[Edited by author with list of benefits added at end]
Like stirring water in a kids pool, the rotational energy is stored.

The foldable fluid flywheel, is a circular pipe, with a hydraulic motor (pump) which could cause the fluid to flow in the pipe, or, when needed could be used to restore
the energy from the turning fluid.
Perhaps the pump should be on an external pipe, so that when not needed the flow is continuous.

This FluidFlywheel could use fuel as its fluid, e.g. in a helicopter or in cars, so practically no extra weight is needed. In bicycles, it could be filled with water, and rigged up when riding, around the wheels, but when done, could be emptied and maybe even folded away. In case of bicycle wheels and the likes, it could even be used without a pump, just 1 or maybe several valves, that close (or let the fluid only forward) during storage, open when not used, and close again when restoring energy from the wheel. In helicopters it could be pumped into the rotor circumference and back out, according to the need, and would allow for folding wings. In the case of a helicopter, it does not even have to consist of a wheel, but rather fluid weights, sent out to storage at the tips of the propeller wings.

Safety valves can be installed, so that if the energy gets too high all you get is a spray and loss of fluid, rather than the heavy casing needed for solid flywheels against explosion.

Details:
Flywheels store kynetic energy "in the rotation".
The drawbacks of a flywheel are:

The bigger the mass (the more it weighs) the better.
The larger it is, the better
Wheels usually (not all wheels) catch up the space from center to the circumference.
Flywheels can explode from overload, so they need a heavy encasing.

For many uses, people want a lightweight solution, and would want it not to catch too much space.

Benefits
1.Folding, 2.Weight as needed. hence 3. Boats can use more or less of it at will. 4.If fuel is used, the flywheel will not take up extra weight. (Helicopter,air-craft etc) 5. does not need center so space around vehicle "walls" can be used. 6. Safer - valves control fluid, hence also 7. Lighter, because no heavy safety encasement is needed.

---

This idea has NOTHING to do with the Water-Runner which comes up when you google for 'water flywheel'.

(-) Friction. Flywheels don't need mass, they need SPEED. Energy goes with mass times the square of the velocity. Modern flywheels are lightweight and CRAZY fast. Carbon fiber disks in vacuum chambers with magnetic bearings all to get the best power to weight. You are going in the exactly wrong direction to create something that will stop itself in minutes.

//hmm. How is it better? I think whenever we propose a new solution we should face squarely with this question.//

While in a technical or journalistic setting this question is perfectly valid, I think it is not the right kind of question for the half bakery. In this place it is distinctly possible, if not even expected that the proposed Idea will in fact not be better and in some ways the worse it is the better the halfbaked idea can be. I think the pertinent question is how is this more INTERESTING for the application desribed.

For this idea in particular i think [pashute] has missed a big oportunity to create somthing really fun and interesting, like say a lazy river fluid flywheel, which would entertain during the day while generating power at night, or say using the wastewater from tall buildings to create a toroidal flow that could be used to turn a generator or something. As presented there is nothing really interesting here.

Thanks [QED]. a. What happens to fluids that are pumped at fantastic speed. Why cant that be accomplished? (I suppose giving the water a twist would help). b.What about the option to fill the "weights" at the circumference with fluid. You would still have a "high speed" solid like flywheel, but could still empty it when not used. Could be good for boats...
c. Your right about the valves with fuel, but then the valves could release the fuel back into the fuel tank, and not out into the air.

And [WcW]: Its better: 1.Folding, 2.Weight as needed. hence 3. Boats can use more or less of it at will. 4.If fuel is used, the flywheel will not take up extra weight. (Helicopter,air-craft etc) 5. does not need center so space around vehicle "walls" can be used. 6. Safer - valves control fluid, hence also 7. Lighter, because no heavy safety encasement is needed.

Helicopters, airplanes and boats are not applications where there is a lot of waste energy could benefit from mechanical storage. As for bicycles the added weight never seems to be worthwhile. Lets remember that even with a fluid mass*speed=energy. If it is lightweight then it must turn fast. Fast speed=equally high centrifugal force and high friction even if the torus stays still.

The idea of a flywheel that can be moved easily but which can be filled with a fluid on-site is kind of cool.

