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Hi Al,
If a "full-strength" Streamline duct were tested under the conditions of 9.5"H20 at the entranced to the inlet then at the widest part of the duct you should theoretically measure 9.5*.84 = 7.98". I recall Tracy Crook getting 5.6" at 120 MPH in front of his core - certainly he had a different configuration, but just a data point. So your 3+" H20 is certainly a bit on the low side. So what could cause that? I see three possible causes:
1. Duct not properly shaped (but, based on your sketch it looks fine to me)
2. Exit area insufficient (you don't mention the ration of inlet to exit area) - but you do show some negative pressure in the rear which could mean there is possibly some restriction in air flow out of the duct exit. In other words, if you were looking at the pressure with no core in the duct then the exit pressure should be ambient. However, it being only - 1+", I don't think this is the major source of your problem
3. Boundary layer ingestion. With no standoff from fuselage for you inlet, it is possible (likely?) that a percentage of your air into the duct could be composed of the boundary layer. That the air pressure right before you duct inlet is 20% lower than the free stream potential indicates to me that the probe was at least partially into the boundary layer where the air flow has already slowed.
So finding what causes the 20% delta from airstream dynamic potential of 12" to 9.5" H20 might be significant.. Could this all be attributable to boundary layer effect?
Since there is no standoff from the fuselage with your inlet this means you are likely ingesting some boundary layer - how much and is it significant is, of course, the question. IF the inlet is , then with the boundary layer moving considerably slower than free air stream, and IF you are ingesting a considerable amount (as a percentage of your free air flow) then you would naturally not get as much pressure recovery in the duct as you might expect from all Free-flow entering the duct inlet..
My only point here is while it could be the sharp curvature of your duct (if you sketch is anywhere to scale then I don't think the curvature is too much) OR it could be you are not getting the airflow into the duct that you believe you are. So before I would start too much cutting and shaping of the duct I would give some thought to whether boundary layer ingestion might be the problem. As you know, it does matter how good your duct is, if you are not getting the airflow into it you require, you are not going to cool.
That's the only things I can think of that might explain both the lower inlet pressure and the low duct pressure - good luck on solving the problem.
Best Regards
Ed
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.dmack.net/mazda/index.html
you would as a result get less
----- Original Message ----- From: "Al Gietzen" <ALVentures@cox.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Friday, July 13, 2007 1:02 AM
Subject: [FlyRotary] FW: Oil cooler air flow
OK, all you aerodynamicists; here's the scoop - what I did and what I
measured;
(Pressures measured in inches of water.)
1 - sealed the strake/wing crack in front of exit fairing - - No effect
2 - Installed Gurney flap at back edge of exit fairing (see photo 1) - Very
little effect.
3 - Set up manometer to get some pressures. (photo 2)
4 - Dynamic pressure in front of scoop, ½" from surface (photo 3) -- 9.5" at
160 mph. Nothing wrong with that. Free stream away from surface would be a
bit over 12".
5 - Static pressure ½" from face of core - 3 ¼" (end of tube was pointing
slightly away from direction of flow at about 60 degrees to the flow; flow
velocity is pretty low)
6 - Static pressure just behind the core --- 0"
7 - Static pressure at behind exit fairing, ¾" from surface (photo 4) - ¾"
8 - Installed VGs about 2 1/2 ft in front he exit fairing - No noticeable
effect.
So .. . plenty of dynamic pressure in front of scoop and only 3 ¼" pressure
in front of the cooler core. This would suggest that the scoop/duct isn't
working.
Photo 5 is a reasonably accurate x-section of the scoop and duct. Shaping
was limited because duct entrance is right behind the spar, and by the
limited thickness of the wing root. I'm just guessing that the somewhat
abrupt curvature of the upper wall at the entrance is causing flow
separation, turbulence; and whatever.
You'll also note in photo 4 that I had cut the center section of the exit
fairing to allow exit further forward - that didn't help. I also did soap
drops on the surface which way air was going both in front of; on top of;
and behind the fairing. In all places the flow was relatively straight
back. I'm also guessing that there isn't much chance of developing a
greater negative pressure at the exit; but, hey, whatever works
All I need to know is a simple fix:-)
Al
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