# Turbulence Required For Lift

person123
For the wing of a plane, as an example, I think that the circulation around the wing due to faster flow on the top is balanced by the circulation of a vortex which is formed off of the tail. Since the greater speed on the top is necessary for the lift, would that also mean the turbulent flow which has the formation of vortices is necessary?

## Answers and Replies

Science Advisor
Summary:: I'm wondering if turbulence is required to produce lift.

For the wing of a plane, as an example, I think that the circulation around the wing due to faster flow on the top is balanced by the circulation of a vortex which is formed off of the tail. Since the greater speed on the top is necessary for the lift, would that also mean the turbulent flow which has the formation of vortices is necessary?
Do you mean that some vortices have to form somewhere behind the plane? I would say yes, because the plane accelerates some of the air downwards.

But a plane in an open airmass is one special case. The above doesn't have to be true for all cases of lift, for example for enclosed flow in a pipe.

russ_watters
Science Advisor
Gold Member
Summary:: I'm wondering if turbulence is required to produce lift.

For the wing of a plane, as an example, I think that the circulation around the wing due to faster flow on the top is balanced by the circulation of a vortex which is formed off of the tail. Since the greater speed on the top is necessary for the lift, would that also mean the turbulent flow which has the formation of vortices is necessary?
This is a regular favourite! The 'speed' over the top of an aerofoil may be greater but the vertical displacement of air by the wing thickness returns to zero when it comes back down. At low airspeeds, there is, apparently, no reason why the two merging airflows should be out of step so there's no reason for turbulence to occur - certainly not in the region over the top where the lift force appears.
because the plane accelerates some of the air downwards.
. . . and that (downward) change of momentum of the air is the overall reason for the upward lift force. (Using the bigger picture and allowing Newton to enter the argument)
A very significant mass pushed downwards per second to lift a 1T aeroplane.

Science Advisor
A very significant mass pushed downwards per second to lift a 1T aeroplane.
The point was that only a small part of the atmosphere is pushed down, which induces vortices in the wake.

Mentor
Wait a minute. Are vortices turbulence? I thought those were different things.

DaveE and person123
DrStupid
. . . and that (downward) change of momentum of the air is the overall reason for the upward lift force.

It is controversal if this is cause or effect. It is at least not sufficient to explain lift.

DrStupid
The point was that only a small part of the atmosphere is pushed down, which induces vortices in the wake.

That would just mean that vortices are a side effect but not that they are required. A.T. already mentioned a case that could be used to check that. In a laminar fow within a pipe there should be no vortices. If they would be required for lift there should be no lift anymore. Is that the case?

Science Advisor
Gold Member
It is controversal if this is cause or effect. It is at least not sufficient to explain lift.
Unless one is wedded to the religion of Bernouli I can't see a reason for not considering Momentum change. Even the staunchest Bernouli = flight supporters are happy to use Momentum when describing the action of a helicopter blade so where does that stop?
The partial vacuum above a wing is there, of course and Bernouli explains it for an aerofoil but it's only half way towards the full argument. Particularly when you try to explain planar wings and upside down flight. In those examples, the Bernouli effect is allowed not to be relevant but in all examples, there is net displacement of air downwards. I don't understand why this is so hard to acknowledge. Can we have a reactionless force? I'm still looking for one of them.
As for cause and effect, the 'cause' could be the pilot filling up with fuel before the flight.
P.S. Sorry but I enjoy poking this particular wasp nest.

person123
Wait a minute. Are vortices turbulence? I thought those were different things.
I sort of imagined if vortices form, it means the flow is turbulent, but I guess that isn't necessarily the case. Could flow still be laminar with vortices?

Do you mean that some vortices have to form somewhere behind the plane? I would say yes, because the plane accelerates some of the air downwards.
But I'm wondering if it not only needs to accelerate the air downward, but also induce angular momentum at the trailing edge.

DrStupid
I don't understand why this is so hard to acknowledge.

Same procedure as for the vortex above: prevent the downwash by a tube and see if there is still lift or not.

sophiecentaur
Mentor
Could flow still be laminar with vortices?
I don’t know. That is why I asked. Hopefully one of the other members will be able to answer that.

Science Advisor
Gold Member
But the tube will 'weigh more'.
Question: Has anyone ever designed an aeroplane that causes no down wash?

person123
DrStupid
I sort of imagined if vortices form, it means the flow is turbulent, but I guess that isn't necessarily the case.

I agree that turbulent means that vortices form but not the other way around. I would limited it to chaotic formation of vortices at different scales. But I'm not an expert.

Science Advisor
But the tube will 'weigh more'.
Question: Has anyone ever designed an aeroplane that causes no down wash?
Just to clarify: In post #2 I didn't mean a flying tube/pipe.

