Wing aero

[CBeck113]

Here's a great video showing the vortices on the tips of airplane wings as it lands into a fog bank. This is what the aero engineers want to avoid in some areas (i.e. upper tips of the rear wing) and create in others (i.e aroung the floor).

http://youtu.be/QZkggFzAtEc

And it is cool to boot.

[Tommy Cookers]

tip vortices are always present (they cannot be eliminated)
unless there is zero lift, or unless the wing has an infinite span
usually they are only strong enough (enough pressure reduction) to be visible (as 'fog') at high AoA
as shown in the clip, when the plane is briefly flown at high AoA to briefly produce higher lift to reduce the plane's descent
a 300 ton plane will need maybe 350 tons of lift then, maybe each tip vortex shown is equivalent to 75 tons of lift
they remain active some time after the plane has landed

some manouvres have continuous high AoA, and the plane can hit its own vortices/wake even 1 minute after it made them
more often, at airports there is a possibility of an aircraft hitting the wake vortices of the previous landing aircraft
seperation distances and heights are mandated, intended to prevent this happening

[shelly]

Very nice video. To me, a good name for the thread would be "tip vortices" as they are crucial in f1 aerodynamics, and it would be nice in my opinion to discuss them a bit on f1technical.

Taliking about vortices, i think most forumers think about the wingtip vortices of the rear wing, which, as the previous poster says "can not be eliminated".
There is much less discussion about all the other tip vortices: from the front wing, from the diffuser footplate, from the t-tray, from the bargeboards... there are lot of vortices on a f1 car: they are not only a source of drag to minimize, but they are used to generate downforce and to influence the flow field.

One should not look at liners, but at fighter planes with delta wing or lex to see airplane application of vortex lift: the key concept is that there is low or sometimes very low pressure inside a vortex

The front wing is a good example to start from: the endplate shape is designed to get advantage from the wing tip vortex of the front wing. Most teams use a small arched section to house the tip vortex - with this trick they get the a good amount of extra downforce.

There are many other examples, I hope that we can discuss them here

[hollus]

I would love to discuss it too. How about you provide a drawing? You seem quite knowleadgeable and in this particular case a picture can explain things much better than words. When you say the end plates benefit from the wing tip vortex... Why is it better with and arched guide? How does it affect the vortex itself, as it has no immediately adjacent air anymore? Etc, etc... My in-brain CFD has a bad vortex model beyond their existance, size and direction... A pic would help.

[Kiril Varbanov]

Some words from me, copy-paste from my blog:
Vortex in aerodynamics is any fluid or gas formation which usually has turbulent flow. What is typical for a vortex is the low pressure at its core, which rises progressively as we go away from the center to the outer edges where the pressure is very high. This is one of the reasons why aero people generally would like to avoid creating vortices - the high pressure would mean lower velocity of the surrounding layers, thus drag.

Other reason why vortices are generally avoidable is because of the possibility of "vortex burst" - this is the moment where the formation literally breaks and creates even more turbulent and uncontrollable flow. This is less likely to happen with weak vortices and respectively, usually seen with strong vortices, where the core sometimes disintegrates into few smaller vortices.

The reason why, however, vortices are sometimes deliberately induced is to wake or re-energize the boundary layer - the small portion of air which is very close to a surface, where due to skin friction and resistance the velocity of the air is very low. We would like, as aero people, to have less drag, so we create a vortex generators - small winglets, which often induce normally rotating, weak vortices. The trade-off is the smaller portion of drag induced due to the shape of the device, but it's more beneficial in terms of L:D coefficient.
In F1 world we often hear the term referred to as "wingtip vortices", as seen on the picture below. The reason for creation of such formations is the natural tendency of the air to move from high to low pressure regions, being a continuous function. Here, since we operate with negative lift (downforce), the direction of the vortices is upwards.
These wingtip vortices, on the other hand, create lift-induced drag and drag is unwanted in any of its forms in motor sports, where speed matters.

