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Physics of deep ocean


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OK. I really want to know something about equalizing pressures at deep ocean.

Specifically what happens if you are in a hydrolock (as I'll nickname it, think of an airlock) and you fill the hydrolock with water and absolutely no airbubbles remain, then you open even a small valve to the outside pressure to "equalize the pressure".

 

What I described is a torpedo tube. And it stands to reason that nothing happens.

But my mind keeps going back to the engineering principle of waterhammer.

If you have water at 1atm and you have water at 100atm and you bring them together, how do you NOT get a waterhammer?

So in the torpedo tube example what is the forces inside the tube when you have it completely flooded at 1atm, then expose it to 100atm.

Water is non-compressible so in theory NOTHING happens. But then:

1) you have an equalizing valve (why?)

2) the noncompressibility is precisely the waterhammer problem on pumps when you take water at lower pressure and stop its flow cause a spike in pressure.

Can someone discuss the nuances in this example?

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3 hours ago, IDNeon said:

OK. I really want to know something about equalizing pressures at deep ocean.

Specifically what happens if you are in a hydrolock (as I'll nickname it, think of an airlock) and you fill the hydrolock with water and absolutely no airbubbles remain, then you open even a small valve to the outside pressure to "equalize the pressure".

 

What I described is a torpedo tube. And it stands to reason that nothing happens.

But my mind keeps going back to the engineering principle of waterhammer.

If you have water at 1atm and you have water at 100atm and you bring them together, how do you NOT get a waterhammer?

So in the torpedo tube example what is the forces inside the tube when you have it completely flooded at 1atm, then expose it to 100atm.

Water is non-compressible so in theory NOTHING happens. But then:

1) you have an equalizing valve (why?)

2) the noncompressibility is precisely the waterhammer problem on pumps when you take water at lower pressure and stop its flow cause a spike in pressure.

Can someone discuss the nuances in this example?

It is not strictly true that water is non-compressible. Liquids are about as compressible as solids, which is to say hardly at all by comparison with gases. But they still compress a bit. If you suddenly expose the water in a torpedo tube to 100atm, you exert that pressure on the water inside and thus on the walls of the tube as well. The water will compress a tiny bit and the walls of the tube will stretch and expand a tiny bit as well. (Because it is only a tiny bit, very little work is done, so there will be very little stored energy in the compressed and stretched materials.)

Water hammer is a shockwave caused by abruptly blocking the path of a moving mass of water, thereby causing rapid change in momentum. This change of momentum requires a certain impulse (F x t) and because t is so small (because it happens fast), F has to be great. So that means water hammer creates large forces and hence pressures - a pressure wave. 

When you open a torpedo tube that is full of water, you do not have this, because the water on both sides of the opening is static and no change of momentum occurs. So it won't cause water hammer, just a bit of stretching of the walls of the tube. 

As for the equalising valve, I suspect that will be because when you flood a torpedo tube in practice you most certainly do have trapped air, which will compress to 1% of its volume, storing a lot of energy and causing water to flood in as it is compressed - with momentum. So there can be large forces and energies created in that scenario, which you do not want for safety reasons. So you flood it progressively rather than instantaneously.

At least, that would be my best guess as to what is going on.   

 

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45 minutes ago, IDNeon said:

What are the stresses imposed on any object when equalized this way? It seems there would be very little?

If the pressure is equalised throughout the object, i.e. with no spaces at a different pressure, then merely a bit of compression of the materials of which it is made. 

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1 minute ago, exchemist said:

If the pressure is equalised throughout the object, i.e. with no spaces at a different pressure, then merely a bit of compression of the materials of which it is made. 

Awesome. My intuition is correct but I wanted to bounce it off more physics minded persons.

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Posted (edited)
1 minute ago, IDNeon said:

Awesome. My intuition is correct but I wanted to bounce it off more physics minded persons.

Yes, this is how it is possible for divers to go down so far, as long as the gas mixture they breathe is at the pressure of the surrounding water. 

