User talk:Mr swordfish/Bernoulli principle

Comments? Mr. Swordfish (talk) 19:47, 29 May 2012 (UTC)

Hi Mr swordfish. Firstly, thanks for inviting us to comment on your new proposal. This is certainly the most collaborative approach to significantly expanding an article. Secondly, are you aware that instead of using your User page for this purpose it is a simple matter to create a personal sandbox for yourself? I presently have two personal sandboxes and I find this very useful because I usually have at least two projects or new articles in progress – see User:Dolphin51/Sandbox and User:Dolphin51/Sandbox2.

I have some concerns with your proposed new section:

1. Since Bernoulli’s principle only applies along a streamline ... It is true that BP applies along a streamline but it is incorrect to say BP only applies along a streamline. A more accurate statement is that BP applies throughout any region of irrotational flow. In external flows, such as the flow around an airfoil, the flow is irrotational everywhere except in the boundary layer and in the wake left by the airfoil. In the case of a plume of air, such as the one flowing across one side of the piece of paper, the Bernoulli constant will be the same along all streamlines within the plume. Outside the plume the flow is presumably stationary.
2. In the situation of a plume of air raising a piece of paper, as with the flow around an airfoil or just about any phenomenon in physics, there are two or more legitimate explanations. Once the audience has been identified it may be true to say that one particular explanation will be more meaningful for that audience. Where there are two legitimate explanations it is unreasonable to dismiss one of them as incorrect. Notwithstanding that the authors of some peer-reviewed physics articles are willing to dismiss BP and say it is incorrect as an explanation of the rising piece of paper I would be most unhappy for the Wikipedia article on BP to say the same thing. My reasons follow.
3. I haven’t yet looked at the four sources you quote to support the statement that the explanation based on BP is incorrect. I will look at those that are web-based and let you know my thoughts, within the next day or two.
4. You are proposing to say a curved streamline will develop a pressure gradient perpendicular to the direction of flow. I agree that this is a true statement but it isn’t one of the principles of fluid dynamics, or even one of the principles of physics – it is an application of none other than Bernoulli’s principle (or Euler equations, which are even more fundamental than BP.) Approaching the center of a free vortex, the static pressure falls and the fluid speed increases; this interchange of static pressure and fluid speed is precisely what BP is all about. Consequently, it would be foolish to say the explanation based on curvature of streamlines is correct and the explanation based on BP is incorrect.
5. Here is my explanation, using BP, of why the paper rises when I blow a plume of air across the top. As the air leaves my mouth it is at atmospheric pressure and it is linear flow – moving in a straight line. When the plume reaches the piece of paper it becomes vortex flow – moving along a curved path to conform to the shape of the paper. (Depending on one’s point of view, this phenomenon may be the Coanda effect.) The outer edge of the plume of air is in contact with the atmosphere so it is constrained to remain at atmospheric pressure. The inner edge of the plume is in contact with the paper so it is not constrained to remain at atmospheric pressure. Vortex flow always displays a faster speed on the inside of the curve and a slower speed on the outside of the curve, so the air in contact with the piece of paper is actually moving slightly faster than when it left my mouth! The outer edge of the plume is moving at the same speed as when it left my mouth. The faster flow adjacent to the piece of paper is accompanied by a reduced static pressure, as explained by BP. On the upper side of the paper the pressure is slightly below atmospheric, while on the lower side the pressure is atmospheric.
6. Here is my explanation, using BP, of why the paper rises when I blow a plume of air across the bottom. As the air leaves my mouth it is linear flow, but it becomes vortex flow when it moves adjacent to the piece of paper. One edge of the plume is in contact with the atmosphere so it is constrained to remain at atmospheric pressure, and at the same speed as it left my mouth. The other edge of the plume is adjacent to the paper so it displays static pressure a little higher than atmospheric, and speed a little lower than when it left my mouth. All this is exactly as explained by BP.
7. I’m sorry I don’t have sources to quote to support my explanations above, but I will see what I can find in the next day or two. Dolphin (t) 00:05, 30 May 2012 (UTC)

Dolphin, Swordfish is using these sandboxes in order to separate work, and more importantly, refs, from each other, they sometimes get tangled up in a sandbox. And Sword, sorry I can't say anything about this (too advanced for me), but I'll go over with an automated tool and fix a few |manual of style errors. Buggie111 (talk) 00:10, 30 May 2012 (UTC)

Dolphin,

Thank you for your detailed and specific comments. Feedback like this is what makes for a better article. First I'll address the sourcing followed by your numbered items.

