Airpressure in Rama? ( physics question )

[zaraza]

I'm reading Clarke Rama again and i wonder...
How is air distributed in this giant space ship?
We know that there is air in centerline axis, because guy with light aircraft can fly to the other end of Rama and can breathe without aids.
However, in one of first descents into Rama, one of the crew "tries" the air when "air pressure is 1/3 of Earths airpressure".

[AndreH]

...
How is air distributed in this giant space ship?
....

If Clarke doesn't tell we only can speculate:

Taken from http://en.wikipedia.org/wiki/Clarke's_three_laws
ETA:

3.Any sufficiently advanced technology is indistinguishable from magic.

"Clarke's Third Law codifies perhaps the most significant of Clarke's unique contributions to speculative fiction. A model to other writers of hard science fiction, Clarke postulates advanced technologies without resorting to flawed engineering concepts (as Jules Verne sometimes did) or explanations grounded in incorrect science or engineering (a hallmark of "bad" science fiction)[citation needed], or taking cues from trends in research and engineering (which dates some of Larry Niven's novels). Accordingly, the powers of any future superintelligence or hyperintelligence which Clarke often described would seem astonishing."

[Swift]

I'm reading Clarke Rama again and i wonder...
How is air distributed in this giant space ship?
We know that there is air in centerline axis, because guy with light aircraft can fly to the other end of Rama and can breathe without aids.
However, in one of first descents into Rama, one of the crew "tries" the air when "air pressure is 1/3 of Earths airpressure".

<<< SPOILER ALERT >>>


Great book, one of my favorites.

Clarke does touch on the increase in air pressure. When they first enter, it is very low at the hub (too low to breath) and increases as they descend, but it is still pretty low (as you note, I didn't recall the numbers).

After the thawing of the sea, the microorganisms put out a lot of oxygen into the atmosphere. He also mentions the humidity goes up a lot, presumably because of the increased vapor pressure of the now warmer water. And IIRC, even after this, the pressure at the hub is still lower than at the surface, it is just that it is now up to breathable pressure and oxygen content.

He is also talks about the storms that develop (for which they need to temporarily leave). I would guess those would circulate the air.

But other than that, I don't recall anything specifically about air circulation.

[AndreH]

Ok, lets speculate:
If there would be no consumption of any of the gases I would speculate diffussion alone would be enough. Maybe with higher concentration of heavier gases at the "bottom".
Even with consumption of any component thsi process might be enough
I was asking myself anyway if the air would rotate together with the hole cylinder or if it would do something completely different. I think this could be simulated today. No idea if anyone had done so far.

[zaraza]

After the thawing of the sea, the microorganisms put out a lot of oxygen into the atmosphere.

Yes, i had the same thought.
But, here is what's on Chapter 22

"And although Laura Ernst - now doubling as research scientist as well as ship's doctor - had proved that they definitely generated oxygen, there were far too few of them to account for the augmentation of Rama's atmosphere. They should have existed in billions, not mere thousands"

http://www.scribd.com/doc/3937725/Cl...h-Rama#page=71

[zaraza]

I was asking myself anyway if the air would rotate together with the hole cylinder or if it would do something completely different.

I think it must rotate with the cylinder. If not, there will be massive storms on 'ground' level because of rotation speed.

Question: If it's rotating with cylinder i suppose it is affected by inertia? In that case ground air pressure would increase during the air ( or oxygen ) insertion - more air - more mass - more pressure.

Of course, unless entire air volume was there from the beginning.

[eburacum45]

The air pressure in a rotating habitat would decrease towards the axis, as the air would rotate with the cylinder. However Coriolis effects would be quite significant, so I'd expect it to get quite breezy at times. Perhaps a fully open habitat might not be a good idea- a series of baffles could be used to slow down any persistent vortices in the airflow.

[AndreH]

I think it must rotate with the cylinder. If not, there will be massive storms on 'ground' level because of rotation speed..

Thats pretty much what inhabitants would like it to do. The question is: Would it?

Question: If it's rotating with cylinder i suppose it is affected by inertia? In that case ground air pressure would increase during the air ( or oxygen ) insertion - more air - more mass - more pressure.

I agree

Of course, unless entire air volume was there from the beginning.

Than the pressure at the "bottom" would still be higher. But no increase.

[AndreH]

The air pressure in a rotating habitat would decrease towards the axis, as the air would rotate with the cylinder.

Would it really? I think in the long term yes. But what would happen if you have a non rotating cylinder and start rotation? Would the air rotate with the cylinder from the beginning on? Or will there be a kind of intermediate state with heavy winds blowing at the "bottom"?

However Coriolis effects would be quite significant, so I'd expect it to get quite breezy at times. Perhaps a fully open habitat might not be a good idea- a series of baffles could be used to slow down any persistent vortices in the airflow.

IIRC Clarke describes "Coriolis" storms in the book (Heck, its more than 10 years since I've read it last). Baffles will be a good idea indeed.

[IsaacKuo]

Would it really? I think in the long term yes. But what would happen if you have a non rotating cylinder and start rotation? Would the air rotate with the cylinder from the beginning on? Or will there be a kind of intermediate state with heavy winds blowing at the "bottom"?

The air would initially lag, but it would soon catch up. The minimum energy state is for the air to co-rotate with the shell. Think about it--if there are winds blowing trees around and stuff, where is this energy coming from? Unless the air is a superfluid (it isn't), even just the action of the turbulent air motion will dampen air motion down to the minimum energy state of air co-rotating with the shell.

