How Hard To Blow To Stall A Wing?

[ringo]

Can't really compare a nearly vertical f1 wing to a simple low camber wing.
To stall it, it would have to be blown very hard, the momentum under the low presure side is very high. The air coming through the slot doesn't have enough dynamic pressure.

I think it alters the wake more than it does stalling. There is a detached flow area in the centre of the wing on the back side. I think this area is manipulated by the ducting. wish i could find a pic now.

[vonk]

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.

I have to agree with Oqami. Circulation and the bound and staring vortexes are abstract concepts belonging to an abstract model of a physical process. They represent theoretical analogies to observed physical phenomena. But they must obey conservation of mass, energy, and momentum.

An aircraft starting down a runway generates lift (and “circulation”) gradually from zero. That means the “bound” vortex also develops gradually, imparting a rotational momentum to the wake it leaves behind. This continues during lift-off and flight. In the physical world, that rotational momentum is eventually dissipated by viscosity in the wake.

The Lifting Line Theory includes no model of a viscous wake. But it needs to balance the rotational momentum generated by the “bound” vortex. Ergo, the abstract concept of the “starting” vortex. (Helmholtz' vortex theorems require equal and opposite vorticity). I don’t know why it’s called “starting” vortex, because it needs to be around in the abstract model so long as the “bound” vortex does its thing.

BTW, an aircraft starting its takeoff ground roll leaves no starting vortex behind. Those who say they have observed it have never come close to the jet engine blast at take-off. But small aircraft have been flipped over by the trailing vortexes of “heavies” if they didn't delay their own takeoff long enough.

[kilcoo316]

BTW, an aircraft starting its takeoff ground roll leaves no starting vortex behind.

It does.


But it is that weak the viscous forces quickly damp out the eddy momentum.

[wrcsti]

Has anyone considered they are not blowing 90 degrees? Thus making the vortex smaller.

[cornermarker]

Has anyone considered they are not blowing 90 degrees? Thus making the vortex smaller.

Yeah, but there's no way to tell at the moment. But to reduce the size of the vortex, you'd just reduce the pressure from the slot jet. Plus, to angle the jet would require a nozzle sandwiched in the wing, and that's just more things to fail. I'd put money on it just being hollow in there.

Kelpster

Here's some f1 porn for you, to give you an idea of what they're working with. I'm just gonna leave this up for a day or so.

[xpensive]

Can't really compare a nearly vertical f1 wing to a simple low camber wing.
To stall it, it would have to be blown very hard, the momentum under the low presure side is very high. The air coming through the slot doesn't have enough dynamic pressure.

I think it alters the wake more than it does stalling. There is a detached flow area in the centre of the wing on the back side. I think this area is manipulated by the ducting. wish i could find a pic now.

I agree, it all seeems like a funny gimmic to me, leaked out as the "dead zone" some months ago, to send he compatetition in the wrogh direction, that litttle scooop doing all that?

[slimjim8201]

To help visualize what stall looks like...

[youtube]http://www.youtube.com/watch?v=Ti5zUD08w5s[/youtube]

[n_anirudh]

Hi guys,

Just carried out a simple simulation in ANSYS of a normal and blown wing. It was at relatively low speed of 16m/s and a low aspect ratio wing typically used in FSAE cars

Boundary conditions and the mesh were appropriate. The wall Y+ returned values below 20.
The blown velocity was 75% of the free stream and at an angle of -45deg to the global co-ordinate system.at about 60-70% of chord distance. Here are the velocity contours between the same levels. the wing is at 0 AoA. Boundaries were sufficiently far to account for any blockage in the simulations.

The airfoil used is a single element one and hopefully quite relevant to the topic discussed here. I request Mods to move it to appropriate thread.

Normal airfoil


Blown airfoil 45



From the converged lift and drag values of the the 2 simulations,

NORMAL AIRFOIL
LIFT -235N
TOTAL DRAG = 62N
L/D -3.77


BLOWN AIRFOIL
LIFT -81N
T.DRAG = 18
L/D -4.5

As seen, stalling a wing is effective at low speeds as well, and just these 2 cases were tried.
The induced drag on the blown wing was 3.5 times less as seen on the normal wing.I have not considered endplate factor for this calculation. [2*H/B, I presume]
The simulations are 2D and simple wing theory calcs were used for approximations. The blown wing is effectively a 3D phenomenon.Look forward to any opinions/suggestions.

