Simulating Gravity In a Fluid

[Patroklos]

I have been contemplating the idea of aquatic species and their likelihood to develop technology, including eventual space flight. Specific to this question, I am speaking of aquatic species that are generally suspended, not in general contact with the ocean floor (those species would just use normal rotation sections similar to us, just with water as the atmosphere). Reading through discussions online regarding assumptions on what such a species requires, there seems to be a general notion that an aquatic animal doesn't require gravity and that this represents a significant advantage in developing space flight. One of the few relative to a terrestrial species (similarly, raising aquatic life for food by real world human travelers is easy and can similarly be accomplished in micro gravity is often an idea encountered).

Recent experiments on the ISS disprove that. Even though buoyancy and other factors may change an aquatic organisms perception and experience of gravity, fish still suffer bone deterioration in micro-gravity just as humans do. More species specific to some aquatic species, some adaptations like swim bladders require gravity to work correctly.

So given that aquatic species need gravity, how do you simulate it for their circumstances?

I was watching The Expanse recently because it has some good visualizations of artificial gravity. Especially relevant is the depiction of birds essentially being near weightless when flying over people and objects experiencing simulated gravity. In this situation the bird is not flapping its wings so much to stay elevated (unless it just recently took off), it is doing so to catch up with the objects rotating away from it. Am I missing something?

So relevant to artificial gravity in a fluid, and an individual suspended in it, I understand things thus:

1.) In a ring habitat, rotated to simulate Earth gravity, if it is filled with air and the surfaces of the walls and floor and ceiling are perfectly smooth a suspended object would feel effectively no simulated gravity.

2.) In the same habitat, but with a more realistic array of equipment and other friction causing irregularities with the habitat surfaces in line with what you would expect with a spacecraft, there will be some spin forces imparted to the air which will simulate a fraction of the surface simulated gravity to a suspended object.

2a.) The character of the surface elements imparting acceleration to the air will drastically vary simulated gravity inside the fluid medium. If the habitat is a ring subdivided by walls, essentially all the air from floor to ceiling is getting some acceleration via pushing and the effect to the suspended object will be stronger, perhaps perceptibly uniform, between the surfaces and an air suspended object (I am not talking about Coriolis effects, just the actual magnitude of acceleration)

2b.) If the habitat is a cylinder( so no ceiling), and if the objects on the floor and the wall are not so large that there exists a significant void space in the center, the simulated gravity forces present in the air closer to the surface will fall off precipitously as you move cloer to the center axis of the cylinder to the point that if the radius of the void space within the cylinder is big enough simulated gravity falls off to near zero as you approach the axis of rotation.

3.) Applying this to a water filled habitat, essentially all of this is the same except due to the density of water over air, the acceleration effects are more effectively transmitted away from any acceleration imparting surfaces or objects, meaning stronger simulated gravity relative to distance from the surface than in air. In a cylinder, you could presumably have a significant void radius where there is no weightless zone unless you were perfectly balanced around the axis of rotation.

Question: If the habitat is not being actively rotated, ie its just accelerated to provide surface and then left to spin, wouldn't the friction with the fluid (air or water) eventually slow it to a stop? Can there never be equilibrium because while the solid habitat structure won't encounter friction in a vacuum as long as its suitably rigid, the fluid interior atmosphere will forever be in friction with the interior surfaces as long as they are moving. Like the difference between spinning a hard boiled or raw egg. So an atmosphere filled habitat using rotation for artificial gravity will always need some sentimental spin force? Or will an equilibrium eventually be achieved?

So after all of that, assuming I am correct, the chief difference to an aquatic species over a terrestrial one regarding gravity and spaceflight is not that they can do without simulating gravity at all, but rather that once gravity is simulated they are more adopt at utilizing a three dimensional environment without wasting void spaces, just like in any other situation. To me this means more usable space per volume enclosed, so a better habitat mass/crew ratio.

[Elheru Aran]

*Disclaimer: Not an expert, nor even a scholar, this is just off the cuff here*

As for the fluid-filled space habitat thing... wouldn't the fluid inside it be moving *with* the habitat? Without gravity affecting it externally, only its internal gravity generated by rotation, it should be moving simultaneously with the rest of the habitat around it. Even if it's subdivided by airlocks or whatever, that won't be an issue. The only reason it would stop moving is if there was a sudden deceleration of the habitat; then inertia would do its thing, and the fluid would start sloshing around. Otherwise, it should remain static, artificially induced currents for freshness aside.

