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I think we should call that a roof extension, and yes, the transition should be gentle. Prof. Kamm discovered that it is best to have a sharp transition to a flat back, after roof extensions, etc, have been carried as far back at a reasonable taper as other considerations permit. That sharp transition is one of the few places we don't want a curve in this game. On another thread, someone wondered about a saw-tooth edge, copied from nozzles that need silencing. That does not increase efficiency, it just puts the sound into mutual interference. The proper refinement of a kamm back edge is just a mild lip, which can trip the wake into a single large, stable vortex instead of many competing ones.
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