[flyboy2160]

....Here, since we operate with negative lift (downforce), the direction of the vortices is upwards.
...

K man, you had one too many or were in Australia when you wrote this. Aircraft wing vortices creating lift have vortices that roll upward. Car down force wing vortices must roll down. I think I've seen some car vortex pictures showing this down roll. I'll look for them

[shelly]

kiril, in your post there are a lot of good inputs, but what you do nto talk about is exactly what i hinted at in my previous post.
You talk about wing tip vortices of the rear wing and their drag, and you talk abiut small vortex generators that are used to fix the local flow and energeize the boundary layer: you are not mentioning vortices being used as downforce contributors and as general flow conditioners - and this is their biggest relevance in f1 aero.

The picture you posted is very helpful in discussing vortices because it give us an example of a vortex tube made visible by its condensation trail (water dew point is lower at loew pressure). You can visualize the vortex as a tube, a bit conical a the beginning, than almost cylindrical, that extends for a certain length before it weakens or bursts.

The rw tip vortices are the most visible because are stronger, but there are a lot of other voritce detaching from the car, On this forum some good pictures /mvies were posted: one of a caterham in winter test 2012 and one of a force india in spa 2011 showing vortices being shed from the front of the car, visible because of the damp/cold.

Let us focus of the front wing as a start (i will try to put a drawing hollus): under the arched sahpe on the extreme left in the picture a vortex tube similar to the rw visible one is running. We do not see it, but we can easily imagine that it must be there (there is a high-low pressure interface at the front tip also). The arched shape is used to have it guided in a consistent position - the outer lip influencing its strength as tip winglets do on passenger airplanes. th advatage i downforce is immediate: the low pressure of the vortex core acts against the arch and sucks it down towards the road .
The other two raised portion of the front wing leading edge are raised because of two other vortices: the inner enplate vortex and the vortex formed at the intersection between a more loaded (in front of the wheel) and a less loaded (inner) wing part.
The sections are raised because of the modified angle of attack induced by the presence of these two vortices, and are arc-shaped to house thes voritces and exploit their low pressure.

I hope I have been not too obscure - will try to provide drawings

[flyboy2160]

There already is an excellent Thread here "Reducing the drag of a two element wing through stall.) See page 69 for ringo's excellent pics. (I couldn't figure out hoe to link that post here.)

But you are agreeing with me here. You said leading edge of the wing, so the vortices interact with the surfaces as they go downstream.
At the end plates of a wing, those are the last surfaces on the car, nothing else is downstream.
I know vortices have uses, but it's mainly for pressure mixing. Most of the time they are detrimental, so efforts are made to eliminate them if nothing is downstream.

The slots now, i am not sure about. I don't know what is happening even though i have seen the flows themselves in cfd.
They look like they cancel the vortices, that is what i would think.

http://i1010.photobucket.com/albums/af2 ... gslots.jpg

On a high cambered wing like this, the pressure distribution is so strong that it's much taller than the wing element. High pressure fills up everywhere in that pocket between end plates. So much so, some of the high pressure spills over the top of the end plate. This drives the vortices as it curls over the plate. The same can be said of the low pressure area behind the car and that flowing along the endplates.
The slots are shaped in such a way, upward chanelling louvres, that the high pressure between the end plates pass through and move upward. This upward, high pressure, movement on the outside of the end plate acts to miminize the flow coming over from the top (yellow arrows).

[shelly]

Yes rw tip vortices and drag have been discussed a lot here - on this thread i would like to discuss the downforce effect of the voritces under the front wing, the floor etc

[Pierce89]

Yes rw tip vortices and drag have been discussed a lot here - on this thread i would like to discuss the downforce effect of the voritces under the front wing, the floor etc

I think we should discuss the DF enhancement vortices under the front wing along with the y250 . We should also mention the way bargeboard vortices go under the floor and reduce pressure. That's why the pre 09 cars had the smaller lower forward bargeboards under the nose.This image shows them just in front of the large main bargeboard.

