I am curious why the person you spoke to said they would prefer a more blunt face in front of the turbine, as in your F-15. The type of compressors we use on models are not as intolerant to turbulent flow as a full scale axial flow turbine. But I see no reason to not use reasonable design rules, and minimize turbulent flow creation. Typically, any kind of quick or sudden expansion in the duct will create a turbulent flow. I would prefer to design with smooth contours inside, and slowly reduce the diameter of the centerbody to more of a teardrop shape, as the above posters have shown or mentioned. ( Mach1's pic "C" ) [8D]

I assume your turbine is inside a bypass, and you are using a fully ducted system. If not, it probably doesn't matter. A short blunt aft facing area would be lighter, and you could probably slide the turbine forward a bit more. But it wouldn't be a cleaner flowpath.

Looking forward to seeing your project fly - next time with turbines! [8D]