REFLECTOR:Vortex Generators on XL

Brian Michalk reflector@tvbf.org
Sun, 7 Dec 2003 11:33:39 -0600


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I have an excellent minds eye picture of what VG's do, and why they are
good.  So far, all of the technical discussion of VG's do not contradict how
I see them working in my mind.

I came up with my concept of VG's one day when I was a riding in an MD80 (I
think).  I was seated above the wing, and was examining the engine nacelle,
and wondered why they put a huge VG down there on the nacelle.  As luck had
it, that day was very humid, and the VG generated a visible vortex at a high
AOA.  Really cool, the vortex went over the wing, and kept airflow attached
to the huge flap that is very close to the fuselage.

Here's how I came to see VG's functioning.  First, look at the airfoil
without any aid to keep the airflow attached.  Somewhere above the airfoil,
the air comes detached, and tumbles around in this little area.  It became
detached because the air molecules wanted to go forward rather than getting
sucked down by the airfoil.  Kinda like cavitation, or very mild stall.
Like the large red spot on Jupiter, there's this small part near the
trailing edge that doesn't stay attached.  In a way, it is a vortice, but
it's center of rotation is parralell to the wing.

Now, we want the airflow to stay attached.  What we need is to move a lot of
air molecules closer to the wing, but first I want to explain what I think a
vortex is.  It's a rotating structure of air where the air pressure at the
center is very low.  As distance increases from the center of the vortex,
air pressure increases, and then again decreases to the point the vortex
cannot be detected.

Back to attaching airflow to the airfoil; assume we could make a vortex
several inches (or some optimum) distance away from the airfoil.  Now, the
vortex provides a higher pressure air that was displaced from the center of
the vortex.  The center of this vortex is now substituted for the tumbling
vortex of the original airfoil.

Yeah, this is difficult to explain.  I'm no aerodynamicist, but this minds
eye view works for me.
 Brian Michalk  <http://www.michalk.com>
Life is what you make of it ... never wish you had done something.
Aviator, experimental aircraft builder, motorcyclist, SCUBA diver
musician, home-brewer, entrepreneur and barely single


  -----Original Message-----
  From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On Behalf
Of Alex Balic
  Sent: Saturday, December 06, 2003 6:45 PM
  To: reflector@tvbf.org
  Subject: RE: REFLECTOR:Vortex Generators on XL


  Keith,
  You are correct about the way that VG's work for the most part, and I was
hoping not to start a really big discussion about this, but I suppose that
is the intent of this news group.  The reason that laminar flow is lost, is
because skin friction slows the airflow within about .01 or so the surface
the surface of the wing. This flow, lacking kinetic energy (relative to the
surface of the wing) is drawn into the negative pressure area above the
wing, separates from the surface, and becomes turbulent.  Vortex generators
are designed to mix the air that still contains energy (which fortunately is
close to the wing surface) with the low energy air at the wing surface, and
thereby maintaining laminar flow further back into the negative pressure
area than would be possible without this mixing.  If you look at a wind
tunnel test you will see the vortices moving wildly back across the wing,
but upon close inspection, there is an energized layer of laminar flow
against the skin. This extended laminar flow is what the vortices make
possible, and what keeps the wing flying at a higher angle of attack.

