Today, a great Victorian engineer. The University
of Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
Have you ever noticed how,
when you walk on wet sand, it momentarily dries out
around each footprint? That odd behavior was
explained in 1885 by a British engineering
professor at the University of Manchester. He was
Osborne Reynolds, and he wanted to explain
nothing less than the very structure of the
In his later years, Reynolds tried to replace the
then-current ether theory with the idea that the
universe is filled with tiny granular particles;
hence the interest in sand. Well, we've forgotten
his granular theory, but not the rest of his work.
Reynolds came from a long line of Anglican clergy
in Northern Ireland. He was drawn into his father's
hobby of math and mechanics, and he was talented.
He was 25 when Manchester made him a professor. We
know him best for his work on the flow of fluids --
like water, air, oil or steam. His Reynolds
Number is a ratio that shows the effect of
viscosity in a flow. Compute it, and you'll know
the nature of the flow before you see it. The
Reynolds number for a BB sinking in honey is less
than one. For a reed in the breeze it might be a
hundred. For a golf ball in flight it's over
From those numbers you expect to see no hint
of irregular fluid motion as the BB creeps through
viscous honey. You expect the reed to sway
rhythmically as air peels off in vortices behind
it. The golf ball is dimpled because, at that
Reynold's Number, dimpling enhances turbulence. It
narrows the wake and increases the ball's travel by
Reynolds' genius gave us much more. He also worked
in electricity, magnetism, and astrophysics. But no
book on fluid flow fails to mention Reynolds' name
early and often. It was he who explained how an
oiled bearing lets a shaft ride on a cushion of oil
without touching metal. Reynolds first figured out
how to use mathematics to describe the flow of oil
between the shaft and the sleeve.
A large painting hangs in the engineering
building at Manchester. It shows an aging Reynolds
with something like a reserved smile playing about
his lips. In his lap he holds a shallow bowl filled
with marbles. It's a demonstration of his ideas
about the granular substrate of the cosmos. But his
gift to us lies elsewhere -- under the hood of your
car, in the motion of a supertanker, in the human
So what about the way wet sand dries out when you
walk on it? Reynolds realized that lapping waves
bring sand to an optimal packing with grains as
close together as they can get. Stepping on it
disturbs the packing and creates more empty space.
Water flows away from the surface to fill the
That behavior is important when we handle any kind
of particulate matter, but Reynolds wasn't thinking
about commerce. He simply had a mind that stayed in
constant motion -- a mind that quite literally saw
the world in a grain of sand.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
Kargon, R. H., Reynolds, Osborne., Dictionary of
Scientific Biography, Vol. XI, New York: Charles
Scribner's Sons, 1975, pp. 392-394.
I am grateful to listener Ben Mathes for urging me
to do a program on Reynolds.
To calculate the Reynolds Number, Re,
multiply the velocity of a fluid flow relative to a
body by a characteristic dimension of the body (a
width or a diameter) and divide by the kinematic
viscosity of the fluid.
The drag coefficient for a sphere as a function of
Reynolds Number, as given by
Schlichting, H. Boundary Layer Layer Theory.
4th ed., McGraw-hill Book Co., 1960.
Note added March 18, 2011. A relative of Osborne Reynolds,
Deborah Reynolds, writes to point out an error in my text
above. Reynolds came from a long line of clergy, but not
Irish clergy. His father was British, and only in Northern
Ireland for seven years when he was born there.
Reynolds original apparatus for demonstrating the
onset of turbulence
being operated by the author at the University of
Manchester in 1975.
Example of Reynolds dryout phenomenon on a beach.
(Photo by John Lienhard)
The Engines of Our Ingenuity is
Copyright © 1988-2000 by John H.