The dimples on a golf ball debate loosened a memory or two.
Honda two or three years ago, produced a few motorcycles for F1 and
World Superbike competition that had the front fairings drilled with
lots of little holes - too small for significant airflow, but rather,
intended to give the dimpled-ball effect. The idea had been tested
extensively in the wind tunnels in Japan, and cost little for one-off
race bikes. Didn't make a significant difference in the real world.
Yamaha came up with a new engine about the same time that produced
about ten horsepower more than Honda's, and they smoked Honda. At
least, that's the way I remember it. I don't know whether the aero
effect was reduced in the turbulence and changing aspect angle of a
real race bike, but I don't think they've continued to do the drilling.
OTOH - the keel of the racing yachts in the America's cup all have
water-sized (larger, shallower, elongated) dimples to reduce drag.
I believe the dimples do rely on boundary-layer effects. It is not
always advantageous to carry that boundary layer with you though.
As far as I know, no high-performance supersonic jets have deliberately
roughened surfaces, in fact, I've heard that the flat gray paint
used on Navy jets these days actually costs several tens of knots in
top speed, but make sense for low visibility. The hydroplane race
boats are waxed before each day of racing. I would hazard a guess
that the dimples make sense at lower speeds, but lose most of their
benefit as fluid flow rate increases.
Anyone at the drags try back-to-back runs with a clean and dirty truck?
Jim
"Any government that is big enough to give to us everything we want, is big
enough to take from us everything we have!" -- President Gerald R. Ford,
speaking to the Republican National Convention
in San Diego, CA on Aug. 12, 1996.
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