The Babinsky article was published in Physics Education, which is published by IOP. Their Peer Review policy is here: http://atom.iop.org/atom/usermgmt.nsf/refereeservices - the short answer is that Physics Education is a peer-reviewed publication.

The Eastwell article is in Science Education Review. Their review policy is here: http://www.scienceeducationreview.com/contributors.html "Each manuscript is typically forwarded to 6 reviewers..." I think this qualifies as peer review.

Smith's article was published in The Physics Teacher. Their publication policy is here: http://www.aapt.org/publications/tptauthors.cfm I cannot determine from this webpage whether this is peer-reviewed or not, moreover since the article is 40 years old current review policy may not apply to this article. I will look into this farther.

The other citations are not (to my knowlege) peer-reviewed, and I don't think I implied that they were.

1. I agree that the statement ...Bernoulli’s principle only applies along a streamline ... is is perhaps overly broad and will try to find a more nuanced way to say this.
2. Agree that there is more than one way to (correctly) explain things. That doesn't mean we should accept clearly incorrect explanations. I'll elaborate further later.
3. I see that you have read the sources available on the web. Unfortunately, Smith's article is not available that way. Shoot me an email at mr_swordfish@gmx.com and I'll forward you a copy. It's worth reading as it was the first scholarly paper to seriously question how elementary texts explain lift - even if you disagree, you should appreciate its historical importance.
4. I'm glad to see that you agree that curved streamlines will develop a pressure gradient, but I'm perplexed to read that you don't think it's fundamental. It's derivation is a simple one-liner based on centripetal acceleration (V^2/R) and Newton's 2nd law. It is an immediate and direct consequence of Newton's 2nd law - it doesn't get much more fundamental than that! See Babinsky for the complete derivation with picture (it's the last two paragraphs.)
   Curved streamlines create a pressure gradient. Straight streamlines do not. One can use the Bernoulli Principle to prove this, but it's far far simpler to derive it directly from Newton's 2nd law.
5. I think your explanation is correct. You are very careful to say that the air leaves your mouth at atmospheric pressure. Then the airstream follows the curve of the paper, which induces vortex flow. Accordingly, there is an increase in speed and a consequent drop in pressure. The air speeds up and goes faster than it was going when it left your mouth.
6. Unfortunately, not all (and I'd say most) explanations are not this careful and misrepresent what the BP states. More about that below.
7. I’m sorry I don’t have sources to quote to support my explanations above... Me too, because a careful and nuanced explanation along the lines of what you provided is exactly what I'd like to have in the article. Hopefully, we can find something published.

My own explanation based on pressure differences (which doesn't meet wiki standards since it's mine, not from a published source) would begin like yours. But once we establish that the air follows a curved path, we can just apply the fact that curved streamlines create pressure gradients and we're done. If we want, we could mention that the air speeds up due to the BP, but that's unnecessary to explain why the paper rises.

Now, how about those other explanations? For instance, here's what the Univ of Minnesota says http://www.physics.umn.edu/outreach/pforce/circus/Bernoulli.html:

Bernoulli’s principle states that fluids in an area moving faster than the the surrounding area possess less pressure. Faster-moving fluid, lower pressure.... When the demonstrator holds the paper in front of his mouth and blows across the top, he is creating an area of faster-moving air. The slower-moving air under the paper now has higher pressure, thus pushing the paper up, towards the area of lower pressure.

The clear implication is that the faster moving air has lower pressure. But we both know (and it's well established fact) that the air leaves the demonstrators mouth at atmospheric pressure. This explanation doesn't say that the air speeds up, or that it curves. Reading this explanation, one would think that faster moving air always has lower pressure, and that's simply wrong. One would also think that straight airflow can develop pressure gradients, which as we've seen above is also untrue.

Now, I don't think I've particularly cherry picked this example. Howlers like this are everywhere, as a little bit of Googling will show.

My own take is that we have another situation quite like the equal transit time fallacy. Sloppy and incorrect reasoning is repeated over and over in "science" explanations, leading to widespread misunderstanding of the underlying physics. Yes, it is possible to explain lift (of an airplane wing, or the paper in this demonstration) using BP, but the overwhelming majority of treatments butchers it. I think your explanation is essentially correct, but I've yet to read such a careful and nuanced treatment elsewhere. Usually it's crap like the quote above. Mr. Swordfish (talk) 20:19, 30 May 2012 (UTC)

Hi Mr swordfish. Thanks for taking the time to wade through all my words. My objection can be summarised in one short sentence: I object to your use of the word "incorrectly".

