Laminar Flow
Why Physics Stack Exchange recommends it
Physics Stack Exchange users cite it as a vivid demonstration that low-Reynolds-number (Stokes) flow is reversible, in answers about mixing, entropy and diffusion.
Recommended by 1 people across 1 different months, 2013 to 2017.
What it is
A 2-minute demonstration filmed at the University of New Mexico physics department showing laminar flow in a highly viscous fluid. Dye stirred into the fluid by a cranking apparatus can be unmixed by turning the crank backward. The device was built by John DeMoss and Kevin Cahill.
Cited in 2 Wikipedia articles, including Stokes flow, Laminar flow.
What people said
Recommends CuriousOne · 2016-07 · Physics Stack Exchange · on “What happen when you stir a cup of coffe?”
An experiment is worth a thousand words: [this video] Laminar flow in a highly viscous liquid can be "unmixed". If you wait for a while, the machine couldn't do it, anymore because the diffusion would have been irreversible. Slightly more theoretical: [LINK] (Hamiltonian), which formalizes the notion that a phase space volume element can be smeared our over the entire phase space, even if its volume stays the same. This, however, requires Hamiltonian dynamics, which diffusion does not obey.
Cites hackpert · 2013-07 · Physics Stack Exchange · on “What is the reason behind Laminar Flow and the ability to return to near-original state in Laminar fluids?”
Laminar flow here refers to the smooth disruption of fluid in layers, in a medium. A peculiar trait in a few examples of laminar flow is the ability to return to near-original state, as demonstrated at [this video] . Why is this not considered as a violation for the laws of thermodynamics, for it appears to be a reversal of entropy. What is the (deepest possible) reason behind such flow and restructuring, and how is it different from turbulent flows?
Cites CR Drost · 2015-09 · Physics Stack Exchange · on “Is blowing bubbles a decent means to stir a liquid?”
I'd recommend testing with dye, indeed, but the usual wisdom is that a fluid near its boundaries moves approximately as those boundaries move: so, for example, if you're modeling a big parking lot during a hurricane, the wisdom says that the proper boundary condition for the wind speed on the pavement is to set it to zero, and let the simulation form its own "boundary layer" and whatever. Following that strategy, your bubbles will indeed drag these "boundary layers" of fluid with them and create a convective transport upwards. It's a lot more explanation to get to this, but whether the transport "mixes" the fluid or not is not a given simply due to transport. ( Here is a YouTube video [VIDEO
Cites tpg2114 · 2017-03 · Physics Stack Exchange · on “Why is Stokes flow reversible?”
And if you want an example of "real life Stokes flow," here is something that shows it is, in fact, reversible: [this video] Of course, only to an approximation -- there is molecular diffusivity, and many other things that you can't see with your eyes but actually happened. But from a model perspective, it was completely reversed.