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Originally Posted by theclencher
I don't know what you read, but it has long been accepted that a sharp cut-off of the trailing edges generally causes the airflow to continue on in more laminar flow past the car than if the trailing edges are radiussed. That is one of the main premises behind Kamm's theory and all Kamm-backed vehicles.
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That's also my understanding, and you may be right that the problem is semantics, ambiguity, or my misunderstanding of Barnard's writing. The quote(s) in question (
Road Vehicle Aerodynamic Design, an Introduction, 2nd ed., MechAero Publishing 2001):
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Strongly unfavourable pressure gradients at the rear should be avoided; some taper and rear-end rounding should be used. - p 98
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This one doesn't specifically make it clear whether by rear-end rounding he's referring to the side-rear corner, or to plan-view tapering.
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Radiusing and tapering the rear end produces significant drag reductions. Hucho quotes Cd reductions of 4-8 per cent for radiusing, and up to 20 percent for side panel and roof tapering, with a combination of taper and radiusing producing up to 22 per cent reduction - 108
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This one seems less ambiguous, however it is in a section about commercial vehicles (transports/vans), where the roof line is generally straight from front to back.
Maybe the distinction is that there's some aerodynamic benefit to radiusing the side/rear corner where there is otherwise no taper in the shape in plan/profile view.
But my gut tells me that if you've already achieved an proper tapered/boat tailed shape, the side/rear corner should be sharp, a la Kamm back.
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There could be multiple reasons for this: the stylists wanted it that way and the aerodynamicists said it wouldn't hurt much; the bumper skin could be a huge injection molded plastic part and for internal mold-flow and filling reasons radiussed corners worked better than sharp ones; it is easier to paint parts with more generous radii (again, fewer rejects and re-works); and/or perhaps it is the knowledge that a generous radius on a part distributes stresses better thus making the part stronger and less likely to break there.
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These are all good points that could lead to a compromise vs. proper aerodynamics.
I've realized aero is a much more complicated subject since I decided to make an effort to go beyond "lay" or "pop" aerodynamics (which I like to describe as a general understanding of the fundamentals which I can mis-apply in specific situations
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Good, reply clencher - thanks.
Still don't know the answer for sure though. Perhaps time to track down an expert and ask directly.
a new policy....I intend to ignore the nescient...a waste of time and energy. 
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