Abrasive flow finishing

[William T Jenkins]

On Wed, 17 Dec 1997, Dan wrote:

> 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

   I had always read in aerodynamic texts that the cause of reduced drag
for a dimpled versus a smooth golf ball was due to the dimples keeping the
airflow attached to the ball a bit more than if it was perfectly smooth.
As far as its applications to airliners, if it proves significant, I would
think that it would certainly be something all of the big companies would
look into.  As for its use in intake manifolds, that is an interesting
idea.  It would be neat to see a flow comparison between a dimpled and
non-dimpled intake of the same basic dimensions.  But I keep looking at a
bird's wing, and it looks pretty smooth to me...