Vortexes: how close before they merge?

[hollus]

This weekend we had a deluge of vortexes. In principle, every little winglet or sharp edge could throw out its own vortex, but in practice we see fewer, larger vortexes and they all have to live together in a single circulation pattern, so there must be a limit of how close two co-rotating vortexes can be before they either annihilate each other or merge together.

I tried to illustrate graphically how I imagine it works in my head, looking along the vortex axes. It might be very wrong! I would imagine that in situation 2 the vortexes could still deform somewhat to coexist, but eventually they come so close together that...

Is it like what I drew? Do they annihilate each other instead? Maybe it depends?

Also, is there a quantitative rule to predict this?

[gixxer_drew]

Yes basically (except that one center small vortex I have never seen it like that unless you induce it on purpose) they resist each other at first. Two parallel vortices rotating say clockwise are flowing opposite directions to each other at any point they would touch, so there are forces to both join and push them apart.

Over time and therefore distance they dissipate energy as the air approaches equilibrium. Once the forces drawing them together are stronger than the ones pushing them apart.... poof.

[n_anirudh]

"Vortices" do merge when left long enough. So far only a 2D view is considered.. Look up Crow instability and related mechanisms.

[shelly]

I willl try to retrieve some info about vortex coalescence and post it here. There are sveral places in the car where vortices are close and interact, it would be nice to have some idea about that

[horse]

Ted Kravitz once used the word "vortice" in one of his BBC articles so I gave him some stick on twitter.

"Vortexes" is a correct, if not common, pluralisation, however. See Cambridge Dictionaries Online.

[Just_a_fan]

I would use "vortices" as the plural. "Vortexes" just sounds a bit clunky to my ear.

[Huntresa]

I would use "vortices" as the plural. "Vortexes" just sounds a bit clunky to my ear.

Isnt Vortices the actual plural form of vortex ?

[shelly]

This article is veery technical, deriving eqaution it finds again the Crow instability cited by adiruth and mentions a "soliton solution" whcih could be interesting to look at:
http://math.arizona.edu/~zakharov/567Ar ... Vortex.pdf

This other link is much more readable:
http://massey.dur.ac.uk/resources/ngparker/chapter8.pdf

[riff_raff]

Since these vortices are only formed in wake flows, and since they tend to follow a flow path that continually spirals outward as they lose energy and velocity, it would seem unlikely for the conditions to occur in F1 cars where two of these vortices would couple or merge in any meaningful way.

The principle of using vortex generators on the front wing end plates to create energetic vortex flows directed along the side gaps between the underbody and track surface, that spiral in a direction which discourages underbody airflows from flowing laterally outward, has long been common practice. While this is an instance where wake vortices are intentionally created and used to benefit, most of the wake vortices you see trailing from F1 cars are unintentional, and are simply a visual representation of wasted energy.

Lastly, anyone that has landed a small aircraft at an airport where large commercial jets operate will tell you the hazard that wake vortices represent. The wake vortices produced by a commercial jet are still powerful enough a minute or two after landing that they can flip a light aircraft passing into them.

[CBeck113]

Lastly, anyone that has landed a small aircraft at an airport where large commercial jets operate will tell you the hazard that wake vortices represent. The wake vortices produced by a commercial jet are still powerful enough a minute or two after landing that they can flip a light aircraft passing into them.

The difference is that in F1 the vortices are being used as bodywork (positive effect that outweighs the losses) , controlling flow around the car. For plane, they're just (a) drag [sorry, couldn't resist] - they are reduced to a minimum to reduce fuel costs.

BTW, the wake voritces are the main reason that there is such a long pause between launches of all aircraft. The A380 causes a 3 minute pause after launch, almost negating the increased loading capacity by forcing fewer take-offs (standard is around 1:30).

On topic, I think that letting (or forcing) vortices to merge would cause more damage than it would help, since the flow would most likely break up, causing even more drag, and the sealing effect would be lost. I am guessing this based on the fact that merging any two systems usually only works when they are syncronized, in this case air speed and vector, otherwise you generate losses (slow the air, changes in direction). If it could be timed that there is a positive change to the speed, adding more energy and reinforcing the vortex, then great - but this would need to work in changing conditions, and I'm not sure if it would be worth it for a window of 20km/h or so (just an example!).

[Tommy Cookers]

Lastly, anyone that has landed a small aircraft at an airport where large commercial jets operate will tell you the hazard that wake vortices represent. The wake vortices produced by a commercial jet are still powerful enough a minute or two after landing that they can flip a light aircraft passing into them.

BTW, the wake voritces are the main reason that there is such a long pause between launches of all aircraft. The A380 causes a 3 minute pause after launch, almost negating the increased loading capacity by forcing fewer take-offs (standard is around 1:30).

the aircraft will be producing little lift and little vortices until it goes to large AoA near the end of the runway
vortices move with the air (often in the modern world a crosswind, so leaving the runway, giving clean air for a following plane)
so vortices are more important on the approach than on the takeoff ?
(takeoff seperation is almost always controlled, driven by the possibility of any takeoff problem for the incumbent)
throughout approach the AoA is large and the vortices strong, though a crosswind will take them from the path of a following aircraft
and the seperation is to a greater or lesser extent down to its pilot

in some more frivolous modes of flying it is possible and even commendable to run through one's own (previously made) vortices
they do persist, particularly in calm air

[shelly]

Another interesting link:
http://www.lr.tudelft.nl/fileadmin/Facu ... 4_1_05.pdf

it is old (2004) but shows some good theory recap

[riff_raff]

...The difference is that in F1 the vortices are being used as bodywork (positive effect that outweighs the losses) , controlling flow around the car. For plane, they're just (a) drag [sorry, couldn't resist] - they are reduced to a minimum to reduce fuel costs.....

Trailing wake vortices from the front wing and sidepods can be used to improve the aero performance of the underbody. But wake vortices produced by the rear wing are not so easy to take advantage of. All wake vortices are the product of a transfer of energy to the passing airflow.

As for merging vortex airflows, this is not always a good thing to do. Rotary wing aircraft can lose lift and crash during a VTOL landing due to a phenomena called a "ring vortex state". This is where the vortex produced by the rotor downwash reflects off of the ground, spirals back upward, and then gets drawn back into the rotor airflow causing the rotor to lose lift.

[olefud]

The difference is that in F1 the vortices are being used as bodywork (positive effect that outweighs the losses) , controlling flow around the car. For plane, they're just (a) drag [sorry, couldn't resist] - they are reduced to a minimum to reduce fuel costs.

While vortices are often wasteful, when provoked in the boundary layer they are useful to maintain boundary layer attachment. As the shear between laminar layers across the velocity profile increases, vortices formed between layers can function as “rollers” that decrease shear friction within the boundary layer.

[shelly]

But besides that, vortices are used for downforce because of their lower pressure

So vortices =
1) drag (e.g. rw tip vortices
2) flow conditioners /"virtual bodywork" in free stream (e.g. bargeboard upper vortex)
3) boundary layer mixers near the surfaces to delay separation (small vortices induced by small vortex generators, like 1 cm high)
4) low pressure stream tubes that produce downforce if run below surfaces (e.g. front wing vortices, bargeboard lower vortex)