Key Statements by Aerodynamics Expert Doug McLean

Here are key statements (marked by headings starting with "On") by aerodynamics expert Doug McLean recorded in the preceding post, in his book Understanding Aerodynamics, in Physics Teacher and proposed update of Wikipedia article on Lift, with my comments in italics in parenthesis. Comments by McLean will be posted as soon as they arrive. Headlines from articles in NYT 2003 and Scientific American 2020 claiming that "nobody can explain flight", are also included.

(The net result will show to be that there is no scientific theory of flight commonly agreed to be correct, which is demonstrated by the fact that no such theory is presented in the literature, only a bunch of theories which are all viewed to be incorrect/incomplete/empty. This is a major failure of modern physics/aerodynamics, unbelievable to the general public and fiercely denied by aerodynamics experts. It is the ultimate expression of Nobel Laureate Hinshelwood's characterisation of modern fluid mechanics as being split into practical fluid mechanics (hydraulics) observing phenomena which cannot be explained (lift), and theoretical fluid mechanics explaining phenomena which cannot be observed (zero lift of potential flow), in other words a complete collapse as science. )

On Standard Theory of Flight:

So in one sense, the physics of lift is perfectly understood: Lift happens because the flow obeys the NS equations with a no-slip condition on solid surfaces. On the other hand, physical explanations of lift, without math, pose a more difficult problem. Practically everyone, the nontechnical person included, has heard at least one nonmathematical explanation of how an airfoil produces lift when air flows past it. Such explanations fall into several general categories, with many variations. Unfortunately, most of them are either incomplete or wrong in one way or another. And some give up at one point or another and resort to math. This situation is a consequence of the general difficulty of explaining things physically in fluid mechanics, a problem we’ve touched on several times in the preceding chapters. (from the book Understanding Aerodynamics: Arguing from the Real Physics by McLean) 

The conventional mathematical theories can all be traced back to established laws of physics and have evolved over the years, from potential flow with the Kutta condition, through boundary-layer theory, lifting-line theory, and so forth, to the present RANS/DES state of the art. In the aeronautical/scientific community there is a broad consensus that these theories model lifting flow correctly to their respective levels of physical fidelity.

The qualitative physical explanations are something else altogether. We devise them to help us with our intuitive understanding and to communicate with non-technical audiences, but they're not an essential part of our scientific understanding. I don't even like to refer to them as "theories". No one yet, to my knowledge, has devised objective criteria for choosing which aspects of the physical phenomenon to include in such an explanation, and which to omit, leaving the choice largely to subjective taste and to perceptions of what the target audience will understand. Given the complexity of the phenomenon, the subtlety of the cause-and-effect relationships involved, and the subjectivity of decisions as to how to proceed, I'm not at all surprised that numerous explanations have been circulated, that some of them are wrong, and that not everyone agrees on which one, if any, is actually correct. ... this state of affairs doesn't justify the conclusion that "no one knows what keeps airplanes in the air." The early mathematical theories settled that question a century ago, and the current state of the art carries on the tradition.

I don't think seeking qualitative physical explanations implies that our real scientific explanation based on no-slip NS needs any "fix". In my previous note I made clear how I see the distinction between the science and the qualitative explanations. 

By “physical explanations without math” I mean explanations that appeal to physical principles but don't depend on solving equations or making any other kind of quantitative determination. I'd agree with you that such explanations are, in a sense, "not true physics". In my previous note I tried to provide some rationale for why we pursue qualitative explanations at all, but I also argued that they aren't essential to our scientific understanding and that they shouldn't even be called "theories".

(Doug sends here a double message: (i) There is a scientific explanation/theory of flight. (ii) There is no satisfactory scientific explanation of flight. Doug makes a distinction between (i) quantitative explanation by math (NS with no-slip) and (ii) qualitative physical explanation without math, with the double message that (i) is settled, while (ii) is not settled. But NS with no-slip is uncomputable so (i) is empty and so none of (i) and (ii) is settled. Besides, the very idea of physics without math is acknowledged as not true physics. Why then pursue such an idea? It can only be done if there is no scientific explanation of flight.)

