On the nature of "dirty" air

[hollus]

We often hear that a car in the wake of another car loses downforce because it is in "dirty" air. This effect supposedly extends a good 50 meters or more behind a F1 car. We also hear that it reduces front downforce more. Considering a car with the front wing 30 meters behind, the rear is like 34 meters behind, not a dramatic difference, so I interpret (wrongly?) this front bias as meaning that the wings suffer a greater loss of performance than the floor-diffuser combo.
What exactly is "dirty" about this air that costs so much downforce to the trailing car?
Is it just turbulent, with random lateral velocity components? The rear wing vortices themselves don't seem to survive much beyond 10 meters...
Is it the vertical velocity added to the air? It is not like that is going to result in a vacuum below it, a new mass of air moves in to fill the potential gap...
Is it the speed added to the air in the direction of movement reducing the relative air speed for the car behind?
Is it all of the above? Is it something else?

[Pierce89]

We often hear that a car in the wake of another car loses downforce because it is in "dirty" air. This effect supposedly extends a good 50 meters or more behind a F1 car. We also hear that it reduces front downforce more. Considering a car with the front wing 30 meters behind, the rear is like 34 meters behind, not a dramatic difference, so I interpret (wrongly?) this front bias as meaning that the wings suffer a greater loss of performance than the floor-diffuser combo.
What exactly is "dirty" about this air that costs so much downforce to the trailing car?
Is it just turbulent, with random lateral velocity components? The rear wing vortices themselves don't seem to survive much beyond 10 meters...
Is it the vertical velocity added to the air? It is not like that is going to result in a vacuum below it, a new mass of air moves in to fill the potential gap...
Is it the speed added to the air in the direction of movement reducing the relative air speed for the car behind?
Is it all of the above? Is it something else?

It is a bit of all of the above combined with the fact that f1 cars are designed in wind tunnels in laminar flow. So the turbulence itself is partly to blame along with taking the car out of their design conditions.

[Lycoming]

Rear wing vortices don't seem to extend much beyond 10 meters you say? Perhaps not visible ones, but think about this: Commercial jet airliners require separation on the order of 6-10 minutes, and their wings have much higher aspect ratios (and therefore relatively less intense vortices) than an F1 car. They are much larger though.

That aside, vortices produce upwash in between them, which does not do wonders for the following car. Additionally, if the ambient air is turbulent as opposed to laminar, it will separate earlier than normal. The effect of this on the following car of course depends on how close the, say, front wing is to separation when the incoming flow is laminar. If the flow separates early from the underside of the wing, it's basically stalled and not producing much downforce.

I can't speak for why the front wing is said to be more affected. It may just be journalist's theories, possibly corrupted from offhand comments from engineers, but I simply don't know.

[thisisatest]

i believe the front wing is more affected because it is designed to be passing through calm air. the rear wing already expects some level of turbulence. also, the front of the car is all shaped to steer the air to the rear wing. it's all still doing that, so in a way the air is "conditioned" before hitting the rear wing...

[riff_raff]

...so in a way the air is "conditioned" before hitting the rear wing...

That's an excellent description! The direction/local velocity of the airflows at the underbody and rear wing LE have been altered by the front wing and other body aero surfaces. The front wing's aero performance is more affected by wake turbulence than any other part of the car. However, the aero performance of the underbody or rear wing can also be affected by the change in trailing wake airflows caused by a car following very close behind.

[Diff-user]

These are the reasons I believe it so.

1) As earlier mentioned, turbulence has a big role to play. The ground effect front wings lose roughly 20 - 25 percent downforce just by being in turbulent air instead of the transition free flow they are designed for (the numbers come from a paper in open literature). I can give you references, if interested, that explain this in great detail.

2) More importantly, the upwash created by the diffuser, beam-wing and the rear wing results in a lesser angle of incidence of air on the front wing. Remember seeing those large rooster tails behind the cars in rain or on a dusty track? That's upwash.

3) Lastly, the wake effects reduce the effect of all those intricate tiny flow conditioners that you see on the front wing, affecting the flow over the rest of the car, affecting overall downforce.

The wake effects are larger in the central portion of the wing. Experiments have show that this is the region where the flow separates first and produces lesser downforce than the outboard ends, when in the wake of a bluff body. This is the reason for the F1 tech regs having a near neutral aerofoil section in this region. It tries to reduce the aerodynamic disadvantage of the trailing car to some extent.

The rear wing is designed to work in the upwash of the front wing. In other words, it is designed to work in upwash while the front is not. And also, since it is not in ground effect, the effects of turbulence are not to separate the flow but to keep it attached.

[lebesset]

i believe the front wing is more affected because it is designed to be passing through calm air. the rear wing already expects some level of turbulence. also, the front of the car is all shaped to steer the air to the rear wing. it's all still doing that, so in a way the air is "conditioned" before hitting the rear wing...

isn't there therefore a case for the front wing to be reduced in size even more than it has been for next year ? with the 2014 length the wings will still be able to turn air flow around the front tyres to varying degrees depending on the steering angle ; reducing the wing even further would not only improve the relative performance in dirty air but would decrease the incidence of front wing damage in close racing , surely something we all want to see

[riff_raff]

isn't there therefore a case for the front wing to be reduced in size even more than it has been for next year ? with the 2014 length the wings will still be able to turn air flow around the front tyres to varying degrees depending on the steering angle ; reducing the wing even further would not only improve the relative performance in dirty air but would decrease the incidence of front wing damage in close racing , surely something we all want to see

This is something the IndyCar has been working to address with their rules over the past couple years. They have changed the regulations for bodywork to allow cars to race nose-to-tail with less effects from wake turbulence.