Rear Wing Vibrations

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thepowerofnone
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Re: Rear Wing Vibrations

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turbof1 wrote:Does the loss of the beam wing has anything to do with this? Loosing the rigidity it gives to the rear wing?
Removing the rear beam wing would have a huge impact here, that beam would have at least tripled the natural frequency of the rear meaning a much larger energy required to obtain the same magnitude of "wobble". If I had to guess I'd say some bump or something near the beginning of the straight caused the flow to briefly detach or deviate on one side of the upper wing causing an asymmetric loading, then this small pertubance grew as an unstable oscillation from side to side as the car went down the straight. If I remember correctly this was most apparent on Hamilton at the end of the back straight, which pretty much rules out any effects due to dirty air in front of the car and is significant because that straight is so damn long that oscillations have a while to grow.

Someone asked if structurally this is ok - answer its certainly undesirable, CFRP isn't meant to flex at all a lot and some delimitation probably occurred. Clearly the wing didn't fail, but the aerodynamicists won't like the wing shaking about like that and anyone looking at the telemetry probably won't have enjoyed what it did to their readings. This isn't like a flexing front wing where there is some obvious aero benefit, I doubt this is intentional.

jfeu
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Re: Rear Wing Vibrations

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turbof1 wrote:Does the loss of the beam wing has anything to do with this? Loosing the rigidity it gives to the rear wing?
That loss of ridigity can lead to a lot of aeroelastics issues, like flutter for instance. Nonetheless in this case I suspect this wobble is not caused by aero forces, but just because of a poor structural damping.

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turbof1
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Re: Rear Wing Vibrations

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It might not be a rear wing, but anyone remembering Massa's front wing back in 2011 during the Indian GP? This looks very similar.
[youtube]http://www.youtube.com/watch?v=Ql0n1DsIGsE&sns=em[/youtube]
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zonk
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Re: Rear Wing Vibrations

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zonk
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Re: Rear Wing Vibrations

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CBeck113
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Re: Rear Wing Vibrations

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Another example, starting at 3:10

[youtube]http://youtu.be/WaWoo82zNUA?[/youtube]
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thepowerofnone
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Re: Rear Wing Vibrations

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A comment on each of the above three videos:
video 1 - turbof1: That example is a little different in that those front wings were from the time where everyone was trying to exploit flexing wings to get them closer to the ground and improve the ground affect. I believe that flutter was caused by the wing flexing towards the ground, then getting too close and choking the flow, losing lift and flexing back up, then back down again etc etc. Basically the flex was intentional but went too far. The difference in this rear wing case is that the flutter side to side is either caused by shearing aero forces on the rear wing, which are going to be really small, aero forces on the uprights, which should also be small, or some vortex shed further up the car causing a periodic oscillation on the uprights, which I think is most likely. The only other explanation is that the uprights experienced some kind of small global buckling which started the whole thing but I find that unlikely.

video 2 - zonk: Consider that this flutter is caused by aero loading by the wing itself, causing it to flutter, whereas in the rear wing case the movement is orthogonal to the rear wing section. Its quite a different case.

video 3 - zonk: Again, in the first half the tailplane suffers catastrophic failure, again the loading is in the same plane as the displacement, although it is an excellent video showing how a small oscillation can grow to massive failure. The bridge is again a little different - whilst the bridge section doesn't produce a great amount of lift, its the wake that causes the flutter more than anything. Here is a standard video of a circular tube in a wind tunnel - symmetric shapes don't produce lift but if you look at the wake you will see it oscillates with a regular frequency which we can calculate with the Strouhal number. https://www.youtube.com/watch?v=SJ3w4bg5Tx8 (there are better videos for the actual local flow but this one shows the wake oscillation nicely).

The point I am trying to make is that in all three videos the loading and the displacement occur in the same direction, as is common for aero-elastic effects. In the rear wing case the loading is mostly vertical but the displacement is horizontal, so something else is probably at work.

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turbof1
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Re: Rear Wing Vibrations

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That's nice info, cheers mate. Yeah the 2011 front wing case is indeed a bit different. It was also back then a completely new wing philosophy. Very strangely was that Fernando Alonso didn't had the same issue. Maybe a bad batch of parts?
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zonk
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Re: Rear Wing Vibrations

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Thanks also!