On No-Slip vs Navier's Friction Boundary Condition:

I understand that a BC enforcing a relationship between wall shear stress and slip at the wall is mathematically permissible, but I don't think it's an actual "physical BC" because slip at the wall is a fiction. No-slip, on the other hand, is a physical BC imposed on us by the physics at the microscopic level. Of course forcing the fluid to have zero velocity at the wall requires some applied force, but the required force arises naturally from the solution to the viscous-flow equations. There's no need for the BC to address force explicitly, and no need to revert to Navier's condition.

(We read that Doug knows that slip is fiction, but in Fig 4.1.14 in his book he displays a turbulent boundary layer with a sublayer which effectively is slip. Doug states that the physical force required to keep a fluid particle at rest (no-slip) naturally arises from the solution of viscous equations as if mathematics has physical power. Strange!)

On New Theory of Flight:

RANS/DES doesn't do as well as we'd like on cases with massive separation, though it's improving as our DES capabilities and turbulence models improve. So maybe your New Theory can make a contribution there. (Here is a little opening to the need of something new. Good!)

On Computability of NS with No-Slip:

Of course I don't claim DNS is computable for an airplane, as I explain on p. 51 of my book. I'm referring to RANS. And that's what I meant here. I apologize if my choice of wording confused you. So in one sense, the physics of lift is perfectly understood: Lift happens because the flow obeys the NS equations with a no-slip condition on solid surfaces = RANS.

(Doug claims that lift is perfectly understood because the flow obeys RANS. But RANS includes both wall and turbulence models and so is not true flow physics and so the claim of perfect understanding is empty. It does not explain lift better than the model $L=C\alpha$ with $L$ lift and $\alpha$ angle of attack and $C$ a constant to be determined by observation for one $\alpha$ and $L$.)   

(Navier-Stokes with slip without wall/turbulence models = DNS for an airplane requires more than $10^{16}$ mesh points way beyond present computational capacity.)

On Separation:

The standard theory does not lead to "early separation (at the crest of wing)", provided the boundary layer is turbulent. (What if the boundary layer is laminar?)

The idea that laminar-bubble reattachment is a fiction that we dreamed up because we need it is also off-base. The existence of laminar bubbles with turbulent reattachment is amply documented experimentally. They're typically associated with separation at low R_x and so don't show up on airliner wings at cruise, but sometimes appear near leading edges of deployed slats and flaps, and on wings of smaller airplanes at lower speeds (gliders, HPAs, etc.) (So laminar bubble reattachment does not explain lift of wings.)

On Proposed Update of Wikipedia article on Lift force:  

New version of Simplified Physical Explanations of Lift by McLean to appear on Wikipedia:

The flow around a lifting wing is a complex fluid-mechanics phenomenon that can be understood on essentially two levels:

1) The level of the mathematical theories, which are based on established laws of physics and represent the flow accurately, but which require solving partial differential equations.

2) The level of qualitative physical explanations without math. Correctly explaining lift is difficult because the cause-and-effect relationships involved are subtle. A comprehensive explanation that captures all of the essential aspects is rather long. There are also many simplified explanations, and most readers will likely already have been exposed to one or more of them. But simplifying the explanation of lift is inherently problematic, and no simplified explanation has been devised that's completely satisfactory. Each of the simplified explanations presented below is therefore accompanied by a discussion of its shortcomings or errors.

Over the last hundred years or so, many different simplified explanations have been proposed. Most follow either of two basic approaches, based either on Newton's laws of motion or on Bernoulli's principle. But neither approach, by itself, is a completely satisfactory explanation.

(We read that McLean/Wikipedia sends the message all the people on Earth traveling by air that there is no scientifically satisfactory explanation of lift. This is nothing but a monumental failure of modern aerodynamics, unique in the history of science. But McLean also serves as a key authority for Wikipedia to give the false impression that everything is OK: 

• There are several ways to explain how an airfoil generates lift...For example, there are explanations based directly on Newton's laws of motion and explanations based on Bernoulli's principle. Either can be used to explain lift.

