<Line to sacrifice to the cut-off gods>
First off, this is on topic, we are talking flow through the TB here.
Secondly, the effect you are referring to is called the "boundary layer".
It happens in both fluids and gasses. The article was correct, a mildly rough
surface generates a slip flow so that fluid/air next to the surface moves more
slowly than the majority that passes by it, now on a "cushion" of fluid/air,
instead of the surface (avalanches and rockslides demonstrate this effect on a
large scale). Absolutely smooth surfaces only slow the air slightly which
*can* disrupt the total airflow and slow the total air flow down. Short
duration surface obstacles in the fluid/air flow are better off being smooth
than rough as they don't have a chance to develop the slip flow effect.
Many attempts have been made at controlling the boundary layer. You'll
notice on the SR-71 Blackbird (the fastest plane in the world) there's a
series of grooves in it's wings. This grooves actually "suck" the boundary
layer air into the plane (it goes to the engines if I'm not mistaken) so that
the other air can slide on by the boundary layer with no turbulence. The
Austrailian yacht that won the America's cup (the first time) from the US had
a 3M coating of grooved panels that channelled the water past the yacht
inducing laminar flow on the boundary layer. A neat trick and good for quite
a bit less friction, enough to make up for that new huge wing they had under
the water so the boat could attack the wind more intensely and not get blown
over. The first production attempt at killing the boundary layer was done on
the P-51 Mustang by grinding down it's rivets flat so the air wasn't disrupted
by them. This was also the first attempt at laminar flow around an airplane.
It's one of the reasons why the derned thing was so fast, even though the F-4U
Corsair had a more powerful engine: aerodynamics.
For our throttle bodies, I would think a smooth surface finished with some
1000 grit sandpaper honing lines parallel to the air flow would be the best we
could do. Anything else would require some serious machining or redesign of
the TB itself.
Shaun H.
---original message---
Date: Wed, 24 Feb 1999 14:01:15 -0700
From: Gary Shook <gary.shook@mci.com>
Subject: DML: aerodynamics (off topic), was: intake and exhaust aerodynamics
(long)
>Well, I remember reading in a science mag a coupla years ago about how
rough
>surfaces are aerodynamically more efficient than smooth surfaces. A coupla
>guys were experimenting with different surfaces and measuring liquid flow
>(velocity) through a pipe. The pipe with the smooth surface had low
>velocity because liquid running against the surface would shear and roll up
>(like rolling a carpet up) until it got so big it would break away from the
>surface and create turbulence, thus slowing overall velocity.
I couldn't help responding to this, reminded me about the Trans Alaska
Pipeline System (TAPS)... they discovered this very phenomenon was limiting
flow through the 4' diameter pipe that carries crude oil 800 miles through
Alaska... their solution was introduction of a long-chain polymer that they
call DRA (Drag Reduction Agent) which essentially makes the crude act like a
jello plug, moving through the pipe without turbulence... works very well,
put them over the 1.0M bbl per day mark... but it has to be re-injected
after each pump station... the massive turbine pumps destroy the long-chain
polymers, making them useless.
just a bit of trivial information, but I don't think it's real applicable
here, unless you could inject some DRA into your TB.. :-)
Gary Shook
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