Variations of vortices: vicious or virtuous? [article discussion]

[jjn9128]

New article from Vyssion and myself on vortices around a Formula 1 car and where they are beneficial or detrimental to the whole car's aerodynamic performance.
Variations of vortices: vicious or virtuous?

Please discuss/critique/ask questions below.

[Tommy Cookers]

the usually seen aircraft con-trails are exhaust condensate not vortical condensate ...
and usually flying is for efficiency operating at a low lift coefficient so giving only weak vortices

btw whoever compiled the F1 rules & stats 1950-59 has slipped up
having shown as 1952-3 F1 rules the F2 rules to which the 1952 & 1953 WDC GPs complied
though iirc those had a 500cc limit for supercharged engines not 750cc as shown

[Vyssion]

the usually seen aircraft con-trails are exhaust condensate not vortical condensate ...
and usually flying is for efficiency operating at a low lift coefficient so giving only weak vortices

btw whoever compiled the F1 rules & stats 1950-59 has slipped up
having shown as 1952-3 F1 rules the F2 rules to which the 1952 & 1953 WDC GPs complied
though iirc those had a 500cc limit for supercharged engines not 750cc as shown

Aircraft Contrails actually have 2-3 definitons, or rather, there are 2-3 different types; one of which is exhaust emissions. These are formed by the water vapour in the exhaust gasses condensing due to the low ambient temperature at altitude and the impurities in the exhaust emissions, such as sulfur compounds, serve as a surface where droplets can then form and then freeze - which produces the contrail.

However, the second type are formed due to decreases in pressure. As a wing generates lift, it causes a vortex to form at the wingtip. The reduction in pressure and temperature across each vortex can cause water to condense and make the cores of the wingtip vortices visible; that is, to form a pressure drop contrail.

One thing you are correct in implying is that pressure-contrails do not happen typically at altitude - which is implied by the line "Aircraft contrails are a familiar sight to anyone who has so much as glanced at the sky" because they are typically seen at lower altitudes where the humidity is much higher - e.g. take off and landing. But it doesn't sound as poetic to say "look at a plane taking off"...

As for the "efficiency" of lift generation vs. the strength of the vortex, at low CLs, your induced drag is lower than at high CLs... So when you are travelling at the necessary speed for level flight (i.e. Lift = Weight), then at a low CL, you are travelling faster and so your velocity curl rate is pushed over a longer axial distance which reduces the strength of the vortex. However, these types are typically visible at low speeds and form due to humidity so depending on weather, you may still see a "strong" core. On some aircraft though, there is a pressure based contrail which forms off the nacelle and is used to keep the flow attached around the pylon for the engines though and that can be seen quite well under, once again, specific atmospheric conditions.

As for the rules and stats, I'll get someone to check that and make a change if required - thanks mate.

edit I just found an awesome image of contrails and had to share it

[Big Tea]

Sorry, off topic

[hollus]

Awesome article.

Regarding the Y250 vortex: It was forced (probably unintended) on the teams by adding the neutral section in the center of the front wing. As usual, what was meant to hinder performance, ended up being used to increase performance by using it to get cleaner flow to the middle and rear of the car.

A couple of questions here:

1) Do the benefits of the Y250 vortex actually counteract the drawbacks of being limited in the use of the front wing?

2) If FIA today deleted the Y250 sections of the rule book, would the teams still try to create their own versions of Y250 vortexes, now that they discovered their benefits? (equivalent vortexes, likely in a different, optimized position). Or would they just forfeit the Y250 vortex, make lots of front downforce, try to balance it in the back, and resort to a different way of pushing the dirty air away from the front wheels? In that case, which way?

[jjn9128]

A couple of questions here:

1) Do the benefits of the Y250 vortex actually counteract the drawbacks of being limited in the use of the front wing?

2) If FIA today deleted the Y250 sections of the rule book, would the teams still try to create their own versions of Y250 vortexes, now that they discovered their benefits? (equivalent vortexes, likely in a different, optimized position). Or would they just forfeit the Y250 vortex, make lots of front downforce, try to balance it in the back, and resort to a different way of pushing the dirty air away from the front wheels? In that case, which way?

Interesting. On 1, the benefits of the y250 definitely outweigh the drawbacks - if it were a negative teams would try to kill it rather than enhance it.

With 2... it would depend how the y250 was prevented - if it was by going back to pre-2009 front wing designs then the direction of the front wing tip wake would completely overhaul the flow-field around the front wheels, fundamentally changing the shape of the tyre wake. Another way to kill it would be to force teams to create a smooth, continuous surface from the main wing to the FIA section... which would lose significant downforce - maybe not a bad thing...

That said teams have used bargeboards and lifting wings under the chassis for years before the y250 so it's likely they were using the same concepts, just significantly less powerful because they had more freedom ahead of 2009, they placed Viking horns and bargeboards and turning vanes all over the place. As with anything it depends how the rules are worded and what is left out that can be exploited - the most efficient way to control the tyre wake is with a vane behind the wheel like the Formula E car or the 2021 concept sketches.

[n_anirudh]

A quick addition to a very nice article:

Helicity is used primarily to identify vortices usually aligned to the flow - used to identify counter-rotating vortices in the flow. Helicity is the scalar product of vorticity and velocity.

isosurfaces of Q-criterion are predominantly constructed from an instantaneous flow - such as DNS/LES/URANS, while helicity is used to see the time-averaged structures.

[Vyssion]

A quick addition to a very nice article:

Helicity is used primarily to identify vortices usually aligned to the flow - used to identify counter-rotating vortices in the flow. Helicity is the scalar product of vorticity and velocity.

isosurfaces of Q-criterion are predominantly constructed from an instantaneous flow - such as DNS/LES/URANS, while helicity is used to see the time-averaged structures.

Most detection methods were created under the assumption that steady flow fields and vortices move much more slowly than the average particle in the flow does.

Helicity is not Galilean Invariant. A detection method is Galilean invariant if it produces the same results when a uniform velocity is added to the existing velocity field, which means that methods which do not directly depend on velocity (such as pressure or vorticity) are Galilean Invariant. Because Helicity does not satisfy this, it means that it is not as good at resolving time-accurate vortical flow features. The underlying assumption is that near vortex core regions, the angle between the velocity vector and the vorticity vector is small. Pushing that definition to the extreme, i.e. when H = +/- 1 (with sign defining clockwise or anticlockwise rotation), it means that a streamline going through the core region has zero curvature and so there arise "sometimes" circumstances where the extracted core line doesn't strictly follow the actual vortex core. So as you say, it is better at steady-state.

Q-criterion, yes you are correct, is typically only able to trace vortex core lines in instantaneous flow calculations; except in very specific conditions where a statistical time-average over a "time packet" is used rather than the full time-average spectrum.

Either way, there are quite a lot more ways to follow vortex cores in steady-state simulations than in time-accurate simulations.