Edited by exchemist
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2 hours ago, exchemist said:

Yes, this is how it is possible for divers to go down so far, as long as the gas mixture they breathe is at the pressure of the surrounding water

I'm familiar with that but not with the taking a diver from 1atm then exposing them to say....20atm suddenly.

And I'm curious if that's physiologically possible.

For instance if you put a free diver out of a "hydrolock" then do they have their ears rupture? Gasses still inside them burst out rather than squeeze out over time as they descend?

Another way of asking this (more practical in physics) is...

If I have a pressure vessel at 1atm with water filling it, and an outside pressure of 10atm.

If I open a valve....will it take a period of time for the pressures to equalize or will it be instantaneous? 

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A 'vessel' filled completely with water at 1 atm, submerged under water at 10 atm, will experience some compression stressess, and on opening a valve to equalize pressure, there will be very little, if any, movement of water since the 'vessel' is already filled.
However, as the pressure is now equalized, the 'vessel' will now relax from the compression stresses, and, depending on materials used, there could be some expansion ( back to normal ) of the 'vessel', and so, a limited inrush of water.

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So what would you think the time frame is for equalization?

If you open a very small valve on a gas filled pressure vessel the equalization would take some time?

But not with liquids?

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16 minutes ago, IDNeon said:

So what would you think the time frame is for equalization?

If you open a very small valve on a gas filled pressure vessel the equalization would take some time?

But not with liquids?

How long is a piece of string? 

The time will depend on the pressure differential, the size of the valve aperture, the viscosity of the fluid passing through and the size of the space the fluid has to fill.  

 

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2 minutes ago, exchemist said:

ow long is a piece of string? 

The time will depend on the pressure differential, the size of the valve aperture, the viscosity of the fluid passing through and the size of the space the fluid has to fill

No fluid has to pass through. 

I didn't mean that the pressure vessel was empty.

The conditions apply as before.

If you had a balloon of air in water in this pressure vessel at 1atm.

Then you open a valve to 10atm.

What happens to the balloon in the water? How fast does it get affected by the 10atm?

Generally. Not specifically.

Does it happen nearly instantly because the water is almost incompressible?

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8 hours ago, IDNeon said:

No fluid has to pass through. 

I didn't mean that the pressure vessel was empty.

The conditions apply as before.

If you had a balloon of air in water in this pressure vessel at 1atm.

Then you open a valve to 10atm.

What happens to the balloon in the water? How fast does it get affected by the 10atm?

Generally. Not specifically.

Does it happen nearly instantly because the water is almost incompressible?

This is a silly question.

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1 minute ago, exchemist said:

This is a silly question.

Not silly.

I'm having trouble figuring out if a diver needs to experience a compression time when exiting a pressure hull or if once the chamber is flooded they are good to go.

Essentially I'm trying to figure out if a chamber must be first pressurized by gas to equalize ambient pressure outside pressure hull or can that stage be disregarded as it is for a torpedo.

A torpedo isn't sensitive to forces in the same way a gas filled meat-sack is.

So...I can't assume that what works for a torpedo tube works for a diver.

But it makes sense that it would work except a bit of discomfort when the water pressure goes from 1atm to ambient pressure at depth.

So can you suggest which method must be followed to exit a pressure hull? 

Slow compression with gas then flood tube? Or just flood tube as only step?

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2 hours ago, IDNeon said:

Not silly.

I'm having trouble figuring out if a diver needs to experience a compression time when exiting a pressure hull or if once the chamber is flooded they are good to go.

Essentially I'm trying to figure out if a chamber must be first pressurized by gas to equalize ambient pressure outside pressure hull or can that stage be disregarded as it is for a torpedo.

A torpedo isn't sensitive to forces in the same way a gas filled meat-sack is.

So...I can't assume that what works for a torpedo tube works for a diver.

