# Just curious about many things like hydrostatics

How did you find PF?: I was Googling something and this forum popped up answering a completely different question. It was still very interesting. I wasn't sure what this forum was about and I was prompted to sign up while trying to figure that out. I then received an explanation and am glad I did sign up.

I have so many questions. I feel like I should have a mentor because they are all on the level of homework questions. But I'm 53 and not in school (and probably won't be going back). What is the best way to ask my questions?

Is this a place to ask a question? I have many, but I posted this one to Facebook awhile back and not a soul could help me...

We start with a platform 20 meters high at sea level with a pipe suspended from it with three open valves (one at the bottom, one just above 10 meters from the ground and one at the top) with the bottom of the pipe submerged in a pool of water. (Fig 1)

Then we close the valve at the bottom of the pipe and fill the pipe with water. (Fig 2)

After the pipe is filled with water, we close the top valve. (Fig3)

Then we open the bottom valve.
I believe atmospheric pressure leads to the water boiling to the 10 meter mark. I believe that water vapor takes up less room than liquid water. I believe that the pipe will be cooler due to liquid losing heat energy when it converts to gas. Am I correct so far? FIig 4)
How much cooler would that top chamber be (I realize that it depends on the volume, I'm looking for a formula)?
Would any impurities in the water separate from the vapor now that it's not in a liquid solution? Which? Metals and other minerals? Would it depend on their density?
Please ignore the vacuum question for the moment. I believe that I can better ask that question in reference to an upcoming diagram.

Then we close the middle valve. (Fig 5)

Then we open the top valve. (Fig 6)
What happens in that top chamber?
Is there a vacuum? If so, is there a formula to show the force of the vacuum created relative to its volume (what is it)?
Does the vapor condense into liquid?
Is it a fairly passive reaction (like a sudden mist appearing and gently forming into larger droplets and collecting on the bottom of the chamber) or violently (like with thermal shock)?
Would it be uniform with kind of a cascade forming on one end end moving towards the other (like tapping a bottle of super cooled water and watching it freeze across the bottle)? Or would pockets develop in seemingly random parts of the chamber nearly instantly pulling and pushing on each other creating chaotic looking turbulance? Or something else altogether?
Would anything that came out of solution come back into solution, or would this distill some of the impurities out of the water?
What would it look like?
If the top was a cap instead of a valve and quickly removed instead of opened, would the vacuum created displace the water vapor sending it into the air above? If so, how much of that water vapor would be lost to the air above it (is there a formula based on the dimensions of the pipe)? Would all the water thrown into the air condense into droplets or would some evaporate immediately (is there a formula to know how much would become droplets in the air or evaporate based on the volume spewed, wind speed, temperature and relative humidity?

Would any of this be different if the pipe was less than a drop of water in diameter?

If instead of there being a top chamber, it was a sealable removable section, and we did the same experiment then took off that top section full of water vapor...
How would temperature effect the vapor inside? Would it condense if it became cold enough? If at what temperature (formula for solving that question)? Because the liquid would take up more room, how much pressure would be created? Would it end up being equal to the pressure of the water before the atmospheric pressure turned the liquid into water vapor initially?
If that section was weighted and dropped into a lake (or otherwise effected by higher outside pressure), would the vapor condense? When (what formula would I use to find out)?

I have no real application in mind. Desalination and water purification seem like obvious next things to ponder based on what the answers to these questions are. And what can be achieved using this different effects in general. But, essentially, I'm just trying to wrap my mind around what happens.

I have many other questions, but this one was already prepared due to my recent Facebook post.

Thanks.
Happy to be here.
- Ed

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berkeman

Mentor
What is the best way to ask my questions?
Welcome to PF.

I've moved your New Member Introduction thread to the Mechanical Engineering forum, which is probably the best match for now.

Thank you

berkeman
I have so many questions.
Your stream of questions regarding the operation of a water-filled barometer can be answered, and each step explained by the wonders-of-science.
Most of your commentary is reasonable, but there are a couple of misunderstandings, probably because you were working by yourself.
How do you want to proceed?

gmax137, hutchphd and berkeman
Thank you for getting back to me, Baluncore! I didn't see the notification that anyone had. I'll be sure to check and not wait from now on.

I'm open. Whichever method seems best. Maybe where I am wrong in my assumptions. I am so happy to be on the verge of having a better understanding!

Mentor
I didn't see the notification that anyone had. I'll be sure to check and not wait from now on.

@ChicagoDad -- Click on USER at the top right of the page, and select Preferences. I don't get e-mails for replies in my threads, but you can enable that:

I'm having trouble locating that. Is it because I'm using the app and Chrome on my phone?
I can post screenshots of all the places that I've looked. Buy I thought that I should just probably ask the question first.

Mentor
If you're having trouble seeing the USER link at the top of your screen, try turning your phone sideways to get more width in. Does that help?

Yes (feeling foolish). Thanks. That worked perfectly.

berkeman
Mentor
Yes (feeling foolish).
No need to feel foolish! It's a non-obvious trick that we have to use semi-often on phones.

It is difficult to answer, in an orderly way, the complex braided questions in the long original post.

The first thing to do is reduce the complexity of the analysis. Perform the analysis slowly, so everything has time to reach a state of thermal equilibrium. You can then ignore time and temperature, to consider only the “hydrostatic” relationship between height, pressure, mass, and volume.

