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Old Sep 13, 2012, 11:50 AM
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if you dig in the sand with a spade, you dislodge sand grains. If you do this at an angle, the sand will build up on one side of the spade and air will fill in the space left empty on the other side. but sand is not a fluid, while air is. the molecules both attract and repel each other. You can't simply separate them and create a vacuum or compress them to a solid. Their behavior is similar to a mass of small objects attached through springs that while keeping the distance between them within a small range still allows them to slide relative to each other. When a solid body enters into play, some of these particles get displaced by it. they press against their neighbours, which in turn press against their neighbour and so on, in a chain reaction that moves at the speed of sound. Some springs get compressed, and some get stretched. Since this difference in tension creates an unbalanced system, the particles slide and rearrange themselves to balance these forces out. However, this can only happen at a relatively small speed, at most at the speed of sound, but other particles are in the way. the zones where the springs are stretched are areas of low pressure, where there are fewer molecules of air per volume compared to the total average of the control volume. the areas where the springs are compressed are high pressure areas. this strange situation also results in some odd, slightly unexpected behavior for those that believe a wing acts like a shovel in the sand. At high angles of attack, the flow of air hits the underside of the wing behind the leading edge. a portion of air keeps moving along the bottom of the wing toward the trailing edge. Another portion moves towards the leading edge (backwards compared to the airflow motion), towards the low pressure area above the wing, and runs on the top portion of the wing towards its trailing edge. The low pressure area on top of the wing is actually strong enough to suck the high pressure air forward.
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Old Sep 13, 2012, 12:11 PM
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Originally Posted by richard hanson View Post
Did I pass the test?
Am I missing a basic understanding -in your opinion?- If so - please enumerate .
By the way I have worked with fluidic control circuits which rely on Coanda effects etc for switching -- this stuf always seemed straightforward me .
My problem with what you've said so far is that I still haven't seen you explain why there is a pressure difference when a wing moves through the air. Basically, the same thing I have been asking the whole time. I'm sure that you understand why. I just wanted to know if you could explain the physical mechanism that creates this pressure difference.
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Old Sep 13, 2012, 12:13 PM
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Some wing setups work like the fluidec switching arrangements we once used

example :
air at a given speed , flows into a circuit which has a Y path
- the air will always follow path A (one leg) because the surfaces/ diameter .are the easiest for the air to follow. (air is lazy - it always seeks the easiest paths)
surface tension if you will makes it stay there.
The path can be changed by adding a small stream of air which disturbs this flow and the air takes the next easiest path ( other leg of the Y). and sticks there
So, the idea of air being "sticky" can be easily demonstrated .
Also the concept that air ALWAYS takes the easiest path- becomes evident.
In a nutshell - this nice spongy medium always seeks equalibrium.
The nasty old wings disturb this condition and in doing so, surrounding pressures change and the useful part of this happening (higher pressure on one side) - will try to lift the wing to the low pressure region.
This is probably a lousy example for some - but my point is that depending on the size, shape and speed of the wing moving thru the air, the paths taken will vary.
Nevertheless, the one constant is that a pressure differential does the work.
always
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Old Sep 13, 2012, 12:23 PM
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Originally Posted by Brandano View Post
if you dig in the sand with a spade, you dislodge sand grains. If you do this at an angle, the sand will build up on one side of the spade and air will fill in the space left empty on the other side. but sand is not a fluid, while air is. the molecules both attract and repel each other. You can't simply separate them and create a vacuum or compress them to a solid. Their behavior is similar to a mass of small objects attached through springs that while keeping the distance between them within a small range still allows them to slide relative to each other. When a solid body enters into play, some of these particles get displaced by it. they press against their neighbours, which in turn press against their neighbour and so on, in a chain reaction that moves at the speed of sound. Some springs get compressed, and some get stretched. Since this difference in tension creates an unbalanced system, the particles slide and rearrange themselves to balance these forces out. However, this can only happen at a relatively small speed, at most at the speed of sound, but other particles are in the way. the zones where the springs are stretched are areas of low pressure, where there are fewer molecules of air per volume compared to the total average of the control volume. the areas where the springs are compressed are high pressure areas. this strange situation also results in some odd, slightly unexpected behavior for those that believe a wing acts like a shovel in the sand. At high angles of attack, the flow of air hits the underside of the wing behind the leading edge. a portion of air keeps moving along the bottom of the wing toward the trailing edge. Another portion moves towards the leading edge (backwards compared to the airflow motion), towards the low pressure area above the wing, and runs on the top portion of the wing towards its trailing edge. The low pressure area on top of the wing is actually strong enough to suck the high pressure air forward.
Okay, that seems like it might be a decent description. But let's now consider a thin cambered airfoil with no angle of attack. Actually, let's make it a cambered plate with basically no thickness. You could think of it as a parabolic arc.

