How Hard To Blow To Stall A Wing?

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vonk
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How Hard To Blow To Stall A Wing?

Postby vonk » Tue Apr 27, 2010 4:28 am

Can anybody point me to a formal treatment of the following question? Does anybody know how hard the F1 wings have to be blown to stall?
Thanks in advance.

Image
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Jersey Tom
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Re: How Hard To Blow To Stall A Wing?

Postby Jersey Tom » Tue Apr 27, 2010 5:28 am

If anyone here is privy to the specifics of F1 airfoils.. I don't think they will be answering this question publicly.
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cornermarker
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Re: How Hard To Blow To Stall A Wing?

Postby cornermarker » Tue Apr 27, 2010 9:09 am

Too hard for it to be a practical application on a formula one wing. This is what happens when you blow air through a slot in crossflow on a wing:

Image

You get a vortex. A vortex created by a jet of air has actually been studied only to keep flow attached, surprisingly, not to cause separation. So the question becomes, how hard would you have to blow to not get a vortex, but to get something else.

So hard, it's just not worth it, if it can be done at all.

If you want to create separation, blowing air transversely like that is the last thing you'd want to do. That's what all the research says.

Kelpster

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Re: How Hard To Blow To Stall A Wing?

Postby autogyro » Tue Apr 27, 2010 10:22 am

Surely the closer to break away from conventional reasons the airflow becomes, the easier it is to assist break away by blowing?
With a very high angle of attack the wing must become close to boundary layer break away as speed increases anyway?

cornermarker
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Re: How Hard To Blow To Stall A Wing?

Postby cornermarker » Tue Apr 27, 2010 11:01 am

autogyro wrote:Surely the closer to break away from conventional reasons the airflow becomes, the easier it is to assist break away by blowing?
With a very high angle of attack the wing must become close to boundary layer break away as speed increases anyway?


In researching the purpose for the slot on the McLaren, I found the opposite to be true. Counter-intuitive, yes, but it is the case.

"Surface static pressure measurements and oil-flow visualization results from the wind tunnel tests indicated that transverse grooves, longitudinal grooves, submerged vortex generators, vortex generator jets (VGJ's), Viets' fluidic flappers, elongated arches at (+)alpha (positive angle of attack), and large-eddy breakup devices (LEBUS's) at (+)alpha placed near the baseline separation location reduce flow separation and increase pressure recovery."

http://adsabs.harvard.edu/abs/1992PhDT........50L

Placing most devices close to the normal separation area, including slot blowers, reduces separation, it doesn't increase it.

But if you wanted a controlled separation, you could use a laminar bubble, which effectively changes the profile of the wing. Just remember, blowing transversely= vortices of some kind or another. That's just the nature of fluids. So if you do want separation, the question is how large should the laminar bubble be (how much pressure from the jet), and where should it be placed on the wing.

Kelpster

kilcoo316
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Re: How Hard To Blow To Stall A Wing?

Postby kilcoo316 » Tue Apr 27, 2010 1:33 pm

vonk wrote:Can anybody point me to a formal treatment of the following question? Does anybody know how hard the F1 wings have to be blown to stall?


All dependant on just how sensitive the wing is to begin with.


With the camberline of F1 rear wings, it wouldn't take much.



With regards the crossflow jet inducing a vortex. Yeap, no issues with vortex induction. But where does that vortex travel to in the region of the suction surface of a very highly cambered aerofoil?

Usually it will be off the suction surface long before getting to the wake region. This breaks the bound vortex and thus "stalls" the wing.

Giblet
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Re: How Hard To Blow To Stall A Wing?

Postby Giblet » Tue Apr 27, 2010 1:38 pm

I don't think the wing ever completely stalls anyways. It stalls it to a point, effectively lessening it's angle of attack.
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kilcoo316
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Re: How Hard To Blow To Stall A Wing?

Postby kilcoo316 » Tue Apr 27, 2010 1:46 pm

Giblet wrote:I don't think the wing ever completely stalls anyways. It stalls it to a point, effectively lessening it's angle of attack.


Yeah, you'll never separate at least the first 50% of chordlength of the first element of an F1 rear wing (as currently designed).


Break the bound vortex though, and the method of downforce production changes. So, depending on how you define stall, it is or isn't.

vonk
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Re: How Hard To Blow To Stall A Wing?

Postby vonk » Tue Apr 27, 2010 2:26 pm

kilcoo316 wrote:
Giblet wrote:Break the bound vortex though, and the method of downforce production changes. So, depending on how you define stall, it is or isn't.

Can you elaborate on the bound vortex?
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cornermarker
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Re: How Hard To Blow To Stall A Wing?

Postby cornermarker » Wed Apr 28, 2010 3:21 am

I'm not certain the problem of camber is an issue here. Though F1 wings vary, the upper element's camber actually isn't that extreme on any of the cars, just because of that problem (separation). Then we'd have to take into account the element gap, which on the wing effectively acts as a (somewhat) tangential flap blower, meaning it wouldn't be extremely sensitive to separation.

Low pressure slot means a small vortex, and as the studies show, this would just keep the flow attached. If it is highly pressurized, however, yes we would get something like this:

Image

Again, effectively changing the profile of the wing. But how hard would you have to blow to get this effect at 200mph and despite the influence of the element gap? Pretty hard.

But we should ask the OP: With the understanding that you're going to have a vortex in the mix, is the image above what you're looking for?

Kelpster

kilcoo316
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Re: How Hard To Blow To Stall A Wing?

Postby kilcoo316 » Wed Apr 28, 2010 1:23 pm

vonk wrote:Can you elaborate on the bound vortex?


Erm... for practical reasons, I can't go into too much really.

It is one of 4 vortices that make up the flow around a (subsonic) wing.


