Love to see this topic raised from the dead
I would like to correct/ increase definition on some things said before.
*First take a look at the link Rich gave us which is very helpful.
*Consider looking at the right side of a car (ie right is front, left is back) and for the ease of explanation consider the car as 2d (the overall profile on the centreline)
*Stagnation point is a theoretical point ahead of smthng that moves through
and the air starts to split at this point in order for the part to move through the air. WE, OUR CARS move THROUGH the air, not the other way...
1. As we move toward left the car starts to show its profile. our bumper being the foremost point, it has a stagnation point just ahead of it. Since the bumper isn't solid as it moves through air, the air suddenly gets sucked by the rear of the bumper, creating a vacuum effect and spiraling vertices.
2. next is the handle on the front hood. lets say it just splits the air left to right, so it is negligible at this moment(we could use it if talking 3d)
3. next is the profile of the front hood. specifically on the 1303 and less on the older bugs it has a hump facing forward. this has another stagnation point that the spirals from the bumper interfere with.
THERE is the first actual problem area due to the LOW PRESSURE POINT BEHIND the bumper and the HIGH PRESSURE POINT IN FRONT of the hood.
IN GENERAL every clean profile that the air gets to "see" first is a HIGH PRESSURE AREA, analyzed in HIGH PRESSURE POINTS and the opposite is for the LOW PRESSURE/VACUUM AREA/POINTS
Evil, the reason the auto makers put the air intake between the windshield and the hood is that the create a strong vacuum zone by raising the rear lip of the hood (to further promote the effect) so air can be SUCKED in.
Also the fan at the back actually works, because it lays in a low pressure area (engine compartment) that is connected to the "laminar" air flow at the rear with the grills below the rear window.
Don't confuse ram air that can work with carbs, efi with the actual fan
that is if we could put a big hose supplying air directly into the fan the fan would spin, but there would be no air flow.
same applies to turboprop engines in aircraft where u have a turbine spinning and a gearbox actually a huge reduction box that the prop is connected on to. if the ratio of turbine/prop was 1:1 the prop would go supersonic and the effect will be the same as spinning the same prop in space at zero pressure
this a real example from the cessna super cargomaster caravan aircraft where the turbine runs at 33000rpm @ 75% thrust and the prop at a mere 1750rpm.this is why u cannot go supersonic with a prop and only with a jet
sorry for the non germanlook example, it was the most recent i had from a personal experience
Chris