No they cannot and this is acknowledged = DoubleSpeak.) 

On Need of New Theory:

1. The science of lift is not in dispute.
2. Correctly explaining lift qualitatively isn’t easy.
3. Over the last 100 years or so, many different explanations have been put forward for various audiences, and the apparent incompatibilities among the different approaches has been a source of confusion and controversy.
4. In the aeronautical/scientific community there is a broad consensus that these theories model lifting flow correctly to their respective levels of physical fidelity.
5. Your take on the standard theories of aerodynamics is outside the mainstream, as is your proposed New Theory.
6. Do I think there's any need for a new theory? Not at a conceptual level, but perhaps at the practical prediction level.
7. Even if this New Theory were also correct, there's no way that it's the first.
8. I don't think seeking qualitative physical explanations (of lift) implies that our real scientific explanation based on no-slip NS needs any "fix".
9. I don't think Euler/NS with slip is useful for cruising flight with attached flow, but it might be useful for modeling massively separated flow.
10. Calculating a no-slip TBL with a good turbulence model represents the physics in a physically realistic way, which is preferable to an ad hoc fix like a slip BC. It seems to me almost guaranteed that a slip BC won't get it right.
11. RANS/DES doesn't do as well as we'd like on cases with massive separation, though it's improving as our DES capabilities and turbulence models improve. So maybe your New Theory can make a contribution there.

On Headlines of Scientific American and NYT: Nobody can explain flight:

Those headlines are sensationalistic nonsense. Immediately after the NYT 2003 article came out, I wrote to Kenneth Chang at NYT to try to set the record straight, but he didn't reply. 

The NYT and SciAm headlines were written by people under the same misapprehension as you are, i.e. that the qualitative explanations reflect the state of the science as a whole. In aero engineering circles those headlines are considered to be nonsense.

(Doug acknowledges above that nobody can explain flight, and thus that the title is not nonsense, but anyway makes efforts like Wikipedia to cover up this undeniable fact. This is serious and will be the topic of the discussion with Wikipedia on the next level beyond Talk page. )

Closing Words by Doug:

OK, let me back up and comment on one part of your question: "How can something which is well understood be difficult to explain and boil down to confusion?" Well, the part that's well understood, in my opinion, is that a lifting flow at high Re obeys the equations of continuum fluid motion with turbulence accounted for, say by RANS. This is a set of field PDEs that enforce the relevant physical principles locally, point-by-point. The local balances that are enforced are pretty simple. Determining how the flowfield behaves, on the other hand, requires solving the set of PDEs. Aspects of a solution (pressure distributions, drag, etc.) can be compared with experiment to evaluate the quality of the simulation it provides. A solution can also be interrogated at as many points as you like to verify that the physical balances embodied in the equations were honored, point-by-point. From a pure science perspective, I would argue that this is all the "science" we need, and, given the generally high quality of the simulations, I think it justifies my statement that the science of lift is well understood.

But of course our natural curiosity pushes us to go beyond what the actual science requires and to try to devise global, qualitative explanations that answer questions such as "why is the flow above and below the airfoil deflected downward?" or "Why is the pressure reduced in a region above the airfoil?" With such questions we're really asking how the solution to a complex set of field PDEs behaves, and we're asking for answers that illuminate physical cause-and-effect. Extrapolating from local principles to global behavior is naturally difficult (Doing it rigorously requires solving PDEs, after all). And the cause-and-effect relationships involved are subtle. It's not surprising that such qualitative explanations have been error-prone. But, as I've argued before, the qualitative explanations aren't essential to the science, and their faults don't contradict my assertion that the science is well understood.

In this connection I would point out that the proposed New Theory is similar to RANS in the sense that it requires solving a set of PDEs. It's also similar to RANS in the sense that solutions don't provide intuitive qualitative explanations for global flow patterns. The New Theory and RANS are thus equally "deficient" in the sense of failing to provide to a "qualitative" understanding of flow patterns. (No: The New Theory comes with an explanation of the generation of large lift at small drag as a consequence of the general explanation of slightly viscous incompressible bluff body flow as potential flow modified by 3d rotational slip separation).  

