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Sometimes, superficial turbulence is induced via "turbulators" in order to improve lift or reduce drag.Summary:: I'm wondering if turbulence is required to produce lift.
Please, see:
https://www.mh-aerotools.de/airfoils/turbulat.htm
Sometimes, superficial turbulence is induced via "turbulators" in order to improve lift or reduce drag.Summary:: I'm wondering if turbulence is required to produce lift.
Suggest looking at the web book by Dr. John S. Denker, "see how it flies": https://www.av8n.com/how/
Check chapter 3.
Also checkout his web page on physics topics.
Very good article. I have read quite a bit of Denker's works. Also, emailed him a couple of times. The first time an instructor put me into a spin my first thought was "this could kill me", it looks like you are going straight down, you aren't but it seems like it. You certainly don't have time to analyze the situation - just react.
I thought that was what was referenced by "circular flow". My understanding is that it isn't actually physically true. That circular flow is just a conceptual model of relative flow velocities once you remove the average flow velocity. I am not convinced the conceptual model is correct at the trailing edge as the 2 streams join there. The real flow is always downstream however. It has been a useful concept historically though.Yes, that's exactly what I'm referring to.
It actually does. It's common to represent lift and a pressure profile over/under the wing, along with the associated velocity profile per Bernoulli's principle.Also, my understanding is that Bernoulli does not account for the sum total lift measured in practice.
Denker specifically mentions the physical laws are not cumulative, that is, you don't get some lift because of one law and some from another. All the physical laws are operating simultaneously. So you have event called lift and when that event, a fluid motion problem, is in effect you have Bernoulli's equation available and Newton's various laws. You choose the equations that make solving a particular problem easiest. As a corollary you could look at an electrical circuit - you don't get some current in a branch due to Kirchhoff's voltage law and some due the current law; they are both in effect simultaneously and you use whatever combination of associated equations needed to solve the problem. Denker does show a nice example of Newton's conservation of momentum as related to a wing but that in no way affects the Bernoulli effects - both are just there simultaneously.I thought that was what was referenced by "circular flow". My understanding is that it isn't actually physically true. That circular flow is just a conceptual model of relative flow velocities once you remove the average flow velocity. I am not convinced the conceptual model is correct at the trailing edge as the 2 streams join there. The real flow is always downstream however. It has been a useful concept historically though.
Also, my understanding is that Bernoulli does not account for the sum total lift measured in practice. It seems to me this and accelerating the flow downwards both contribute to the total lift force.
That would mean Bernoulli is wrong. Lift always needs (or is) a force acting between airfoil and air in addition to buoyancy. This force requires (or is equivalent to) a pressure difference between top and bottom side of the airfoil in addition to the static pressure difference and corresponds to different velocities according to Bernoulli. This is always the case as long as the Bernoulli equation holds.Also, my understanding is that Bernoulli does not account for the sum total lift measured in practice.
Whereas there always needs to be a pressure difference according to Bernoulli, there doesn't need to be a downward acceleration of the air. This is just the usual case for airplanes because there is nothing below or above the airfoil that excerts additional forces on the air. In that case the force from the airfoil is equal to the net force acting on the air around it and therefore accelerates it downward according to Newton 2. But this is not the only possible case. Exceptions without or with reduced downwash have already been mentioned above (flying within a tube or near the ground). In these cases the downward acceleration of the air doesn't correspond to the force between air and airfoil.It seems to me this and accelerating the flow downwards both contribute to the total lift force.
I have seen a few lectures where a contra view is presented. Who should I believe and why? Certainly, lift can be explained by the pressure gradients across surfaces. Can you mathematically show that those pressure gradients are totally explained by Bernoulli's principle, even for a symmetrical wing near CLcrit? The reason I ask is that the difference in length of the 2 surfaces is only very slightly different due entirely to the change in stagnation point on the LE with AOA. The rest of the curve is the same. If the flow splits at the stagnation point and re-joins at the TE, then the 2 flows have almost the same mean velocity.It actually does. It's common to represent lift and a pressure profile over/under the wing, along with the associated velocity profile per Bernoulli's principle.
