Is Turbulence Necessary for Lift on a Wing?

[Dale]

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.

Sorry, I guess I misunderstood your approach above. When you said

It can't bob up and down because other air is preventing it from doing so. If we followed the logic up, it would have the entire column of air, up to space, bob up and down together.

I thought you were doing the analysis in a frame where the bulk air is still and the wing is moving. In that frame I can see how the air bobs up and down, but there is other air in the way both to the front and to the back. I can see your point about it getting squeezed, but in that frame it seems to me like it should be squeezed forward and backward equally.

 

[russ_watters]

Sorry, I guess I misunderstood your approach above. When you said
I thought you were doing the analysis in a frame where the bulk air is still and the wing is moving. In that frame I can see how the air bobs up and down, but there is other air in the way both to the front and to the back.

Well, the results have to be identical, but mea culpa; I do tend to switch back and forth, describing the situation from whichever frame presents (imo) the best visual/mental picture of what happens.

I can see your point about it getting squeezed, but in that frame it seems to me like it should be squeezed forward and backward equally.

In the frame where the air is still and the wing moving? I don't see how it can be expected to move forwards. Where could the air in front of the wing go?

Expanding on the last visual I gave (sorry for the multiple late edits...), let's say you have a box of air, with an airfoil in the middle. The airfoil also represents an airfoil-shaped hole in the air, in the center of the box. Now move the airfoil one chord length forward and stop it. Now you've moved the airfoil-shaped hole in the air forward and where that hole was previously, is filled with air. This has to be true because the front wall of the box prevents the air from being pushed forward out of the box, and the hole in the air where the airfoil started can't just be an unsupported vacuum. When the airfoil is in motion, the air in front rushes back to fill the void left behind it.

 

[russ_watters]

Here's a crude sketch of what I described in Post #52:
The red is the air, arranged in columns, including an airfoil-shaped parcel.

 

[boneh3ad]

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).

The simple answer is that nothing says it has to stay attached to the surface. I tends to on account of the fact that "nature abhors a vacuum," but under the right conditions, separation occurs and you end up with flow that doesn't stay attached to the surface and a recirculating bubble forms. It's all part of the same air stream, though.

 

[boneh3ad]

Expanding on the last visual I gave (sorry for the multiple late edits...), let's say you have a box of air, with an airfoil in the middle. The airfoil also represents an airfoil-shaped hole in the air, in the center of the box. Now move the airfoil one chord length forward and stop it. Now you've moved the airfoil-shaped hole in the air forward and where that hole was previously, is filled with air. This has to be true because the front wall of the box prevents the air from being pushed forward out of the box, and the hole in the air where the airfoil started can't just be an unsupported vacuum. When the airfoil is in motion, the air in front rushes back to fill the void left behind it.

Here's a crude sketch of what I described in Post #52:
The red is the air, arranged in columns, including an airfoil-shaped parcel.

View attachment 269934

There is effectively no way that an "airfoil shaped parcel" passes over the actual object and the result on the other size is another "airfoil shaped parcel" as you have drawn, especially given that this is in a closed box. The movement of the object will have to push the air in front of it out of the way, which will tend to create two circulating regions in the box: one above the chord of the foil rotating clockwise and one below it rotating counterclockwise. The effects of viscosity dragging air along with the airfoil will reinforce this.

Additionally, the air moving over the top of the airfoil will be moving faster relative to the surface and so the parcel that was originally foil-shaped will leave the trailing edge highly distorted and with nonzero vorticity.

 

[russ_watters]

There is effectively no way that an "airfoil shaped parcel" passes over the actual object and the result on the other size is another "airfoil shaped parcel" as you have drawn, especially given that this is in a closed box...

Obviously, my 15 second post-it note sketch is very simplistic, and obviously some of the air that was in front will pass above and some below the airfoil, at least breaking up and distorting the parcel before the air moves behind the airfoil. "Parcel" was probably the wrong word since it tends to be used to describe a very small amount of air, which all moves together (though it does include deformation, and two parcels would probably work). The point of the exercise is that there is a volume occupied by the airfoil in the first sketch, which is then occupied by air in the second, and vice versa, per continuity. It doesn't actually have to be the same molecules of air, but it does have to be the same volume and mass of air (in incompressible flow).

Additionally, the air moving over the top of the airfoil will be moving faster relative to the [bottom?] surface and so the parcel that was originally foil-shaped will leave the trailing edge highly distorted and with nonzero vorticity.

What, you couldn't tell the sketch was of a symmetrical airfoil at zero aoa? What you are analyzing goes way outside the scope of the thought experiment.

 

[Arjan82]

@russ_watters, regarding post #48, I've tried to infer your way of viewing lift. Am I correct in my description of your thoughts below?