You have written Is anybody saying that the BP is incorrect? I know I'm not. In your new section False Demonstrations of Bernoulli’s Principle you include the following sentence:

• There are several common classroom physics demonstrations that are sometimes incorrectly ascribed to Bernoulli's Principle (my emphasis)

If this use of the word "incorrectly" could be eliminated (and the words "False Demonstrations" could be eliminated from the title) we might be able to make progress. Dolphin (t) 22:40, 30 May 2012 (UTC)

UPDATE: Regarding whether The Physics Teacher is a peer reviewed journal, I contacted its managing editor, Pam Brown, and she wrote back that "Yes, The Physics Teacher has always been a peer-reviewed publication."

In other news, I've significantly re-drafted the section. Comments appreciated. Mr. Swordfish (talk) 13:46, 6 June 2012 (UTC)

So far I have only had a quick look at the re-draft. I can make a couple of comments.

There is a paragraph begining A correct explanation would say: When the airstream encounters the paper, it follows the curve of the paper and is deflected downward. This downward deflection causes a pressure gradient ... There is an emphasis on the airflow being deflected downward - this would be appropriate if it were used in an application of momentum principles to explain lift. Considering the section is an application of Bernoulli's principle to explain lift I think the emphasis should be on the pressure gradient being caused by the curved shape of the streamlines. Curvature of the streamlines indicates the presence of a centripetal force acting on the airstream as it follows the curvature of the paper, and hence a reduction in pressure adjacent to the piece of paper.

In accordance with Wikipedia's policy on titles, the article Bernoulli's principle is spelled with the first word in upper case but all subsequent words in lower case. The draft section contains a mixture of upper and lower case for the word principle. Dolphin (t) 04:01, 8 June 2012 (UTC)

The latest version of the draft is a great improvement! It still gives the impression it might be written for a textbook or students' notes rather than an encyclopedia. The first two paragraphs are unsourced and give this impression very strongly.

There is the expression all other things being equal. This is colloquial. If the explanation is dependent on one or two parameters remaining unchanged, these parameters should be identified explicitly. If Wikipedia were to say "all other things being equal" it suggests Wikipedia doesn't actually know what the other things are.

There is also a sentence This explanation leaves something to be desired. This expression looks like someone's opinion so it is not a good one for an encyclopedia. Wikipedia draws from reliable published sources but it doesn't express an opinion of its own.Dolphin (t) 23:09, 19 June 2012 (UTC)

Thanks for your comments. I think the Lardner quote is a great "hook" to get the reader's attention, but upon reflection I realize that "hooks" are not really appropriate for an encyclopedia article. So, the first paragraph is out. Agree that my statement of what BP actually says is too loose - I'll tighten up the language. Mr. Swordfish (talk) 13:31, 20 June 2012 (UTC)

Thanks for those changes. I think the new section is really beginning to take shape. A couple of comments:

As you know, the plume of air leaving the demonstrator's mouth follows the curve of the paper so there is a variation in speed across the width of the plume according to the relationship between speed and radius within a free vortex. The momentum within the plume remains unchanged so the speed along the median streamline remains constant; the speed along streamlines of greater radius than the median decreases; and the speed along streamlines of smaller radius than the median increases. You are proposing to say This change in pressure is associated with faster speed in accordance with Bernoulli's principle. Thus Bernoulli's principle says that the air speeds up as it flows along the surface. I suggest something like the following:

The air adjacent to the surface of the piece of paper is moving slightly faster than the mean speed of the plume, and the air on the outside of the plume is moving slightly slower than the mean. The change in pressure is associated with the variation in speed across the width of the plume as the plume follows the curvature of the piece of paper. The reduction in pressure acting on the top surface of the piece of paper, and the increase in speed of the air flowing adjacent to the top surface, are related by Bernoulli's principle.

At the end of the new section you are proposing to use the words ... are sometimes explained in a similarly misleading manner. I suggest you clarify exactly what is intended by adding a few extra words to say ... are sometimes explained in a similarly misleading manner by saying "faster moving air has lower pressure".

Dolphin (t) 23:17, 20 June 2012 (UTC)

I agree with adding a few extra words. Done.