Your take on the standard theories of aerodynamics is outside the mainstream, as is your proposed New Theory. You maintain that Prandtl was wrong about BL physics and that K and J were wrong about circulation theory. I disagree. Nothing in this discussion has convinced me that there's anything wrong with the standard theories. Nor has anything you've written convinced me that your New Theory has any more than a possible peripheral niche application calculating massively separated cases. At this point, I don't know what kind of resolution you're hoping for. I don't expect that you'll convince me or convince the editors (or arbitrators) at Wikipedia to see things your way. With regard to Wikipedia, if you had a growing group of followers writing peer-reviewed papers based on your approach, it would be a different story, but that doesn't seem to be happening. (Yes it does!)

So I've answered your questions, and I think my answers have been devastating to your side of the argument. But you don't really seem to pay attention to my arguments. Whenever I point out what I think is an error in your reasoning, you change the subject instead of offering a rebuttal. Given how all of this has devolved, I really don't see any point in further discussion. I ask you please to stop the emails. If you carry on the discussion on Wikipedia, I may join in.

Summary:

Doug says that:

1. Flight is perfectly understood because fluid flow obeys the RANS equations.
2. Qualitative explanation of lift is not essential to the science, and their faults don't contradict my assertion that the science is well understood.
3. No-slip is enforced in physical terms because the required force arises naturally from the solution to the viscous-flow equations.
4. New Theory and RANS are equally "deficient" in the sense of failing to provide to a "qualitative" understanding of flow patterns.
5. Headline of Scientific American 2020 sensationalistic nonsense.  
6. There is no need of New Theory, even if it happens to be correct, because there are already many theories (for different audiences) which are (even if not correct) by the aeronautical/scientific community agreed in broad consensus to model lifting flow correctly to their respective levels of physical fidelity. 
7. New Theory can make a contribution for separated flow.

(1 is empty: Perfect understanding does not come from looking at RANS numbers. 2 is misunderstanding of the purpose of science. 3 is misunderstanding of the difference between real physics and mathematics. 4 is misunderstanding of New Theory. 5 is cover-up. 6-7 There is and there is not any need of New Theory.)    

From Staying Aloft: What Does Keep Them Up There?: 

NYT 2003: To those who fear flying, it is probably disconcerting that physicists and aeronautical engineers still passionately debate the fundamental issue underlying this endeavor: what keeps planes in the air?

''Here we are, 100 years after the Wright brothers, and there are people who give different answers to that question,'' said Dr. John D. Anderson Jr., the curator for aerodynamics at the Smithsonian National Air and Space Museum in Washington. ''Some of them get to be religious fervor.''

The answer, the debaters agree, is physics, and not a long rope hanging down from space. But they differ sharply over the physics, especially when explaining it to nonscientists.

''There is no simple one-liner answer to this,'' Dr. Anderson said.

From No One Can Explain Why Planes Stay in the Air:

Scientific American 2020: In December 2003, to commemorate the 100th anniversary of the first flight of the Wright brothers, the New York Times ran a story entitled “Staying Aloft; What Does Keep Them Up There?” The point of the piece was a simple question: What keeps planes in the air? To answer it, the Times turned to John D. Anderson, Jr., curator of aerodynamics at the National Air and Space Museum and author of several textbooks in the field.

What Anderson said, however, is that there is actually no agreement on what generates the aerodynamic force known as lift. “There is no simple one-liner answer to this,” he told the Times. People give different answers to the question, some with “religious fervor.” More than 15 years after that pronouncement, there are still different accounts of what generates lift, each with its own substantial rank of zealous defenders. At this point in the history of flight, this situation is slightly puzzling. After all, the natural processes of evolution, working mindlessly, at random and without any understanding of physics, solved the mechanical problem of aerodynamic lift for soaring birds eons ago. Why should it be so hard for scientists to explain what keeps birds, and airliners, up in the air?