Now here's a puzzling question. I understand that hotter exhaust gasses produce more noise; I understand that turbofans work by surrounding the hot gasses with a stream of cooler gasses, which additionally absorb heat from the core; what puzzles me is that there was at least one proposal for a nose reducing mechanism that had the exhaust velocity lower in the middle of the exhaust stream and higher around the edges?
Why does that work? That sounds like the worst thing you could do!