Bernoulli's Principle Question

HIB · 10-08-2009, 10:27

I came across this site today and it has left me a little puzzled.

http://www.physics.umd.edu/lecdem/se...osf5/f5-03.htm

I have seen the Bernoulli principle demostration of blowing between two sheets of paper before and it always made sense to me. As I understand it, as the velocity of the air increases, the pressure decreases and so the pressure outside the sheets of paper is greater than that between the sheets and the sheets of paper move together.

They offer a conflicting/alternative explanation here. My understanding of their explanation is -> The movement is a result of the bend in the sheets : As the air moves away from the sheet, a reaction force moves the sheets together.

They give two examples which they say prove that Bernoulli's principle is not an adequate explanation. But from what I can see, a pressure difference could still explain the two scenarios they demonstrate. As in the original 'sheets of paper experiment', the air flow over one side of the sheet is faster than the 'still' air on the the other side. A pressure difference is created (as per Bernoulli's principle) and the sheet moves.

Is there something wrong with the way that I am reasoning this out?

**Professor_Fink** · 10-08-2009, 21:26

Ignore the bend explanation. It's misguided. If fluid dynamics worked in that way, planes wouldn't be able to fly (the wings are shaped in exactly the wrong way). I saw it given in a TV show recently as well. I guess they must have gotten it from that website.

HIB · 11-08-2009, 14:46

http://www.terrycolon.com/1features/fly.html

Actually, I'm not sure that it is that simple. Take a look at the link above. There's a link at the bottom to 'Bernoulli, Coanda & Lift' as well. Both good pages.

There is a nice demonstration of the Coanda effect using a spoon and a stream of water from a tap. He then goes on to show that the classic 'blow across a piece of paper' experiment is another example of the Coanda effect. The Bernoulli principle may also be at work but the dominant effect is the Coanda effect (in the stream of water case it would have to be all Coanda effect, I think?)

The description of how planes fly also dispels the simple picture that I had in my head: air flowing more quickly over the curved surface, Bernoulli effect, pressure difference, lift.
Instead, it is explained as a net movement or pumping of air from the top of the wing to the bottom and the reaction force on the wing is the lift.

As regards the original experiment in the first link: what a pity they didn't try blowing the air across the other side of the sheet! If it is a Coanda effect then the sheet should still move in the same direction even though the pressure difference has been reversed.

**Professor_Fink** · 11-08-2009, 16:07

Quote:

Originally Posted by HIB

http://www.terrycolon.com/1features/fly.html

Actually, I'm not sure that it is that simple.

No, it isn't. In reality you need to use the Navier-Stokes equations to determine what is actually happening, and it doesn't admit a particularly simple explanation. It also depends on things like Reynolds number, something which is not obvious in the explanations you point to.

An example of where the curvature explanation fails is for example with a spinning golf ball. A spinning sphere behaves similarly o an aerofoil and experiences lift.

This is why I recommended ignoring the stuff about bends. It really isn't a proper explanation.

SkepticOne · 12-08-2009, 17:22

Quote:

Originally Posted by Professor_Fink

Ignore the bend explanation. It's misguided. If fluid dynamics worked in that way, planes wouldn't be able to fly (the wings are shaped in exactly the wrong way). I saw it given in a TV show recently as well. I guess they must have gotten it from that website.

I think it does work as an explanation. Not a complete explanation and not a mathematical model, perhaps.

http://farm3.static.flickr.com/2434/...a6d73f58_o.png

Intuitively we can see that the air clings to the upper surface, and is directed downward.

A propeller is only a moving wing yet it is clear that works by directing air from in front of the propeller and forcing it to the rear.

10-08-2009, 10:27	#1
HIB Registered User Join Date: Jul 2009 Posts: 29 Adverts \| Friends	Bernoulli's Principle Question I came across this site today and it has left me a little puzzled. http://www.physics.umd.edu/lecdem/se...osf5/f5-03.htm I have seen the Bernoulli principle demostration of blowing between two sheets of paper before and it always made sense to me. As I understand it, as the velocity of the air increases, the pressure decreases and so the pressure outside the sheets of paper is greater than that between the sheets and the sheets of paper move together. They offer a conflicting/alternative explanation here. My understanding of their explanation is -> The movement is a result of the bend in the sheets : As the air moves away from the sheet, a reaction force moves the sheets together. They give two examples which they say prove that Bernoulli's principle is not an adequate explanation. But from what I can see, a pressure difference could still explain the two scenarios they demonstrate. As in the original 'sheets of paper experiment', the air flow over one side of the sheet is faster than the 'still' air on the the other side. A pressure difference is created (as per Bernoulli's principle) and the sheet moves. Is there something wrong with the way that I am reasoning this out?

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10-08-2009, 21:26	#2
Professor_Fink Moderator Join Date: Feb 2005 Location: Oxford Posts: 709 Adverts \| Friends Mod: Physics & Chemistry	Ignore the bend explanation. It's misguided. If fluid dynamics worked in that way, planes wouldn't be able to fly (the wings are shaped in exactly the wrong way). I saw it given in a TV show recently as well. I guess they must have gotten it from that website.

11-08-2009, 14:46	#3
HIB Registered User Join Date: Jul 2009 Posts: 29 Adverts \| Friends	http://www.terrycolon.com/1features/fly.html Actually, I'm not sure that it is that simple. Take a look at the link above. There's a link at the bottom to 'Bernoulli, Coanda & Lift' as well. Both good pages. There is a nice demonstration of the Coanda effect using a spoon and a stream of water from a tap. He then goes on to show that the classic 'blow across a piece of paper' experiment is another example of the Coanda effect. The Bernoulli principle may also be at work but the dominant effect is the Coanda effect (in the stream of water case it would have to be all Coanda effect, I think?) The description of how planes fly also dispels the simple picture that I had in my head: air flowing more quickly over the curved surface, Bernoulli effect, pressure difference, lift. Instead, it is explained as a net movement or pumping of air from the top of the wing to the bottom and the reaction force on the wing is the lift. As regards the original experiment in the first link: what a pity they didn't try blowing the air across the other side of the sheet! If it is a Coanda effect then the sheet should still move in the same direction even though the pressure difference has been reversed.