Optimum shape for a pressurized structure in a pressure gradient

  • #1
DaveC426913
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Main Question or Discussion Point

I came across this pie-in-the-sea concept:

city-solutions-seasteading-buildings-float-ocean-fish-tower_57413_600x450.jpg


(Obviously, the pictured structure would be extremely susceptible to complete catastrophic failure, having no apparent internal means of water-tight seals to prevent complete implosion. Which is why you'd more logically build a city in tube-and pod fashion, full of bulkheads.)

Setting aside the materials requirement for a moment, consider such a very large open space structure - city-sized - large enough that it experiences quite a pressure gradient from top to bottom.

Would a sphere be the optimal shape? Or would you get something ... I dunno ... inverted pear-shaped?
 

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  • #2
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The "ideal shape" (constant wall thickness?) for local forces would have a curvature that depends linearly on the depth. I'm not sure if such a shape even exists.
 
  • #3
berkeman
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(Obviously, the pictured structure would be extremely susceptible to complete catastrophic failure, having no apparent internal means of water-tight seals to prevent complete implosion. Which is why you'd more logically build a city in tube-and pod fashion, full of bulkheads.)
What is that thing, Dave? Where did you find it? It seems to only have a little bit to do with your overall question, right?
 
  • #4
256bits
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Underwater art.
Proof of concept futuristic habitat.
Fish amusement park.
Insides of a smurf ball - who knew.
 
  • #5
DaveC426913
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What is that thing, Dave? Where did you find it? It seems to only have a little bit to do with your overall question, right?
:shrug: It's an underwater city, like, a half mile across - and deep.

It came up on another forum. The image was uploaded, so it has no [word meaning continuity of ownership] (and I didn't bother reverse Goggling it).


I am wondering, if such a thing were built, would it be spherical? Or would its shape be influenced by the fact that different parts of it are in different pressures - so to make the compressive strength equal across any given area, maybe the shape would be different. Maybe it would be narrower at the bottom and wider at the top.
 
  • #7
Baluncore
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An inverted cone, point down would meet the hydrostatic pressure requirement.

If wall thickness could be varied the structure would have thicker walls at depth and could be of almost any external shape.
 
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  • #8
DaveC426913
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Cool! Thanks!
 
  • #9
Baluncore
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Cool! Thanks!
Just when it was getting interesting.

A spherical tensile film surface is great when internal pressure is greater than external, but how do you stop an atmospheric pressure sphere or tube collapsing at an ocean depth of one mile?
There is a wall structure analogy with vacuum balloons. They require a light weight rigid skin, a skin that can be made by building a double layer tensile structure tied internally with a tensile honeycomb filling. The skin is then pressurised with air sufficiently to maintain the external spherical surface. The same could be done in the marine environment, with pressurised seawater, within reason.

Why do submarines not use the pressurised skin? Is it actually possible to use the double wall at depth when a single thicker wall results in sufficient mass to achieve neutral buoyancy?

The problems in engineering such a deep structure will be hydrostatic pressure differential and lack of light. I have never seen water as clear as the artists impression in post #1.

It would really be so much easier to engineer a neutral buoyancy planar structure close to the sea surface, below the wave action and shipping. It would be necessary to flow with the local current, as such a large structure could never be moored. There would be no points on the structure strong enough to anchor the structure. Remember that dynamic pressure is half the fluid density * velocity2. Tidal and ocean currents would break the lines, or collapse and destroy it like any beached ship.

Maybe it could be dirigible, with many wind turbines above and many small underwater propeller pods below. Then it could gently “sail” into the wind while solar PV and wave energy were harvested above.

It could be flexible and have large holes that allowed whales and dolphins to surface without visiting the human inmates.
 
  • #10
DaveC426913
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The problems in engineering such a deep structure will be ... lack of light. I have never seen water as clear as the artists impression in post #1.
It is not very apparent in the artist's diagram, but I think the top of the sphere is open to the surface.

Whereas horizontal light penetration near the bottom would be near zero, it should not be too difficult to arrange mirrored surfaces to reflect light through open areas of the structure.
 
  • #11
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Then why isn't the optimum a shallow disc, that lets in lots of natural light? It would need to sit in a lagoon surrounded by a reef.

The deeper it goes, the more problems. Benefits of deep???
 
  • #12
Baluncore
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Yes, the optimum is a shallow disc, sitting in a lagoon or lake.
There is an extreme alternative to going deep, where light is not a problem and hydrostatic pressure is a positive asset.
Wflmi.jpg
 

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  • #13
DaveC426913
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Then why isn't the optimum a shallow disc, that lets in lots of natural light? It would need to sit in a lagoon surrounded by a reef.
Because then we'd be talking about a floating city instead of a submerged city. Floating cities have been imagined to-death. They're just like regular cities, except with more water features.

A submerged city has different possibilities. The most obvious of which, is that you could look out instead of down, and you could see deep sea critters, such as sperm whales, in their more natural habitat. And if you don't like perpetual twilight, you'll probably be happier staying at the floating city down the road.

OK, sure not thee compelling reason to build an underwater city, but this is blue-skying. Think of the tourist possibilities.
 
  • #14
DaveC426913
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:shrug: It's an underwater city, like, a half mile across - and deep.

It came up on another forum. The image was uploaded, so it has no [word meaning continuity of ownership] (and I didn't bother reverse Goggling it).


I am wondering, if such a thing were built, would it be spherical? Or would its shape be influenced by the fact that different parts of it are in different pressures - so to make the compressive strength equal across any given area, maybe the shape would be different. Maybe it would be narrower at the bottom and wider at the top.
Provenance.

The word is provenance.

And now I can free up some processor cycles.
 

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