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Originally Posted by doma
Do you mean the NACA ducts?
Very interesting - I didn't know they had a name and a defined geometry. I had seen the shape before on cars (the F40 has them all over the body-work even) but didn't realize there was an aerodynamic reason for the shape.
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Yep. They have the shape for a reason.
The NACA (the National Advisory Committee for Aeronautics or something like that) was a predecessor to NASA. They pretty much defined the various airfoil shapes, etc.
NACA ducts function by "bleeding" off the boundary layer off the surface with air flow passing over it. Basic boundary layer theory is that the air at the surface of an object is not moving. At some distance from the surface, the air is moving at the speed of the passing air flow. The space in between is called the boundary layer (hint: it's very thin). If you disturb the boundary layer by sticking something into the flow, you can make the boundary layer thicker (increasing drag), and even make it "break away" from the surface, which usually leads to turbulent airflow (tumbling) and lots of drag.
Having an "air scoop" stick into the air flow, causes quite a bit of drag and disturbed air flow around the scoop - you get the air intake, but with side effects. A NACA duct works differently. As the boundary layer encounters the narrow opening of the NACA duct, it follows the surface into the duct - this "bleeds" the air into the duct. As the opening widens, the air that misses the "front" of the duct, instead "rolls" over into the duct as it encounters the opening. The result is that the boundary layer is not disturbed, and just follows the duct down into the duct. When the air flow encounters the rear of the duct, a new boundary layer is begun along the rear surface that is the same as what would have been there anyway without the duct opening.
The end result of a NACA duct is that you have a "scoop" for air that supplies air flow into the surface, without any increase in aerodynamic drag or any disruptions in the air flow. A pretty neat solution. That's why you will find them on things like the under-tray of the Elise or on F40s, not to mention lots of air craft, space craft, etc. There are also lots of imitations where the ducts are just there for looks.
Oh yea, just having the opening in the tray of the Elise instead of the plastic duct won't work the same. The air will flow into the opening, but the boundary layer will become separated - causing turbulent air flow resulting in more drag, and less actual air flow into the opening (and spoiled aerodynamics under the car)...