Thanks

[vonk]

This is great stuff! Thanks. =D>
It would be interesting to see what happens with injection normal to the wing's surface and everything else the same. I'm interested in the minimum injection pressure to get separation and how sensitive that is to free stream speed.

[n_anirudh]

This is great stuff! Thanks. =D>
It would be interesting to see what happens with injection normal to the wing's surface and everything else the same. I'm interested in the minimum injection pressure to get separation and how sensitive that is to free stream speed.

No worries. Just an attempt to see how the concept works at low speeds as well.
In my opinion at the same angle of blowing, The flow speed effectively required to stall should be greater than 50% of free stream..esp at this low speed..otherwise the jet coming out would simply be washed away in my opinion. More scientifically that min injection pressure can also be opined as a velocity %age of the freestream..


With the blown wing, we now have a few parameters:

Mass flow rate,
Angle of blowing

any more??

I bet FSAE/FS teams who run aero packages must be trying out something similar already..

[vonk]

With the blown wing, we now have a few parameters:

Mass flow rate,
Angle of blowing

any more??

Free stream speed
Angle of attack

[jla06]

Cornermarker,

sorry for not replying sooner, had a busy week!

I am using a low speed, open return wind tunel to test the model I've designed. As you can see from image 1 below, the rear outboard section (i.e. region corresponding to where is normaly located the aileron) has been slightly modified in order to obtain a nearly separated flow at the location of the jet holes. Obviously, the closer to separation, the smaller the external force (blowing) is required. This is important in aviation since the blown air is bled from the engine (compressor stage). A reduction in engine mass flow rate causes a reduction in thrust, so it's a compromise between thrust and control capabilities.

I designed my model in such a way that it allows for this outboard region to be exchanged with other parts. Each exchangable part differs in a unique variable so it makes it possible to study several parameters at a relatively low variable cost. Bare in mind that the parts are Rapid Prot. so it is kind of expensive. The variables I decided to look at are 3:

1. Chordwise location of jet holes (around 80% +/- 2%)
2. Blowing angle in the chordwise plane (normal to local surface, directed forwards) - not skewed sideways as this induces the formation of vortices which energize the BL
3. The equivalence to a conventional aileron (i.e. creating a sort of equivalence table between mass flow rates and deflection angles of a typical aileron)

Here are some images you might find interesting.

Image 1: Full assembly. Exchangeable component shown in red.
http://img534.imageshack.us/img534/8826/dibujo13l.jpg

Image 2: Exchangeable component used to as a convetional aileron reference
http://img687.imageshack.us/img687/7290/dibujo12p.jpg

Image 3: Flow visualization - Fluorescent oil + UV lighting
http://img683.imageshack.us/img683/5207 ... 7small.jpg
http://img6.imageshack.us/img6/6164/dscf1153small.jpg

This will serve as my final year project of my aeronautical degree which I will hopefully complete in a few weeks time!

[jla06]

This is great stuff! Thanks. =D>
It would be interesting to see what happens with injection normal to the wing's surface and everything else the same. I'm interested in the minimum injection pressure to get separation and how sensitive that is to free stream speed.

No worries. Just an attempt to see how the concept works at low speeds as well.
In my opinion at the same angle of blowing, The flow speed effectively required to stall should be greater than 50% of free stream..esp at this low speed..otherwise the jet coming out would simply be washed away in my opinion. More scientifically that min injection pressure can also be opined as a velocity %age of the freestream..


With the blown wing, we now have a few parameters:

Mass flow rate,
Angle of blowing

any more??

I bet FSAE/FS teams who run aero packages must be trying out something similar already..

One of the guys I work with is a highly regarded engineer in the field of aerodynamics. He told me that the blowing speeds should be roughly of the same order as the free stream velocity.

When you say angle of blowing, which plane are you referring to? Are you trying to separate the flow or delay separation?

[cornermarker]

Cornermarker,

sorry for not replying sooner, had a busy week!