I'm not an expert, but you're talking about some seriously strong materials there. For bigger flywheels, you may need some unobtainium. And if you had that, you could just spin it faster.

//What happens to fluids that are pumped at fantastic speed. Why cant that be accomplished?// It can, but at great cost due to frictional contact of the fluid with the walls of the container. From your example of a kid’s pool, try it and tell me how long the water continues to spin. The answer is in minutes and the energy to get it to spin is not tiny.

//What about the option to fill the "weights" at the circumference with fluid// It would "work". Certainly better than having the fluid circulates inside the pipes, because in general bearings have low friction, but you are missing the main point, what you need is SPEED not mass. A 1lb flat titanium disk, in a vacuum chamber with magnetic bearing can store thousands of times more energy than a 10lb circular pipe spun on normal bearings with rotating seals for filling and purging.

E=I*w^2

Let's say you have 100 times the inertia, I can guarantee you mine can spin 1000 times faster and that 1000 gets squared. The flat disk will also store that energy with very little loss, magnetic bearings aren't perfect and neither is the vacuum we can assume, but yours will be a nightmare to balance as the weight can shift and destabilize the whole system with any disturbance.

Think of it like this, which you would rather have hit you, a 100lb weight going 1mph or a 1lb weight going 100mph. The first will annoy you, the second will annihilate you, because E=m*v^2. Case 1 = 100*1^2 = 100. Case 2 = 1*100^2 = 10000

[Pashute], Interesting idea, but fluid friction will kill this immediately. Friction with the pipe walls, and friction in the fluid itself.

In a liquid, stresses are proportional to the rate of shear. Ergo, as long as you have wall friction, you will have shear. And as long as you have shearing in your fluid, you will have mondo friction. If you eliminate wall friction, your shearing stops and you have a good flywheel. You will never eliminate wall friction for any real liquid. Liquids are rather good at dispersing energy. Think of it this way. Get a pan with 1l <1kg> of water, and a 1kg disc on bearings. Spin them both to a roughly equal speed <swirl the water until it's moving fairly laminarly for optimuim resuls>. Which one comes to rest first? You won't have even noticed any reduction in the speed of the flywheel <disc> by the time all organised motion of the water has ceased. Once again, the entropy monster has come out to play.

It's simple fact that tricks can be done with solid flywheel systems such as vacuum, magnetic bearings, etc that reduce friction even further. There don't seem to be any equivalent modifications possible for your liquid flywheel.

I'd suggest reading up on fluid dynamics, especially the sections regarding pipe friction, shear/velocity profiles, and also laminar versus turbulent flow.

Pipe friction is a bear. A flywheel based on flowing liquid is going to die fast. We discussed that once before, here, anyhow.

A portable or temporary flywheel, with a normal rotating hub, that is ballasted at the rim with discrete containers of non-flowing water, on the other hand, would work, even if the density is less than other wheels (if you could find a use for it). But that's obvious enough, and not what the first part of the idea's about.

//I wonder if liquid helium could work.
Doesn't it have low friction? // A
flywheel filled with sufficiently cold (and
therefore superfluid) liquid helium
would be interesting. Below a certain
speed, the fluid would remain stationary
as the toroidal container turned. Above
a certain speed, the fluid would rotate,
but not necessarily in step with the
container.

Superfluids exhibit a phenomenon
which is basically a macroscopic
manifestation of quantum behaviour,
and is known to physicists (particularly
those who work in the field of quantum
hydrodynamics) as "bloody odd".

High speed flow: Are all of you sure there's no way to have the fluid reach very high speed flow? Perhaps some way to get the fluid "in sync" with movement similar to the Vacuum Vortex Tube (heater/cooler), which was found to work by waves of movement at the ultrasound level, would help the fluid go fast and with less friction. Of course this is so with the Quantum Vortex in Superfluids, but I'm asking about water. So my question is: what happens at extreme high speeds of flow in a pipe. Has this been tried?

I didn't find any information about that in books or sites about hydrodynamics. They only deal with low speed flow. I'm asking what happens under accelerated conditions?

[pashute], I can only relate to a similar case of gas in a pipe (injection of oxygen using a 1 1/4 inch pipe into liquid steel), and the limit there seems to be the speed of sound in that media. At that speed the flow basically chokes off, and more pressure doesn't give more flow.

