APU Exhaust shapes
A tech question but as I am not a professional I ll post it here
BAck in the when the MD80 graced our skies the early ones sported an APU/Exhaust in the conventional cone shape while the newer ones were changed to a boat tail or wedge shape. Boeing went on to use the wedge shape on the T7 whereas the 767 and 747 used the conical version . I assumed that was an aerodynamic advancement to go to the wedge but along comes the 787 and its back to the cone again.
Any one know the reasons/aerodynamics fro the two styles and why the change back?