Turbulence Required For Lift

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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?
 

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  • #2
A.T.
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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.
 
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  • #3
sophiecentaur
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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.
 
  • #4
A.T.
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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.
 
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Wait a minute. Are vortices turbulence? I thought those were different things.
 
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  • #6
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. . . 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.
 
  • #7
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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?
 
  • #8
sophiecentaur
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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.
 
  • #9
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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.
 
  • #10
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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.
 
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  • #11
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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.
 
  • #12
sophiecentaur
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But the tube will 'weigh more'.
Question: Has anyone ever designed an aeroplane that causes no down wash?
 
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  • #14
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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.
 
  • #15
A.T.
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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.
 
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  • #16
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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.
 
  • #17
russ_watters
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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!
 
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  • #18
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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).
 
  • #19
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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.
 
  • #20
russ_watters
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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.
 
  • #21
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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.
 
  • #22
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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.
 
  • #23
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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?
 
  • #24
russ_watters
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#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?:
xy4tc.png


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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