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I agree, Thomas - if you are forced to truncate the
"streamline duct" you want to start at the core and go toward the
inlet. However, I decided that to see if pinching the duct before
the bell shape would work since I have such a short distance.
In fact, when I calculated out the opening by
simply extending the streamline duct coordinate out 3-6" from the core - the
opening was 60-75% (I forget exactly) the size of my core. I decided that
would up the cooling drag quite a bit - so decided to try the pinched shape
which restricts the size of the opening to near that of the streamline duct but
then tries to compensate for the short length by speeding up the boundary layer
for great penetration toward the core. Again, my cooling is fine and I can
top out at 200 mph in my draggy RV-6A, so I think the drag is down a
bit.
But, again that is just my theorizing about why I choose
to pinch the curve.
Ed
----- Original Message -----
Sent: Tuesday, February 27, 2007 1:52
PM
Subject: [FlyRotary] Re: Pinched ducts
was : [FlyRotary] Re: cowl openings for water radiators
Hmm,
if I remember right from reading what I have
from K&N - if you do not have the optimal lenght available you start to
cut it from the intake - take radiator and apply the optimum duct, measure
from the radiator towards the intake whatever distance you have available and
cut the duct there....
Dave,
are you a Navy-flyer or a medic?? :)
Thomas
----- Original Message -----
Sent: Tuesday, February 27, 2007 1:34
PM
Subject: [FlyRotary] Re: Pinched ducts
was : [FlyRotary] Re: cowl openings for water radiators
Hummm, Dave, perhaps my understanding of what it takes
to keep the boundary layer attached to the duct wall is
flawed.
From what I believe I understood regarding
airflow in a duct is that the pressure recovery both aids and hinders
the boundary layer's attachment to the duct
wall. The pressure build up (area of slower molecules) tend
to push and keep the boundary layer pushed against the wall of the
duct as it curves out - at the same time it is slowing down the boundary
layer. So its the point of separation is (at least in part)
contingent on how much speed the boundary layer has enabling it to push how
far into the pressure recovery area - before it ultimately separates.
The further the better is my understanding.
My understanding is that in a duct - it is
the recovery pressure which builds in the expansion area just
before the core. This "high" pressure area will "push"
back on the boundary layer causing it slow and eventually to separate
from the wall. . However, if you keep the boundary layer speed
up it pushes further into the pressure recovery area following the duct
curve before the "back pressure" slows it enough to cause it to
separate.
Also the speed of a molecule in all random
directions is much, much higher than the component imparted by the airspeed
- about 1100 ft/sec at sea level as I recall compared to about 40 ft/sec in
the duct. So my interpretation is that (at least in a duct) its the
back pressure of the recovered pressure that causes the separation - not
necessary the curve of the duct alone although that certainly contributes to
the pressure recovery. That being said,its clear that the three
factors (duct curve, expansion area and separation)
really go hand in hand. The greater the curve the more
pressure recovery occurs and the greater the tendency for
separation. The higher the velocity of the boundary layer the
further it can penetrate into the higher pressure area before being slowed
and separation occurs.
There is NO doubt that having a longer duct would
improve the situation. However given I only had 3 -6" my take
was that speeding up the air (and boundary layer energy) would ensure
it penetrated deeper into the bell shape before the pressure recovery caused
separation. But, as I have often stated - I could be completely wrong
about what I think I understand.
You are after-all the Navy flyer and I know
they cram a lot of areo into Navy pilot's heads. Me- I'm a electrical
engineer, so what I know about aerodynamics is what I have read (and
think I understand).
But, regardless these pinched ducts have provided
the best cooling with the smallest opening that I have achieved - so,
Dave, if you stay quite it may not learn the truth
{:>)
Ed
----- Original Message -----
Sent: Tuesday, February 27, 2007
11:46 AM
Subject: [FlyRotary] Re: Pinched
ducts was : [FlyRotary] Re: cowl openings for water radiators
Ed,
Good discussion about streamline ducts. No doubt that they are
superior although I have a slightly different take on what quality makes
them work best. I also agree that it is wall separation that we are
trying to avoid.
But IMHO the important way to get there is "avoiding sharp
turns." I think of the air molecules as little race cars
coming in the duct. The less turning they do, the better. If
they need to turn, the radius of the turn needs to be as large as
possible. And just as important, the turn radius is distributed so
that more of the turn is done after the air has started to slow down (near
the face of the radiator). In other words, the turn radius is a
function of speed. Just like with a car, don't turn it much before
slowing down or it will separate.
With that in mind, see why the "conventional duct" is so
terrible. There is a single sharp turn right at the end of the
straight-a-way. Separation occurs there and the whole plenum becomes
turbulent.
With the bell shaped duct (K&N), it is easy to see why we need
length. The longer the duct, the larger the turn radius can
be throughout the whole distance.
Given our limited space however, there will undoubtedly be a point
where the necessary turn radius becomes too small for the speed of the air
and it separates. But at least get the air to expand as much as
possible before that happens.
With your restriction in the neck you are setting yourself back
before you start the necessary expansion. You have created
less distance over which to average the turning radius, you have increased
the speed - meaning the air can tolerate less of a turning radius
before separating (lower velocities are known to maintain laminar flow
much better than high velocity), and you have increased the total amount
of expansion the streamlines need to undergo (narrower starting
point). So my guess is a rather dismal effect on cooling
compared to what you could have.
