Boundary Layer Separation

SpaceShipNine

New Member
Ok guys, I got these info from wikipedia, please help me put it together or shoot them down as you see fit.

The overhead question is: Why does air separate from laminar flow to turbulent flow as we increase the AOA?
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wiki:
Flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient that the speed of the boundary layer falls almost to zero.


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wiki:
An adverse pressure gradient occurs when the static pressure increases in the direction of the flow. This is important for boundary layers, since increasing the fluid pressure is akin to increasing the potential energy of the fluid, leading to a reduced kinetic energy and a deceleration of the fluid.

SpaceShipNine: what does this mean? are we increasing the Potential Energy of the fluid when we increase the AOA? If so why?
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wiki:
Reynolds number Re is a dimensionless number that gives a measure of the ratio of inertial forces (
) to viscous forces (μ / L)

laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion, while turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce random eddies, vortices and other flow fluctuations.

SpaceShipNine: How is increasing the AOA changing the Reynolds number? If im close in my understanding, increasing the AOA means increasing the inertial forces that slows down the boundary layer, until it slows down to a point and it just... separates?

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I think I just got even more confused trying to ask this question...
 

tgrayson

New Member
First, you're confusing laminar flow with separation. Different things. Separation happens regardless of whether the flow is laminar or turbulent.

Secondly, the kinetic vs potential energy thing is based on the Bernoulli equation. What that equation basically says is that the energy in the airflow is a constant; part of it is in bound up in the velocity of the airflow and part of it is in the static pressure and you can convert one form into the other by constricting or expanding the cross-sectional area of the channel the fluid is flowing through.

After the air moves over the top of the airfoil, the air is less constricted so its velocity starts to decrease and the static pressure increase. Eventually, the velocity may reach zero and at any point after that, the air will flow backwards. This flow is said to be separated (stalled).
 

SpaceShipNine

New Member
So when you increase the AOA, making the airfoil steeper, the air has a bigger area to "fill in" and slow down?

so that at the critical AOA the airfoil is so steep and the area so big that immediately after the air goes over it tries to fill in that big area and BAM increase static pressure - flow backwards -stall?

is this right?

btw, thanks a bunch!
 

tgrayson

New Member
So when you increase the AOA, making the airfoil steeper, the air has a bigger area to "fill in" and slow down?
Essentially, yes, but it's not generally that sudden, at least on a relatively thick airfoil. If you get some separation towards the rear, you won't see a huge impact on the airfoil. The lift coefficient will keep increasing with AOA until the separation because significant. The stalls are "soft".

Thin airfoils, such as you might see on a jet, have a tendancy to have sharper stalls, because the flow separates toward the leading edge rather suddenly.
 

SpaceShipNine

New Member
so on the same note, how does lift increase with increase in AOA?

Its not the "air has to go up and over" so distance is increased -> speed increase -> more lift right?

another CFI used a jeep running down a hill analogy, since air has weight, gravity draws it towards the earth, so when AOA goes up, the hill becomes steeper and the air/jeep accelerates, producing more reaction force upwards.
 

fish314

Well-Known Member
so on the same note, how does lift increase with increase in AOA?

Its not the "air has to go up and over" so distance is increased -> speed increase -> more lift right?

another CFI used a jeep running down a hill analogy, since air has weight, gravity draws it towards the earth, so when AOA goes up, the hill becomes steeper and the air/jeep accelerates, producing more reaction force upwards.

Hey SS9, the post that Tgrayson linked to has a great explanation, (or check out www.boundvortex.com which is our very own Tgrayson's site, and it's pretty good!)

...But your CFI buddy's explanation is totally out to lunch. Gravity doesn't have any real effect on the flow of air around an airfoil. The way he's trying to explain it "seems like it should make sense" because everyone is used to thinking about aircraft in basically straight and level flight, but if you imagine an airplane doing a loop and realize that those same forces are at work the entire way around the loop, you can see that his explanation doesn't work.
After all, lift is increased by increasing AOA (until critical AOA, that is), even for the airplane with it's nose pointed straight down on the back side of the loop, or inverted at the top of the loop. But if that gravity explanation made sense, then you would think that the air would be flowing backwards or separating earlier in straight nose down attitudes. Or you might think that the airflow would "fall off the wing" in inverted attitudes. In fact, if gravity were a factor, wouldn't the air "fall off the bottom of the wing" in straight and level flight?
The fact is, the actual force that gravity exerts on the individual air molecules is miniscule in comparison to the forces that the airfoil exerts on the molecules, or the forces the molecules exert on each other.
 
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