Abrasive flow finishing

Michael Baxter MBaxter at compuserve.com
Mon Jan 12 05:58:40 GMT 1998


Sorry for the late reply. It bounced the 1st time I posted it and dashed
back to the line.

Chris Conlon <synchris at ricochet.net> writes:

>> Perhaps there's a difference between drag on an object in free
space (planes, golf balls) 
and mass flow through a tube.
<<

>> Supposedly when you have laminar flow over a surface the flow speed
right at the surface 
is near 0, increasing (as distance squared) to whatever max value "far"
from the surface. I 
*think* that this nice, smooth flow over the surface also provides
maximum drag
<<

>> The bumps cause turbulent flow, which results in little pockets of
air where the 
"sandpaper" isn't touching you... and thus less drag.
<<

The least skin friction happens when the flow is laminar. The most when it
is turbulent. The 
dimples on a golf ball delay boundary layer separation further back along
the surface and you
have to take into account the golf ball is rotating making the
dimples dynamically random.
Boundary layer separation is the point of transition between laminar flow
and turbulent flow.
The more laminar flow over an airfoil, the less the drag.

>> Someone with a better understanding of aerodynamics please feel free
to correct and 
clarify this!
<<

At least this is what I've been taught. The FAA recently changed
some percentages in the 
"official" airfoil lift theory after god only knows how many years of
pounding the old 
parameters into aviator's brains. Aerodynamics is still black magic, IMO.
All I really know
is if I rotate to 7 degrees nose-up, it flies and goes where I point it
within reason :-)



bmccord at whittman-hart.com writes:

>> Every thing I studied was for water. So it my be a little different
for air.
<<

Air is a fluid and obeys all the laws of fluid dynamics.


Terry Martin <terry_martin at mindlink.bc.ca> writes:

>> Not less "sticky", which implies adhesion, but a micro laminar flow on
the dimple or bump,
which causes less turbulence in the main flow. In other words the reduction
of turbulence by
introducing a bump, exceeds the drag of the bump itself.
<<

I agree. I believe the random dimples must be reducing the size/number
of eddies. It's a 
great idea and I hope it works for the automotive world both under the
hood and on the sheet 
metal. 

I'm not sure we'll be seeing random dimples showing up on aircraft wings
like golf balls. We 
rely on turbulent air flow (boundry layer separation occurs between
25-33% back depending on 
the shape of the airfoil) for control effectiveness. Because of this, we
won't see mechanics
grinning from ear to ear with a ball peen hammer in one hand :-).

When the controls surfaces don't have enough area during initial
flight testing, the 
engineers add little metal tabs, called vortex generators, at random
angles. Technically 
correcting a design deficiency. If random dimples show-up on aircraft
wings, it'll be on new
designs after much testing in a wind tunnel.


 -- Michael Baxter, MBaxter at Compuserve.com
 http://ourworld.compuserve.com/homepages/MBaxter
 From Reno, NV USA on 11-Jan-1998



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