Swimming pool in a rotating space station

[DaveC426913]

TL;DR Summary

How does the Coriolis Force affect the currents and water levels in a toroidal swimming pool?

The Exodus thread got me thinking about swimming pools in a rotating space station.

Assume two scenarios: two toroidal pools that circumscribe the station, one is continuous and one is divided into segments by barriers.

(Sorry, typing on my phone is very arduous for these old thumbs, so I can't describe in as much detail as Id like.)

In continuous pool #1 I think surface water will flow antispinward and bottom water will hug the bottom. Yes?

I think this will break up naturally into eddy cells. Yes?

The cells will rotate vertically - I.e. Axis parallel to ship's rot. axis. The number of cells around the stations circumference will be dependent on rot. speed and the 3 physical dimensions of the pool.

Scenario 2 has barriers preventing complete flow, so one endless pool is now, say, 6 long pools.

Will CF act within each cell so that the water level is not flat? I.e.will the waterline be noticeably higher at the spinward end of any given pool than at the antispinward end?

 

[Ibix]

I'd expect pool 1 to be static water. Otherwise I could put a water wheel on the bank and extract energy. In the case of an ideal frictionless channel the water could flow at a steady rate and my water wheel would slow the water relative to the ground until it didn't flow anymore. In a case with frictional loss, the water will sync with the station naturally - assuming it wasn't poured into the channel already rotating to start with[1].

Pool 2 just has serious frictional losses.

Bad things can happen in toroidal pools, though. Arthur C. Clarke's Rama has one and it generates awesome tidal waves when the ship manoeuvres. I've not seen maths supporting this, but uneven forces on a spinning body of water is likely to result in some serious sloshing.

[1]Obscure reference: "It must be this river on the map!" "What's it doing on the map? Put it back in its banks!"

 

[DaveC426913]

the water will sync with the station naturally

Sorry, are you dismissing the difference between the forces at the waters surface (nearer the ships axis) from the forces at the pool bottom (farther from the ships axis)?

I expect vertical eddy cells.

Pool 2 just has serious frictional losses

Pool 2 is six pools that do not communicate. So just examine one of them. It's just a regular, long pool that goes a short way round the station.

I am wondering if CF would manifest even minimally as an asymmetrical water surface - i.e. slightly higher at one end, for the same reasons as in pool 1.

 

[jbriggs444]

I am wondering if CF would manifest even minimally as an asymmetrical water surface - i.e. slightly higher at one end, for the same reasons as in pool 1.

No. Not in the slightest.

The Coriolis force only manifests if the water in the pool is moving relative to the rotating frame. The water in these pools is subject to no forces that would cause it to move relative to the rotating frame.

Now if you had a temperature difference to drive a convection cell, then you could get a circulation going.

 

[Ibix]

The water had better be stationary if the station is in a steady state. If it isn't stationary I can extract energy until it is, so either it's stationary in the steady state or I can extract energy in the steady state and we have a PMM on our hands...

 

[DaveC426913]

The water had better be stationary if the station is in a steady state. If it isn't stationary I can extract energy until it is, so either it's stationary in the steady state or I can extract energy in the steady state and we have a PMM on our hands...

That's a very good point. Impossible to argue.

My argument was going to be:
Surely a particle of water at the surface experiences a tangential force different than a particle at the bottom, resulting the surface particle getting ahead of the bottom particle, setting up a convection cell.

Ok clearly I don't have a handle on this yet.

 

[hmmm27]

Coriolis effect doesn't kick in until you try to change latitude, which - in this case - simply means changing distance from the axis (which is the same on Earth, but without that pesky 3rd dimension obcuscating things).

In the reference frame of somebody enjoying a dip, toss some water (or a ball) "upwards" and it moves spinwards ; pee off the diving board and it will move antispinwards.

Bear in mind that from an external POV the temporarily free falling objects will be moving in a straight line.

I suppose convection cells would have a slight tendency towards warm, up and spinwards ; and cool, down and antispinwards.

 

[jbriggs444]

Surely a particle of water at the surface experiences a tangential force different than a particle at the bottom, resulting the surface particle getting ahead of the bottom particle, setting up a convection cell.

The particle at the surface has a tangential velocity different than a particle at the bottom. Resulting in a relative motion just enough so that they orbit each other once per rotation of the wheel they ride on. Which is to say, both are stationary relative to the rotating pool.

The particles have different centrifugal forces (or centripetal accelerations if you prefer), but that just means that the pressure gradient is steeper toward the rim, not that there is circulation.

 

[TeethWhitener]

As others have mentioned, a completely calm pool would look the same as it does on earth. But a splash fight might look really strange.

 

[DaveC426913]

Thanks all. Good discussion.

 

[Klystron]

For future reading, checkout SF short stories by John Varley including "Blue Champagne". The title refers to the unique shape of an interplanetary swimming resort visited by wealthy clients. One swims in the water wearing breathing mask and fins, not at or on a surface.

In my novels the water reserves / swimming 'pools' also serve as radiation shield, oxygen and hydrogen reserves, water supply and heat sink. No surface swimming but interesting conduits that confuse visitors requiring guides such as in "Blue Champagne".

