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Old Sep 13, 2012, 08:28 PM
"...certainty is absurd."
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This has always been my favourite simple explanation of lift, complete with an experiment you can do in your bathtub with some pepper for flow visualization:

http://www.arvelgentry.com/origins_of_lift.htm

The circulation vortex not only explains lift, but is also the key to all vortex panel methods of airfoil analysis that can very accurately predict the pressure and velocities around an airfoil by modelling the big circulation vortex as the sum of a lot of little ones representing the airfoil surface. I have personally measured the pressures around an airfoil in a wind tunnel using mercury manometers and lots of tubes, and compared that to the predicted pressure distribution from vortex panels in a pretty simple program I wrote on punch cards way back when.

You can see it all in your bathtub, starting vortex and all.

Kevin
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Old Sep 13, 2012, 08:56 PM
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Originally Posted by richard hanson View Post
Well-- Newtonian physics explain some of the reasons flying objects do what they do- NOT all the info- which makes sense-- Newton had no aircraft to use in hands on testing.
The interaction of air and a wing in a confined space or where air is sucked across a plate or blown across a plate IS different as compared against a wing in unconfined space
I have not seen helicopers being tested in a tunnel--for example

Wind tunnels can provide info about air in reasonably close contact with the wing but pressures above and below need to include unconfined space.
basically , wind tunnels and Newton etc., explain some of what is involved - NOT all of it.
.
Been done a lot!
Lockheed's AH-56 was in the tunnel at Ames when a rear support broke, the vehicle fell off the supports and went around the tunnel a few times...
Adding more scars to the sides.
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Old Sep 14, 2012, 02:15 AM
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Originally Posted by kcaldwel View Post
The circulation vortex not only explains lift, but is also the key to all vortex panel methods of airfoil analysis that can very accurately predict the pressure and velocities around an airfoil by modelling the big circulation vortex as the sum of a lot of little ones representing the airfoil surface.
Sort of like this... with the punch cards replaced by a touch pad...
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Old Sep 14, 2012, 09:12 AM
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You really think this is a good definition of lift?
I It may be appropriate for someone in the field of teaching and applying aerodynamics but as a basic statement I will go with
Lift is basically a condition caused by pressure differences .
The various explanations as to where this applies - fills many a library.
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Old Sep 14, 2012, 10:35 AM
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Kevin, I like that page. It's pretty similar to how I'm going to describe it, but I'll post this anyway because it goes into a little more of the details of where circulation comes from, physically.

Anyway, so let's go back to the thin curved airfoil at zero angle of attack, in a steady free stream flow. I'm going to ignore separation for now, because it just complicates things and isn't needed to describe how lift is produced.

Because the flow cannot go through the airfoil, it must follow the airfoil's curvature. We also know that the flow will not be able to turn sharply around the trailing edge, so after the flow has developed, the streamlines just above and below the airfoil should leave the trailing edge smoothly. This is called the Kutta condition, and in nature it is enforced by viscosity. Based on the Kutta condition and the requirement that the flow must go around the airfoil, we can imagine the streamlines very near the surface to look something like the first picture below.

Notice in the first picture that the streamlines have an overall clockwise curvature in the vicinity of the airfoil. So, by placing this curved airfoil in a steady free stream, we have introduced a net turning of the flow. Aerodynamicists like to call this circulation. This word used to scare me because it seems sort of abstract, but it's really not. It just means there is an overall turning of the flow, which we know must occur based on the boundary condition and Kutta condition described above.

Because we know the airfoil is responsible for introducing the circulation, aerodynamicists like to represent thin airfoils as a line of vortices that produce this circulation (turning), kind of like in the second picture. Again, this seems kind of abstract and non-physical, but I'll describe in the next post where it comes from physically. For now, it's sufficient to recognize that the airfoil is turning the flow.
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Old Sep 14, 2012, 10:36 AM
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In the last post, I talked about circulation introduced by the airfoil. It's all well and good to accept that this circulation exists, but where does it actually come from? That's the point of this post, and also to explain how the circulation introduces a pressure difference between the top and bottom surfaces.

