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Old 02-21-2018, 01:36 AM   #1
Dalillama
 
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Default Weather in a Dysonish sphere

Picture a sphere ~500 AU in diameter, filled with air (oxynitro mix at approximately 1.25 atm of pressure). The material of the sphere has no gravitational masd. Knots of dirt, rock, and water drift with the winds, as do cohesive balls of flaming gas (assume butane). In the center 9f the sphere is a ring of fire .5 au in diameter which burns and goes out on a 20 hour cycle, 10 hours on, 10 hours off. Assuming physics like real life on a macro scale except as specified, what kind of weather is to be expected?
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Old 02-21-2018, 04:02 AM   #2
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Default Re: Weather in a Dysonish sphere

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Originally Posted by Dalillama View Post
Picture a sphere ~500 AU in diameter, filled with air (oxynitro mix at approximately 1.25 atm of pressure). The material of the sphere has no gravitational masd. Knots of dirt, rock, and water drift with the winds, as do cohesive balls of flaming gas (assume butane). In the center 9f the sphere is a ring of fire .5 au in diameter which burns and goes out on a 20 hour cycle, 10 hours on, 10 hours off. Assuming physics like real life on a macro scale except as specified, what kind of weather is to be expected?
The mass or otherwise of the sphere doesn't matter - if it's uniform there's zero gravitational force from the sphere inside it.

Does the central ring of fire have any mass, in which case things will drift towards it, or is the whole space effectively in zero-G, in which case the flaming gas will soon run out of oxygen to burn due to lack of convection?
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Old 02-21-2018, 06:37 AM   #3
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Default Re: Weather in a Dysonish sphere

Weather is a way for planets to move heat around. Hot air rises only because there's gravity. Winds occur mostly because hot air rises (needed that low pressure to get filled in at ground level from somewhere else), and rotate direction mostly because the planet spins (Coriolis effect). Rain comes from evaporating water (absorbing heat where it's warmer) condensing again (releasing heat where it's cooler).

So the weather is all going to be driven by that central fire and the drifting balls. But with no gravity, there's no wind, and little way other than conduction and radiation to move the heat away from the fires. Warmer and colder as the drifting balls of fire get closer or further, warmer near the center and cooler toward the outer rim, in a pretty smooth gradient.

Are the "knots of dirt, rock, and water" large enough to have their own gravity?
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Old 02-21-2018, 07:44 AM   #4
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Default Re: Weather in a Dysonish sphere

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Does the central ring of fire have any mass, in which case things will drift towards it, or is the whole space effectively in zero-G, in which case the flaming gas will soon run out of oxygen to burn due to lack of convection?
I was envisioning zero G except where the rocks or globules are big enough to have gravity.

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Are the "knots of dirt, rock, and water" large enough to have their own gravity?
Many of them. The blazing gasballs might have some as well.
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Old 02-21-2018, 08:55 AM   #5
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Default Re: Weather in a Dysonish sphere

The blazing gasballs need some propulsive force to keep them moving into oxygenated air, or they'll go out.

At which point I suspect you basically end up saying "magic physics", and you might as well choose the weather system you fancy. This isn't necessarily a bad thing; it's just that making painstaking physics solutions to problem A when problem B is solved by magic seems like wasted effort.
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Old 02-21-2018, 09:09 AM   #6
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Default Re: Weather in a Dysonish sphere

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The blazing gasballs need some propulsive force to keep them moving into oxygenated air, or they'll go out.
Would setting the ring to spinning like a coin on its edge get me some decent winds going?
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At which point I suspect you basically end up saying "magic physics", and you might as well choose the weather system you fancy. This isn't necessarily a bad thing; it's just that making painstaking physics solutions to problem A when problem B is solved by magic seems like wasted effort.
Probably it is, but since I'm doing Larry Niven's setting into Spelljammer, it feels like I ought to have at least some idea of what should be happening in a setup like this.
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Old 02-21-2018, 01:38 PM   #7
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Default Re: Weather in a Dysonish sphere

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Originally Posted by RogerBW View Post
The mass or otherwise of the sphere doesn't matter - if it's uniform there's zero gravitational force from the sphere inside it.

Does the central ring of fire have any mass, in which case things will drift towards it, or is the whole space effectively in zero-G, in which case the flaming gas will soon run out of oxygen to burn due to lack of convection?
The atmosphere inside the sphere, though, will self-gravitate towards the centre, unless a uniform pressure of 1.25 bar is part of the exception to physics. So where is this 1.25 bar we're told about? At the shell? In that case the central pressure is enormous and the mass of the internal atmosphere is huge. Or is 1.25 bar at the centre? In that case the pressure at the shell is negligible. Or is 1.25 bar maintained throughout in defiance of physics?

Somebody ought to look up the physics of Jeans Collapse. Bags not me.
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Old 02-21-2018, 01:49 PM   #8
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Default Re: Weather in a Dysonish sphere

The shell is going to be pretty cold out there 250 AU from a gas ring. You can estimate its temperature from its area (vast) and the heat output of the gas ring by using Stefan's Law, or you can calculate the required heat output to maintain a given temperature the same way. That's because all the heat produced by the ring must leave the sphere by radiation of the outer surface of the shell, whether it arrives by radiation to the inner surface of the shell or by convection in the atmosphere.
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Old 02-21-2018, 01:57 PM   #9
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Default Re: Weather in a Dysonish sphere

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Somebody ought to look up the physics of Jeans Collapse. Bags not me.
This isn't a close case. Density is approximately 1.5 kg/m^3, radius is 250 AU or 3.7e+13m, volume is about 2e+41 m^3, so mass is about 3e+41 kg or 15 billion times the mass of the sun. The schwarzchild radius is 2MG/c^2, or 2 * 6.67e-11m^3⋅kg^-1⋅s^-2 * 3e+41 kg / 9e+16m^2⋅s^-2. Negating units, we get 2m * 6.67e-11 * 3e+41 / 9e+16, or 2m * 2e+31 / 9e+16, or 4.5e+14m, so we actually have a supermassive black hole.
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Old 02-21-2018, 02:10 PM   #10
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Default Re: Weather in a Dysonish sphere

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This isn't a close case. ... so we actually have a supermassive black hole.
I'm gearing up to run a Hard (ish) sci-fi game with GURPS, TL10, reaction drives. I made a crib sheet to calculate travel times for reaction drives (inside my spaceships sheet).

I am STILL not ready for this kind of physics. :-D
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