Two longitudinal trailing vortices, one from each wingtip.

One bound vortex (which is the real source of the velocity differential between pressure and suction surfaces of a subsonic wing).

One starting vortex, which is left at the point where the wing first starts to move through the air.

Image

Grab a text book on the fundamentals of aerodynamics and have a look, it'll explain it in more depth than I'll ever do here. Or have a little look at:

http://www.onemetre.net/Design/Downwash ... ftline.htm


(or similar)

Ogami musashi
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Re: How Hard To Blow To Stall A Wing?

Postby Ogami musashi » Wed Apr 28, 2010 5:53 pm

Please pay attention that vortex (especially the central one) which is value of the integral of circulation is not physically a vortex. Rather this is an elemental method to calculate forces on an airfoil;

In reality it correponds to the integral of curl of an elemental volume of air which by the same virtue corresponds to the divergence of velocity of that elemental volume along the a curve.

My main point for my post is to distinguish calculation tools (circulation,vortex sheets) from the actual physics.


As for the stalling thing, I read i think on forumla 1.com that they blow at the gap between the two elements. Usually the vortex from the first element and the air that flow into the gap helps maintaining the boundary layer on the second element and then they blow it so that it stalls.
So yes it will create a lower AOA.

But the main question is how the recirculating flows behave.
If you take your illustration with the wings, the region where the flow are separated includes actually a low pressure region at the aft of the wing.
This mean the profile drag is very high (high pressure forward of the wing, low pressure at the back); However when the flow actually separate you have an increase in pressure, i guess this is what they want.

Diminishing the AOA will only lower induced drag; on cars the induced drag is almost constant because they do not lower their AOA (thus they increase their downforce) like planes so maybe that's the goal of the mclaren wing even at the expense of more profile drag.


Of course a fully stalled wing would be the ideal case i think.

kilcoo316
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Re: How Hard To Blow To Stall A Wing?

Postby kilcoo316 » Wed Apr 28, 2010 10:17 pm

Ogami musashi wrote:Please pay attention that vortex (especially the central one) which is value of the integral of circulation is not physically a vortex. Rather this is an elemental method to calculate forces on an airfoil;


In what way is it not physical?


The starting vortex has been observed in experiments. Everything in nature has an equal and opposite reaction. The observation of the starting vortex is a key part of establishing the horseshoe theory in physical reality.


The bound vortex exists (perhaps a better term would be circulation) - we know that through the velocity differential between pressure and suction surfaces on any aerofoil.

Yes, for Prandtl lifting line etc, the idea is used as a tool for development - but that is an extrapolation from physics, not a numerical technique constructed only from mathematics.




Anyway - my main point is that - you break the circulation, you kill the most efficient method of downforce production. Crossflow injection in an already strong adverse pressure gradient situation will almost certainly do that.

jla06
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Re: How Hard To Blow To Stall A Wing?

Postby jla06 » Wed Apr 28, 2010 11:06 pm

cornermarker wrote:Too hard for it to be a practical application on a formula one wing. This is what happens when you blow air through a slot in crossflow on a wing:

Image

You get a vortex. A vortex created by a jet of air has actually been studied only to keep flow attached, surprisingly, not to cause separation. So the question becomes, how hard would you have to blow to not get a vortex, but to get something else.

So hard, it's just not worth it, if it can be done at all.

If you want to create separation, blowing air transversely like that is the last thing you'd want to do. That's what all the research says.

Kelpster


Very interesting topic. I am actually carrying out some experimental investigations related to this priciple. I´m looking at varying the incidence, the chordwise location and its equivalence to an aileron deflection. I have modified the wing slightly so that the flow just upstream of the blowing slot is about to separate, therefore requiring a relatively small Vjet to induce separation. From what I've read, ideally you want Vjet of the same order of magnitude as Uinfinity.

By the way, Cornermarker, where did you find the image of the velocity field?

cornermarker
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Re: How Hard To Blow To Stall A Wing?

Postby cornermarker » Thu Apr 29, 2010 12:24 am

jla06 wrote:
cornermarker wrote:Too hard for it to be a practical application on a formula one wing. This is what happens when you blow air through a slot in crossflow on a wing:

Image

You get a vortex. A vortex created by a jet of air has actually been studied only to keep flow attached, surprisingly, not to cause separation. So the question becomes, how hard would you have to blow to not get a vortex, but to get something else.

So hard, it's just not worth it, if it can be done at all.

If you want to create separation, blowing air transversely like that is the last thing you'd want to do. That's what all the research says.

Kelpster


Very interesting topic. I am actually carrying out some experimental investigations related to this priciple. I´m looking at varying the incidence, the chordwise location and its equivalence to an aileron deflection. I have modified the wing slightly so that the flow just upstream of the blowing slot is about to separate, therefore requiring a relatively small Vjet to induce separation. From what I've read, ideally you want Vjet of the same order of magnitude as Uinfinity.

By the way, Cornermarker, where did you find the image of the velocity field?


Cool, will you be using a wind tunnel or water chamber? If using a program, a good way to calibrate your work would be to see whether a slot jet actually helps flow stay attached. If not, it won't represent what researchers have found:

"Surface static pressure measurements and oil-flow visualization results from the wind tunnel tests indicated that transverse grooves, longitudinal grooves, submerged vortex generators, vortex generator jets (VGJ's), Viets' fluidic flappers, elongated arches at (+)alpha (positive angle of attack), and large-eddy breakup devices (LEBUS's) at (+)alpha placed near the baseline separation location reduce flow separation and increase pressure recovery."

http://adsabs.harvard.edu/abs/1992PhDT........50L

As for the velocity field, you can find the link in one of my earlier posts in the thread, probably the first one it's displayed in.

Kelpster.




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