Up until now, I have only briefly mentioned viscosity, but I haven't really described how it comes into play for the airfoil, except for that it enforces the Kutta condition.

In a real flow around an airfoil (before stall, anyway), it turns out that most of the effects of viscosity are localized in a thin layer near the surface called the boundary layer. Right at the surface, viscosity causes air particles to "stick" to the surface; their velocity will be 0 relative to the airfoil. However, not too far away from the surface the flow will reach its full velocity. This means that in this boundary layer there is a velocity gradient, which I tried to show in the picture below.

Now let's think about the boundary layer in terms of circulation (turning). For now, we'll just focus on the top surface of the airfoil. Right at the surface, the velocity is 0, because of viscosity, but just above outside of the boundary layer, there is some velocity that we'll call Vu, which is on the same order of magnitude as the free stream speed. If we take just a little section of the boundary layer of length ds, this change in velocity introduces a net turning Vu times ds, in the clockwise direction. We then come to the conclusion that viscosity in the boundary layer is actually responsible for causing the circulation described in the previous post.

On the bottom of the airfoil, a similar thing happens, but we'll call the velocity Vl, and notice that the turning on the bottom of the airfoil is in the counterclockwise direction. If we wanted to find out the net clockwise circulation caused by the airfoil, we'd just add up all the Vu * ds pieces on the top and then subtract all the Vl * ds on the bottom. We know there must be a net clockwise circulation (as explained in the previous post), so that means on average Vu must be greater than Vl.

Now we have reached the conclusion that the velocity on the top surface must be greater, on average, than on the lower surface, without resorting to any nonsense explanations like equal transit time or saying the flow "feels like" or "wants to" do anything. Once this conclusion has been reached, it's just one small step to show that the pressure must be lower on the top surface than the bottom surface, by applying the law of energy conservation. The Bernoulli equation is just one way to state conservation of energy, so often that is used to figure out the pressure difference once the velocity difference is known. The total lift is computed by simply adding up all the pressure differences * ds on the airfoil and taking the component perpendicular to the free stream direction.
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Old Sep 14, 2012, 11:58 AM
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Originally Posted by Montag DP View Post
Kevin, I like that page. It's pretty similar to how I'm going to describe it, but I'll post this anyway because it goes into a little more of the details of where circulation comes from, physically.

Anyway, so let's go back to the thin curved airfoil at zero angle of attack, in a steady free stream flow. I'm going to ignore separation for now, because it just complicates things and isn't needed to describe how lift is produced.

Because the flow cannot go through the airfoil, it must follow the airfoil's curvature. We also know that the flow will not be able to turn sharply around the trailing edge, so after the flow has developed, the streamlines just above and below the airfoil should leave the trailing edge smoothly. This is called the Kutta condition, and in nature it is enforced by viscosity. Based on the Kutta condition and the requirement that the flow must go around the airfoil, we can imagine the streamlines very near the surface to look something like the first picture below.

Notice in the first picture that the streamlines have an overall clockwise curvature in the vicinity of the airfoil. So, by placing this curved airfoil in a steady free stream, we have introduced a net turning of the flow. Aerodynamicists like to call this circulation. This word used to scare me because it seems sort of abstract, but it's really not. It just means there is an overall turning of the flow, which we know must occur based on the boundary condition and Kutta condition described above.