Consider the classic demonstration of centrifugal force-- putting water in a small bucket on the end of a rope and then spinning it around quickly, even vertically. The water won't start sloshing around unless your centrifugal force starts slowing down too much, it'll more or less stay in one place. Same thing, I think.

Now where there might be friction is if the habitat is being rotated around some kind of central, static hub. Then that's where you would see any deceleration from friction, but the change in inertia should be extremely minimal if the friction is being managed well. If the entire thing rotates, then I would expect no issues in the gravity ring itself since it should keep rotating till kingdom come or they stop it rotating by using rockets or whatever.

[Esquire]

I'd be more interested in the psychological advantages of a water-dwelling species for space travel - obviously removing water pressure from the environment would mess with the physiologies of our spacefaring fishpeople, but I imagine they'd still be better mentally-suited to it, on average, than a human who wasn't 99th-percentile-in-literally-everything and also massively trained beforehand. We have yet to see anything resembling an 'average' human in space, as far as I'm aware. Spin gravity works just as well if the spin section is full of water, in any case.

[SpottedKitty]

I was watching The Expanse recently because it has some good visualizations of artificial gravity. Especially relevant is the depiction of birds essentially being near weightless when flying over people and objects experiencing simulated gravity. In this situation the bird is not flapping its wings so much to stay elevated (unless it just recently took off), it is doing so to catch up with the objects rotating away from it. Am I missing something?

My WAG is partly that, partly the bird having learned that it still needs to flap now and then to overcome a small amount of air friction.

It really is a very unintuitive situation. I'm reminded of a scene in Babylon 5, which had a railcar system running along its length close to (but not quite at) the station axis, so the spin gravity is very low but not quite zero. Our Hero jumps out of a car just before a bomb in the next seat explodes; but because the gravity is artificial, not real, he isn't falling. He's moving through the few hundred yards of air between the car-shattering kaboom behind him and the rotating inner surface, with only the speed he kicked off from the car. He's not in danger from the "fall", but he will get the great-grandaddy of all road rashes when he gets side-swatted by the ground (stated in-episode to be trundling along at about 60mph). So he's being hit by a truck instead of falling off a tall skyscraper.

Spoiler: he gets saved by a literal deus ex machina.

FWIW, I'd say the effects of using water instead of air would be very similar, except the overriding force on moving objects would be friction instead of centripetal/Coriolis forces.

[Sea Skimmer]

I'd be more interested in the psychological advantages of a water-dwelling species for space travel - obviously removing water pressure from the environment would mess with the physiologies of our spacefaring fishpeople, but I imagine they'd still be better mentally-suited to it, on average, than a human who wasn't 99th-percentile-in-literally-everything and also massively trained beforehand. We have yet to see anything resembling an 'average' human in space, as far as I'm aware. Spin gravity works just as well if the spin section is full of water, in any case.

I don't see why water dwelling species would have any advantages mentally. Not as a rule at least. Species which are largely stationary in the water might have an advantage, but those which normally swim around fair bit would probably feel much more limited and cramped then humans do confined inside a spacecraft. Water species in real life also tend to have very strong senses of smell, which might create it's own problems concerning lack of natural smell stimuli vs smelling a bunch of industrial coatings.

As far as water pressure goes, nothing would stop you from having pressurized water inside a spacecraft, however it would increase the hull mass. But the mass issue is already horrendously worse for a water species anyway. A 3x3x6ft block of water, a space a human can comfortably stand in, would already weigh around 3,370lb, which is just about 1.5 metric tonnes.

The equipment to pump and purify the water will also consume more mass and electrical power, and while the water itself may have some advantages for temperature control they of course mean you need complete electrical isolation of all systems at all times....which is a bloody nightmare. Operations and maintenance would be complicated to say the least. Any kind of noise, from for example engines, will also be much harder to mitigate though how much this matters depends on what kind of ship or space station we are talking about. This is a non trivial issue in it's own right, and one that isn't just about comfort but actual physical safety if we start talking about noise sources from high voltage switch gear or large engine systems.