The point considering the flow in a pipe with some airfoil across its whole diameter is to test if lift can be generated without vortices at all, not if that is applicable to airplanes.

russ_watters
DrStupid
Just to clarify: In post #2 I didn't mean a flying tube/pipe.

Maybe I also need to clarify that in #9 and #10 I didn't mean a flying tube/pipe but an airplane flying within a pipe.

Mentor
That would just mean that vortices are a side effect but not that they are required.
I would agree with that - a negative side effect that designers often work hard to minimize.

Guys - can we please stay off the lift fallacies/what causes lift argument?! The OP didn't ask about it and it isn't necessary to derail every_single_lift_thread arguing about it!

Dale, sophiecentaur and person123
DrStupid
Guys - can we please stay off the lift fallacies/what causes lift argument?! The OP didn't ask about it and it isn't necessary to derail every_single_lift_thread arguing about it!

I'm afraid we can't answer the question without discussing the generation of lift itself. If lift is seen as the reaction to the downstream there is no vortext required. But a vortex is required if lift is explained with the Kutta–Joukowski theorem (for example).

person123
That would just mean that vortices are a side effect but not that they are required. A.T. already mentioned a case that could be used to check that. In a laminar fow within a pipe there should be no vortices. If they would be required for lift there should be no lift anymore. Is that the case?
Assuming that there is still lift, what would be going on (or in any case in which lift is generated with vortices)? If the flow is indeed faster on the top, it seems the fluid has a net angular momentum around any pivot point. In order for angular momentum to be conserved, it seems to me there would have to circulating flow in the opposite direction to counter it.

Mentor
But a vortex is required if lift is explained with the Kutta–Joukowski theorem (for example).
I don't agree. If you're calling the circulation around the wing a "vortex", that isn't what the OP is referring to.

DrStupid
If the flow is indeed faster on the top, it seems the fluid has a net angular momentum around any pivot point.

That's the circular flow aroud the airfol the Kutta–Joukowski theorem is based on.

In order for angular momentum to be conserved, it seems to me there would have to circulating flow in the opposite direction to counter it.

That's the starting vortex.

DrStupid
If you're calling the circulation around the wing a "vortex", that isn't what the OP is referring to.

#19 reather sounds like it exactly what he is referring to.

person123
That's the circular flow aroud the airfol the Kutta–Joukowski theorem is based on.

That's the starting vortex.
So would that starting vortex be required?

Mentor
#19 reather sounds like it exactly what he is referring to.
Erp - yep, sorry I think you're right. Back to the OP:
For the wing of a plane, as an example, I think that the circulation around the wing due to faster flow on the top is balanced by the circulation of a vortex which is formed off of the tail. Since the greater speed on the top is necessary for the lift, would that also mean the turbulent flow which has the formation of vortices is necessary?
You're referring to this?:

Yes, it is required. Since it happens after the wing, though, the context is odd. Usually when talking about laminar or turbulent flow people are talking about the flow over the airfoil itself, not what happens in the air behind it. But yes, it is required.

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person123
person123
Erp - yep, I think you're right. Back to the OP.

You're referring to this?:
View attachment 264362

Yes, it is required.
Yes, that's exactly what I'm referring to.

Homework Helper
Gold Member
Joseph M. Zias
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.

vanhees71 and Lnewqban
Joseph M. Zias
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.

Lnewqban
Kenwstr
Yes, that's exactly what I'm referring to.

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.

Mentor
Also, my understanding is that Bernoulli does not account for the sum total lift measured in practice.
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.

Lnewqban
Joseph M. Zias
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.
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.

A.T., Lnewqban, vanhees71 and 2 others
DrStupid
Also, my understanding is that Bernoulli does not account for the sum total lift measured in practice.

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.

It seems to me this and accelerating the flow downwards both contribute to the total lift force.

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.

The downward acceleration of the air is not only not a necessary condition for lift. It actually is an unwanted side effect because it reduces the lift. When air flows out of the high pressure zone below the wing or into the low pressure zone above the wing the pressure difference and therefore the lift is decreased. At normal conditions only the direct air flow from the bottom to the top side around the wing tip can be reduced (e.g. by winglets). But at very low altitudes it is also possible do reduce the downwash because the gound prevents the air from escaping downward. If lift would be supported by the downwash, it should be reduced under these conditions. But instead it is significantly increased (ground effect).

Kenwstr
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.

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.

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.

sophiecentaur
DrStupid
Can you mathematically show that those pressure gradients are totally explained by Bernoulli's principle, even for a symmetrical wing near CLcrit?

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?

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.

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.