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The fish is not merely holding position--it's actually accelerating upstream! Reminds me of this Tadashi video: [How do fish swim so quickly?] https://www.yout

The fish is not merely holding position--it's actually accelerating upstream! Reminds me of this Tadashi video: [How do fish swim so quickly?] https://www.youtube.com/watch?v=wYDh5d9pfu8 https://www.youtube.com/watch?v=wYDh5d9pfu8 Tadashi shows that fish turn drag into thrust by swapping the positions of the vortices they shed. Low effort, high output. TFA takes this one step further--fish bodies are shaped to effortlessly harvest thrust from drag. No effort, free energy!

a3w 4y ago
TIL: The first law of thermodynamics has an exception for dead fish. (JK, don't cite me in physics class!)
- munificent 4y ago
  Well, actually... In this case, the fish isn't a closed system. It's harvesting energy from the moving water, which requires external energy from the sun to keep the water cycle going.
- bocytron 4y ago
  Reminds me of Derek's video about a vehicle which is powered against wind: https://youtube.com/watch?v=jyQwgBAaBag https://youtube.com/watch?v=jyQwgBAaBag
  - Maursault 4y ago
    His claim about a sailboat not being able to exceed windspeed sailing directly downwind is deceiving because it is definitely not the fastest point of sail. I believe this is done for dramatic effect to make Blackbird's feat sound more incredible. Sailing close hauled, almost directly into the wind, can achieve double wind speeds near effortlessly on low draft racing multihulls, and possibly on land with whatever those sailing carts are called. Of course, directly downwind is the only course for this experimental propellor craft, thus the only criteria.
    - tobinfricke 4y ago
      But that's the entire point of Blackbird: to show that it is possible to sail directly downwind faster than the wind. It is not immediately obvious that this is possible.
HarHarVeryFunny 4y ago
Pretty counter intuitive that it takes less energy to swim upstream than to move forwards in still water! I wonder if live fish swimming upriver ever do this - just relax and enjoy the ride - rather than putting any effort into it ?
- robbedpeter 4y ago
  It depends on turbulence, though. If it's a wide uniform flow, there are no vortices to exploit, so the intuitive assumption holds. You need the vortices to work against, and certain configurations will have negative impact, so a fish has to pay attention enough to surge or turn when needed. Throw a dead fish in a turbulent river and it's 100% going to end up far downstream, even though it might take a few upstream jaunts on the way.

a3w 4y ago

TIL: The first law of thermodynamics has an exception for dead fish. (JK, don't cite me in physics class!)

munificent 4y ago
Well, actually... In this case, the fish isn't a closed system. It's harvesting energy from the moving water, which requires external energy from the sun to keep the water cycle going.
bocytron 4y ago
Reminds me of Derek's video about a vehicle which is powered against wind: https://youtube.com/watch?v=jyQwgBAaBag https://youtube.com/watch?v=jyQwgBAaBag
- Maursault 4y ago
  His claim about a sailboat not being able to exceed windspeed sailing directly downwind is deceiving because it is definitely not the fastest point of sail. I believe this is done for dramatic effect to make Blackbird's feat sound more incredible. Sailing close hauled, almost directly into the wind, can achieve double wind speeds near effortlessly on low draft racing multihulls, and possibly on land with whatever those sailing carts are called. Of course, directly downwind is the only course for this experimental propellor craft, thus the only criteria.
  - tobinfricke 4y ago
    But that's the entire point of Blackbird: to show that it is possible to sail directly downwind faster than the wind. It is not immediately obvious that this is possible.

HarHarVeryFunny 4y ago

Pretty counter intuitive that it takes less energy to swim upstream than to move forwards in still water! I wonder if live fish swimming upriver ever do this - just relax and enjoy the ride - rather than putting any effort into it ?

robbedpeter 4y ago
It depends on turbulence, though. If it's a wide uniform flow, there are no vortices to exploit, so the intuitive assumption holds. You need the vortices to work against, and certain configurations will have negative impact, so a fish has to pay attention enough to surge or turn when needed. Throw a dead fish in a turbulent river and it's 100% going to end up far downstream, even though it might take a few upstream jaunts on the way.