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Hyperloop lets you travel on a resonant acoustic wave?

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Re: Hyperloop lets you travel on a resonant acoustic wave?

#81

Post by jessriedel »

andrewcooke wrote: post #75
jessriedel wrote: post #19

I can't figure out how the OP proposal differs from Charles Alexander's. Low frequency (long wavelength) waves travel around the tube at the speed of sound. By traveling at the same speed, and correctly choosing a point along the wavefront, one can take advantage of significantly reduced apparent drag. It's implicit in Alexander's suggestion that either (a) the total length of the loop is an integer multiple of the w…

it's slightly less wrong, as far as i can see - http://www.acooke.org/cute/HyperloopW0.html

I read your post, but I don't understand why the first link can't work. Is there a problem with a wave propagating along the tube with essentially infinite envelope? Here's a nice animation:

http://www.acoustics.org/press/151st/WaveVectorDemosmall.gif

In other words: decompose the standing wave into its forward and backward components and just keep the forward one. This is not the same thing as the air having a net velocity, so you don't bleed all your energy into friction with the walls. (You'll still lose some energy as the sound wave is naturally damped by friction due to the residual motion of the air molecules, but this might not be a large effect.)

Re: Hyperloop lets you travel on a resonant acoustic wave?

#82

Post by [deleted] »

speeq wrote: post #8

"Elon Musks final invention will be a time machine. It will only work once before exploding. Before entering it, Elon will turn around and tell us he has done his duty and given us many gifts, and now we must use those gifts to advance mankind. He hopes great things for us. We beg for him to stay and aid us further. He smiles sadly, He looks at us with his tired but happy eyes, shakes his head, and steps into the mac…

[deleted]

Re: Hyperloop lets you travel on a resonant acoustic wave?

#83

Post by pjscott »

enko wrote: post #79
pjscott wrote: post #38

Earlier quoted context omitted.

I refer you to Charles Alexander's idea, which goes through a bit of the math for why traveling with the peak of an intense, low-frequency sound wave would give freakishly low drag. http://charlesalexander2013.wordpress.com/2013/07/18/hyperlo...

Ah, I see. That's an interesting idea. Sounds impractical - if you're going to do all that engineering work, might as well just get rid of the air in the first place - but cool concept.

Do you know how difficult it is to build and maintain an evacuated tube of that size? Not all loopy tubes are created equal.

Re: Hyperloop lets you travel on a resonant acoustic wave?

#85

Post by phreeza »

niels_olson wrote: post #42

As a surfer, let me volunteer that it's a bit like a highway onramp: you want to match speed exactly when the front arrives. Tiny adjustments in speed, pitch, yaw, roll, and center of gravity make the difference between making it, falling off the back, and getting eaten. Mistiming your drop leads to bad outcomes that are directly proportional to the kinetic energy of the wave and are inversely proportional to the dep…

The difference between sound and water waves is that sound waves are longitudinal, ie the local displacement of the medium is parallel to the wave travel direction, while in water it is perpendicular. The wave thus does not really have a 'height', it would just fill the entire tube, and there wouldn't really be a way to miss it.

Re: Hyperloop lets you travel on a resonant acoustic wave?

#86

Post by enko »

pjscott wrote: post #83
enko wrote: post #79

Earlier quoted context omitted.

Ah, I see. That's an interesting idea. Sounds impractical - if you're going to do all that engineering work, might as well just get rid of the air in the first place - but cool concept.

Do you know how difficult it is to build and maintain an evacuated tube of that size? Not all loopy tubes are created equal.

As opposed to building this thing, filling it (perfectly) with ultra-high-power resonant soundwaves of the precise characteristics, and flinging in shuttles and hoping nothing goes wrong? I don't know, the evacuated maglev is sort of sounding like the safe, easy option?

The reason evacuated tubes are expensive and difficult is because of the tube, not because of the evacuation. Once you have a great big tube, reducing the air inside isn't going to be that big a deal. You don't even need to get rid of all the air, just maintaining 90% vacuum would be fine, you could whiz down there at 1000kph as easy as kiss my hand.

Anyway, I'm not a physicist, I am just wondering if people have a full understanding of the forces involved flinging stuff around at 1000kph+ in an enclosed space at full air density. It is some serious shit. People are comparing it to aeroplanes, but at sea level it would be 3x the shockwave a plane has to deal with. High speed trains in Europe and Japan already match or exceed the air-displacement issues faced by airlines - and that's one third the proposed speed of this device. I don't know, man. Sure, in ideal circumstances you might be able to engineer some low drag trick. Still sounds like unicycling along the edge of a cliff to me. Near mach 1 in a fully pressurised tunnel? Dear christ.

Re: Hyperloop lets you travel on a resonant acoustic wave?

#88

Post by radarsat1 »

> the tunnel would be opened up to the outside atmosphere. That would immediately ruin the resonance and all capsules should come to a screeching halt.

Correct me if I'm wrong, but my intuition is that this would also lead to quite a bad explosion. I mean, the amount of energy I imagine it would take to initiate a standing wave between LA and NY should be massive, (and certainly not a small amount to maintain it either, though far less than the initial amount if it truly stores the energy well). But if it is _storing_ this energy, then any rupture should release it quite... quickly. No?

A standing wave is not nothing.. it is a huge amount of pressure oscillating back and forth in an underground cavity. A controlled hole in the cavity wall could release it properly, but a crack in the side could effectively lead to rupture, couldn't it?

Another thing, if the air pressure inside the tunnel is indeed quite high, the train cars would also need to be pressurized. This leads to some other potential dangers.. not to mention the time to pressurize would be added to the total trip time.

Re: Hyperloop lets you travel on a resonant acoustic wave?

#89

Post by andyn »

moomin wrote: post #27

Not to be funny, but it seems like every other day has another "hyperloop uses acoustic waves" article. It's a good guess, but it doesn't even contain new speculation (another article went into detailed theorising about how cars would accelerate and decelerate). Seriously, until we have a solid info, or even a new guess, can we lay off Hyperloop?

It's easy to get excited, but I remember the hype surrounding the Segway.

Re: Hyperloop lets you travel on a resonant acoustic wave?

#90

Post by Tloewald »

If the solution involves a tunnel or tube from SF to LA then I fail to see how it can be as cheap as has been claimed. I suggest it's an accelerator that launches capsules ballistically, "skips" them to maintain velocity, and recaptures a good deal of energy on arrival.
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