Basic Aerodynamic Question

[Cactus466]

Hello Everyone, this is my first post so forgive me if this has been covered before or it may seem like a basic question.

My question is this, I have been watching this video http://www.youtube.com/watch?v=jYaIXWNOa_A

The point that really interest's me is around 3mins 30secs. I understand that air gets passed over the diffuser creating a pressure differential pushing the car into the ground. Just watching the video I notice the way the air leaves the rear wing, as Martin Brundle describes it as a "upside down wing".

The cars passing behind into this dirty air is obviously not a good thing but then at what point does the car behind enter into the slipstream? What kind of distance are we talkimg here from being at one point in dirty air to now being in the car's slipsteam? When does it go from a dis-advantage to an advantage?

Sorry if this sounds like a daft question but I would just like to hear peoples opinions.

[Scotracer]

If the car is in the dirty air, he is in the slipstream. Unfortunately the loss in performance in the corners is not balanced by the gain in the slipstream on the straights. Modern F1 cars don't get much of a slipstream advantage either (despite getting a huge disadvantage from the 'dirty air')...which is made even worse with the current rpm limit.

[n_anirudh]

I so wish the airflow over the rear wing would be as straight as that..Also in the video, no vortices are being shown. There is span wise flow which becomes more prominent closer to the endplate.

Slip streaming would occur at 0.1 L (length of the car) or closer IMO. Beyond this point, vortices generated from the rear wing,beam wing, diffuser and the rotating tires make it quite hard for cars to pass.

[wrcsti]

Next time you drive behind a truck on the highway hold your throtle constant to a speed where you will barely pass him. If you dirve a small car you will feel the car accelerate more rapidly as you near the truck and when you move over to overtake he will push you off to the side then suddendly you have to accelerate more to get by him. Thats a slipstream right there.

[Cactus466]

Thanks for the replies everyone, makes a little more sense now.

[burchenal]

This is actually quite an in-depth question with a basic answer of we don’t know.
Research in the public domain has been limited to downstream vehicle locations of 1 car length. At these kinds of spacing the airflow behind a modern Formula One exhibits large turbulence intensities and velocity deficits with big affects on the aerodynamic performance of downstream vehicles. If you want definite numbers track down the report: The Aerodynamic Characteristics of a Race Car Wing Operating in a Wake – Durham University.

As for my own educated guess of when a car experiences turbulence from a leading vehicle it could be more than 5 car lengths given the decay rate and strength of vortices produced by the rear wing. Naturally the transition from turbulence to slipstream would occur when you enter the velocity deficit of the wake – At a guess this could be between 1 to 3 car lengths.

[Ogami musashi]

the length is dependent on the wake signature; there's no generic answer.

And it is even different depending on the lateral/vertical separation with the car.

[wesley123]

Slip streaming would occur at 0.1 L (length of the car) or closer IMO. Beyond this point, vortices generated from the rear wing,beam wing, diffuser and the rotating tires make it quite hard for cars to pass.

I would say it is way more. I remember a quote from the 2007 era or something that drivers already noticed it when almost 3 car lengths behind, which really is a lot. The 2009 rules reduce this some but i still think it is 1.5-2 car lengths until the drivers notice it.

[n_anirudh]

Wesley123, I agree with you on that. The drivers would first feel the effects around 1-2 car lengths, but we have noticed the cars being "sucked" into each other (as a result of low drag) at very close lengths beyond which they break out of the slip stream and make the overtaking pass.

Having said that, I wish the OWG had started with moveable rear wings as compared to the front wings which did nothing more than controlling tire wear and adjusting car balance.