Fluid flow over part of an aircraft wing

Recently, on a conversation with PhD in Engineering, I came to know that if, instead of the whole wing, air/fluid flows over part of it, then that will cause great turbulence and swirl. When I asked him for reference, he said that it can be found in any book on Aerodynamics. But still today, I haven't been able to find out.

To be more precise, let's start with two imaginary scenario. First, there is an aircraft wind tested in an wind tunnel where everything goes according the Bernoulli's principle and other known factors. Now, instead of wind tunnel, we used a small sized fan that has much less diameter than the length of the wing, does that will cause big turbulence and swirl over the part of the wing that hasn't got any kind of airflow?

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CWatters
Homework Helper
Gold Member
Yes. It's hard to make a jet of air that doesn't interact with stationary air either side of it. Certainly it would be hard to do with just a fan. The jet would tend to diverge creating air flow over unintended parts.

You might be able to solve the problem by adding fences/strakes to contain the airflow over the of of interest.

256bits
Gold Member
So the PhD left you a mystery......

https://en.wikipedia.org/wiki/Stall_(fluid_mechanics)

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So the PhD left you a mystery......
https://en.wikipedia.org/wiki/Stall_(fluid_mechanics)
Can't understand how that is relevant to what I have asked. What I want to know has nothing to do with the angle of attack but rather flow of air/fluid over a fraction of the wing surface instead of the whole wing.
Yes. It's hard to make a jet of air that doesn't interact with stationary air either side of it. Certainly it would be hard to do with just a fan. The jet would tend to diverge creating air flow over unintended parts.
You might be able to solve the problem by adding fences/strakes to contain the airflow over the of of interest.
I know that jets always tend to diverge and it's a common experience we can gather from fans used for everyday cooling. But actually I don't have any problem if the jet tends to divert. What I want to know is whether such partial flow will cause turbulence and swirl or not. My common sense tells me that there isn't any reason for creation of turbulence and swirl for such reason. And, by the way, the jet will flow on both side of the wing just not over the whole part of it.

256bits
Gold Member
Can't understand how that is relevant to what I have asked. What I want to know has nothing to do with the angle of attack but rather flow of air/fluid over a fraction of the wing surface instead of the whole wing.
All I can say is, you sure.
Seems to be a lot of swirl and turbulence as the flow separates from the wing, from front to rear. ( and not side to side )
Interpretation of what the PhD was saying, between you and me, I guess is different.

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CWatters
Homework Helper
Gold Member
PGI.. look up how wing tip vortices are formed. If the pressure is increased below a wing the air tries to escape in all directions, including sideways. It doesn't just flow rearwards.

PGI.. look up how wing tip vortices are formed. If the pressure is increased below a wing the air tries to escape in all directions, including sideways. It doesn't just flow rearwards.
Does that mean formation of turbulence and swirl to a very high degree?

Baluncore
If you only have airflow over part of the length of a wing you will only get lift over that part.
Lift comes from lower pressure above the wing with some higher pressure below the wing. That means still air above the wing will flow along the wing into the moving air lift region, while flowing air below the wing will spread out along the wing into the still region.
In the transition region there will be air departing the trailing edge from above and below with very different velocities = turbulence.

Why would you want to test only part of a wing?

CWatters
Homework Helper
Gold Member
Does that mean formation of turbulence and swirl to a very high degree?
What does high degree mean? It's significant enough to make high aspect ratio wings more efficient than low aspect ratio wings. It's an issue that has affected the design and performance of everything from WW2 fighter aircraft to F1 racing cars. Ditto modern airliners where, I believe winglets significantly reduce drag and improve fuel consumption.

And the situation you describe would be worse. On aircraft you get this effect even though there is airflow over the whole wing. It could be more pronounced with airflow only over part of the wing

anorlunda
Staff Emeritus
@P G I, please clarify your question. Does it pertain to real aircraft or only to the validity of a wind tunnel test?

russ_watters
Mentor
What I want to know is whether such partial flow will cause turbulence and swirl or not. My common sense tells me that there isn't any reason for creation of turbulence and swirl for such reason. And, by the way, the jet will flow on both side of the wing just not over the whole part of it.
Consider a fan alone in an empty room. All of the air blown out the front must re-circulate to the back of the fan to maintain continuity.