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<DIV><FONT face=3DArial color=3D#0000ff size=3D2><SPAN =
class=3D937531017-07122003>I have=20
an excellent minds eye picture of what VG's do, and why they are =
good.&nbsp; So=20
far, all of the technical discussion of VG's do not contradict how I see =
them=20
working in my mind.</SPAN></FONT></DIV>
<DIV><FONT face=3DArial color=3D#0000ff size=3D2><SPAN=20
class=3D937531017-07122003></SPAN></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial color=3D#0000ff size=3D2><SPAN =
class=3D937531017-07122003>I came=20
up with my concept of VG's one day when I was a riding in an MD80 (I=20
think).&nbsp; I was seated above the wing, and was examining the engine =
nacelle,=20
and wondered why they put a huge VG down there on the nacelle.&nbsp; As =
luck had=20
it, that day was very humid, and the VG generated a visible vortex at a =
high=20
AOA.&nbsp; Really cool, the vortex went over the wing, and kept airflow =
attached=20
to the huge flap that is very close to the fuselage.</SPAN></FONT></DIV>
<DIV><FONT face=3DArial color=3D#0000ff size=3D2><SPAN=20
class=3D937531017-07122003></SPAN></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial color=3D#0000ff size=3D2><SPAN =
class=3D937531017-07122003>Here's=20
how I came to see VG's functioning.&nbsp; First, look at the airfoil =
without any=20
aid to keep the airflow attached.&nbsp; Somewhere above the airfoil, the =
air=20
comes detached, and tumbles around in this little area.&nbsp; It became =
detached=20
because the air molecules wanted to go forward rather than getting =
sucked down=20
by the airfoil.&nbsp; Kinda like cavitation, or very mild stall.&nbsp; =
Like the=20
large red spot on Jupiter, there's this small part near the trailing =
edge that=20
doesn't stay attached.&nbsp; In a way, it is a vortice, but it's center =
of=20
rotation is parralell to the wing.</SPAN></FONT></DIV>
<DIV><FONT face=3DArial color=3D#0000ff size=3D2><SPAN=20
class=3D937531017-07122003></SPAN></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial color=3D#0000ff size=3D2><SPAN =
class=3D937531017-07122003>Now,=20
we want the airflow to stay attached.&nbsp; What we need is to move a =
lot of air=20
molecules closer to the wing, but first I want to explain what I think a =
vortex=20
is.&nbsp; It's a rotating structure of air where the air pressure at the =
center=20
is very low.&nbsp; As distance increases from the center of the vortex, =
air=20
pressure increases, and then again decreases to the point the vortex =
cannot be=20
detected.</SPAN></FONT></DIV>
<DIV><FONT face=3DArial color=3D#0000ff size=3D2><SPAN=20
class=3D937531017-07122003></SPAN></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial color=3D#0000ff size=3D2><SPAN =
class=3D937531017-07122003>Back=20
to attaching airflow to the airfoil; assume we could make a vortex =
several=20
inches (or some optimum) distance away from the airfoil.&nbsp; Now, the =
vortex=20
provides a higher pressure air&nbsp;that was displaced from the center =
of the=20
vortex.&nbsp; The center of this vortex is now substituted for the =
tumbling=20
vortex of the original airfoil.</SPAN></FONT></DIV>
<DIV><FONT face=3DArial color=3D#0000ff size=3D2><SPAN=20
class=3D937531017-07122003></SPAN></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial color=3D#0000ff size=3D2><SPAN =
class=3D937531017-07122003>Yeah,=20
this is difficult to explain.&nbsp; I'm no aerodynamicist, but this =
minds eye=20
view works for me.</SPAN></FONT></DIV>
<P><FONT size=3D2>&nbsp;Brian Michalk&nbsp; &lt;<A =
href=3D"http://www.michalk.com/"=20
target=3D_blank>http://www.michalk.com</A>&gt;<BR>Life is what you make =
of it ...=20
never wish you had done something.<BR>Aviator, experimental aircraft =
builder,=20
motorcyclist, SCUBA diver<BR>musician, home-brewer, entrepreneur and =
barely=20
single<BR>&nbsp;</FONT> </P>
<BLOCKQUOTE dir=3Dltr=20
style=3D"PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #0000ff 2px =
solid; MARGIN-RIGHT: 0px">
  <DIV class=3DOutlookMessageHeader dir=3Dltr align=3Dleft><FONT =
face=3DTahoma=20
  size=3D2>-----Original Message-----<BR><B>From:</B> =
reflector-admin@tvbf.org=20
  [mailto:reflector-admin@tvbf.org]<B>On Behalf Of </B>Alex=20
  Balic<BR><B>Sent:</B> Saturday, December 06, 2003 6:45 =
PM<BR><B>To:</B>=20
  reflector@tvbf.org<BR><B>Subject:</B> RE: REFLECTOR:Vortex Generators =
on=20
  XL<BR><BR></FONT></DIV>
  <DIV><SPAN class=3D243531900-07122003><FONT face=3DArial =
color=3D#0000ff=20
  size=3D2>Keith, </FONT></SPAN></DIV>
  <DIV><SPAN class=3D243531900-07122003><FONT face=3DArial =
color=3D#0000ff size=3D2>You=20
  are correct about the way that VG's work for the most part, and I was =
hoping=20
  not to start a really big discussion about this, but I suppose that is =
the=20
  intent of this news group.&nbsp; The reason that laminar flow is lost, =
is=20
  because skin friction slows the airflow&nbsp;within about .01&nbsp;or=20
  so&nbsp;the surface&nbsp;the surface of the wing. This flow, lacking =
kinetic=20
  energy (relative to the surface of the wing) is drawn into the =
negative=20
  pressure area above the wing, separates from the surface, and becomes=20
  turbulent.&nbsp;&nbsp;Vortex generators are designed to mix the air =
that still=20
  contains energy (which fortunately is&nbsp;close to the&nbsp;wing=20
  surface)</FONT></SPAN><SPAN class=3D243531900-07122003><FONT =
face=3DArial=20
  color=3D#0000ff size=3D2> with the low energy air at the wing surface, =
and thereby=20
  maintaining laminar flow further back into the negative pressure area =
than=20
  would be possible without this mixing.&nbsp; If you look at a wind =
tunnel test=20
  you will see the vortices moving wildly back across the wing, but upon =
close=20
  inspection, there is an energized layer of laminar flow against the=20
  skin.&nbsp;This extended laminar flow is&nbsp;what the vortices make =
possible,=20
  and what keeps the wing flying at a higher angle of=20
attack.</FONT></SPAN></DIV>
  <BLOCKQUOTE dir=3Dltr style=3D"MARGIN-RIGHT: 0px">
    <DIV class=3DOutlookMessageHeader dir=3Dltr align=3Dleft><FONT =
face=3DArial=20
    color=3D#0000ff =
size=3D2></FONT>&nbsp;</DIV></BLOCKQUOTE></BLOCKQUOTE></BODY></HTML>

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