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Re: fluid mechanics question



On Sat, 2 May 1998, Teresa Hein wrote:

"When introducing a square block in a steady airstream, why is the
airstream more unstable immediately after passing over the block as
compared to the initial impact of confronting it?"

Might this simply be a matter of inertia? If something stirs the air,
then patterns of disturbance will continue, at least until viscosity damps
the motion. If the mental model involves a massless fluid, then
turbulence will seem confusing.

If the student instead asked: "if a MOVING block is within a volume of
UNMOVING air, why is the motion of the air different after the block has
passed through?", then the answer would be more obvious.

Is the student really asking "what causes turbulence?" This I'm not so
sure about, but I think it involves the growth of tiny vortices with time.
If a tiny vortex becomes larger and larger with time, then we would expect
to see no vortices at the leading edge of the block, and large vortices at
the trailing edge. It's nonlinear behavior, with effects similar to
"supersaturation" and "seed crystals", and chapters on turbulence in books
about chaotic dynamics. My mental trigger for this topic is:

Bigger whirls have lesser whirls which feed on their velocity
and lesser whirls have lesser still, and so on
...to viscocity.

I don't recall the author, but I'm certain it appeared in James Gleik's
popular book CHAOS. I imagine that whenever there is a large velocity
gradient, such as in the boundary layer near an object, then the energy
stored in the velocity gradient can power the growth of vortices, and so
the little poem above runs in reverse, with tiny "whirls" growing,
competing, consuming each other, etc., and leaving a turbulent wake behind
which eventually slows and stops, with warmer air the final result.

If the above demonstrates my confusion, I hope everyone here will feel
free to jump in and correct me!

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William J. Beaty SCIENCE HOBBYIST website
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