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Flow separation leads to stalling?!

by tsimon
Tags: flow, leads, separation, stalling
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tsimon
#1
Feb29-12, 09:27 AM
P: 25
Why does flow separation give less lift?

A separated "suction side" would in my pov give zero pressure (which is the lowest possible).
And low pressure on suction side is, of course, beneficial.

I think I got it now; as the pressure can not drop further a increase in angle of attack will produce less lift as the projected force (integration of pressure difference) in "lift direction" will be smaller.
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boneh3ad
#2
Feb29-12, 10:20 AM
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Quote Quote by tsimon View Post
Why does flow separation give less lift?

A separated "suction side" would in my pov give zero pressure (which is the lowest possible).
And low pressure on suction side is, of course, beneficial.

I think I got it now; as the pressure can not drop further a increase in angle of attack will produce less lift as the projected force (integration of pressure difference) in "lift direction" will be smaller.
First, you can't have "zero pressure" unless you are in a vacuum.

Anyway, when the flow separates, you get a separation bubble which is a local area of the flow where the boundary layer has detached and formed a circulating bubble between it and the surface. In this bubble, velocities are typically much lower than underneath. When you slow a fluid down, the static pressure rises, so now you have a higher pressure above the airfoil than below in certain points. That can lead to a loss of lift, and in some situations, even the opposite effect.
tsimon
#3
Feb29-12, 10:23 AM
P: 25
Ok, so flow separation does not really mean that a volume is evacuated but rather that there is a circulation?

so this is incorrect:
http://upload.wikimedia.org/wikipedi...lFormation.svg

But if so, why would flow separation be negative for a golf ball? If separation increases pressure than the drag due to pressure should decrease?

boneh3ad
#4
Feb29-12, 10:43 AM
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Flow separation leads to stalling?!

No. It means that the shear stress at the wall goes to zero due to an adverse pressure gradient, which effectively moves the boundary layer up off of the wall. Of course there will be air in between the boundary layer and the wall still, as you aren't going to create a vacuum. That air will just be barely moving compared to all the other air moving around and often forms a recirculating bubble.

Quote Quote by tsimon
But if so, why would flow separation be negative for a golf ball? If separation increases pressure than the drag due to pressure should decrease?
Did you reread this? You seem to be contradicting yourself. When flow separates, it leaves a region of relatively low pressure, meaning that when a golf ball separates without dimples, you have a larger region of low pressure than if separation is delayed by the dimples, so you have more drag. That is why the dimples help. They delay separation by creating a turbulent boundary layer.
tsimon
#5
Feb29-12, 10:52 AM
P: 25
Quote Quote by boneh3ad View Post
...Anyway, when the flow separates, you get a separation bubble which is a local area of the flow where the boundary layer has detached and formed a circulating bubble between it and the surface. In this bubble, velocities are typically much lower than underneath. When you slow a fluid down, the static pressure rises, so now you have a higher pressure above the airfoil than below in certain points...
Quote Quote by boneh3ad View Post
...Did you reread this? You seem to be contradicting yourself. When flow separates, it leaves a region of relatively low pressure, meaning that when a golf ball separates without dimples, you have a larger region of low pressure than if separation is delayed by the dimples, so you have more drag. That is why the dimples help. They delay separation by creating a turbulent boundary layer.

What I meant was if separation gives high pressure (as you wrote in your first responce) then separation on the backside of an golf ball would be beneficial.

Finally I want to thank you for answering my questions.
boneh3ad
#6
Feb29-12, 11:33 AM
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Indeed I did contradict myself, haha. My apologies. Let's try this again, this time without forgetting everything I know about aerodynamics.

All I said about the reason separation occurs is correct, and my golf ball stuff was correct.

On an airfoil, when the flow separates, the displacement thickness increases greatly, makin the airfoil appear much thicker. At a certain point, this effect gets so large that the separation bubbles begin shedding and causing great disturbances in the flow, altering the airfoil's ability to deflect flow downward, which is, in the most simple sense, the reason for lift generation.
tsimon
#7
Feb29-12, 11:47 AM
P: 25
A decrease in the airfoils ability to deflect the flow downward implies that the transverse (?) pressure gradient is smaller and thereby the pressure is higher
boneh3ad
#8
Feb29-12, 12:39 PM
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Look at it this way, as the angle of attack gets larger and the separated region grows, the airfoil begins to look more and more like a circular cylinder which sheds vortices, generates large amounts of drag and no lift.
rcgldr
#9
Feb29-12, 03:54 PM
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Stall doesn't mean complete loss of lift, as even the turbulent flow will have some reduction in pressure, but usually means a large reduction in lift and large increase in drag. If you plot coefficient of lift versus angle of attack, as angle of attack increases, so does lift, but at soma critical angle of attack further increases in angle of attack result in reduced lift and this is considered stall condition. Usually this is aggravated by the fast than when lift is reduced, the aircraft or that wing will descend, increasing angle of attack, reducing lift further, possibly resulting in a snap roll.