I think your suggested explanation is correct, but it's a bit hard to read, especially for someope unfamiliar with the material. I'll try to incorporate the ideas using simpler language. I may not get to it until next week. Mr. Swordfish (talk) 20:39, 21 June 2012 (UTC)

I tried using your carefully worded explanation involving different speeds at different parts of the plume, essentially a speed gradient across the height of the plume, but when I showed the article to a few lay persons this language seemed to confuse them. I think it is sound to introduce the idea of the change in speed as a function of radius, but that's probably best done elsewhere in the article, or over at the Lift page. I think that at this point the new section is ready to go live. Nothing is final on wikipedia, so it can always be improved form here. I'll put it up on Sunday or Monday.Mr. Swordfish (talk) 20:42, 22 June 2012 (UTC)

Sources

I have examined, fairly closely, all but one of the cited sources for two of the sentences in the lead paragraph. All but one are available on the world-wide-web. I found these sources do not support these two sentences. My reasoning is as follows:

1. There are several common classroom physics demonstrations that are sometimes incorrectly ascribed to Bernoulli's Principle.

Source: Ref 39: Professor Robert Bauman is Professor of Physics Emeritus and appears to be a credible commentator. The cited document is a reliable published source but I see nothing to suggest it has been peer-reviewed. Bauman defends BP. In his abstract, he acknowledges that authors have sought explanations that are alternatives to BP, and that some of those alternatives to BP are inconsistent with physics. He makes no comment on the phenomenon of blowing across a single sheet of paper, causing it to rise.

I see nothing in Bauman’s document that talks about “demonstrations that are sometimes incorrectly ascribed to BP” so it would be inappropriate to cite this document as a source.

2. peer-reviewed physics articles show that this explanation is incorrect

Sources:

1. Ref 42: Holger Babinsky is a faculty member of the Engineering Department at Cambridge University and appears to be a credible commentator. The cited document is a reliable published source but I see nothing to suggest it has been peer-reviewed. In his Figures 2 and 5, and the accompanying text, he talks about blowing a plume of air over a piece of paper. However, he is using this example to illustrate errors in the popular explanation of lift. He disingenuously describes the experiments, and the explanations, in a misleading child-like way so that he can promptly dismiss the explanations as incorrect. For example, he writes “it is said, this is because the average velocity on the upper surface is greater (caused by blowing) than on the lower surface (where the air is more or less at rest).” Some people might have used this silly description of events but not reliable commentators. We are not saying it is because the average velocity on the upper surface is greater than on the lower surface. We are saying it is because of vortex flow (curved flow) along the top surface and stationary air adjacent to the lower surface. Babinsky is being disingenuous.

On p.498, in connection with Figure 5, Babinsky writes "While it is true that a curved paper lifts when flow is applied on one side, this is not because air is moving at different speeds on the two sides." Reliable commentators are not saying it is because the air is moving at different speeds on the two sides. (Again, Babinsky is being disingenuous.) Reliable commentators are saying it is because of vortex flow (curved flow) along the top surface and stationary air adjacent to the lower surface.

It is true that Babinsky is identifying something as incorrect but it isn’t the professional use of the lifting-piece-of-paper experiment. What he has identified as incorrect is a clumsy description of the experiment, and the explanation, that might have been tried by some popular commentators but hasn’t been tried by professional commentators. Clumsy descriptions and clumsy explanations are always incorrect – there is nothing unique about clumsy descriptions and clumsy explanations of experiments in fluid dynamics.

On p.499 of Babinsky’s paper he states “However, the fact is often overlooked that Bernoulli’s equation applies only along a streamline.” This is misleading and later in the same paragraph Babinsky corrects his error. He says “Sometimes, all streamlines in a flow originate from a region where there is uniform velocity and pressure (such as a reservoir or a uniform free-stream) and in such a case it is possible to apply Bernoulli’s equation throughout the flow.”

2. Ref 43: Peter Eastwell is a secondary school science teacher. He concedes his lesson to a class of upper primary school students was the first time he had been called upon to engage with BP. He is undoubtedly a genuine and enthusiastic science teacher but he can’t be regarded as a credible commentator on the subject of BP. His document can’t be regarded as peer-reviewed and I would challenge it being described as a reliable published source.

On p.7, Eastwell writes “This air moving below the aerofoil will effectively collide with the underside surface, pushing the aerofoil upwards,” This is not a credible description of generation of lift on an aerofoil and it illustrates that Eastwell’s document is not a reliable published source.

3. Ref 44: Norman F Smith, The Physics Teacher, November 1972. No web site cited so I haven’t been able to see it.

4. Ref 45: Klaus Weltner and Martin Ingelman-Sundberg are associated with the Department of Physics at Frankfurt University so they appear to be credible commentators. The cited document is a reliable published source but I see nothing to suggest it has been peer-reviewed. These two physicists are not saying BP is incorrect. They are advocating more effective ways of discussing fluid mechanics in schools and textbooks: “Pressure gradients generated by the deflection of streaming air can be clearly demonstrated by simple experiments which would substantially improve the discussion of fluid mechanics in schools and textbooks.” – (6. Conclusion)

I have read the document but found no comment about blowing air across a piece of paper. Can someone provide detail as to where I can find such comment?

