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1) The tunnel should be as wide as possible. However, two factors limit the width.
a) in F1, you have a bunch of crap in the middle of the car - the driver pod, engine, etc. This limits how close to the center the two tunnels can come.
b) air spillage. If the tunnels are too close to the outside of the car, the air spills out and causes vorticity. This is bad. F1 guys solved this by using sliding skirts (like the rubber piece on the bottom of a house door), and making full-width tunnels.
Designers put vertical slats in the tunnel to keep the flow laminar and moving upwards along the tunnel contour, instead of sideways. Making multiple tunnels (3) is basically the same general thing. The slats can be increased up to a practical limit of maybe 1-2" spacing between them, wherein a slot/edge effect actually impedes air going into the slots.
2) the f1 ones are actually curved. The curvature helps the tunnel get higher in the bumper faster, so therefore is a good thing if you have a limited amount of room for the diffuser section. Obviously, this curvature has an upper limit, beyond that and the air detaches. A gentle curve like in the F1 picture I provided is generally a good compromise.
I'm working on a similar write-up for the aerodynamics of wings, turbo wheels, and a few other things. Thanks for the positive commentary!
-D
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