Using your reasoning, what happens to the pressure on the top and bottom of the airfoil? Does the pressure increase on the bottom and decrease on the top, like we all know it should?

It seems to me that, using your description, there will be a "void" on the front half of the lower surface, so pressure should decrease there. There would also be one on the back half of the upper surface, so pressure should decrease there too. Likewise, pressure would increase on the front of the upper surface and the back half of the lower surface. Since the airfoil has no thickness, these pressure changes on the top and bottom surfaces would be equal and so we'd get no net lift. Is that consistent with your description, or am I interpreting it wrong?
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Old Sep 13, 2012, 12:44 PM
Grad student in aeronautics
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Montag, here is the way I like to break down the questions and answers.

Questions:
1. Given the geometry of the object (camber, thickness, aoa, etc.), why do the streamlines form the way that they do?

2. How do the positions of the streamlines determine the local flow velocities?

3. How do the local flow velocities determine local pressures?

4. How do the local pressures determine the resultant aerodynamic force?

Answers:
1. This is the tough one. My best explanation comes from observing the starting vortex.

2. Conservation of mass. Streamlines act as walls (by definition)

3. Conservation of energy

4. Integration



The point I'm trying to make is that question number one is the hardest and the one that requires the most thought. Unfortunately, it is often tangled with the other questions, making it difficult to have a discussion.
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Old Sep 13, 2012, 12:45 PM
Texas Buzzard
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Back to Newtonian or Classical Physics

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Originally Posted by Montag DP View Post
Oak, but by moving the card, surely you are moving the air as well? In any case, why does moving the card cause the pressure to change?
.................................................. .................................................. .....................

You asked , "Why does the card cause the air to move?"

You admit that air has mass and is not initially moving.
For any stationary mass to be made to move that mass has to be accelerated
by an unbalanced Force. Don't you agree?

Now what is the origin of that force that causes the air to be moved? (Answ. is
the moving wing).

If a force acts through a distance then that Force has done Work ( F x S ).

Therefore can't we say that the air is having WORK done on it caused by the action of the moving wing?

Why is the wing moving in the first place? The wing usually moves because either gravity is changing Potential Energy into Kinetic Energy, i.e., as in a descending glider OR the wing is being pulled ( A FORCE that is doing Work on the wing) by an engine - a jet or an internal combustion engine.

Other than this can you explain how a stationary mass can be made to have work done on it?

Rather simple Dr. Watson - eh?
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Old Sep 13, 2012, 12:48 PM
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On a simple arc shaped plate there could well be a small zone of lower than ambient pressure just behind the leading edge that forms a lower surface separation bubble. Lower surface separation due to extreme curvature is not unheard of. It would depend on the shape of the airfoil.

But keep in mind that the shape of the airfoil produces an upsweep in the airflow ahead of the leading edge and the air tends to then follow the shape of the surfaces. So on a simple circular arc shape you won't see a low pressure spot even at zero AoA. In that way the air simply does not act like a stream of sandblasting particles which would produce the voids that you describe.
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Old Sep 13, 2012, 12:57 PM
Grad student in aeronautics
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Quote:
Originally Posted by Texas Buzzard View Post
.................................................. .................................................. .....................

You asked , "Why does the card cause the air to move?"

You admit that air has mass and is not initially moving.
For any stationary mass to be made to move that mass has to be accelerated
by an unbalanced Force. Don't you agree?

Now what is the origin of that force that causes the air to be moved? (Answ. is
the moving wing).

If a force acts through a distance then that Force has done Work ( F x S ).

Therefore can't we say that the air is having WORK done on it caused by the action of the moving wing?

Why is the wing moving in the first place? The wing usually moves because either gravity is changing Potential Energy into Kinetic Energy, i.e., as in a descending glider OR the wing is being pulled ( A FORCE that is doing Work on the wing) by an engine - a jet or an internal combustion engine.

Other than this can you explain how a stationary mass can be made to have work done on it?

Rather simple Dr. Watson - eh?
By definition lift does no work. Drag does, lift doesn't.
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Old Sep 13, 2012, 01:23 PM
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Drag sometimes does work.
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Old Sep 13, 2012, 01:50 PM
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Quote:
Originally Posted by BMatthews View Post
On a simple arc shaped plate there could well be a small zone of lower than ambient pressure just behind the leading edge that forms a lower surface separation bubble. Lower surface separation due to extreme curvature is not unheard of. It would depend on the shape of the airfoil.