When the flow separates enough to result in a stalled condition, the area under the detached flow is filled with vortices (as opposed to some gigantic stagnant separation bubble). I'm not sure what happens to the boundary layer, since the flow near the wing maybe just reversed (it's moving forwards at the suface of the wing).

Youtube video of real aircraft wing with streamers. Stall region first occurs at 55 seconds into video, with the streamers moving forwards (due to the vortice). The wing has washout (less angle of attack at the tips), so the root reaches stall condition first.

http://www.youtube.com/watch?v=bIsWseMbDQU&hd=1

In these video, although the flow is claimed to remain attached in some of them, it's actually seperating somwhat, but not enough to be "stalled". It's not clear how much of those small voids of separation are filled with a boundary layer or smalll vortices. These are relatively slow speed wind tunnels, so the stall regime occurs at very large angles of attack. For real aircraft, stall regime usually occurs between 8 to 22 degrees angle of attack, depending on the aircraft (delta wing's swept back leading edge can take advantage of small leading edge vortices, so they can reach 20 degrees or so angle of attack without stall).

http://www.youtube.com/watch?v=6UlsArvbTeo

http://www.youtube.com/watch?v=xW63SZ1LAqo&hd=1

http://www.youtube.com/watch?v=J-xxCkebdZs

higher speed lower angle of attack stall regime:

http://www.youtube.com/watch?v=5wIq75_BzOQ
boneh3ad
#10
Feb29-12, 04:47 PM
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Quote Quote by rcgldr View Post
Stall doesn't mean complete loss of lift, as even the turbulent flow will have some reduction in pressure...
Excellent post and description, hopefully alleviating the confusion I likely caused with my initial incorrect post. However, this first line is not relevant and, generally, not correct.

Turbulence on a wing is often independent of separation (though separation will almost certainly trip the boundary layer to turbulence). In fact, if the boundary layer is turbulent before separation occurs, it will delay the onset of separation due to the fact that the turbulent mixing does a much better job of redistributing momentum to resist adverse pressure gradients than does a laminar boundary layer, hence the golf ball example.

Additionally, if you could somehow have 100% turbulent flow over a wing (not technically possible) that is not separated, you would still have approximately the same lift as the same case but fully laminar. Drag would be the quantity that is most affected.
tsimon
#11
Mar3-12, 12:39 PM
P: 25
Quote Quote by rcgldr View Post
Stall doesn't mean complete loss of lift, as even the turbulent flow will have some reduction in pressure, but usually means a large reduction in lift and large increase in drag. If you plot coefficient of lift versus angle of attack, as angle of attack increases, so does lift, but at soma critical angle of attack further increases in angle of attack result in reduced lift and this is considered stall condition. Usually this is aggravated by the fast than when lift is reduced, the aircraft or that wing will descend, increasing angle of attack, reducing lift further, possibly resulting in a snap roll.

When the flow separates enough to result in a stalled condition, the area under the detached flow is filled with vortices (as opposed to some gigantic stagnant separation bubble). I'm not sure what happens to the boundary layer, since the flow near the wing maybe just reversed (it's moving forwards at the suface of the wing).

Youtube video of real aircraft wing with streamers. Stall region first occurs at 55 seconds into video, with the streamers moving forwards (due to the vortice). The wing has washout (less angle of attack at the tips), so the root reaches stall condition first.

http://www.youtube.com/watch?v=bIsWseMbDQU&hd=1

In these video, although the flow is claimed to remain attached in some of them, it's actually seperating somwhat, but not enough to be "stalled". It's not clear how much of those small voids of separation are filled with a boundary layer or smalll vortices. These are relatively slow speed wind tunnels, so the stall regime occurs at very large angles of attack. For real aircraft, stall regime usually occurs between 8 to 22 degrees angle of attack, depending on the aircraft (delta wing's swept back leading edge can take advantage of small leading edge vortices, so they can reach 20 degrees or so angle of attack without stall).

http://www.youtube.com/watch?v=6UlsArvbTeo

http://www.youtube.com/watch?v=xW63SZ1LAqo&hd=1

http://www.youtube.com/watch?v=J-xxCkebdZs

higher speed lower angle of attack stall regime:

http://www.youtube.com/watch?v=5wIq75_BzOQ
Great responce, thanks. I still (pardon me being a retard) having a hard time grasping why separation is negative for a golf ball. A detached flow filled with vortices should in my pov increase pressure and this should give less resistance for the ball.
rcgldr
#12
Mar3-12, 03:26 PM
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Quote Quote by tsimon View Post
I still (pardon me being a retard) having a hard time grasping why separation is negative for a golf ball. A detached flow filled with vortices should in my pov increase pressure and this should give less resistance for the ball.
For a golf ball, if the flow detaches sooner, the resulting wake is larger, accelerating more air in the direction of the golf ball, resulting in higher drag.