The lift can be calculated by integration of the pressure over the surface (and than subtracting buoyancy). But in order go get the pressure profile according Bernoulli the velocity profile must be given. If you are looking for a general proof, not only for a special case with known velocity profile, then Bernoulli alone is not sufficient. I'm not even sure if such a proof is possible because we don't have general solutions of the Navier-Stokes equations. But we can check it for every special case. Do you have an example for a velocity profile that doesn't match the pressure profile according Bernoulli?Can you mathematically show that those pressure gradients are totally explained by Bernoulli's principle, even for a symmetrical wing near CLcrit?
Why should that be a problem? Lift results from the velocity profile over the whole surface of the airfoil and not from the velocities at two special points only and with different velocities you get different forces even if top and bottom side of the profile have the same length.The reason I ask is that the difference in length of the 2 surfaces is only very slightly different due entirely to the change in stagnation point on the LE with AOA. The rest of the curve is the same. If the flow splits at the stagnation point and re-joins at the TE, then the 2 flows have almost the same mean velocity.
No, I don't have example velocity profiles. I am just going by statements in lectures and technical notes that I have come across over the years. The nature of this is that I do not have references either. I am not insisting anything here, it just seems to me a point of some controversy that I’d like to better understand. In truth, I probably don’t have a high enough technical background to do so but why not ask and see if I can learn something at least.The lift can be calculated by integration of the pressure over the surface (and than subtracting buoyancy). But in order go get the pressure profile according Bernoulli the velocity profile must be given. If you are looking for a general proof, not only for a special case with known velocity profile, then Bernoulli alone is not sufficient. I'm not even sure if such a proof is possible because we don't have general solutions of the Navier-Stokes equations. But we can check it for every special case. Do you have an example for a velocity profile that doesn't match the pressure profile according Bernoulli?
Why should that be a problem? Lift results from the velocity profile over the whole surface of the airfoil and not from the velocities at two special points only and with different velocities you get different forces even if top and bottom side of the profile have the same length.
Same time is wrong. The upper time is usually shorter, despite greater distance....Well, that whole concept of upper surface greater distance in the same time, just always seemed too schoolboy simple and convenient. ...
It is at least strongly misleading. Of course higher speed results in greater distance at the same time. But the flows on top and buttom side don't pass the profile in the same time.Well, that whole concept of upper surface greater distance in the same time, just always seemed too schoolboy simple and convenient.
Without seeing those lectures myself, I can't respond to them directly - I don't know what they said or how it was interpreted. What I will say is that it seems a relatively recent internet phenomena (10-15 years) where teaching lift by attacking fallacies has become popular, and unfortunately when you focus so strongly on fallacies without teaching the realities, it creates new, opposite fallacies. Bernoulli has been incorrectly tied to the most popular fallacy, which has lead people to believe even if just by implication that Bernoulli's principle itself is a fallacy. It isn't.I have seen a few lectures where a contra view is presented. Who should I believe and why?
Of course. Here's an explanation of how it works:Certainly, lift can be explained by the pressure gradients across surfaces. Can you mathematically show that those pressure gradients are totally explained by Bernoulli's principle...
What does that have to do with it? Bernoulli's principle doesn't explain why the speed changes, it only explains how speed and pressure are related. That's it. Anything else is a mis-application/over-reach....even for a symmetrical wing near CLcrit?
That's too qualitative to be easy to respond to, but it sounds vaguely like the equal transit time fallacy. But regardless of the cause of the velocity profile being what it is, the relationship between velocity and pressure follows Bernoulli's principle..... But sure, if the velocity profiles aren't much different, the pressure profiles won't be much different.The reason I ask is that the difference in length of the 2 surfaces is only very slightly different due entirely to the change in stagnation point on the LE with AOA. The rest of the curve is the same. If the flow splits at the stagnation point and re-joins at the TE, then the 2 flows have almost the same mean velocity.
You don't seem to be mentioning that the airflow above the wing gets "squeezed" by this process, not unlike in a Venturi tube (but without the exact area ratio to calculate it). So the air over the top ends up going faster than just the "path length" difference would imply. But again, how the velocity profile is established has nothing to do with Bernoulli.The reason I ask is that the difference in length of the 2 surfaces is only very slightly different due entirely to the change in stagnation point on the LE with AOA. The rest of the curve is the same. If the flow splits at the stagnation point and re-joins at the TE, then the 2 flows have almost the same mean velocity.
