Yes, any disruption of the smooth shape will increase drag. The drag caused by the shape of something is called "profile drag". However, there is also a "boundary layer" close to the surface of the shape that is caused by the air flowing over the leading edge. Within the boundary layer, the viscous effects of the air are high, which means the velocity of the flow is low. Have you ever noticed that they don't bother smoothing every rivet on the outside of an big passenger airplane? That's because the boundary on a big airplane is quite large, and the those little rivets are inside that B/L, therefore they don't add much if any drag. However, there is another type of drag called "friction drag" that accounts for that same viscous effect of the air being slowed near the surface. All of the different drag components are added together to determine the overall coefficient of drag (Cd) of an object.
I don't know how tall the boundary layer is at the center of the hood of a typical car, it depends on whether the flow is laminar (smooth) or turbulent at that point, and whole bunch of other factors. You really only need to consider the part of the scoop that is sticking above the boundary layer. The "NACA duct" you see on many cars is supposed to be the optimal shape for feeding outside air into the interior of the vehicle. You'll notice it doesn't stick-up at all, thus it is entirely within the boundary layer. You still have drag due to the other factors like the viscous drag, pressure drag, etc - but it's the best you can do.
Do you recognize that one? That's on a Veyron.
HOWEVER - all of that is probably just unimportant nonsense, because...
The question is NOT whether the hood scoop increases drag (it does) - the question is whether it is significant. If the drag increase is small compared to the overall drag of the vehicle, then it really doesn't matter - and that's certainly the case with an AC Cobra. Those big rotating tire meats and that open top probably create much more drag than the air scoop - and feeding air efficently to the engine will generate much more horsepower than any impact of the scoop drag.
Gee - that was fun - just like the old days of my former life...
Good explanation, that explains the design of modern sportscar vents pretty well.
I read in a book somewhere, a car is even more complex to design than an airplane because it works in contact with two mediums. It runs on the road, and runs in the air. Hence, its a much more complex problem to solve because you don't want a car to behave like an airplane that flies off, and still want it to go as fast as possible with minimal power.
For comparison, I was amazed as a kid when I saw this video (I think it was discovery channel). A small plane, with a single cylinder engine per wing, could easily overrun a 12 cylinder sportscar....
Edited by proton007 - 1/22/13 at 9:16pm