CWatters
Gold Member
I am very confused by all of this. The specific example given, which is a fan blowing over a portion of a wing but not all of it, is kind of nonsensical when the metric by which you evaluating the results is minimizing "swirl", which I will henceforth call vorticity. A fan, by default, adds considerable vorticity to the flow downstream of it; after all, it is using rotating blades to impart motion to the flow.

If you instead just use a hypothetical blower upstream of your wing that creates a laminar jet blowing over a portion of the wing, then you still have vorticity that is generated, this time around the edges of the jet. You have a jet at one speed and a free stream at a different speed, which sets up a shear layer. That shear region generates vorticity and is subject to the Kelvin-Helmholtz instability. In fact, this happens regardless of whether there is a wing in the jet or not.

berkeman and CWatters
If you only have airflow over part of the length of a wing you will only get lift over that part.
Lift comes from lower pressure above the wing with some higher pressure below the wing. That means still air above the wing will flow along the wing into the moving air lift region, while flowing air below the wing will spread out along the wing into the still region.
In the transition region there will be air departing the trailing edge from above and below with very different velocities = turbulence.
Sorry for the inconvenience, but at present I can't clearly define (actually not even able to give some hint about that. So, as per you, the the vortex will appear as the higher pressure stagnant air from below will come up and mix with lower pressure air above. But, if somehow that's prevented, then the possibility of formation of turbulence can be minimized.
If you instead just use a hypothetical blower upstream of your wing that creates a laminar jet blowing over a portion of the wing, then you still have vorticity that is generated, this time around the edges of the jet. You have a jet at one speed and a free stream at a different speed, which sets up a shear layer. That shear region generates vorticity and is subject to the Kelvin-Helmholtz instability. In fact, this happens regardless of whether there is a wing in the jet or not.
Thanks for correcting me, you have correctly said that to get a laminar flow over a part of a wing, a blower is needed. And I must admit that I have forgot to mention the laminar flow. Whatsoever, I think you too want to say that the vorticity/turbulence will be caused when the higher pressure air from below will come up and mix with lower pressure air above the wing, right?

Gold Member
Thanks for correcting me, you have correctly said that to get a laminar flow over a part of a wing, a blower is needed. And I must admit that I have forgot to mention the laminar flow. Whatsoever, I think you too want to say that the vorticity/turbulence will be caused when the higher pressure air from below will come up and mix with lower pressure air above the wing, right?

No, what I am saying is that any time you have a stream of air moving through other air that has a different speed, there will be vorticity generated regardless of whether a wing is present or not.

No, what I am saying is that any time you have a stream of air moving through other air that has a different speed, there will be vorticity generated regardless of whether a wing is present or not.
Does such phenomenon occurs in our everyday fans that we use for cooling?

Gold Member
Does such phenomenon occurs in our everyday fans that we use for cooling?

Again, every time a fluid with one speed moves by the same or a different moving at a different speed, it generates shear and vorticity. This does include just a fan on a desk.

Do this effects slow down the fluid flow?

Gold Member
Yes.

Is there any formula from which we can detect the loss in velocity?

Gold Member
Fluid mechanics does not have a lot of examples of being able to use simple formulae to represent what is otherwise and extremely complex physical phenomenon. Your best bet is to look up the various sources on the behavior of jets. There are various models for how jets behave. In the simplest example, you can simply look at how it diverges and use the area of the jet at a given location along with conservation of mass to determine the average velocity across it at that location. More detail than that requires more careful consideration of viscosity.

anorlunda
256bits
Gold Member
When I asked him for reference, he said that it can be found in any book on Aerodynamics
we used a small sized fan that has much less diameter than the length of the wing
Experiment has been performed countless times since the early days of winged flight.
The propeller is smaller the wing length.
One has the slipstream of the propeller and wing interaction, and if that is what the PhD is talking about, is it actually mentioned in any book on aerodynamics??. Rather the stall characteristics of wing are mentioned more likely for pilot flight school and beginning aerodynamic courses,

If you are interested here is a mild discussion of slipstream/ wing interaction.
https://www.researchgate.net/publication/27341924_Propeller_Wing_Aerodynamic_Interference