My conclusion. The sources described above are aimed at the teaching of young people and newcomers to fluid dynamics. None of them are aimed at clarifying state-of-the-art physics or fluid dynamics. None appears to qualify as a peer-reviewed physics article. I am happy to be proved wrong, but until I have been proved wrong these sources shouldn’t be described as peer-reviewed. That would create a misleading impression about their status as commentary on state-of-the-art fluid dynamics. Dolphin (t) 03:18, 30 May 2012 (UTC)

Again, thanks for your thorough reading of the article and the sources. I do appreciate the care you take with accuracy and fairness. Replying to some of your comments:

Some people might have used this silly description of events but not reliable commentators. Agree completely. Unfortunately, I have been unable to find very many "reliable commentators" while I've seen lots and lots of silly descriptions, some of whom one would think would know better. In particular, I've yet to see an explanation (other than yours above) that explains the phenomena correctly using BP.

I do strongly disagree that Babinsky is being disingenuous or misrepresenting the popular but incorrect explanations. See the Univ of Minnesota quote above. See the hundreds of citations of the Equal-Transit-Time Fallacy from otherwise reputable sources. See the following "silly descriptions" that I found in a few minutes of Googling:

"This surprising result is due to Bernoulli's Principle, which states that moving air exerts less pressure." e how . com /how_4474941_demonstrate-bernoullis-principle.html (site blocked by wiki)

"This is because the air you blow is moving faster than the air under the paper. That means there is more pressure on the underside than the top." http://www.cedarville.edu/personal/lee/project/labs/lab-bernoulli.pdf

"When you blow air on top of the paper, the paper will go up because the pressure is greater from the bottom of the paper (slower moving air), than the top (faster moving air)."http://sites.google.com/site/sed695b/projects/discrepant-events/bernoullis-principle

"Your fast moving breath has less push (less air pressure) than the still air below the paper." http://www.sciencekids.co.nz/lessonplans/flight/flightintroduction.html

"When air is moving fast past the paper, its pressure is lowered. The air pressure under the paper is now greater than the air pressure on top of the paper and the paper is moved upwards."http://www.fi.edu/flights/own2/lift-paper.html

What they all have in common is the misunderstanding that the air has lower pressure because it is moving faster, rather than the (correct) understanding that the lower pressure is associated with a change in speed. It is this misunderstanding that I am trying to correct via the new section.

Eastwell writes “This air moving below the aerofoil will effectively collide with the underside surface, pushing the aerofoil upwards,” This is not a credible description of generation of lift on an aerofoil and it illustrates that Eastwell’s document is not a reliable published source. Agree that this is an incomplete and simplistic description of lift. But it is not actually incorrect and I would not use it to ignore the rest of the article.

These two physicists are not saying BP is incorrect. Is anybody saying that the BP is incorrect? I know I'm not. I do take issue with mis-statements and mis-applications of it. Their article is relevant because they state "...some of these experiments are explained erraneously". Agree that they do not address the particular example, but a few results from the paper such as "...the static pressure is the same as in the surrounding atmosphere." contradict the sloppy faster air -> lower pressure argument so common in the popular explanations. Hopefully I can make this clearer as the article progresses.

My conclusion The sources described above are aimed at the teaching of young people and newcomers to fluid dynamics. None of them are aimed at clarifying state-of-the-art physics or fluid dynamics. None appears to qualify as a peer-reviewed physics article. I am happy to be proved wrong, but until I have been proved wrong these sources shouldn’t be described as peer-reviewed. That would create a misleading impression about their status as commentary on state-of-the-art fluid dynamics.

I don't think you are going to find much "state of the art" commentary in peer reviewed journals on this topic. It's an old topic, and the people writing about it in my lifetime are mostly educators teaching young people and newcomers about fluid dynamics. Thus, the only scholarly journals concerned with it are the likes of Physics Teacher, Physics Education, etc. I think this goes with the territory, and it is not fair minded to dismiss these journals on the basis that they're not state of the art. If we insist on that standard, we'll have nothing to source.

As explained above, at least two of the articles are peer reviewed, and represent current scholarly thinking absent peer reviewed articles stating the contrary.

I'll work on the revision a bit in coming days. Thanks again for your comments. Mr. Swordfish (talk) 21:29, 30 May 2012 (UTC)