There is no actual controversy in the field. It is well understood that both ideas (Bernoulli's and Newton's) have value. And they're related anyway; both use the velocity of the air.While I have an interest, I can not work the proof myself so must rely on technical articles etc. If they disagree on this point, it seems a controversial topic with no clarity as to the truth.
The others pointed out that that's wrong, and I'll name it: that's the "equal transit time" fallacy, which is often incorrectly associated with Bernoulli's principle.“Why should that be a problem?....” Well, that whole concept of upper surface greater distance in the same time, just always seemed too schoolboy simple and convenient.
You need to really emphasize the "-ish" part of this, because according to most modern thinking on the topic, turbulence is entirely deterministic but essentially a form a spatiotemporal chaos, i.e. it is so sensitive to initial conditions that modeling it stochastically is the only tractable solution in most cases.Turbulence:
Even harder to really pin down. But generally speaking it is a random-ish fluctuation of velocities on top of a mean flow. It often starts from an instability in the flow, meaning that a small perturbation grows into large flow features. I'm actually not sure if a vortex is inherently instable such that it will always generate turbulence, or that laminar vortices exist...? But a shear layer (two layers of flow on top of each other with each a different direction) I believe is inherently instable and thus transitions to turbulence rather quickly (but I'm not a genius in stability theory...). Let's not bother with the question about what happens if the fluctuations become as large or larger than the mean flow.
Less efficient? At its most fundamental level, turbulence is dissipative and therefore will tend to decrease efficiency. The caveat here is that turbulent boundary layers are also more resistant to separation, so in cases where they delay separation, the losses due to inducing turbulence can sometimes be less than the gains due to delaying separation (or in the case of wing stall, the losses are an acceptable price to pay for keeping the plane in the air).So, is turbulence required for lift? Hmm, turbulence in the boundary layer is not necessary since fully laminar flow around a wing can also generate lift, albeit (much?) less efficient.
This isn't always doomed to be turbulent, though. You can certainly have everything laminar and maintain lift.But, directly behind the wing, at its trailing edge there is a shear layer generating turbulence. This is however not needed for lift I guess.
Well that is easily answered using ##\vec{F} = m\vec{a}##. You need a force to accelerate the flow, and that force is due to pressure on a fluid element. An element that is accelerating must have a higher pressure upstream of it and so be entering a region of lower pressure. The converse is also true.
- Bernoulli, although absolutely correct and useful, is the most misleading and misused law there is in fluid dynamics, certainly for the laymen. It does not generate any intuition to understand flow. Pressure decreases if velocity increases... I've wondered very long about why this would be true, maybe that's why I studied fluid dynamics later on :).
It actually has nothing to do with the Coandă effect, which is based on the phenomenon where a jet clinks to a nearby surface. This is not what is occurring around an airplane wing. It is more appropriate when discussing planes whose engines are embedded in the wing and produce a jet of air that blows over the wing. See: Boeing YC-14
- The more interesting question about wings is this: why would the flow not separate from the upper side? Why would it follow the surface at all? This has more to do with the Coanda effect. So a plane doesn't fly because of Bernoulli, but because of Coanda! :)
That's not "longer path", that's "equal transit time". It is generally true - not a fallacy - that the parcel taking the longer path arrives at the trailing edge first. But it is popular to mix them together inaccurately (including the oft-cited NASA lift explanation for kids). This is part of what I pointed out earlier: overly aggressive debunking of "equal transit time" that takes other concepts down with it.Oh, one last soapbox remark:
The longer path fallacy is indeed a fallacy. The path length doesn't matter since if you would follow two parcels separating at the stagnation point at the leading edge, one taking the upper route, the other the lower one, they do *not* meet at the trailing edge. So the increase in velocity is not explained by the longer path.
You're absolutely right, that's indeed why I added the '-ish'. But I wasn't going to give a Turbulence lecture here :) Maybe I should have said 'seemingly random' to be more clear.You need to really emphasize the "-ish" part of this, because according to most modern thinking on the topic, turbulence is entirely deterministic but essentially a form a spatiotemporal chaos, i.e. it is so sensitive to initial conditions that modeling it stochastically is the only tractable solution in most cases.
