> OK, all you aerodynamicists; here?s the scoop ? what I did and what I
> measured;
>
> 1 ? sealed the strake/wing 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?
>
> 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
>
> Well two things Al. You are measuring dynamic pressure with the front
> hose. It is basically a pito tube but it says nothing about what
> is going into the duct. The back hose is not exit pressure.
> It is being affected by air currents passing over the open end
> of the tube. If you want to measure static pressure you need
> to hot glue porous foam on the end of the tubes. Ordinary kitchen
> foam works fine. Also drip irrigation tubing is smaller than what
> you have there and would cause less disturbance.
>
> The front foam pressure tap needs to be inside the duct in contact with
> the front face of the core. You should have four or five in there.
> the rear foam pressure tap should also be in the exit duct in contact
> with the aft face of the core. Again four or five.
>
> IMHO your test set up is faulty.
>
> I think your assessment of the duct shape is probably correct.
> I can see no simple fix but good pressure data in four spots
> on the front face of the core would help. You need more U tubes.
>
> You could spend a lot of time getting the duct shape just right with
> good data and still not get it to cool really well.
>
> I know this is a challenge for you and you have some
> creative individualism invested :)
>
> If it were mine I would suck it up and take the core out of the
> wing and place in in the cowl long ways and feed it with a Kays & London
> wedge shaped diffuser connected to a P38 type ram air scoop.
> Similar to what Perry Mick is doing. Last picture.
>
> Paul Lamar ...No rotor no motor.
>
>
> >
> > Oh; yeah. I also tried VGs about 2 1/2 ft in front of the exit
fairing
> --
> > no noticeable effect. I had previously tried smaller VGs on top of the
> > exit
> > fairing for no joy.
> >
> > Al
>
> 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-
>
> Brilliant Matt. I like it. Perhaps with some boundary layer energizers
> ahead of it. Why did I not think of that :)
---------------------------------------------------------
> > Here is a better way of dealing with the boundary layer Al.
> > This is from Hoerner Fluid Dynamic Drag.
> > --
> > Paul Lamar ...No rotor no motor.
Okay, let's all play nice now. Al is giving us an opportunity exercise
our noggins a bit, and it should be fun (and hopefully productive).
My take on the core layout which has the fins nearly aligned with the free
stream flow (as depicted in the HFD scan Paul sent out - divergent flow?)
is that it requires quite a bit of room in the longitudinal (fore-aft in
the
airframe) axis to make it work - otherwise the turns get so tight that
maintaining attached flow through the duct is very difficult. From
previous
discussions, my impression is that the critical dimensions are the aspect
ratio of the duct - how much the flow has to diverge (expand) and how much
room there is to do it.
Canting the core may allow a slightly hybrid approach between the wedge
diffuser and the divergent duct, and allow things to be more compact. I
can't visualize how at the moment though.. Placing the exit duct on the
top
may help a bit too.
Applying boundary layer/flow correcting devices is a band aid solution for
making a slightly cramped duct work. This may be the easiest method for
getting Al's cooler to work properly. Nothing wrong with that. But, each
band aide that is applied closes the gap between the performance of the
divergent duct and the wedge diffuser.
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.
Regards,
Matt-
I don't consider boundary layer control a band aide. I consider it
essential.
The Hoerner plot shows recirculation at the top of the core which is a
very
bad thing. Hot air goes around and around and nobody knows where it comes
out.
Kays & London, after spending 30 million of our tax money, discovered that
forcing the air to turn 90 degrees caused turbulences in the core which
enhanced
the heat transfer. The airplanes that are cooling the best right now
use the Kays & London wedge diffuser.
Paul Lamar
Okay.. Fair enough on the boundary layer control.. I suppose even the
vaunted P51 has a boundary layer control in that the scoop is mounted away
from the fuse bottom so as to not ingest any boundary layer...
Regards,
Matt-
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