I don't understand why helicopters and aircraft need flywheels. ...shouldn't they use just regular airplane and helicopter wheel;s? In fact, I didn't even know flys had wheels in the first place....I thought they had legs.

In a constant velocity system a liquid flywheel would operate as you are thinking it would, everything spining at a constant speed is everything happy. The problem comes in when you attempt to extract or impart energy onto the flywheel, when this happens the liquid will begin to shear and friction will take over and begin to damp the system. In a completely open circumperantial tube the water would literally contnue to spin inside the tube as you attempted to slow the wheel down(extract power), obviously adding baffles would help with this but then you begin to add weight to the system which was one of the reasons you proposed the idea, additionally if you fail to completely fill any cavities you will experience surging(ask anyone who's ever driven a smooth bore tanker truck such as a milk truck about surge.

The only application in which I percieve a benefit here was the aforementioned transportation of said flywheel to a remote location and then filling on site, the use of a honeycomb or open cell foam system to eliminate shear would also help with this. I guess the hard part is figuring out what on earth you might need a flywheel for that would benifit from this feature.

//In a liquid, stresses are proportional to the rate of shear.//

Say you have a conventional water pressure tank with a head of compressed air at the top. Would there be significant energy loss to shear forces if the water was blown out of a small aperture to spin a pelton turbine?

Where does this energy "go"? Does it heat the liquid? make noise? Flow out of the system into his noodly appendage? I've never understood this.

If I recall Shear and viscosity are closely linked, the more viscous a liquid the greater the amount of shear that it will experience. Also I believe that different effects apply to differnt situations IE discharge through a nozzle as opposed to travel in a pipe. I know that Pipe diameter and rate of flow are related and interdependent as different sizes of pipes may have different allowable rates of flow without creating cavitation or turbulence a smaller pipe may be able to more efficiently flow that the equivalent larger pipe for a given set of conditions.

Certainly one of the effects of shear is the generation of heat, this is particularly noticeable in automatic transmissions which require a cooler to maintain their operating temperature and carry away excess heat due to shear in the converter. As to discharge from a nozzle the design of the nozzle is critical to the flow it will allow as a nozzle than allows a laminar/smooth flow will flow more than one which creates a turbulent flow even though the cross section of the throat is the same. This is visible if you view the water coming out of the end of a hose, as the flow increases the flow will appear smooth but as it increases further the flow will begin to scatter and spray , eventually you reach a maximum flow rate for a given nozzle at which point increasing the pressure does not increase the flow, this occurs when the shear forces become sufficiently high that they create back pressure in opposition to the flow

I'm just going to drop in something where I have no clue what I'm talking about. (I mean, even more so than usual.) Assume for a moment that you have some fluid which could undergo a phase change between regular fluid and superfluid. Put it into something toroidal-ish with vanes on the inside, and while in the regular fluid state, spin it up, imparting a lot of momentum to the fluid. Then wave magic wand / chill / do whatever to put the fluid into its superfluid state. It should still have all that angular momentum, but with no way to get it back out. Then, later, reverse the effect to harvest the energy. Are there any fluids that would be candidates? Am I completely off what I think I understand as a superfluid?

//Say you have a conventional water pressure tank with a head of compressed air at the top. Would there be significant energy loss to shear forces if the water was blown out of a small aperture to spin a pelton turbine?Where does this energy "go"? //

I just wrote, and chose to delete, a rather long discourse on shear in liquids, pipe and flat surface flow, etc. I don't think you want me handing out lessons, so I deleted it.

First point - in terms of shearing, for a "normal" ie newtonian fluid, the pressure in the pressure tank is only important in terms of how fast the fluid flows out the nozzle - and the formation of cavitation voids. We're getting our concepts mixed up a bit here.

Anyhoo, I assume (without seeing any numbers) that we'll be dealing with turbulent flow out of a nozzle. So you're starting with potential energy in terms of pressurised air and probably some fluid head height as well. With nozzle flow, you'll get a mix of coherent velocity (what I gather you're after), and wasted energy in the form of heat (noncoherent velocity).

