tensile strength only comes into play after atmospheric pressure has been overcome.

[NanoByte]

Zen:

OK, so I meant to quote zut as saying “less than zero absolute pressure”. What’s the difference? I don’t understand why there’s any reason to bring in any reference to whether the pressure is measured with respect to a vacuum or otherwise, where the effect discussed has only to do with a difference in pressure, along with other forces.

And I don’t understand any parts of the discussion where a lower container for the liquid through the siphon is claimed to affect the siphon insofar as getting water to the lower end of the tube. After water gets to that end, no doubt it can be somewhat impeded by contained water blocking its exit. In my world, a siphon does not require any outlet container; so, as to a question of how a siphon works, why must we consider a catch basin at all?

Ray (Now, let’s move on to a siphon using Klein bottles.)

[jdnewmil]

*Originally posted by NanoByte *
**Zen:

OK, so I meant to quote zut as saying “less than zero absolute pressure”. What’s the difference? I don’t understand why there’s any reason to bring in any reference to whether the pressure is measured with respect to a vacuum or otherwise, where the effect discussed has only to do with a difference in pressure, along with other forces.

The reason absolute pressure is important has to do with the negligible tensile strength of the fluid. I used the analogy before of stacks of “cigar boxes” to show that it is important to know how far you are from absolute zero pressure (or its vapor pressure), which determines whether the fluid is going to start cavitating or not.

As for the difference in pressure… that is the driving force in moving the water. Of course it is important… just for different reasons than the absolute pressure is important.

And I don’t understand any parts of the discussion where a lower container for the liquid through the siphon is claimed to affect the siphon insofar as getting water to the lower end of the tube. After water gets to that end, no doubt it can be somewhat impeded by contained water blocking its exit. In my world, a siphon does not require any outlet container; so, as to a question of how a siphon works, why must we consider a catch basin at all?
**

You don’t, really, though it is nice to know that air bubbles aren’t going to start rising up the tube from that end. It also provides another reference point of zero velocity fluid at the surface level C’.

[ZenBeam]

In my world, a siphon does not require any outlet container; so, as to a question of how a siphon works, why must we consider a catch basin at all?

I added the outlet basin so there would be a well defined difference between the water levels on the inlet and outlet sides. Without the outlet basin, the physics at the end of the outlet tube is more complicated. Worrying about what’s going on there doesn’t add anything to understanding how the siphon itself works.

You don’t need to have the outlet basin. I thought I was simplifying the problem, but if it makes it more complicated for you (and/or others), just ignore it.

[zut]

More points:

*Originally posted by hussman *
Now, let’s let the hose go up a meter then down, the pressure difference is the same for similar drops in elevation (minus friction losses in the tube). Why? Because the overall energy change is the same at the endpoints, and if the fluid is to rise in the pipe while conserving energy, it must be ‘pulled’ up the tube without a loss of energy.

This seems to be the crux of your argument, and I agree, but it doesn’t seem to have anything to do with tensile strength. Unless you’re arguing that air pressure does not push, and “tensile strength” pulls? If that’s the case, you’re gonna have to explain why air pressure suddenly disappears.

*Originally posted by ZenBeam *
On the outlet side, the water may or may not be in tension. This will depend on how quickly the water is flowing, how tall the siphon is, and on how long the outlet tube is… Lengthen the outlet side of the hose and drop the outlet-side tank far enough (or do away with it completely), and the water near the top of the outlet side can be under tension.

I think I agree. I was thinkin’ about this a little more last night, and I think that, ignoring frictional effects, the water is never under tension unless either the inlet side or the outlet side is 34’ above it’s respective water surface level (or the end of the tube, if the outlet side has no pan). Frictional effects will shorten the effective maximum inlet height, but lengthen the effective maximum outlet height. I’m honestly still not quite sure about this, though.

*Originally posted by ZenBeam *
Without the outlet basin, the physics at the end of the outlet tube is more complicated.

I disagree, it should actually be easier: with no basin, water exits the tube at amospheric pressure, in parallel streamlines. However, that’s a minor side issue that doesn’t affect the basic workings of the siphon.

*Originally posted by ZenBeam *
…tensile strength only comes into play after atmospheric pressure has been overcome.

Yes, this is exactly my point.

*Originally posted by NanoByte *
I don’t understand how zut concludes, apparently by definition, that tension only occurs where there is " [less than] zero absolute pressure". Tension in a material surely occurs at any time there are components of force/pressure that oppose each other, in an away-from sense, while applied at different points of the material (here, on the downward side, gravity opposed to cohesive molecular forces). How does absolute pressure get involved in an issue involving only difference of forces/pressures?

