Understanding Undercut/Overfloor flow Performance

[NoDivergence]

Lots of people have been talking about how "tight" sidepods are and how aggressive the undercuts are under them.

Generally, to generate downforce, you want high pressure above the floor and low pressure below the floor. Correspondingly, you want high velocity flow under the floor but slow moving air above the floor.

Why is it that maximizing mass flow above the floor is such a huge concern, I would think under the floor would be the only thing that would matter? Is it for pressure recovery? I would think this would assist with drag, but how does this help with downforce? Does it "drive" the diffuser?

[hollus]

Flow over the floor literally drives the diffuser. This fast air entrains air from below the diffuser exit, helping extract it from there. Now, extracting air from the wide (in the vertical direction) part of the diffuser forces air from the narrow part of the diffuser, and from under the floor in general, to speed up to provide enough mass flow that a vacuum doesn't form.
Actually, for performance that vacuum would be even better, but the laws of physics are such that instead flow accerelates and you only get low pressure via Bernoulli.

[DaveKillens]

Ground effects are here to stay, even though the regulations inhibit it's application severely. The sliding skirts are now banned, but that does not prohibit the engineers from finding an alternate solution. Air leaking from the sides allow the air pressure under the floor to climb, thus every attempt is made to seal the side of the floor. The Lotus forward exhaust was one of those attempts. But generally, the sidepods lower the air pressure along the sides of the car, and the sharp edges of the floor inhibit the air from easily flowing under the floor. Then, as Hollus describes, that airfow maximizes what little diffuser effect is allowed.

Before 1995 the cars rode so low that they could seal the sides of the car, and minimal leakage occurred. The cars back then had straight sidepods. But once the FIA raised the floor and mandated the dimensions of the floor, the engineers found that if they lowered the air pressure along the sides of the sidepods, they could claw back some downforce.

[skgoa]

Uh, you can actually see an early cokebottle rear on that Benetton.

[NoDivergence]

Okay, take a look at the McLaren's rear suspension. That's more along the lines of what I was thinking of. High pressure from the suspension arms (helps for downforce), then low pressure wake which causes increased pressure differential between the wake and diffuser, driving the diffuser. Very nice solution.

[DaveKillens]

Uh, you can actually see an early cokebottle rear on that Benetton.

And I also see straight sides along the sidepods, and a transition from side to bottom that does very little to hinder the flow of air from side to the underbody. The Benetton rode almost touching the ground, and thus there was little "leakage". But imagine raising that chassis two or three inches, and all of a sudden that sealing effect is gone.

[Huntresa]

Uh, you can actually see an early cokebottle rear on that Benetton.

Seeing as the first coke bottle was in 1982/1983 it would be weird if the Benetton didnt have it