Pseudo-transonic Equation with a Diffusion Term
Now we examine the claim, mentioned by Crighton, that the "flat" part of an elliptical aircraft nozzle jet radiates less noise than a similar circular jet. We can see no instability patterns, or lack of them, on the surfaces of the jet to explain a lower radiation from the wide part of the jet, and indeed Maestrello and McDaid found very little difference in nearfield noise emission from various sides of a rectangular jet. What could be happening, however, is the 90-deg shift in major axis shortly after leaving the aircraft, which then presents a smaller width radiating source to observers on the ground than would a circular jet of the same power. This then could explain the possible noise benefits of elliptical nozzles, although more elaborate theories have been proposed.