I am using a low speed, open return wind tunel to test the model I've designed. As you can see from image 1 below, the rear outboard section (i.e. region corresponding to where is normaly located the aileron) has been slightly modified in order to obtain a nearly separated flow at the location of the jet holes. Obviously, the closer to separation, the smaller the external force (blowing) is required. This is important in aviation since the blown air is bled from the engine (compressor stage). A reduction in engine mass flow rate causes a reduction in thrust, so it's a compromise between thrust and control capabilities.

I designed my model in such a way that it allows for this outboard region to be exchanged with other parts. Each exchangable part differs in a unique variable so it makes it possible to study several parameters at a relatively low variable cost. Bare in mind that the parts are Rapid Prot. so it is kind of expensive. The variables I decided to look at are 3:

1. Chordwise location of jet holes (around 80% +/- 2%)
2. Blowing angle in the chordwise plane (normal to local surface, directed forwards) - not skewed sideways as this induces the formation of vortices which energize the BL
3. The equivalence to a conventional aileron (i.e. creating a sort of equivalence table between mass flow rates and deflection angles of a typical aileron)

Here are some images you might find interesting.

Image 1: Full assembly. Exchangeable component shown in red.
http://img534.imageshack.us/img534/8826/dibujo13l.jpg

Image 2: Exchangeable component used to as a convetional aileron reference
http://img687.imageshack.us/img687/7290/dibujo12p.jpg

Image 3: Flow visualization - Fluorescent oil + UV lighting
http://img683.imageshack.us/img683/5207 ... 7small.jpg
http://img6.imageshack.us/img6/6164/dscf1153small.jpg

This will serve as my final year project of my aeronautical degree which I will hopefully complete in a few weeks time!

Great work! This is the kind of thing I've been waiting for so thanks for your input. Those shots are beautiful So, help me understand what I'm seeing in the uv photos. What I got out of this is that it is in fact possible to use blowing to force separation, but the blowing slot must not arranged as not to create vortices, which, as we know, delays separation.

I know your model isn't meant to approximate the f1 wing, but what does your understanding of the phenomenon tell you when you look at the McLaren wing above? What is your hunch about what is happening on the wing at high speed, given the pressures and velocities likely to be involved. Vortices, no vortices? How important is it that the slot be blowing forward to get separation? That's something I had not considered previously, but now I realize it's something they might do if the object were to force separation. How hard is it to avoid the formation of vortices?

Above, there is a quote from an abstract which suggests that all kinds of devices placed near the area of separation (including such simple things as grooves) help to delay that separation. I'm sure they didn't point their jets forward, but otherwise does everything gel?

Now, back to your research, what kinds of applications do you see this having in aviation? Have you found anything completely unexpected? Of course, I'd be interested in seeing any more pictures or data you'd be willing to share Congrats, and what will you be doing with your degree?

Thanks again.

Kelpster

[n_anirudh]

This is great stuff! Thanks. =D>
It would be interesting to see what happens with injection normal to the wing's surface and everything else the same. I'm interested in the minimum injection pressure to get separation and how sensitive that is to free stream speed.

No worries. Just an attempt to see how the concept works at low speeds as well.
In my opinion at the same angle of blowing, The flow speed effectively required to stall should be greater than 50% of free stream..esp at this low speed..otherwise the jet coming out would simply be washed away in my opinion. More scientifically that min injection pressure can also be opined as a velocity %age of the freestream..
With the blown wing, we now have a few parameters:
Mass flow rate,
Angle of blowing
any more??

I bet FSAE/FS teams who run aero packages must be trying out something similar already..

One of the guys I work with is a highly regarded engineer in the field of aerodynamics. He told me that the blowing speeds should be roughly of the same order as the free stream velocity.

When you say angle of blowing, which plane are you referring to? Are you trying to separate the flow or delay separation?

The blowing was at -45 to the horizonatal axis in the 2 c/s images. The idea was to run a parameter study, blowing at diferent speeds to obtain an approximate value. I was aiming for flow separation.

here is the velocity contours for blowing at -90 deg [vertically down] at the same flow speed mentioned and from the same chord location.


The lift and drag were -56N and appx 18N Drag
L/D = -2.35 appx

the entire system stalled..the flow from the lower surface seem to have a feedback mechanism and affected the flow on the upper surface.As earlier I used a steady state simulation.