Thank you for the clarification. I was trying not to get into the details, but what I was referring to is when the classical approach of separating the mean flow and the turbulence fluctuations break down. Sometimes it is not clear how to separate the mean flow from the turbulence fluctuations and these fluctuations cannot be called 'small' anymore such that you can neglect these terms.It always starts with some form of instability in the flow, though sometimes the initial perturbation is so large that it breaks down essentially immediately in a process often called bypass transition. Generally speaking, fluctuations do not grow anywhere near the magnitude of the mean flow before a boundary layer breaks down. In subsonic flows, the unstable waves reach perhaps a few percent in magnitude in velocity fluctuations. At the other end of the spectrum, they can reach ~50% in terms of pressure in hypersonic flows.
Ok, you're right, a laminar boundary layer causes less frictional drag making it more efficient. I indeed had the separation issue in mind.Less efficient? At its most fundamental level, turbulence is dissipative and therefore will tend to decrease efficiency. The caveat here is that turbulent boundary layers are also more resistant to separation, so in cases where they delay separation, the losses due to inducing turbulence can sometimes be less than the gains due to delaying separation (or in the case of wing stall, the losses are an acceptable price to pay for keeping the plane in the air).
I really wasn't trying to say that todays me doesn't understand how Bernoulli works, I do. But back in the day, when I still didn't know about this I was puzzled by it. And then Bernoulli simply does not help with getting an intuitive understanding of how wings work. It just replaces one unknown (why does a wing generate lift) with another (why does higher velocity means lower pressure).Well that is easily answered using ##\vec{F} = m\vec{a}##. You need a force to accelerate the flow, and that force is due to pressure on a fluid element. An element that is accelerating must have a higher pressure upstream of it and so be entering a region of lower pressure. The converse is also true.
So how would you explain why the flow over the top part of the wing stays attached onto the surface? Maybe the original Coandă effect is just about jets, but the reason a jet clings to a surface is comparable to why flow over a wing stays attached. It has a lot to do with the wall normal direction of momentum transport. If there is enough of that the flow clings to the surface, if not you have separation (or the jet does not cling to the surface).It actually has nothing to do with the Coandă effect, which is based on the phenomenon where a jet clinks to a nearby surface. This is not what is occurring around an airplane wing. It is more appropriate when discussing planes whose engines are embedded in the wing and produce a jet of air that blows over the wing. See: Boeing YC-14
I think the argument usually goes like this: two air parcels arrive at the leading edge, one goes up, the other goes down, when they arrive at the trailing edge the parcel that took the upper route had to cover a longer distance compared to the lower one and therefore the velocity of the upper parcel had to be higher. Higher velocity means lower pressure, hence lift.That's not "longer path", that's "equal transit time". It is generally true - not a fallacy - that the parcel taking the longer path arrives at the trailing edge first. But it is popular to mix them together inaccurately (including the oft-cited NASA lift explanation for kids). This is part of what I pointed out earlier: overly aggressive debunking of "equal transit time" that takes other concepts down with it.
If you thought I was denying that one path is indeed longer than the other, mea culpa, that is not what I intended to say. The idea here was more that you don't need the longer path to generate lift. A flat plate can also generate lift, planes can fly upside down, etc. That's what I meant by calling it the "longer path" fallacy.I can't imagine how confusing it must be for kids to read/see something so obviously true be claimed as false.
See, now both of those common/popular examples are, in fact, fallacious. If you look at the actual airflow paths you will see that due to the moving stagnation point and redirection of the air up toward/at the leading edge, you will see that the air over the top does indeed take a longer path. And I particularly dislike the flat plate example because a flat plate makes a really poor airfoil. We should not be teaching how lift works based on bad examples/exceptions except where necessary for the math (it is part of aerodynamics calculations to break the airfoil into plates, starting with 1).If you thought I was denying that one path is indeed longer than the other, mea culpa, that is not what I intended to say. The idea here was more that you don't need the longer path to generate lift. A flat plate can also generate lift, planes can fly upside down, etc. That's what I meant by calling it the "longer path" fallacy.
Why not driven forwards somewhat?the top surface is "squeezed" somewhat, and is driven backwards
The "other" air is in the way of that. The parcel of air eventually has to go from in front of to behind the airfoil. If it gets slowed down, it piles up in front of the airfoil.Why not driven forwards somewhat?