But it makes sense that it would work except a bit of discomfort when the water pressure goes from 1atm to ambient pressure at depth.

So can you suggest which method must be followed to exit a pressure hull? 

Slow compression with gas then flood tube? Or just flood tube as only step?

Then the question becomes a different one, viz. what limits are there to the rate of pressure change the human body can withstand, or something like that. 

So far as I know, the most sensitive part of the body from the point of view of pressure changes is the ear.  The middle ear equalises pressure with the environment via the eustachian tube, which is linked to the nasal sinuses and is very narrow, with a sort of semi non-return valve in it, to encourage the removal of any mucus discharges etc. To pressurise the  middle ear, air must go the "wrong way" through this non-return valve, leading to the sensation of deafness, relieved by swallowing, that we are all familiar with in a descending aircraft. Swallowing allows the non-return valve to open  and allow air in "the wrong way".

If you increases the pressure too fast, you experience deafness, as the eardrum becomes stretched by the pressure difference, followed by pain as it is further stretched. The eardrum can easily rupture if enough time is not allowed for the ears to "pop" and thus transfer the increased pressure via the eustachian tube to the other side of the eardrum. Some people have difficulty equalising the pressure, especially if they have a cold or other inflammation of the sinuses. To you have to go carefully or you can inflict considerable pain and make people go deaf.    

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Posted (edited)
6 hours ago, IDNeon said:

Not silly.

I'm having trouble figuring out if a diver needs to experience a compression time when exiting a pressure hull or if once the chamber is flooded they are good to go.

Essentially I'm trying to figure out if a chamber must be first pressurized by gas to equalize ambient pressure outside pressure hull or can that stage be disregarded as it is for a torpedo.

A torpedo isn't sensitive to forces in the same way a gas filled meat-sack is.

So...I can't assume that what works for a torpedo tube works for a diver.

But it makes sense that it would work except a bit of discomfort when the water pressure goes from 1atm to ambient pressure at depth.

So can you suggest which method must be followed to exit a pressure hull? 

Slow compression with gas then flood tube? Or just flood tube as only step?

In exiting out a submarine's escape trunk, for example, you only need to flood the trunk.

Forbes had a good article about it that may be of interest. Explains everything in good detail.

https://www.forbes.com/sites/davidhambling/2021/04/26/how-to-escape-from-a-submarine-stranded-on-the-sea-bed/?sh=231aa94f1227

Edited by Endy0816
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Posted (edited)

Really deep diving requires special procedures and special equipment i.e. decompression chamber otherwise diver will have decompression sickness which might even end up with death. You should read entire articles:

https://en.m.wikipedia.org/wiki/Decompression_sickness

https://en.m.wikipedia.org/wiki/Diving_chamber

Hydrostatic pressure at sea level is 1 atm. 

Approximately 2 atm at depth 10m.

Approximately 3 atm at depth 20m.

and so on, so on, every 10m.

Whether diver will survive getting out of submarine which sunk depends on depth at which he/she get out of it and availability of decompression chamber waiting for them..

Edited by Sensei
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3 hours ago, Sensei said:

Really deep diving requires special procedures and special equipment i.e. decompression chamber otherwise diver will have decompression sickness which might even end up with death. You should read entire articles:

https://en.m.wikipedia.org/wiki/Decompression_sickness

https://en.m.wikipedia.org/wiki/Diving_chamber

Hydrostatic pressure at sea level is 1 atm. 

Approximately 2 atm at depth 10m.

Approximately 3 atm at depth 20m.

and so on, so on, every 10m.

Whether diver will survive getting out of submarine which sunk depends on depth at which he/she get out of it and availability of decompression chamber waiting for them..

Technically also depends on how long you spend at that pressure/depth.

600 feet(183 meters) is supposed to be survivable though have to get back to atmospheric pressure fast.

Ironic problem of having too much air in you, when escaping not having enough air.  Better off really waiting for the Rescue Sub in almost every case.

 

 

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