Jeff Spitzer wrote:

Normally I wouldn't offer this little tidbit.  But because we are so
fascinated with diffusers and Paul's little plate trick here goes.  The
spool trick that Vance cited is an example not of "lift" but of a Radial
Diffuser.  When the cardboard is suspended above the spool, the static
pressure of the flow at the O.D. of the spool must be atmospheric.  Because
the area is a function of radius for two circular parallel flat plates the
area gets smaller as you move in.  Therefore, in the outflow direction this
device is a diffuser.  Therefore, the pressure inboard of the rim MUST be
lower than atmospheric...hence the suction.  There is also some positive
pressure at the center due to the 90 degree turn..hence the equilibirum.

BTW, this was the original theory behind the Supertrapp muffler.  A
diffuser that doesn't act as a sound megaphone.  In practice the Supertrapp
diffuser kinda sucks due to low cost manufacturing constraints.  Radial
diffusers can be extremely effective compact diffusers.  One such example
can be found on our Altus II 65,000 ft UAV where the tube from the turbo to
the intercooler practically reverses direction and effectively diffuses to
the size of the air/water intercooler in about the size of a shoebox.

Jeff Spitzer

Thanks Jeff for enlightening us on that. Very interesting.
A more every day example is the parrallel wall diffuser
in turbo chargers.

Here is an illustration from Advanced Engine Technology
by Heinz Heisler. Published by the SAE.

parrdiff.jpg

Paul Lamar
 
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