Abrasive flow finishing

[Dan]

I read an article recently in Design News concerning skin
friction drag reductions.  They quote another article in
Nature where skin friction drag was reduced by 13% on
aerodynamic surfaces by adding random small bumps to 
the surface (vs a smooth surface).  Design News was considering
the fuel reduction costs possible for typical commercial aircraft.
A 13% skin friction drag equates to about a 6.5% overall
drag reduction.  Big numbers when a 1% drag reduction saves
a typical commercial aircraft about $100,000 annually in fuel
costs.

The idea is similar to the dimples used on golf balls which
enable the golf balls to fly farther than undimpled
balls.  I wonder what this might mean for engine intakes and
exhausts?  Maybe an extrude honing type smooth surface is
not the ideal surface even when injectors are near the valves.

The dimples have to be in a random pattern, else you increase
drag.  The theory is that random dimples (or chevrons) reduce
bursting near an aerodynamic surface.  Busting is caused by
low speed air streaks near the wall and is believed to be
responsible for creating intense turbulence.

Comments?

Dan L