[Sky Captain]

I think it is unlikely a water based species would develop advanced technology. For example species living in water will have trouble using or even discovering fire and fire is essential in a path to anything more advanced than sharpened sticks and rocks. Unless those species are partially aquatic like frogs there will be very hard to overcome barriers in a path to advanced technology. If the planet is water world then it is even more problematic because there is no land surface to base advanced industry required to develop space flight.

[Elheru Aran]

I think it is unlikely a water based species would develop advanced technology. For example species living in water will have trouble using or even discovering fire and fire is essential in a path to anything more advanced than sharpened sticks and rocks. Unless those species are partially aquatic like frogs there will be very hard to overcome barriers in a path to advanced technology. If the planet is water world then it is even more problematic because there is no land surface to base advanced industry required to develop space flight.

I want to say I saw a concept one time which discussed in lieu of fire, an aquatic species using underwater lava as their source of high heat...

[madd0ct0r]

So, if you fill a large bowl with water and soap for bubbles and place it on a lazy susan and start turning it...

1) liquid has zero shear strength so a film of water along the inside will get dragged along with the turning wall. The bubbles a little off the wall wont be spinning, at least initially. You are right that if you stop inputting enegy at this point the wall will slow down until the whole mass is rotating at the same uniform rate.

2)in a small diameter if you keep inputting energy to keep the wall turning at same rate the mass will gradually speed up until it matches the wall rotation
3) in a large one where some critcal ratio of enrgy input to diameter to viscosity is passed shit gets weird. You get counter rotations forming, like orbital gears turing against the layer that is being dragged along by the wall. Then you get a counter counter rotation in the center, and horrific slip stream currents between the gears. Things that"d tear a hyper intellectual jellyfish in half. One real world example is Saturn's hexagonal polar storm. A variant of this effect occurs in wide rivers. Peak stream speed is normally 0.1 depth below center of surface.
4) as a varaiant of 3) you get unstable gear sets that grow, fly out of line and whirl off in a dissapation of turbulent energy.

The extra mass and viscosity of a liquid means certain effects are more important then in air cylinders

[Ziggy Stardust]

Basically any time there is a density differential in a large enough closed container of fluid, you will start to see some weird effects. There's a reason fluid dynamics often uses modelling techniques derived from chaos theory to explain such behavior.

[Sky Captain]

I think it is unlikely a water based species would develop advanced technology. For example species living in water will have trouble using or even discovering fire and fire is essential in a path to anything more advanced than sharpened sticks and rocks. Unless those species are partially aquatic like frogs there will be very hard to overcome barriers in a path to advanced technology. If the planet is water world then it is even more problematic because there is no land surface to base advanced industry required to develop space flight.

I want to say I saw a concept one time which discussed in lieu of fire, an aquatic species using underwater lava as their source of high heat...

Maybe, but anything involving hot stuff will be extremely difficult in water. Water is just so damn good at taking heat away. Now imagine you want to forge some tool out of iron meteorite you found on seafloor nearby active underwater volcano. Stick it into lava (that also cools rapidly possibly generating deadly steam explosions in the process) then pull out and it will cool back down in seconds before useful work can be done.

Setting up any kind of smelting operation underwater with Stone Age tools seems nearly impossible.

[Esquire]

<snip>

Apologies - I'd been imagining there might be a psychological advantage in being more used to thinking and operating in three-dimensional spaces, but on further reflection I think that's just a brainbug I picked up from somewhere. I can't believe the water weight issue didn't occur to me, it really should have.

[Sea Skimmer]

Apologies - I'd been imagining there might be a psychological advantage in being more used to thinking and operating in three-dimensional spaces, but on further reflection I think that's just a brainbug I picked up from somewhere. I can't believe the water weight issue didn't occur to me, it really should have.

I'm still waiting for that day when we take a bird into space. But the water 3D thinking thing is a common brainbug, found among other places in the TNG tech manual which claimed the ship was carrying gene spliced killer whales and dolphins to work in an astrometrics lab.

