7. Low-Reynolds-Number Flows

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Recommends tpg2114 · 2012-12 · Physics Stack Exchange · on “Name of experiment”

I don't know if there is a formal name for it, but my favorite search engine likes to call it a Reverse Entropy Machine. The main fluid is glycerin and the dye is food coloring. You can see an example of the setup here [LINK] . You can also watch a video that describes it [this video] along with some lecture notes [LINK] where it is called Kinematic Reversibility. It works because the motion of the inner cylinder is relatively slow and the glycerin is very viscous so the flow is laminar. Molecular diffusion, an irreversible process, is negligible over these time scales (although if you let it sit a really really long time it would smear). So the entire process itself is reversible and the ink blo

Recommends tpg2114 · 2015-02 · Physics Stack Exchange · on “Dependence of streamlined flow on viscosity”

All of the NCFMF videos [LINK] are awesome, but here is one [this video] that explains and demonstrates the low-Re flow in an experiment! Also, I very highly recommend seeking out a copy of An Album of Fluid Motion [LINK] which MIT has a PDF scan of some parts [LINK] (luckily, the creeping flow and laminar flow is in it!)

Recommends Jokela · 2015-06 · Physics Stack Exchange · on “Interaction between body and fluid”

Can someone give an outline about this or give a reference where I can read more? Low reynolds number (Laminar) flows are reversible. There is a really good old Video about the issue: [this video] The most interersting stuff to your question is shown at 21:00 -> The precise answer at 27:00 -> Short, because the laminar flow is reversible the normal propusion through turbulence is simply not possible. The forces and Torgues inside are indeed zero. Thus the only way to create a viscous propulsion is to move in a way which overcomes this reversibility. All this is shown in this video very understandable way.

Recommends Jokela · 2016-04 · Physics Stack Exchange · on “Question about propellers”

Also this question shows you are about to ask your self through to the solution of turbulence. The answer here doesn't make the point. If you remain on low reynolds number, you would be able to do it with almost no losses. the video at 27:00 -> [this video]

Recommends Jokela · 2019-02 · Physics Stack Exchange · on “How to start moving in at $Re\ll1$”

You are right. Swimming is impossible if Re<1. The forward motion is neutralized by the backward motion. The only way to create propulsion is the screw. That's why the small swimmers has a long tail, they are rotating it, instead of flapping it back and forth. This all is explained well in this old National Committee for Fluid Mechanics video (at ~28:30 is the demonstration) [this video] This picture below is from the video notes [LINK] : [LINK]

Cites Deep · 2020-11 · Physics Stack Exchange · on “From where does irreversibility arise in the Navier-Stokes momentum equation?”

Which "reversibility" are you referring to? If you mean "thermodynamically reversible" (a flow which does not generate entropy) then viscous dissipation ( $\mu\nabla^2\mathbf{u}$ ) always ensures ir reversibility, whatever the Reynolds number. But perhaps you are referring to "kinematic reversibility", which implies reversal of flow in every detail upon reversal of external forces acting on it - that is, upon reversal of external forces, every fluid particle would retrace its trajectory backwards. Low Reynolds number flows, called "creeping flows", indeed display kinematic reversibility as Reynolds number ( $Re$ ) $\to 0$ (see G.I. Taylor's demo [this video] ). Here, the non-linear advection term