Otto:
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That said, the nose of the Porsche could stand some improvement, and various spoilers and splitters have been designed for it, with anecdotal reports as to respective efficiency. Many on this website have made significant aerodynamic improvements to their vehicles by home-made experimentation and some pretty good intuition. From this, we can surmise that the factory original is not necessarily the final word on design.
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I agree. However, just because *some* factory originals are not optimal doesn't mean *all* are not optimal. Again, be careful and be prepared to do some testing. However, my primary point was not that the front *can't* be improved--it was that rear improvement (if possible) would likely help more, though that's a generalization and may not apply to your particular car (I'm not familiar with the 944 and its aerodynamic history).
What I said about mixing distance came from Road Vehicle Aerodynamics, mentioned in my intro. To my knowledge it is not available for viewing on the net--you'd have to find it in a library, probably a local college library. It has quite an extensive section on how airlfow through the engine compartment is designed--apparently, until recently, this has been where the *majority* of wind tunnel time has been spent! The designers work really really hard to make sure the darned ICE is gonna stay cool (and warm in cold temps) and airflow through the grill, the engine compartment, and under the car is obviously critical (as well as very complex).
In general, the air under a car is at a lower pressure (because it's moving at a higher speed) than the air at the front--air moves from high pressure to low, so, yes, the air flow under the car has a lot to do with getting the air through the engine compartment.
The problem with the boundary layer (and I'm just learning this stuff) is this: it is the layer of air that is trying to "stick" to the car, and is trying to move with the car while the air farther from the car is happy to sail on by. The boundary layer forms because air is viscous. At Reynolds numbers for road vehicles, the boundary layer becomes turbulent very quickly; however, the boundary layer may or may not separate. Now from an energy point of view the vehicle will lose less energy to a laminar (non-turbulent) boundary layer than it will a turbulent one, but the real killer in terms of energy loss is when the boundary layer (turbulent or laminar) separates. It turns out that a turbulent boundary layer separates less easily than a laminar one, so sometimes turbulence (at the micro scale) is good. Vortices, which are "turbulence" at a macro scale are something else. Now, you probably knew all that, but I mention it for the following:
One problem (please notice I said *one*) with the boundary layer below the car is that it eventually thickens and meets the ground. The boundary layer (which is probably already turbulent and hopefully is still attached) is trying to move with the car but when it meets the ground, additional turbulence is created because the ground tries to retard the boundary layer. That additional turbulence (which may be "macro" turbulence--I'm not clear on this yet) represents a (small) loss of energy so the Cd has, in effect, just risen. If you can keep the boundary layer as thin as possible for as long as possible, you keep it from meeting the ground (i.e., you've increased the mixing distance) and you thereby lessen the gound-induced turbulence which means you've lessened the energy lost to turbulence.
There are other things going on as well, so this isn't the whole story--but it may help you understand the mixing distance and its effect on drag.
--Steve