[Sea Skimmer]

Maybe, but anything involving hot stuff will be extremely difficult in water. Water is just so damn good at taking heat away. Now imagine you want to forge some tool out of iron meteorite you found on seafloor nearby active underwater volcano. Stick it into lava (that also cools rapidly possibly generating deadly steam explosions in the process) then pull out and it will cool back down in seconds before useful work can be done.

The problem with using any kind of volcanic heat source is this is a highly contaminated heat source. Literally it's blowing out sulphur and other gross impurities that will make physical voids in forged metal and castings besides leading to hopeless weakness of the microstructure. Impurities are basically THE difference between modern steel and old timey stuff. It's also why you use charcoal or coke and not wood or coal to conduct metalworking. The later types of fuel are too impure to be effective. Magma is way worse. Throw some metal into magma and you might as well forget about it being usable.

Setting up any kind of smelting operation underwater with Stone Age tools seems nearly impossible.

Seems to me it's far worse then that, because you don't even have wood to make a stick to poke stuff with.

The clear compromise solution is walking catfish metal working. All the swim you need, but the ability to do the metal working on sandbars or something. Throw in some mangrove swamps for a wood source and a place to build early walking catfish fortresses.

Then write a story about how burning down the mangroves to build spearpoints in a war against crabs also happened to allow a tsunami to wipe out the protohumans of the world and cede total control to the catfish empire.

[Esquire]

You know what? I'd pay good money for that story.

[Patroklos]

*Disclaimer: Not an expert, nor even a scholar, this is just off the cuff here*

As for the fluid-filled space habitat thing... wouldn't the fluid inside it be moving *with* the habitat? Without gravity affecting it externally, only its internal gravity generated by rotation, it should be moving simultaneously with the rest of the habitat around it. Even if it's subdivided by airlocks or whatever, that won't be an issue. The only reason it would stop moving is if there was a sudden deceleration of the habitat; then inertia would do its thing, and the fluid would start sloshing around. Otherwise, it should remain static, artificially induced currents for freshness aside.

Consider the classic demonstration of centrifugal force-- putting water in a small bucket on the end of a rope and then spinning it around quickly, even vertically. The water won't start sloshing around unless your centrifugal force starts slowing down too much, it'll more or less stay in one place. Same thing, I think.

Now where there might be friction is if the habitat is being rotated around some kind of central, static hub. Then that's where you would see any deceleration from friction, but the change in inertia should be extremely minimal if the friction is being managed well. If the entire thing rotates, then I would expect no issues in the gravity ring itself since it should keep rotating till kingdom come or they stop it rotating by using rockets or whatever.

I don't think it works the way you describe. If all the mass in the habitat is stationary relative to each other, and you impart spin onto the hapitat ring or cylinder, you have to transfer that to the fluid, air or water, inside it otherwise the habitat will just rotate around it. In our bucket example the reason the water doesn't sill out is because there sides of the bucket are pushing it perpandicular to the radius of the circle its rotating around, This would be analogous to bulkheads inside a habitat ring.

So in a situation where force is being transfered from the rotating habitat by something less than barriers completely traversing the volume, possibly just friction between the fluid and the habitats surfaces, will that be enough to accelerate the entire fluid to a meaningful degree?

I don't see why water dwelling species would have any advantages mentally. Not as a rule at least. Species which are largely stationary in the water might have an advantage, but those which normally swim around fair bit would probably feel much more limited and cramped then humans do confined inside a spacecraft. Water species in real life also tend to have very strong senses of smell, which might create it's own problems concerning lack of natural smell stimuli vs smelling a bunch of industrial coatings.

I think this is a good observation regarding some species like dolphins or whales that live in the water column (pay attention David Brin), but for something lat lives in a reef environment this should be a problem. A reef fish, shrimp, or octopi essentially lives in a 3D honeycomb, spending their lives drating in and out of cramped crevices and chambers within living and dead coral formations often with no regard to up and down orrientations. Its a very analogous situation to a cramped shipboard environment.

As for scents and whatnot, yeah, but I assume they would have the same issues in their terrestrial industrial environments as well.

As far as water pressure goes, nothing would stop you from having pressurized water inside a spacecraft, however it would increase the hull mass. But the mass issue is already horrendously worse for a water species anyway. A 3x3x6ft block of water, a space a human can comfortably stand in, would already weigh around 3,370lb, which is just about 1.5 metric tonnes.

