
Demonstration of Reversibility of Laminar Flow [video] - ColinWright
https://www.youtube.com/watch?v=_dbnH-BBSNo
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OopsCriticality
G. I. Taylor did it better!

See around the 13 minute mark:
[https://www.youtube.com/watch?v=51-6QCJTAjU](https://www.youtube.com/watch?v=51-6QCJTAjU)

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VikingCoder
How well does this really work? It occurs to me that this could be used as a
form of steganography.

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sparky_z
Good luck transporting it without shaking it up. Even if you leave it in place
for your counterparty to find, I'd bet that diffusion would render your
"message" unreadable after just a few minutes.

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kyberias
Maybe one could freeze it. :)

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gus_massa
Better title: "Demonstration of Reversibility of Laminar Flow"

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semi-extrinsic
Nitpicking: it's not because the flow is laminar (which it is), but because
it's approximately a Stokes flow.

If you sit down and non-dimensionalize the Navier-Stokes equations, you end up
combining viscosity, density, a representative velocity and some
representative distance into what we call the Reynolds number (Re). There's
two smart places you can put it, and for very slow flows that's in front of
the nonlinear term. When Re << 1 you can neglect that term, and you get the
Stokes equation, which is time-reversible. If you can grok what the convective
derivative really means, this starts to make intuitive sense.

I said there's another place you can put the Reynolds number, and that's 1/Re
in front of the viscous term. So if you have an extremely fast flow, you can
neglect viscosity, which also makes intuitive sense. Such inviscid flows are
the gateway into understanding turbulence.