 

[DaveC426913]

For future reading, checkout SF short stories by John Varley including "Blue Champagne". The title refers to the unique shape of an interplanetary swimming resort visited by wealthy clients. One swims in the water wearing breathing mask and fins, not at or on a surface.

Yup. Read it.

And the 2016 film "Passengers" has an example of what happens to a swimming pool (and its hapless occupants) in an AG system that spins down without first enacting proper safety protocols, to-wit: "Get out of the pool or you will drown".

 

[jbriggs444]

the 2016 film "Passengers" has an example of what happens to a swimming pool (and its hapless occupants) in an AG system that spins down without first enacting proper safety protocols, to-wit: "Get out of the pool or you will drown".

A fanciful imagining, in my opinion. Contiguous blobs of water that big would tend to break up and would not prevent swimming far enough to get one's head out.

 

[DaveC426913]

A fanciful imagining, in my opinion. Contiguous blobs of water that big would tend to break up and would not prevent swimming far enough to get one's head out.

I'd say though, that's not the problem; this is the problem:

1. There's no buoyancy to aid you. Wherever you want to go, you'll have to paddle there.
2. There's no "up". You have to pick a direction toward air and hope you picked a short trip.
3. There's nothing to settle the water. That air gap you are swimming toward won't be there when you arrive. It's sloshing all over the place.
4. Even if you reach air, the water will stick to you. If you don't take the time to clear the water off your face, you will inhale water.
5. Repeat #3: Even if you get a breath, it's quite likely the air gap will move off (perhaps more like another blob of water will hit you).
6. Since - as you say - the water will break up into smaller globs, you are quite likely to inhale large, small or even tiny globs of water.
7. Finally, 1 thru 6 are a lot of things to discover real quick and not make a mistake about in the minute or so you have left.

 

[jbriggs444]

I'd say though, that's not the problem; this is the problem:
There's no buoyancy to aid you. Wherever you want to go, you'll have to paddle there.

That's fine. Neutral buoyancy is the same whether there is gravity or not.

There's no "up". You have to pick a direction toward air and hope you picked a short trip.

Right. So you open your eyes and look. And try to swim straight. It's not like the new surface is more than ten feet in any direction. Swimming underwater is not difficult.

There's nothing to settle the water. That air gap you are swimming toward won't be there when you arrive. It's sloshing all over the place.

Yes, I agree. which is why I called it a fanciful imagining. The blob in the movie stayed contiguous. It wasn't a bunch of small blobs that you have to keep away from your mouth.

I was not clear on exactly what happened in the movie such that zero G resulted. The craft seemed bo be using centrifugal force for gravity. But you can't turn that off suddenly. If it were a failure of some gravity generator then the water in the pool would only have as much energy as the swimmer put into it. So I am thinking a ponderously slow evolution into the chaotic blob-filled room. If one stayed calm, there would plausibly be no problem to face.

Even if you reach air, the water will stick to you. If you don't take the time to clear the water off your face, you will inhale water.

Which you will do, if by nothing else than reflex. Something is in your mouth, so you'll try to wipe it away. Of course, panic is the enemy. You don't want to be injecting energy into the water with your frantic pawing.

Repeat #3: Even if you get a breath, it's quite likely the air gap will move off (perhaps more like another blob of water will hit you).

Right. A possibly dangerous situation, but not dangerous as portrayed in the movie scene.

[*]Finally, 1 thru 6 are a lot of things to discover real quick and not make a mistake about in the minute or so you have left.

I cannot argue with that. Rule 1: Do not panic.

 

[hmmm27]

Surface tension : blobs will combine - excepting ones already stuck to a surface - and try to form a sphere.

 

[DaveC426913]

That's fine. Neutral buoyancy is the same whether there is gravity or not.

But people are not naturally neutrally buoyant.

Right. So you open your eyes and look. And try to swim straight. It's not like the new surface is more than ten feet in any direction. Swimming underwater is not difficult.

Straight - toward what? A moving target.

Yes, I agree. which is why I called it a fanciful imagining. The blob in the movie stayed contiguous. It wasn't a bunch of small blobs that you have to keep away from your mouth.

As one huge blob? Or as a shifting miasma of big blobs? I don't recall specifically.

I was not clear on exactly what happened in the movie such that zero G resulted. The craft seemed bo be using centrifugal force for gravity. But you can't turn that off suddenly.

It was explicitly rotational gravity. The hab section decelerated. As it did so the water started climbing the wall, then the ceiling.

If one stayed calm, there would plausibly be no problem to face.

Easy to say, sitting in one's living room chair. But people do drown.

This would be an unprecedented problem. She had to figure out what's going wrong* and what to do about it while using up the last of whatever breath she had.

*I expect it would not be apparent immediately what, exactly, was happening, and whence comes the danger to her. She is swimming in water, but now the pool is moving under her, and here comes a bulkhead.

Which you will do, if by nothing else than reflex. Something is in your mouth, so you'll try to wipe it away. Of course, panic is the enemy. You don't want to be injecting energy into the water with your frantic pawing.

OK. "Get out of the water or you are likely to drown."