Unless we are talking about a deep dwelling species the pressure isn't ridiculously increased. You have to go down to 33ft to double the pressure over sea level. Given how much sea life lives above that, I don't think the increased pressure needed would be that much of a penalty regarding pressure vessel strengthening.

On the mass of the water, I have been playing with the idea that essentially the habitat module is just another propellant tank. Water is extremely useful in space, and with electrolysis your habitat model becomes a less efficient expansion of your hydrogen propellent storage. But it does allow for duel purposing, which is a good way to achieve mass savings. This gets a bit more complicated in practice because we are not talking about pure water or even fresh water, all those impurities would have to be considered.

Also a positive is that a water filled habitat is its own radiation shield, water being somewhat good medium for that. Real world designs often use water tanks for this purpose, but such a setup would have no need for dedicated water tanks (another mass win). Being able to duel purpose your breathing atmosphere as radiation protection is a giant mass win. I imagine that such a species would arrange their habitat so the least used equipment or stations are on the edge, with the most used like food prep/sleeping quarters/command stations in the center so that average shielding thickness is a certain value, it being monitored closely for dosage over time (btw I am not saying this is necessarily ALL your radiation shielding...).

Another note is that if this is an aquatic species that doesn't actually breath the air, like our dolphins, you don't need a solid water column. Rather, just enough humidity or misting to keep the animal moist.

The equipment to pump and purify the water will also consume more mass and electrical power, and while the water itself may have some advantages for temperature control they of course mean you need complete electrical isolation of all systems at all times....which is a bloody nightmare. Operations and maintenance would be complicated to say the least. Any kind of noise, from for example engines, will also be much harder to mitigate though how much this matters depends on what kind of ship or space station we are talking about. This is a non trivial issue in it's own right, and one that isn't just about comfort but actual physical safety if we start talking about noise sources from high voltage switch gear or large engine systems.

Yeah, I consider these all hurdles that will have to be tackled for any industrial application, so not unique to space flight. Its a big topic so I was trying to cut this down to "This species is achieving space flight, so whatever hurdles needed to get there have been solved. Now that its there, what are the unique ADDITIONAL challenges an aquatic species face moving forward."

I am not minimizing what you are saying here, just that I was attempting to restrict the scope a bit.

So, if you fill a large bowl with water and soap for bubbles and place it on a lazy susan and start turning it...

1) liquid has zero shear strength so a film of water along the inside will get dragged along with the turning wall. The bubbles a little off the wall wont be spinning, at least initially. You are right that if you stop inputting enegy at this point the wall will slow down until the whole mass is rotating at the same uniform rate.

2)in a small diameter if you keep inputting energy to keep the wall turning at same rate the mass will gradually speed up until it matches the wall rotation
3) in a large one where some critcal ratio of enrgy input to diameter to viscosity is passed shit gets weird. You get counter rotations forming, like orbital gears turing against the layer that is being dragged along by the wall. Then you get a counter counter rotation in the center, and horrific slip stream currents between the gears. Things that"d tear a hyper intellectual jellyfish in half. One real world example is Saturn's hexagonal polar storm. A variant of this effect occurs in wide rivers. Peak stream speed is normally 0.1 depth below center of surface.
4) as a varaiant of 3) you get unstable gear sets that grow, fly out of line and whirl off in a dissapation of turbulent energy.

The extra mass and viscosity of a liquid means certain effects are more important then in air cylinders

I figured this was the sort of stuff I was missing. That the lack of a uniform transfer of forces via structure would lead to wacky layers and currents. Do you have any thoughts on how to counter this to get a uniform gravity simulation? Baffles perhaps? Its sounds like these effects make the maintenance of a large open area of water untenable.

I was also thinking much more energy would be needed to maintain the system. In an air based rotating ring, its not actually the case that you can accelerate it and forget about it. The whole concept works by accelerating objects into the rigid structure, using friction to simulate gravity in the fluid medium. Since there is no shear strength in a fluid, and the habitat is accelearing the fluid into itself vice along, the atmosphere is constantly slowing the system down. This is probably negligible for air, but for the increased mass of a liquid it certainly won't be. Again, I am thinking of spinning a hard boiled vice a raw egg. It might not actually work that way in space.

