4 ms·

I was trying to compare the energy required to overcome friction of the air against the tunnel to driving the route conventionally. 80 million is an excessive

by graeham 14y ago

I was trying to compare the energy required to overcome friction of the air against the tunnel to driving the route conventionally. 80 million is an excessive number to expect want to travel at one time, which implies that driving is much more efficient from an energy required perspective.

jacquesm 14y ago
Yes, I got that, but the fact that there isn't room in there for that many carriages means that the friction can't be that high. Unless there is a way in which the friction in a tube is so high that the little bit of air surrounding the vehicle would exert that much drag on the tunnel wall (so one vehicle length + say a few mm distance to the tunnel wall). I'm not sure what the figure should be but this seems very high.
- graeham 14y ago
  When fluids flow, there is a `no slip` condition at surfaces - that is the velocity of fluid at a surface is the same as the surface`s velocity. So in a stationary tunnel, the fluid may be moving at an average of 300 meters per second, but will be stationary at the wall. This makes a shear stress in the fluid and is what causes drag. In fact, making the air gap between the carraige and wall smaller will increase this drag since there is less distance for this transition to happen between stopped and fully moving air. Consider if each carriage is pulling 30m of air a long distance at great speed. Consider if instead of the air moving against a stationary tunnel, a 30 meter section of tunnel was moving at the same distance and speed through stationary air. The drag on this would be high, much higher than a car at highway speeds.
  - lazyjones 14y ago
    What if it isn't air, but an actual fluid, e.g. water? It cannot be compressed, so it's "crash proof" (carriages cannot hit other carriages with water in between). It also has plenty of mass, so once it has started moving, it will push carriages around easily. A torus with permanently moving water (probably a more high-tech fluid), where carriages are injected / removed after acceleration / before deceleration?
    - eru 14y ago
      You'd still have the friction between carriage and wall.
    - graeham 14y ago
      Both liquids and gasses are fluids and drag is worse in water than air, so the energy required would only be higher. Consider: is it harder to push your hand through the air or a tub of water?
      - lazyjones 14y ago
        > Consider: is it harder to push your hand through the air or a tub of water? Consider a tub of water moving at the desired speed...
- deleted 14y ago
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by graeham 14y ago

jacquesm 14y ago

Yes, I got that, but the fact that there isn't room in there for that many carriages means that the friction can't be that high. Unless there is a way in which the friction in a tube is so high that the little bit of air surrounding the vehicle would exert that much drag on the tunnel wall (so one vehicle length + say a few mm distance to the tunnel wall). I'm not sure what the figure should be but this seems very high.

graeham 14y ago
When fluids flow, there is a `no slip` condition at surfaces - that is the velocity of fluid at a surface is the same as the surface`s velocity. So in a stationary tunnel, the fluid may be moving at an average of 300 meters per second, but will be stationary at the wall. This makes a shear stress in the fluid and is what causes drag. In fact, making the air gap between the carraige and wall smaller will increase this drag since there is less distance for this transition to happen between stopped and fully moving air. Consider if each carriage is pulling 30m of air a long distance at great speed. Consider if instead of the air moving against a stationary tunnel, a 30 meter section of tunnel was moving at the same distance and speed through stationary air. The drag on this would be high, much higher than a car at highway speeds.
- lazyjones 14y ago
  What if it isn't air, but an actual fluid, e.g. water? It cannot be compressed, so it's "crash proof" (carriages cannot hit other carriages with water in between). It also has plenty of mass, so once it has started moving, it will push carriages around easily. A torus with permanently moving water (probably a more high-tech fluid), where carriages are injected / removed after acceleration / before deceleration?
  - eru 14y ago
    You'd still have the friction between carriage and wall.
  - graeham 14y ago
    Both liquids and gasses are fluids and drag is worse in water than air, so the energy required would only be higher. Consider: is it harder to push your hand through the air or a tub of water?
    - lazyjones 14y ago
      > Consider: is it harder to push your hand through the air or a tub of water? Consider a tub of water moving at the desired speed...
deleted 14y ago
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