Later, once you understand the static relationships between variables, you can go back to analyse time, and the rate of temperature change, due to dynamic changes in pressure resulting from the individual transitions between states, that make up the essay long process.

Thank You Baluncore,

I can see how I could be asking too much at once. I was trying to set up the conditions for one question, but was also trying to leave the window open to be corrected about any misconceptions that I may have. I am completely self taught (aside from YouTube) and have no one to correct me if I'm mistaken about a concept along the way.

So let's begin (if it's alright with you) with my primary question. In the last picture that I posted, a 20 meter(-ish) pipe was previously filled with water with the bottom of that pipe submerged while a valve at the bottom was shut (preventing the water from falling straight through). Then the valve at the top of the filled pipe was closed. Then the valve at the bottom of the submerged pipe was opened. At which time, all of the water within the pipe above 10 meters(-ish, assuming the bottom of the pipe is at sea level) boils into water vapor due to atmospheric pressure). Then a valve just above that line of water vapor (roughly just above 10 meters) is closed trapping that negatively pressurized (compared to the outside) water vapor. Then the top valve is opened quickly.

What does that look and sound like?

Assuming there's no wind, does the vapor turn to liquid in the pipe? Is it a cascade effect? Does the pressure difference cause the vapor to form above the pipe? Is it a violent or elegant reaction (like tapping a supercooled bottle of water and watching the ice form through it progressively from one side to the other? Is it cold due to the heat energy lost in the reaction?

Yes, many questions. But really just a single thorough description of the event will answer most of them. I can't imagine the event lasting longer than a few seconds. What are those seconds like?

I do have more questions. But this is the one that inspired the others.

If you would walk me through those 3-5? seconds, I would really appreciate it.

Thank You Again,

At which time, all of the water within the pipe above 10 meters(-ish, assuming the bottom of the pipe is at sea level) boils into water vapor due to atmospheric pressure).
Not exactly.
Boiling is typical of a liquid heated from below, where bubbles of gas move up through the liquid.

ALL the water within the pipe does NOT boil, nor evaporate. The entire column of water fell about half way, leaving water vapour in its place above 10 metres.
The steam tables show the vapour pressure of water at 20°C is 2.336 kPa.

In this case, the surface of the liquid evaporates to fill the space above the falling water column. It is the surface that evaporates because water below the surface is under higher hydrostatic pressure than the surface water, so the surface evaporates first.

The 10-metre level is decided by the Atmospheric pressure pushing up, balanced against the density of water pushing down.

Then a valve just above that line of water vapor (roughly just above 10 meters) is closed trapping that negatively pressurized (compared to the outside) water vapor. Then the top valve is opened quickly.
What does that look and sound like?
The upper section contains water vapour at an absolute pressure of about 2.336 kPa. When the top valve is opened, a bolt of air accelerates into and down the upper tube, hitting the closed mid-valve and being reflected back up the tube. It will sound like a double pop. The first will be a depression wave, as the column of air begins to move, followed by the pressure wave reflection that will be about 20 metres delayed, 60 ms later. What happens with multiple reflections is the sound of an organ-pipe, and needs to be modelled using transmission line theory or numerical simulation. The sound will have a distinct note, I guess somewhere around 1/0.06 = 16.5 Hz, more like a flutter or fart than a sound.

Assuming there's no wind, does the vapor turn to liquid in the pipe? Is it a cascade effect? Does the pressure difference cause the vapor to form above the pipe? Is it a violent or elegant reaction (like tapping a supercooled bottle of water and watching the ice form through it progressively from one side to the other? Is it cold due to the heat energy lost in the reaction?
As the bolt of air accelerates down the tube, the pressure of the water vapour in the tube rises, so the water vapour dissolves in the wavefront of the air. A mist or cloud will NOT form ahead of the accelerating wavefront. Mist may appear just behind the wavefront and in the upper tube during the acceleration, from humidity in the air and the pressure drop at the entrance. The visual effect will be determined by the relative humidity at the time, and the shape of the open-end of the tube.

Mentor
It may interest you to know that this is not just an abstract physics problem, but a real world problem that tripped up at least one engineer. The diagram below shows the system. A tank containing ##TiO_2## slurry with specific gravity about 2, had a pump that supplied a distribution pipe located about 40 feet above the tank liquid level. The line had a valve located just after the pump that opened when the pump was on, and closed when the pump turned off. The discharge end of the line was below the surface of the slurry.

Every time the pump started, there was a KABLAAMM that shook the pipes enough that the pipes were in danger of breaking.

The flow rate and viscosity were such that the entire line ran full when the pump was running. When the pump stopped and the valve closed, the slurry continued to flow until the liquid level in the downward portion of the pipe was about 15 feet above the tank liquid level. There was a vacuum* from that point up to the distribution pipe at 40 feet up. When the pump started up, the flow was helped by the vacuum until the new supply of slurry met the slurry standing in the downpipe with a loud crash.

The solution was to add another valve in the downpipe just above the tank to keep the slurry from running down and creating a vacuum gap. Problem solved.

*More correctly water vapor as discussed by @Baluncore above. But for engineering purposes, it's a vacuum.