Because we know the airfoil is responsible for introducing the circulation, aerodynamicists like to represent thin airfoils as a line of vortices that produce this circulation (turning), kind of like in the second picture. Again, this seems kind of abstract and non-physical, but I'll describe in the next post where it comes from physically. For now, it's sufficient to recognize that the airfoil is turning the flow.
Could you please describe the flow on a flat plate?
It is/is not the same?
I fly models with undercamber thick/thin and flat plates
all work well
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Old Sep 14, 2012, 12:18 PM
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Originally Posted by richard hanson View Post
Could you please describe the flow on a flat plate?
It is/is not the same?
I fly models with undercamber thick/thin and flat plates
all work well
Sure, it works the same for a flat plate at an angle of attack. It's just a little harder to visualize the turning effect. Here's a picture depicting streamlines for the flat plate from Kevin's link:



You can tell from the streamlines that even though the plate is flat, when you place it at a positive angle of attack there will still be a net turning of the streamlines, a.k.a. circulation. After you establish that there's circulation caused by the airfoil, all the rest of the conclusions from the cambered plate in terms of establishing velocity and pressure differences between the top and bottom can still be applied.

One thing about the flat plate is that, if you were to place it at no angle of attack, there would also be no net turning of the flow, and therefore no velocity difference, no pressure difference, and ultimately, no lift.
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Old Sep 14, 2012, 12:32 PM
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Let me just add that the turning of the flow can also be described as sending more of the fluid above the object than below. Whether it's camber, AoA, thickness or no thickness, or a rotating cylinder, it all starts by sending more of the fluid above the object than below, which causes the streamlines above to get closer together and the streamlines below to get further apart.
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Old Sep 14, 2012, 12:50 PM
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Let me just add that the turning of the flow can also be described as sending more of the fluid above the object than below. Whether it's camber, AoA, thickness or no thickness, or a rotating cylinder, it all starts by sending more of the fluid above the object than below, which causes the streamlines above to get closer together and the streamlines below to get further apart.
That's a good simple way to think about it. I like including the boundary layer because it gives the physical mechanism that produces circulation. That said, your explanation would probably be better for explaining to the average person.
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Old Sep 14, 2012, 02:01 PM
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Originally Posted by Montag DP View Post
Sure, it works the same for a flat plate at an angle of attack. It's just a little harder to visualize the turning effect. Here's a picture depicting streamlines for the flat plate from Kevin's link:



You can tell from the streamlines that even though the plate is flat, when you place it at a positive angle of attack there will still be a net turning of the streamlines, a.k.a. circulation. After you establish that there's circulation caused by the airfoil, all the rest of the conclusions from the cambered plate in terms of establishing velocity and pressure differences between the top and bottom can still be applied.

One thing about the flat plate is that, if you were to place it at no angle of attack, there would also be no net turning of the flow, and therefore no velocity difference, no pressure difference, and ultimately, no lift.
If you took that diagram and superimposed a square grid over it
Then assigned some arbitrary number to each section (like stop motion) depecting relative pressure, how would each of the grid sections appear - just some abitrary group of numbers -using the streamlines you show as a guideline.
Would you consider those numbers pertinant in describing what is occuring?
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Old Sep 14, 2012, 02:05 PM
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Originally Posted by richard hanson View Post
If you took that diagram and superimposed a square grid over it
Then assigned some arbitrary number to each section (like stop motion) depecting relative pressure, how would each of the grid sections appear - just some abitrary group of numbers -using the streamlines you show as a guideline.
Would you consider those numbers pertinant in describing what is occuring?
I must say I'm not totally sure what you mean, but the pressure will be highest (and the velocity lowest) in locations where the streamlines are farthest apart. The pressure is lowest in the locations where the streamlines are closest together.
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Old Sep 14, 2012, 02:11 PM
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I must say I'm not totally sure what you mean, but the pressure will be highest (and the velocity lowest) in locations where the streamlines are farther apart. The pressure is lowest in the locations where the streamlines are closest together.
I understand - that is as I understand it but I never see it noted!
To me this is very relevant info -as much so or more so than streamlines. Why is it never shown??
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Old Sep 14, 2012, 02:23 PM
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Originally Posted by richard hanson View Post
I understand - that is as I understand it but I never see it noted!
To me this is very relevant info -as much so or more so than streamlines. Why is it never shown??
In many cases it is. Just do a google search on "airfoil pressure field" and you will see plenty of them.

Edit: or, look at the first entry in my blog, "Unsteady Panel Code."
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