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Holm86
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Re: Rear Wing Vibrations

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I remember reading a study which told that a wing which oscillates in the longitudinal direction of the wing increases lift and decreases drag. I've been trying to find this again but have no success so far. I don't know if the same effect is achieved when the wing has endplates. But then it could be intentionally designed to do so. I've had this thought ever since I saw how much the Williams rear wing oscillates this year.

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zonk
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Re: Rear Wing Vibrations

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Williams rear wing vibrations

[youtube]http://www.youtube.com/watch?v=OmkbkrkbtJ4[/youtube]

some more visible in slo mo from Melbourne

[youtube]http://www.youtube.com/watch?v=Aw-WE0Bcr1A[/youtube]

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turbof1
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Re: Rear Wing Vibrations

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You guys want the topic limited to solely rear wing vibration, or broader? Pm me and I'll see what to do. It's a very interesting topic which I don't necessarily want to keep limited. You guys make the call!
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thepowerofnone
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Re: Rear Wing Vibrations

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Holm86 wrote:I remember reading a study which told that a wing which oscillates in the longitudinal direction of the wing increases lift and decreases drag. I've been trying to find this again but have no success so far. I don't know if the same effect is achieved when the wing has endplates. But then it could be intentionally designed to do so. I've had this thought ever since I saw how much the Williams rear wing oscillates this year.
This comes under the umbrella of passive boundary layer control. I couldn't find a direct paper either, but here is a related one which uses sound to reduce drag http://arc.aiaa.org/doi/abs/10.2514/6.1983-726, same concept of using a vibration to improve efficiency though. Watching the wing vibrate on the Williams as it came into the pits in the videos zonk posted it would seem to imply that the uprights just have a pretty low moment of inertia and anything sets them off into an oscillation. Could well be intentional but if it is it only goes to show how advanced F1 is - passive BL control is tough enough in steady level conditions but adding it to a race car and making it work would be phenomenal.

Just_a_fan
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Re: Rear Wing Vibrations

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flyboy2160 wrote:There are a couple of things that could be involved. If you have separated flow off the back of a bluff body, the vortices shed in that wake can oscillate back and forth. This can couple with one of the natural frequency mode shapes of the structure, giving you that back and forth motion.
A von Karman vortex street. Although I wouldn't have thought the endplate would shed in this way as it's not really a bluff body (the rear edge is bluff but not in comparison to its length).
I suspect, but can't prove because I have no F1 aero experience, that perhaps this oscillation could also be tripped by a temporary disturbance to the airflow to the rear wing (Ninners point) and to the vertical end plates, temporarily separating the flow off them. Once started, the vortex shedding could again couple with one the structural natural frequency vibrations.
I think in the case being discussed that the major energy source for the rear wing oscillation is the track. The vibrations really seem to kick in as they reach the braking zone. A number of tracks seem to suffer from this.
This vortex shedding/structural coupling is what induced the famous Tacoma Narrows bridge collapse in the 30s. One of the reasons bridge decks are "open" now instead of closed like the Tacoma Narrows bridge is to reduce the aero forces on the deck.
Suspension bridge wires are also tuned using tuned mass dampers (Stockbridge dampers) to reduce wind-induced vibration in them. They're very obvious if you look for them. Power lines are also tuned in the same way.
I suspect, but again can't prove, that the beam wing (and monkey seat) helped stabilize the whole back end airflow structure, so I see Turbo's point/question. And yes, the wing end plates without the beam wing support could be less rigid because they are longer.
I would expect the principle benefit from the beam wing was, as you say, that it created a shorter end plate. This makes it stiffer and also alters the frequency at which it is excited.
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flyboy2160
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Re: Rear Wing Vibrations

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Just_a_fan wrote:...A von Karman vortex street. Although I wouldn't have thought the endplate would shed in this way as it's not really a bluff body (the rear edge is bluff but not in comparison to its length)...
"...whereas in the rear wing case the movement is orthogonal to the rear wing section. Its quite a different case...."


I was thinking about the airflow in yaw vis a vis the endplates. That is, a big yawing direction could make them bluff/stall in that direction. But the more I think about it, the less likely that seems.

In any case, this an interesting thread for me.

Flyboy Steve