A study of noise benefit, vis-à-vis thrust penalty, and its correlation to turbulence intensities was conducted for free jets issuing from lobed nozzles. Four convergent nozzles with constant exit area were used in the experiments. Three of these were of rectangular lobed configuration having six, ten and fourteen lobes; the fourth was a circular nozzle. Increasing the number of lobes resulted in a progressive reduction in the turbulence intensities as well as in the overall radiated noise. The noise reduction was pronounced at the low frequency end of the spectrum. However, there was an increase in the high frequency noise that rendered the overall benefit less attractive when compared on a scaled-up A-weighted basis. A reduction in noise was accompanied by a commensurate reduction in the turbulent kinetic energy in the flow field. As expected, increasing the number of lobes involved progressive reduction in the thrust coefficient. Among the cases studied, the six-lobed nozzle had the optimum reduction in turbulence and noise with the least thrust penalty.

A study of noise bene t, vis-à-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 con guration 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 bene t less attractive when compared on a scaled-up A-weighted basis. Increasing the number of lobes involves progressive reduction in the thrust coef cient. The measured thrust loss is shown to be primarily caused by increased amount of low-momentum boundary-layer uid 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 bene t and a rapidly increasing thrust penalty. An analysis, showing that the loss in thrust coef cient 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.