[madd0ct0r]

Spin a raw egg for long enough and itll keep spinning.

Same for the water mass. Like you noted, baffles inside the structure work fine for causing it all to move as effective solid wheel.

[Patroklos]

So the fact that the water mass is constantly being accelerated into the outer shell, tangential to the radius, vice coincident with the curvature of the diameter like something bolted onto or otherwise held to it via friction (person standing on it), won't eventually slow it down?

That's what I am having a hard time getting my head around. As you said fluid has no shear strength. Its going to be accelerated in a straight line away from whatever is pushing it if the entire volume is accelerated uniformly (if not all sorts of craziness happens until this is the case), so anything being accelerated near the outer edge should be causing drag even if it is at the same velocity as the rotating structure unless it achieves some sort of current perfectly conforming to the outer surface. Baffles would seem to make this problem worse in this regard as eventually all of the fluid accelerated in straight lines will hit something at a none 90 degree angle.

I guess to make things simply I get how you can accelerate something solid in a non linear fashion if there is no resistance via a wheel. I am not seeing how this can happen with a fluid with no structure to dictate how force is transferred throughout it.

[SpottedKitty]

So the fact that the water mass is constantly being accelerated into the outer shell, tangential to the radius, vice coincident with the curvature of the diameter like something bolted onto or otherwise held to it via friction (person standing on it), won't eventually slow it down?

Any forces at any given point on the outer shell would be counteracted by forces on the opposite point of the outer shell; net effect zero.

Also, don't forget friction. Unless you're dealing with a supercooled superfluid e.g. liquid helium, friction and turbulence will eventually cause a transfer of angular momentum until the liquid and shell are all rotating more or less together. This will happen faster if there are baffles.

[Sea Skimmer]

I guess to make things simply I get how you can accelerate something solid in a non linear fashion if there is no resistance via a wheel. I am not seeing how this can happen with a fluid with no structure to dictate how force is transferred throughout it.

You need vanes or other suitable structure to make the fluid spin. We already had to figure that out to make artillery shells with liquid payloads like mustard gas work. Without internal vanes the payloads wouldn't spin and the shells wobbled around inaccurately. This is of course a very high rotational situational, but the physics are the same as anything else.

This will also mean the fluid through it's friction and slight compression will exert a braking effect on the spinning metal structure, somewhat like a gyroscope. This will mean additional thrust is needed to keep the RPM of the spinning habitat stable, air would have some kind of braking effect too but it'd be slight in comparison.

[Patroklos]

That is exactly how I understand it to work regarding the braking.

[Feil]

Angular momentum is conserved. There is no magical braking effect that a fluid exerts. A body with a given angular momentum keeps its angular momentum, whether it is compressible or incompressible, rigid or flexible, solid or fluid. Once the fluid is spun up, the fluid-solid system will stay spun up until acted on by an outside force. Any loss of angular momentum is exclusively due to external forces such as gravity, solar wind, or another object with a different angular velocity doing work on it.

It is in this latter case that the two will tend to equalize their angular velocity over time, while strictly conserving their total combined angular momentum. This is the effect described in the case of the artillery shell, in which the shell has one angular velocity (defined presumably by the linear velocity of the shell and the rifling of the barrel) and the fluid has a different angular velocity (increasing over time according to its viscosity and the geometry of the shell's interior).

If you lined up a bunch of trucks (or submarines) inside a spinning cylinder and had them accelerate spinward, you would of course spin down the cylinder. Angular momentum is conserved, and you're giving the trucks/subs more, which means that the rest of the system has to have less. But when they lost enough weight to lose control and had a giant pileup until they all came to a stop relative to the cylinder... as long as the pileup didn't change the location of the cylinder's center of mass... the cylinder will go back to its original angular velocity again at the end when the trucks come to a stop, in spite of all the work being done. It'll just be warmer than it was to start with. Angular momentum is conserved.

You can observe the behavior of a spun-up fluid for yourself by watching a video of someone spinning it up in a flat-sided container. When the fluid equilibriates, its surface will form a parabola. At any point on the parabola*, the slope of the normal line equals the force of gravity divided by the force of 'artificial gravity' at that point in the fluid.

*Except at the interface with the container, where surface tension may dominate.