Think of an analogy: I build a (weightless) flagpole with a platform securely attached to the top. I climb up a ladder and push up on the flagpole with 90lbs. force. Is the flagpole in tension? Yes; absolutely. Next, I get down and you climb up and sit on the platform with 150 lbs force. Is the flagpole in tension? No, it’s in compression. Now I climb back up, while you’re still sitting on top, and push up with 90 lbs force. Is the flagpole in tension? No! it has a compressive force of 60 lbs on it.

Let’s carry this a little farther: Say TheBrain sneaks in during the night and saws the flagpole in half at the base. He’s pretty clever, and he saws it straight across, so we can’t tell by looking. The next day, you sit back down on the platform. You’re pretty agile, so you don’t add any horizontal (“tipping”) forces. What happens? Nothing. The pole is in compression, and just sits there. Now I climb the ladder and push up with 90 lbs. What happens? Nothing. The pole is in compression, and just sits there. Now you get off and I push again. What happens? The pole flies upwards, because it’s in tension.

The point here is that I don’t think the tensile strength of water comes into play at all in a siphon because the water is not in tension, unless the siphon were designed so that the internal pressures go below zero absolute. And that’s a pretty big siphon, on Earth.

[Sleepy_Weasel]

*Originally posted by TheBrain *
I will accept there is a God when he is standing in front of me so I can kick his ass. Not because people THINK there is a god.

Questions of personality conflicts aside, you should probably avoid discussions of theology and stick to scientific studies. At least in those cases, you have some definitive basis for your statements, even if people disagree with them.

God, on the other hand, is by definition superior to mere mortals. Assuming God is real, and not just a figment of our imagination created to comfort us in a long ago age when man needed explanations for things like fire, rain, wind and siphons; God would be immune to an ass kicking from the likes of you or me. On the other hand, for all we know, our universe may be sitting in a petri dish in the lab of some scientist that fills our “God” role, but may actually be just one of billions of his species. In that case, he might not even have an ass, but if he did, it would be REALLY big.

For your sake, if there is a God, I hope he isn’t the type to hold a grudge against people who make arrogant statements like that.

[ZenBeam]

In that case, he might not even have an ass, but if he did, it would be REALLY big.

You better hope God isn’t a woman!

[hussman]

*Originally posted by ZenBeam *
I redid the experiment using water. I used the hose I had on hand, so the siphon only rises about three feet above the water line. The hose is clear, so I can easily see that the water does not coat in any way the inside surface of the tube along the length of the bubble (three to four inches long) with no water flowing (the outlet end was stopped up). Upon unstopping the outlet end, the water immediately began flowing. So, no tensile strength is necessary for the siphon to work.

Wonderful experiment, notice the air is under tension too…it’s just a different fluid.

On the outlet side, the water may or may not be in tension. This will depend on how quickly the water is flowing, how tall the siphon is, and on how long the outlet tube is. If the outlet tube empties into a second tank, whose water level is only a little below the inlet tank water level, the siphon rises less than 34 feet, and the water is flowing slowly, the water will always be under positive pressure, never under tension. Lengthen the outlet side of the hose and drop the outlet-side tank far enough (or do away with it completely), and the water near the top of the outlet side can be under tension. Raise the siphon height above 34 feet, and if the siphon is still working, the water is obviously under tension.

In this experiment, either the device must be placed in a vacuum, or the atmospheric pressure must be accounted for.

Easily done, subtract 14.7 psi (at sea level)

if the fluid is to rise in the pipe while conserving energy, it must be ‘pulled’ up the tube without a loss of energy.

If the fluid in the bladder is under pressure, it will be able to rise in the tube. Surely, hussman, in a barometer, you understand there is nothing ‘pulling’ the mercury up the tube, only atmospheric pressure pushing from below? Please say yes.
[/quote]

I agree the *total system pressure difference * must be positive for fluid to flow. The total system is all of the fluid in the tank and hose. With regards to your other comments, I think you are talking about start-up, applying a vacuum so pressure difference can push the fluid uphill. I agree with you completely, the positive pressure in the bottom is pushing it up the tube.

But I’m talking about steady-state siphon flow. When the fluid is flowing freely from A over B through A’ to C, where A’ at the same elevation as A, the fluid from A to B is under tension going up, reaching a maximum at B. The tension goes up because the pressure in the tube going down by hydrostatics. The tension goes to zero at A’, and then the fluid is under pressure when going below A’.