On the other hand, an energy source like artificial "sunlight" can pump energy into the system to power winds. Convection will cause warm air to rise from places that absorb more sunlight. This motion will be affected by coriolis effects, causing spin-ward winds at higher altitudes.

[Nowhere Man]

[quoting Clarke] "And although Laura Ernst - now doubling as research scientist as well as ship's doctor - had proved that they definitely generated oxygen, there were far too few of them to account for the augmentation of Rama's atmosphere. They should have existed in billions, not mere thousands"

What I took from that was that there had been billions, but once their job of producing O₂ was done, they were broken down again.

It's a great tour of wonders, but it's not really a story. Not much plot, not much conflict, not much climax.

Fred

[zaraza]

IIRC Clarke describes "Coriolis" storms in the book (Heck, its more than 10 years since I've read it last). Baffles will be a good idea indeed.

Yes - and coriolis effect + heat...
Coriolis effect is also mentioned on one of descents. When astronaut does not hold himself for the ladder ( closer to the center axis of course ), they got tendency to drift away.

[AndreH]

Yes - and coriolis effect + heat...
Coriolis effect is also mentioned on one of descents. When astronaut does not hold himself for the ladder ( closer to the center axis of course ), they got tendency to drift away.

To answer the OP question about how the oxygene is distributed I think we can agree RAMA is big enough to have a "weather". (which is described by Clarke). This means oxygene will be distributed by the weather coming from interaction of heating which causes convection together with the coriolis effect.

[AndreH]

The air would initially lag, but it would soon catch up. The minimum energy state is for the air to co-rotate with the shell. Think about it--if there are winds blowing trees around and stuff, where is this energy coming from? Unless the air is a superfluid (it isn't), even just the action of the turbulent air motion will dampen air motion down to the minimum energy state of air co-rotating with the shell.

Meanwhile I came to the same conclusion. My "physics tool box" to approach problems is a bit rusty and sometimes it takes a while to get it open. My thinking was that if at the beginning everything is in rest and the cylinder starts rotation it would start to transmit impulse/energy to the air and sooner or later it must rotate.

On the other hand, an energy source like artificial "sunlight" can pump energy into the system to power winds. Convection will cause warm air to rise from places that absorb more sunlight. This motion will be affected by coriolis effects, causing spin-ward winds at higher altitudes.

ETA: Hmmm...Not counter spin ward?

[IsaacKuo]

ETA: Hmmm...Not counter spin ward?

Yes. Think of the ground level as the "anchor". On average, winds at ground level will be at zero velocity. Winds will actually be flowing in a bunch of directions, generally away from colder spots and toward warmer spots, but as a whole they won't have any spinward or antispinward preference at ground level.

Now consider what happens when air rises from a warm spot at ground level. It starts off with a spinward velocity equal to the ground level velocity. Due to inertia, it will retain this velocity as it rises. But the higher altitude has a smaller rotation arm. This means the air will be moving in the spinward direction faster than necessary to keep up with the habitat's rotation.

[AndreH]

Yes. Think of the ground level as the "anchor". On average, winds at ground level will be at zero velocity. Winds will actually be flowing in a bunch of directions, generally away from colder spots and toward warmer spots, but as a whole they won't have any spinward or antispinward preference at ground level.

Now consider what happens when air rises from a warm spot at ground level. It starts off with a spinward velocity equal to the ground level velocity. Due to inertia, it will retain this velocity as it rises. But the higher altitude has a smaller rotation arm. This means the air will be moving in the spinward direction faster than necessary to keep up with the habitat's rotation.

Ok, I was thinking of the Coriolis force which would act on air moving perpendicular to the rotation. Every movement perpendicular to the rotation would cause a counter spin ward movement, wouldn't it?

Gee...I really would like to see a simulation of this.

[IsaacKuo]

Ok, I was thinking of the Coriolis force which would act on air moving perpendicular to the rotation.

It actually is a Coriolis force effect. I was simply explaining it in terms of the non-rotating frame of reference.

Every movement perpendicular to the rotation would cause a counter spin ward movement, wouldn't it?

No. Inward movement causes a spinward coriolis force, while outward movement causes an antispinward coriolis force.

[AndreH]

It actually is a Coriolis force effect. I was simply explaining it in terms of the non-rotating frame of reference.



No. Inward movement causes a spinward coriolis force, while outward movement causes an antispinward coriolis force.

Seems as we have to clarify definition of directions. By inward you mean from the cylinder surface to the cylinder axis, correct? Outward is the oposite dirction.
A person standing in Rama would interpret inward as "up" and outward as down, right?

I was referring to movements along the cylindre axis (I would assume this to be always present because of temperature differences on the surface for example).

Just tried to make some sketches.....not yet satisfied. Need to think this over.

[IsaacKuo]

Seems as we have to clarify definition of directions. By inward you mean from the cylinder surface to the cylinder axis, correct? Outward is the oposite dirction.
A person standing in Rama would interpret inward as "up" and outward as down, right?

Yes.

I was referring to movements along the cylindre axis (I would assume this to be always present because of temperature differences on the surface for example).

There are no coriolis effects due to movement parallel to the rotation axis.

[AndreH]

Yes.

There are no coriolis effects due to movement parallel to the rotation axis.

Ok, I know there is a Omega x V in the coriolis force. So you must be right. Still thinking about it, though.

[Noclevername]

Maybe it would be easier to think of spinward as "east" and antispinward as "west" respectively.

"East takes you out, out takes you west, west takes you in, in takes you east." Larry Niven, The Integral Trees


ETA: But that's for orbits. Would it be reversed for the interior of a rotating vessel?

ETA Again: No, wait, Ringworld had the "eye storm", so it must be right.