Consider two flow parcels hitting the airfoil at the stagnation point at the LE. One moves along the top, the other along the bottom. When the airfoil generates lift the top route will always be longer than the bottom route. If the geometry seemingly has a top route which is of equal length or even shorter, than the stagnation point will shift until this is still true. The top route is longer because there is more vertical displacement compared to the bottom route. This vertical displacement pushes against air above it and this air above will push back resulting in increased flow velocity (the venturi like effect). This increased flow velocity will result in a lower pressure as per Bernoulli. Hence lift.

Is this correct? If so, I will think about to what extent I agree with it :). I will already say that you are probably correct in that the top route is always longer compared to the bottom route, even for flat plates (looking at the stream lines and treating the separated area as if it was part of the flat plate):

 

[Arjan82]

The simple answer is that nothing says it has to stay attached to the surface. I tends to on account of the fact that "nature abhors a vacuum," but under the right conditions, separation occurs and you end up with flow that doesn't stay attached to the surface and a recirculating bubble forms. It's all part of the same air stream, though.

To me this is a bit unsatisfying. You say 'nothing says it has to stay attached', sure, but it does actually stay attached though... And by the lack of random airplanes falling from the skies there is some physics to it as to why this is the case :). That is what I am looking for.

So 'under the right conditions', what are these conditions? To me it has to do with how well the flow is capable to transport the horizontal directed momentum vertically towards the surface. Turbulent flow is a lot better in momentum transport normal to the stream lines which to me perfectly explains why an airfoil with a turbulent boundary layer can sustain a higher lift coefficient before stalling compared to a laminar boundary layer.

To me the name for this effect is the Coanda effect. Although originally this was for jets clinging to the surface. But it is just a name, we can also name it the Arjan82 effect ;).

 

[Arjan82]

In the frame where the air is still and the wing moving? I don't see how it can be expected to move forwards. Where could the air in front of the wing go?

So at the stagnation point of your symmetric 0 AOA airfoil the velocity has no vertical component. Since the air must move out of the way, it can only go forwards. Somewhat above the stagnation point the flow will probably still move a bit forward, but also upward. When moving upward it will move aft-wards again and eventually end up more aft than where it started. So the trajectory of a fluid parcel just above the stagnation point is shaped more or less like a partial oval, first moving forward a bit, then up and aft-wards and eventually downwards. So there is a part of the flow in your example that has at some point during the movement a forward velocity component.

 

[russ_watters]

So at the stagnation point of your symmetric 0 AOA airfoil the velocity has no vertical component. Since the air must move out of the way, it can only go forwards.

I'm not sure if you mean that literally. A point by definition has zero size. While I guess it is possible that a single molecule is sitting on the leading edge, straddling the stagnation point, I doubt it. Air certainly doesn't pile-up there.

Somewhat above the stagnation point the flow will probably still move a bit forward, but also upward. When moving upward it will move aft-wards again and eventually end up more aft than where it started. So the trajectory of a fluid parcel just above the stagnation point is shaped more or less like a partial oval, first moving forward a bit, then up and aft-wards and eventually downwards. So there is a part of the flow in your example that has at some point during the movement a forward velocity component.

If air molecules get slowed down as they approach the stagnation point, they must be accompanied by air molecules further away (vertically) that speed up, so the average velocity is higher.

 

[russ_watters]

@russ_watters, regarding post #48, I've tried to infer your way of viewing lift. Am I correct in my description of your thoughts below?
...

Yes, that's basically what I'm saying. Also, I really dislike the flat plate since the airflow over the top surface is just such a mess. But at least in that pic you can see the stagnation point down under the "chin" and the airflow curving up to meet the airfoil before it even gets to it.

 

[Arjan82]

Just to be clear, I look at this now strictly from an earth-fixed reference frame, so the foil is moving and the box is standing still.

I'm not sure if you mean that literally. A point by definition has zero size. While I guess it is possible that a single molecule is sitting on the leading edge, straddling the stagnation point, I doubt it. Air certainly doesn't pile-up there.

That there is a forward velocity component doesn't mean the air piles-up. Take an air parcel as close to the stagnation point as you can. At this stagnation point a tangent line to the foil is vertical. What you suggest is that if the foil starts moving this air parcel never moves forward but just directly upward (or downward). I find that unlikely since the wall can only push in horizontal direction (ignoring friction) or, because a parcel has a finite size and the tangent point is indeed a point, the wall will move the parcel nearly only forward. Because of continuity the other parcels around it also need to have a forward velocity component albeit somewhat lower.

If air molecules get slowed down as they approach the stagnation point, they must be accompanied by air molecules further away (vertically) that speed up, so the average velocity is higher.

So, following the frame of reference of the foil the approaching air 'slows down' as you say. But from the frame of reference of the box, the air was already not moving. Thus the approaching foil 'speeds up' the air in the other direction, forward that is :).