Noise, turbulence, and thrust of subsonic freejets from lobed nozzles

A study of noise benefit, vis-a-vis thrust penalty, and its correlation to turbulence intensities is conducted for freejets issuing from lobed nozzles. Four convergent nozzles with constant exit area are used in the experiments. Three of these are of rectangular lobed configuration having 6, 10, and 14 lobes; the fourth is a circular nozzle. Increasing the number of lobes results in a progressive reduction in the turbulence intensities as well as in the overall radiated noise. The noise reduction is pronounced at the low-frequency end of the spectrum. However, there is an increase in the high-frequency noise that renders the overall benefit less attractive when compared on a scaled-up A-weighted basis. Increasing the number of lobes involves progressive reduction in the thrust coefficient. The measured thrust loss is shown to be primarily caused by increased amount of low-momentum boundary-layer fluid over the stretched perimeter. It is inferred that a moderate number of lobes effectively reduce the noise, but increasing the number further results in a diminishing noise benefit and a rapidly increasing thrust penalty. An analysis, showing that the loss in thrust coefficient is proportional to the perimeter-to-diameter ratio and the inverse of the square root of the Reynolds number, provides a guideline for the choice of the lobe dimensions.