But keep in mind that the shape of the airfoil produces an upsweep in the airflow ahead of the leading edge and the air tends to then follow the shape of the surfaces. So on a simple circular arc shape you won't see a low pressure spot even at zero AoA. In that way the air simply does not act like a stream of sandblasting particles which would produce the voids that you describe.
Attached.

Kevin
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Old Sep 13, 2012, 01:51 PM
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The old concept of Yin and Yang is still true --
you just cant do one thing without a complimentary action.
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Old Sep 13, 2012, 02:46 PM
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Quote:
Originally Posted by BMatthews View Post
On a simple arc shaped plate there could well be a small zone of lower than ambient pressure just behind the leading edge that forms a lower surface separation bubble. Lower surface separation due to extreme curvature is not unheard of. It would depend on the shape of the airfoil.

But keep in mind that the shape of the airfoil produces an upsweep in the airflow ahead of the leading edge and the air tends to then follow the shape of the surfaces. So on a simple circular arc shape you won't see a low pressure spot even at zero AoA. In that way the air simply does not act like a stream of sandblasting particles which would produce the voids that you describe.
That's all very true, and that was my point. The explanation offered by Brandano is not correct, because it fails to predict lift on a simple cambered airfoil at 0 angle of attack. It also gets some other important stuff wrong. For example, on the front of the upper surface it predicts the pressure should increase when, in fact, it is decreasing on most of this region.

DPATE, I think your approach is correct.

I have what I believe is a very good explanation of why there is a velocity differential and pressure differential on the upper and lower surfaces, in terms of physical phenomena. I'm hesitant to post it, though, because it involves boundary layers and vorticity and the Kutta condition, and I think some would criticise it as not being simple enough. But, if someone wants me to I will post it.
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Old Sep 13, 2012, 03:34 PM
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I'm glad we are in agreement. I would be interested to see it if you want. Maybe a new thread on 2D lift?
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Old Sep 13, 2012, 05:48 PM
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Quote:
Originally Posted by Montag DP View Post
That's all very true, and that was my point. The explanation offered by Brandano is not correct, because it fails to predict lift on a simple cambered airfoil at 0 angle of attack. It also gets some other important stuff wrong. For example, on the front of the upper surface it predicts the pressure should increase when, in fact, it is decreasing on most of this region.

DPATE, I think your approach is correct.

I have what I believe is a very good explanation of why there is a velocity differential and pressure differential on the upper and lower surfaces, in terms of physical phenomena. I'm hesitant to post it, though, because it involves boundary layers and vorticity and the Kutta condition, and I think some would criticise it as not being simple enough. But, if someone wants me to I will post it.
Apparantly we see the answers differently
You speak of velocity differential and pressure differential

all well and fine.
The forum is MODELING science
- I feel that the most simple yet true explanation is a better approach for this type forum.
We agree that pressure differential is basically what it's all about
how one achieves it - is a different matter .
My background includes providing testimony on technical matters to juries and lawyers etc..in court.
I had it drilled into me that the best explanation is always the simple one and provided in terms the layman can easily follow. I feel that the info relevant for this forum is best addressed in the same manner .
I don't ask that you agree .
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Old Sep 13, 2012, 06:18 PM
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Originally Posted by richard hanson View Post
Apparantly we see the answers differently
You speak of velocity differential and pressure differential

all well and fine.
The forum is MODELING science
- I feel that the most simple yet true explanation is a better approach for this type forum.
We agree that pressure differential is basically what it's all about
how one achieves it - is a different matter .
My background includes providing testimony on technical matters to juries and lawyers etc..in court.
I had it drilled into me that the best explanation is always the simple one and provided in terms the layman can easily follow. I feel that the info relevant for this forum is best addressed in the same manner .
I don't ask that you agree .
I agree with you to some extent. The simplest answer is usually best. In this case, the pressure difference is sufficient to explain why there is a lift force (and there are other equivalent ways of explaining it as well, including momentum balances with an appropriate control volume). The problem is that such explanations don't give insight into the underlying cause. For example, you knew there was a pressure difference but you had a lot harder time explaining what caused the pressure difference.

I'll post my explanation of the physical mechanism that creates the pressure difference, probably tomorrow. It requires a few diagrams and I may spread it out over a few posts to try to make it clear.
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