Hey Al,

I think there's likely separation as illustrated in the first picture.

I think Paul is right about the wedge diffuser, but I don't think you have
to move the cooler to the cowl.  Maybe have a look at the 2nd pic...

What do you think?


Matt-

Al,
I vote for any form of scoop (ala P-51) that is moved far enough off the
surface to get out of the boundary layer. Ingesting boundary layer air is
never efficient regardless of how cosmetic it is. As an alternative, you
oversize the heck out of the cooler and get away with passing less air over
it.
Larry

No doubt that you are right about that.  this configuration began the way it
did because another Velocity builder had put his standard aircraft oil
cooler (for a Lyc) in the same location, and said it worked great without a
scoop ? just the differential pressure above and below the wing was enough.
Go figure.
Actually in the location that it is, even an inch or less away from the
surface would make a big difference.  Keep in mind that the scoop is about
23? wide with about 1 ¼? opening.  How do you get BL diversion with that
configuration?
------------
Matt wrote:
How can we apply instrumentation in the duct so that we can tell where the
flow is becoming turbulent?  Or we can just continue to guess...  Since this
duct is not exactly like any in the books, we can't directly copy any
solution that's out there..  I believe Al's data is valid (it is what it
is), but a few more key data points could help make the picture clearer.
Hopefully we can talk Al into continue playing along.
-------------
I'll look at getting a bit more info.
Meanwhile; I'm looking toward the idea of a vane as what might be a
relatively easy fix.  I think that my scoop opening is large enough, and the
BL thin enough, that if I can get effective diffusion in the duct it should
work just fine.

Note the attached diagram.

Al

A turning vane is worth a try. They usually don't work all that great as
they have form and skin friction drag even if they are not made out of sheet metal
and are perfectly streamlined.

BTW that is an old  diagram WW I era with an obsolete diffuser shape guaranteed
to separate along the duct walls. Folks don't use that shape.

This plot from Hoerner sums up the situation fairly well.
Note again "w" is going to be a function of the actual airspeed flowing through the
core. This can be measured with a pito tube pointing at the aft face of the core.
----------------------------------------------------------------------------
This last plot deserves a bit of explanation since there is no factor
for boundary layer effects. Here they are.

What Hoerner is calling overflowing is now known as external diffusion.
It happens when the rad is not porous enough and can trigger the main flow
to flow around the opening and not in it.

Paul Lamar ...No rotor no motor.

The Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site. http://www.rotaryeng.net
Copyright 1998-2006 All world wide rights reserved.