Finally, the abstract you quoted reads (italics mine),

Liquids at atmospheric pressure which cannot withstand tension will cavitate when and where tensile stresses due to motion exceed one atmosphere.

which supports the contention that tensile strength only comes into play after atmospheric pressure has been overcome. **

Almost, it says the maximum tensile strength is the limit, or maximum stress, the fluid may support. The fluid is under stresses through the tube, tension going up, compressive stresses going down.

I swear our differences are semantic here, what I’m trying to stress (pun not intended originally) is that question asked how does each molecule know that it can go over the hill, against gravity, and I say because it holds itself together as a continuum because of it’s material properties.

[Sleepy_Weasel]

*Originally posted by ZenBeam *
**

In that case, he might not even have an ass, but if he did, it would be REALLY big.

You better hope God isn’t a woman! **

Not that my comments would make a difference in that case… Chicks always go for the badboy types. I’m too nice.

[zut]

*Originally posted by hussman *
I swear our differences are semantic here, what I’m trying to stress (pun not intended originally) is that question asked how does each molecule know that it can go over the hill, against gravity, and I say because it holds itself together as a continuum because of it’s material properties. [bolding mine]

Hummm…I think most of the differences do appear to be semantical except for this last little bit that I’ve bolded above. I would say that the water holds itself together as a continuum because of the air pressure. Material properties (like tensile strength) have nothing to do with it. If you had “magic water” with no tensile strength whatsoever, the siphon would work exactly the same as it does currently, because this material property plays no role in the behavior of the siphon.

[ZenBeam]

I don’t think our differences are solely semantic.

So, no tensile strength is necessary for the siphon to work.

Wonderful experiment, notice the air is under tension too…it’s just a different fluid.

Only if by “tension” you mean “compression”. From a site at britannica.com:

Equal and opposite forces acting on a rigid body may act so as to compress the body (Figure 19A) or to stretch it (Figure 19B). The bodies are then said to be under compression or under tension, respectively.

It was from a page on rigid bodies, so that’s why they specify a rigid body. Here’s another site which says essentially the same thing. Let’s get our terminology straight. The air in the bubble is compressed. It is at less than atmospheric pressure, but it is still compressed.

Regardless, the force exerted by the air in the bubble (due to its compression) on the water in the inlet side of the siphon opposes the flow of the water up that leg of the siphon. The air bubble is not pulling the water. So I will repeat: no tensile strength is necessary for the siphon to work.

[hussman]

*Originally posted by ZenBeam *

Regardless, the force exerted by the air in the bubble (due to its compression) on the water in the inlet side of the siphon opposes the flow of the water up that leg of the siphon. The air bubble is not pulling the water. So I will repeat: no tensile strength is necessary for the siphon to work.
**

Both fluids are under tension, and your bubble experiment would prove it. Is the volume of your bubble increasing as it approaches the top? I’m guessing with a constant mass of air, the pressure should be decreasing (by Ideal Gas Law). (but there would be some gas absorption)

Let’s say you introduce your bubble and try to siphon from leg A to B with an expandable hose, long enough to reach 35 feet (with water as the fluid at 25 degrees Celcius). When you reach 32.8 (the number I calculated earlier), it will start boiling and you cannot siphon anymore. This means that the pressure above it is too low, even though the pressure at the bottom of the B-C leg is higher than A. The vacuum is caused by gravity acting on the B-C leg, putting the rest of the fluid under tension (as long as the fluid is above the level of the vessel).

If you could boil your water, and keep it near 100 C, then you could watch it boil as it goes up after using only 15 ft of tubing or so (7 ft up, 8 ft down). You’d need to insulate the pipe at 100 C, it wouldn’t be cheap.

[zut]

hussman:

I suspect that you have some alternate definition of “tension” than ZenBeam and I do. You appear to be saying: A fluid is under tension when the fluid pressure is less than the ambient pressure.

ZenBeam is saying: A fluid is under tension when the fluid pressure is less than zero absolute presure.

Is that a fair summation?

Now, I think you are technically incorrect; nonetheless, I’d pass the difference off as semantics, BUT you also seem to (unjustifiably, in my opinion) jump from the statement that “the fluid is under tension” to “the fluid is being held together by tensile strength.”

If the fluid was in tension by ZenBeam’s definition, then the jump would be justified. But it’s not: the water is always at a positive pressure (absolute) and tensile strength does not apply. Do you agree? If not, WHY not?