Amusing paper about Reynolds number

[Keith Lofstrom]

The Reynolds number of an object is vaguely the ratio
of inertia to viscous drag, scaled to a dimensionless
number. A huge Reynolds number means that coasting to
a stop occurs over many body lengths.

Space launch engineers must maximize Reynolds number.
As a professional microcircuit designer, I don't use
Reynolds number to design circuits. As a computation-
optimized electromagnetic launch tinkerer and a space
system survivalist, I do.

The Reynolds number for a large ocean ship is around
5e9, for a human swimmer is 4e6, for blood flow in the
brain is 1e2, and for a ciliate bacterium as low as 1e-6.
Among the largest Reynolds numbers occurring in nature
are depth-to-surface convection processes in stars.

I found a readable and amusing 1977 paper, explaining
Reynolds number entertainingly and intuitively enough
that even a transistor jockey like me understands it:

Life at low Reynolds number E. M. Purcell, American
Journal of Physics, Vol. 45, No 1, 1977 January

https://www.damtp.cam.ac.uk/user/gold/pdfs/purcell.pdf

The paper brushes past heavy engineering math with some
memorable examples and examples from nature; for those
pondering nanotechnology, nature's nuts and bolts (and
flagella and motors) are instructive and amusing.

Example: a flagellum-powered microbe moves ~30 μm/s, with
a Reynolds number of around 3e-5. When the flagellum
stops turning, the microbe slows to a stop in about 500
nanoseconds, over a distance of 10 picometers. Optimal
behavior for a tiny brainless object "searching" for
slightly higher concentrations of consumable molecules.

Thinking about this stuff helps me find odd natural
phenomena that I can plagiarize and call "inventions".

Keith L.

--
Keith Lofstrom kei...@keithl.com