BUT, since your cooling is still adequate I am sure you have made a
very nice overall drag reduction. There is no way a conventional
duct with that amount of area would work well. In other works, while
I am very skeptical that the restriction actually helped cooling, big
kudos to you for absolutely minimizing drag and duct area while
maintaining sufficient cooling.
In fact, seeing as how you have proven that it works I am considering
doing that for my oil and intercooler ducts as they are currently getting
more air than they need...
JMHO
On 2/26/07, Ed
Anderson <eanderson@carolina.rr.com>
wrote:
Actually, there is, Joe. But, you are going
to be sorry you asked {:>).
I spent quite a hit of time studying a tome
(Kuchuman and Weber better know as K&W) on air cooling of
liquid cooled engines written back in the hey day of high speed mustangs
lightenings, spitfires, etc. Sort of the liquid cooling
bible. Chapter 12 (the one of most interest to us) showed a
duct that reportedly had the best pressure recovery (84% or thereabouts)
around for a subsonic duct that they had found. It was called a
"StreamLine Duct" (See attached graph - the graph a of the top graph
shows the shape of the duct (or at least 1/2 around the center line -
sorry for the poor quality).
After quite a bit of studying and thinking
about what I had read about cooling ducts, it finally became clear to me
that the perhaps top thing that is clearly detrimental to good cooling
is having flow separation in the duct. Most of the old
drawings of a cooling duct shape followed a sinusoidal shape - rapid
expansion right after the opening. It turns out that "traditional"
shape is probably one of the worst shapes for a cooling duct (the story
why is too long to get into here).
Anyhow, Flow separation leads to eddies and
turbulence which casts a "shadow" of turbulent air on the cooling
core. Like a shadow, the further away from the core the separation
occurs (like near the entrance of the duct) the larger the shadow it
casts on the core area. This "shadow" adversely
interferes with the flow of air through the core and reduces the
effectiveness of the core.
What causes this separation is that as
pressure is recovered by the expansion of the duct, the build up of the
very pressure recover we want - starts to hinder the boundary
layer flow near the wall of the duct. It slows it down and causes
it to lose energy and attachment to the duct wall. At a certain
point the flow separates and starts to tumble/rotate and depending where
(near the duct entrance or near the core) the flow separates, determines
how much of the core area is adversely affected. So if the
boundary layer's energy level (air speed of its molecules) is maintained
at a high level separation is less likely.
So ideally, you would like to prevent any
separation during pressure recovery. The Streamline Duct is the so
called "Trumpet" duct or "Bell" duct . After the opening, there is
a long section of non-expanding duct followed by a rapid expansion into
the "bell" shape just before the core. The long non-expanding part
of the duct maintains the energy (air flow) of the boundary layer and
separation does not occur until well into the "bell" shape
expansion.
In fact, it happens way up in the corner of
the bell/core interface and affects a very small area of the
core.
For full effectiveness the "Streamline duct" from
K&W needs a length of 12-17". Well, that's way more distance
than I had. So I got to thinking that if keeping the speed of the
air molecules near the duct wall helps prevent boundary layer separation
and the cooling killing eddy of turbulent air - what could I do
with my short 3 - 6" (no jokes you guys). We all know from
Bernoulli that if an area is squeezed down that the velocity of the air
flow increases - right?
So I decided to try to maintain or increase the
energy of the air by pitching down the neck just before it goes into the
bell shape expansions in hopes that the increased energy will help the
boundary layer stay adhered to the duct wall until well into the corner
of the bell shape. So that's the story of the pinched ducts.
There is no question in my mind that this is not as effective as if I
could have had the 16" to build the duct - but, in this hobby, you work
with what you've got - right?
Does it work? Who knows - but I seem to fly
with less opening area than most folks and have no cooling
problems. So that's my 0.02 on the topic - see told you, you would
regret asking {:>).
Ed
----- Original Message -----
Sent: Monday, February 26, 2007
8:53 PM
Subject: [FlyRotary] Re: cowl
openings for water radiators
Ed, is there some particular reason that
you necked the inlet down small, then enlarged it again.
Thankyou for the pictures. JohnD
----- Original Message -----
Sent: Monday, February 26, 2007
3:39 PM
Subject: [FlyRotary] Re: cowl
openings for water radiators
John, don't know if these photos will
help. But, like you I only have between 3 and 6" of duct
distance on the radiators. You should do Ok with 20 sq inch on
each opening with a good diffuser/duct. Attached are some
photos of my current ducts. The openings are 18 sq inches
each. I have had one opening down to as little as 10 square
inches - but that was a bit marginal - so opened it back up. I
have a generous exit area for the hot air including a larger 4" x
12" bottom opening as well as louvers on each side of the
cowl. So you mileage could vary - but Tracy has essentially
the same size opening as well as several others.
Ed
----- Original Message -----
Sent: Monday, February 26,
2007 12:12 PM
Subject: [FlyRotary] cowl
openings for water radiators
What size openings do I need for the
water radiators? The Wittman Tailwind cowl I have has
postal slots of 3' x 7 3/4" , which is approx. 22 1/4
sq in. on each side. Sam James for the 160 Lycoming is using
4 3/4' round holes which are 17.6 sq. inches on each side.
My radiators are quite close to the opening and I plan on making
the diffusers trumpet shaped, will the openings be large enough if
I can stay over 20 sq. inches on each side with a decent trumpet
shape. JohnD
hushpowere II on order - hope to start in 2 weeks if weather
cooperates.
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