Google Bernoulli's equation, which is a good starting point for working out what velocities you can expect. A simple fudge factor for "nozzle efficiency" is good enough for all but the most exacting of situations.

Anyhoo, to answer your question - yes there will be significant energy loss, and you'll see it mostly as heat, and probably a bit of divergent flow from the nozzle.

Just so's we're on the same page here - a bit of food for thought. In terms of energy conversion 1000m head height can be converted into 141m/s flow, or a 2.5 degree change in temperature with no losses.

In other words you can get enormous losses to heat, but because of the high specific heat of water you won't see huge changes in water temperature. So you can get huge losses from say a nozzle or a pump or whatever - and you simply will not realise where the energy is going.

I worked at a power station at one point that used huge axial multistage boiler feed pumps. They're nasty buggers. If you dead-head them they pump so much energy into the water that they flash it to steam inside the casing and literally rip themselves apart. messy.

//So you can get huge losses from say a
nozzle or a pump or whatever - and you
simply will not realise where the energy is
going.//

It's a shame that physics has to get in the
way of so many of my good ideas. Thanks
for the explanation, though. Good stuff.
(And I would liked to have read the longer
lesson).

There seems to a some confusion here about rotational dyanmics. Even though this story is old, I thought I'd post some things here, just for future reference.

The formula for kinetic energy was given as mv^2, then corrected to (1/2)mv^2. The formula for *linear* kinetic energy is indeed k = (1/2)mv^2, but that isn't what is going on here, unless in addition to spinning, your flywheel has some translational motion, i.e. moving from point A to point B, as if in a moving vehicle.

For flywheels that aren't translating, we need to use the *rotational* formulas. Rotational kinetic energy, k, is equal to (1/2)Iw^2, where I is the rotational inertia, and w (using w instead of a greek omega) is the rotational velocity, e.g. 46 rpm. Tangential velocity, V_t, equals wR, so w = (V_t/R).

For the case of a flywheel that is a solid, uniform, flat disc, I = (1/2)MR^2. If we substitute for I, and eliminate w in the rotational kinetic energy equation, we get k = (1/4)MV_t^2). So for a given tangential velocity, this spinning flywheel will actually only have 1/2 the kinetic energy as computed if you had incorrectly used the linear equation, a 100% error!

By using a strong vacuum and magnetic bearings we can reduce kinetic friction to low enough levels that we can neglect friction's impact in these simple approximations.

Also, it was mentioned that carbon fiber is used to get the best "power to weight" ratio. I doubt that is the reason, as increased mass *increases* the kinetic energy that could be stored. The reason to use carbon fiber is due to centripetal acceleration, i.e. acceleration towards the center of the circle.

In uniform circular motion, like a rock on the end of a string being swung over your head at a constant angular velocity (i.e. constant rev. per minute), there are tangential and centripetal components to the rock's velocity and acceleration. We can suppose the centripetal velocity is zero in our ideal case. Then the tangential velocity always has the same speed (*uniform* circular motion), but the direction it points changes with time. If you let go of the string, this velocity vector is the direction the rock would fly.

In order for this velocity direction to change, we need a centripetal acceleration, i.e. an acceleration towards the center of the circle. Centripetal acceleration equals the square of the tangetial velocity over the radius, a_c = (v_t^2/R). So, as the tangential velocity increases, the acceleration increases, and as the radius increases, the acceleration decreases.

To provide this centripetal acceleration, we need a force acting towards the center of the circle. Mr. Newton asserts that force equals mass times acceleration, F = ma, so the centripetal force equals the mass times the centripetal acceleration, F_c = ma_c. For a given mass, as the acceleration increases, the force exerted increases. This force is provided by the tension in the string, so T = ma_c.

What this means is that a material with a high tensile strength to weight ratio can handle a large tension without failing, which means a large acceleration, which means a large tangential velocity, which means a large amount of kinetic energy can be stored. Likewise, a material with a low tensile strength wouldn't be to store much kinetic energy.

So the ideal uniform disk flywheel is made of a material with a high tensile strength to weight ratio, like some carbon fibers, spins as fast as possible without explosive disintegration (hopefully with some safety factor!), and has as large a radius as possible to minimize the centripetal acceleration.

That may be more physics than you guys want, but I hope it helps.