Also keep in mind that air drag is proportional to Cd and frontal area. A narrow, low vehicle with moderate Cd can be more aerodynamic than a big vehicle with very low Cd.
Case in point: according to this page, the Cheetah streamlined bicycle has a Cd of 0.055. When it achieved a world speed record of 68.7 mph, the team boasted on a CNN interview: “You put all that technology and a world-class cyclist together and the envelope of speed for mankind has come to a close; we’ve reached the limit.” They may have been right in that the Cd of 0.055 is extremely difficult to beat. But that didn’t matter, because it turned out streamlined bikes could be made much narrower and lower. The Varna Diablo’s Cd is an unimpressive 0.110, but its frontal area is 64% smaller than the Cheetah. The Diablo beat the Cheetah record by a whopping 12mph (81.00 mph).
By the way, I notice none of the record-setting HPVs (human powered vehicles) has a Kamm back. Most modern ones are shaped like this - the nose is long and has a very shallow angle, the tail is also pointed but at a steeper angle. The design goal is to maintain laminar flow as long as possible - the link shows a computer simulation that indicates laminar flow is maintained for 3/4 of the length of the vehicle. Here is another vehicle with a similar profile. But on a production automobile you wouldn’t even attempt to achieve laminar flow because there are too many imperfections and functional elements that disrupt laminar flow. The optimal shape for a turbulent airflow looks very different than an optimal laminar flow body.
Blowing to reduce drag. Yes, this is a useful technique that falls under a general heading of flow control, which I am currently working on. The problem is that trucks necessarily have a very square back. This causes instant seperation and huge vortex sheding behind the vehicle. These oscillating vortices are very large and unstable and use up a great deal of energy. The blowing concept creates a sheet of more stable vorticity rather than the large oscillating vortices.
An easier way of thinking about is that these sheets of vorticity create a virtual surface that extends behind the vehicle and tapers rather than just ending like the vehicle itself. This effect can and has been created using asimpler mechanism. Small angled metal plates protruding from the sides of the vehicle just before the rear can create this vortex sheet as well. They look like this:
These are already commercially available for the rear of trucks, and also at the rear of the cab to bridge the gap from the cab to the trailer.
Is the Kamm back better than a tapered back? No. Things can be done with a the rear of a car once you have accepted a flater back (which is necessary with a trunk and bumper), but it is much better to have it tapered. As compared with the standard sedan look, I am pretty sure the sedan can be better, but it probably depends upon the specifics of the design.
Why is the backards rowboat have a better Cd than forwards? A sharp change in geometry results in large vortices and reverse flow. You are creating an area of dead airspace behind the boat that has low pressure and sucks the thing backwards. Note that the super aerodynamic vehicles, and even wings, get thick rapidly in the front and then slowly taper toward the rear. You don’t want to change geometry too quickly or the flow will become unattached. This being said, the blunt front sucks too, it just sucks less.
Laminar vs turbulent. You are right that you can’t do a care with laminar flow. It requires a very smooth surface, and the door joints, hood joints, etc would trip the flow to turbulent. You wouldn’t want it anyway, as laminar flow becomes unattached (greatly increasing drag) much more easily than turbulent.