Right. A possibly dangerous situation, but not dangerous as portrayed in the movie scene.

With a mere one data point, I'd say "how dangerous it was" is a pretty arbitrary judgment.

 

[jbriggs444]

But people are not naturally neutrally buoyant.

Um, yes, we are. Even more so under zero g.

 

[DaveC426913]

Um, yes, we are. Even more so under zero g.

Speak for yourself. I'm not.

When I take a breath and dive, it is almost impossible for me to stay down without actively swimming downward.
To stay on the bottom of a pool, I have to expel my breath, and even then it's not enough.
If I dive down just a couple of feet, and stop moving, all I have to do to reach the surface is wait.

Are you telling me that if you jumped in the lake and sunk to, say, six feet, you would just sit there until you swam your way to the surface?

 

[Klystron]

But people are not naturally neutrally buoyant.

Um, yes, we are. Even more so under zero g.

Buoyancy depends on your body fat and muscle mass among other factors including water salinity.

After putting on some excess body fat over winter, swimming and floating are easy. As I burn off fat and add muscle from exercise my buoyancy reduces and I work harder swimming.

My gym measures body fat to weight ratio by dunking people in a tub and comparing water level changes. I guess fat floats and bone and muscle sink. Neutral buoyancy implies a happy balance.

 

[DaveC426913]

Buoyancy depends on your body fat and muscle mass among other factors including water salinity.

After putting on some excess body fat over winter, swimming and floating are easy. As I burn off fat and add muscle from exercise my buoyancy reduces and I work harder swimming.

My gym measures body fat to weight ratio by dunking people in a tub and comparing water level changes. I guess fat floats and bone and muscle sink. Neutral buoyancy implies a happy balance.

Yep. I know how fat/muscle affects buoyancy.

I asked if jbriggs would just sit there at at six feet depth and would be forced to swim to reach the surface. Weigh training aside, he has not yet said that he do not float to the surface, given a few moments of inactivity.

If he is doubling down on an unqualified "we are neutrally buoyant." I'm going to have to ask for a citation.

 

[Ibix]

There is no buoyancy in zero g, certainly. The buoyant force depends on the weight of water displaced, which is zero.

 

[DaveC426913]

There is no buoyancy in zero g, certainly. The buoyant force depends on the weight of water displaced, which is zero.

Yes, that's the point.

I originally asserted that one of the problems with finding yourself in zero g in a pool of water is that you are suddenly not buoyant - the tendency for you to float to the surface is absent - which can be quite disconcerting.

jbriggs is asserting that "we are neutrally buoyant" (and therefore, ostensibly, the risk in zero is no greater). I challenge the assertion.

 

[jbriggs444]

Yep. I know how fat/muscle affects buoyancy.

I asked if jbriggs would just sit there at at six feet depth and would be forced to swim to reach the surface. Weigh training aside, he has not yet said that he do not float to the surface, given a few moments of inactivity.

If he is doubling down on an unqualified "we are neutrally buoyant." I'm going to have to ask for a citation.

In zero g, a lead weight is neutrally buoyant. As is a helium balloon.

Personally, in the neighborhood pool, I can exhale to be negatively buoyant and inhale to be positively buoyant. So I count myself as close enough to neutrally buoyant for practical purposes.

If I am underwater in the pool and want to breathe, I will either swim for the surface or push off the bottom. No way would I expect to just sit there and have my head naturally pop up so that I could reach the air. (Wearing a life vest works well for that, but Jennifer Lawrence was not so equipped).

 

[jbriggs444]

Are you telling me that if you jumped in the lake and sunk to, say, six feet, you would just sit there until you swam your way to the surface?

Yes. I've sat on the bottom of the pool before.

 

[DaveC426913]

In zero g, a lead weight is neutrally buoyant. As is a helium balloon.

Yes. We all agree.

Personally, in the neighborhood pool, I can exhale to be negatively buoyant and inhale to be positively buoyant. So I count myself as close enough to neutrally buoyant for practical purposes.

If I am underwater in the pool and want to breathe, I will either swim for the surface or push off the bottom. No way would I expect to just sit there and have my head naturally pop up so that I could reach the air. (Wearing a life vest works well for that, but Jennifer Lawrence was not so equipped).

OK, so some people are neutrally buoyant.

Now that I think about it, my list of dangers only included water-based effects.

With the decel of the hab module, she will experience disorientation/vertigo, further complicating her ability to both find a direction to swim and to keep swimming in that direction.

 

[JT Smith]

I posted this in a different thread, some data from an experiment where people were weighed in air and also submerged in water. For the males in the experiement the average weight in air was 82kg. Submerged in water with lungs filled with air the average weight was -0.84kg. With air expelled the average was +3.5kg. So, like jbriggs444, these men would on average be able to sit on the floor of a pool once they'd exhaled fully.

Women have more body fat. The average underwater weights for the women in this experiment were lower than the mens', proportional to their in-air average weight (68, -1.5, +1.9kg). These women had an average "healthy" BMI (23). Presumably there is a BMI or maybe body fat percentage above which one is buoyant underwater even with emptied lungs.