http://wings.avkids.com/Book/Sports/...r/golf-01.html

http://www.franklygolf.com/golf-ball-aerodynamics.aspx
tsimon
#13
Mar3-12, 04:30 PM
P: 25
So stagnation gives lower pressure? Makes sence for golf ball but not for airfoil as low pressure should be beneficial for an airfoil.
RandomGuy88
#14
Mar3-12, 06:52 PM
P: 363
When the flow separates on an airfoil the pressure in the recirculating bubble will be lower than atmospheric pressure and will usually still be lower than the pressure on the lower surface. But you generally lose the suction peak you get for attached flow. You also won't have the pressure recovery you would get for attached flow so your airfoil is essentially becoming a bluff body and the circulation stops increasing with angle of attack. Similarly if the flow begins by separating at the trailing edge this alters the shape of your airfoil and reduces the circulation which in turn reduces your suction peak so lift goes down.

when the flow separates the pressure doesn't actually change much from the pressure at the point where the flow separated. So when the flow on the golf ball separates the pressure stays fairly low and does not increase back to free stream pressure like it would with no separation. This creates a large region of nearly constant, lower than ambient pressure, on the backside of the ball creating drag. If you were to look at the pressure distribution of a body with a large region of separated flow it would be pretty flat in the recirculating region.

If the boundary layer were completely turbulent on the airfoil this could dramatically change the lift because the displacement thickness is much larger and this alters the effective shape of the body. This usually decambers the airfoil and reduces the lift. I believe a similar thing happens on swept wing that experience attachment line transition. In this case the boundary layer almost immediately becomes turbulent at the attachment line and creates an excessively thick boundary layer dramatically reducing the lift.
boneh3ad
#15
Mar3-12, 10:35 PM
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Quote Quote by RandomGuy88 View Post
If the boundary layer were completely turbulent on the airfoil this could dramatically change the lift because the displacement thickness is much larger and this alters the effective shape of the body. This usually decambers the airfoil and reduces the lift.
Let's not exaggerate. It will almost never "dramatically" change the lift. Even a turbulent boundary layer on a typical airfoil is only often several millimeters thick. The displacement thickness is not that much to begin with so it won't be that dramatic of an effect most of the time. Of course there are always exceptions, but in general the increased turbulent displacement thickness is not that dramatic.

Quote Quote by RandomGuy88 View Post
I believe a similar thing happens on swept wing that experience attachment line transition. In this case the boundary layer almost immediately becomes turbulent at the attachment line and creates an excessively thick boundary layer dramatically reducing the lift.
That is definitely true of attachment line contamination, but again it isn't the huge effect you seem to be implying. The turbulence is much more important due to its effect on drag than lift.
RandomGuy88
#16
Mar4-12, 12:32 AM
P: 363
You are right in general that a turbulent boundary layer has more of an effect on drag then lift and I didn't mean to imply it was the norm although it certainly came out that way. I can think of a few cases I have seen however where early boundary layer transition has reduced the maximum lift by 10 to 20% which is certainly not small. It can happen for certain airfoil sections when the Reynolds number increases and the transition point moves forward. This sort of thing is also a problem when testing multi element airfoils/wings because their performance can be very sensitive to thickness of the boundary layers on the various elements as well as the thickness of the wake of the main element as it flows over the flaps.

Another example I see frequently is when roughness or a trip strip is used. This promotes early boundary layer transition which can have a significant effect on lift. However in this case the roughness or the strip is removing energy from the boundary layer.

For many modern airfoils that have been highly optimized even a change in the effective profile of a few millimeters can be important.
boneh3ad
#17
Mar4-12, 01:04 AM
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And yet modern airfoils are designed with this effect in mind as well. Since it is a known issue, it can be designed around to an extent. They are designed to be more robust than that, otherwise a given plane would be nearly useless outside of its ideal conditions, which really is terrible from a safety standpoint. Imagine needing to descend in an emergency. Density goes up, Reynolds number goes up, transition comes forward... Now factor in 10%-20% loss of lift... bad news.
tsimon
#18
Mar4-12, 04:23 AM
P: 25
Quote Quote by RandomGuy88 View Post
...

when the flow separates the pressure doesn't actually change much from the pressure at the point where the flow separated.

...
Cheers!


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