[shelly]

front wing vortices - endplate vortices number 1,2,3; y250 vortex number 5 let us start from them.

hera we see that at the ned of 2011 rbr used an arched shape also at the y250 change of section, to house the vortex formed by the load difference between the central mandated section and the outer part of the front wing

[hollus]

...What is typical for a vortex is the low pressure at its core, which rises progressively as we go away from the center to the outer edges where the pressure is very high...

I've underlined the part I mean to challenge. Is this true? Is there high pressure in the edge of a vortex? I always though there is low pressure in the center and pressure equal to the surrounding pressure in the edge, that the high and low pressures creating the vortex annihilated each other and we were left with a low pressure of a different origin, consequence of the rotational movement imparted to the fluid by the equalization process.

P.S. a little note to all. Saying that the vortices move "upwards" or "downwards" is very confusing to me. Upwards in which part of the vortex? In my mind they move clockwise or counterclockwise, and even that can be interpreted to mean something only if we state our point of view (viewed from in front of from behind the car?). Is there an engineering standard frame of reference that I am missing? I am a chemist and we do have absolute frames of reference to describe the direction of helicity or whatever it is called, but they are obscure to outsiders and get lost in a 2D picture.

[variante]

These wingtip vortices, on the other hand, create lift-induced drag

Sounds like you are saying that the source of drag is the vortex itself, when i think it is just the symptom of something else creating drag (the wing).

the low pressure of the vortex core acts against the arch and sucks it down towards the road .

This is a crucial point IMO. I'd be interested to know how the outer edge of the vortex, which is at high presssure, doesn't "annihilate" the effect of the low pressure core.

This X-31 at high AoA may help:

[Pierce89]

These wingtip vortices, on the other hand, create lift-induced drag

a
Sounds like you are saying that the source of drag is the vortex itself, when i think it is just the symptom of something else creating drag (the wing).

the low pressure of the vortex core acts against the arch and sucks it down towards the road .

This is a crucial point IMO. I'd be interested to know how the outer edge of the vortex, which is at high presssure, doesn't "annihilate" the effect of the low pressure core.

This X-31 at high AoA may help:
http://ars.els-cdn.com/content/image/1- ... -gr012.jpg

The outer edge isn't "high" pressure but somewhere around freestream pressure so the "mean" is reduced pressure. BTW the red in your picture is reduced pressure not increased if that's where the confusion comes from.

[flyboy2160]

...P.S. a little note to all. Saying that the vortices move "upwards" or "downwards" is very confusing to me. Upwards in which part of the vortex? In my mind they move clockwise or counterclockwise, and even that can be interpreted to mean something only if we state our point of view (viewed from in front of from behind the car?). Is there an engineering standard frame of reference that I am missing? ...

There is a standard terminology. The air in the vortex will try to flow from the high pressure area into the low pressure area. At the wing tip of lifting wings, the air is trying to flow around the edge and back up on top of the wing. This means the vortex helix is 'rotating upward' no matter whether you look at it from the front or the back. If you're doing a math calculation, you obviously need to pick a coordinate system, but the rotation will always be defined going from high pressure into low pressure around the edge.

(Another simplistic, easier to explain way to look at the drag-due-to-lift from the vortices is to consider them as wasting energy due to the wing stirring the viscous air, just like it takes energy to stir your peanut butter in the jar to mix the oil. This 'stirring' energy loss is different from the viscous drag due to sliding the wing surface through the viscous air. This 'sliding' drag is analogous to pulling your spoon straight out of the peanut butter jar. The airplane thrust must do work to overcome both types of viscous forces. In theory, you don't have the stirring losses for an infinite span wing. And it does decrease as the wing aspect ratio increases, which is why sailplanes have those very slender, very high aspect ratio wings.)

Hope this food analogy helps. I would have gone for a beer analogy, but the peanut butter one is more illustrative.