An experimental study of the effects of upstream obstructions upon subsonic jet noise

Abstract

Results obtained for noise produced by two obstructions—one circular (1 inch diameter x 2·84 inch long) and another rectangular (0·4 inch thick x 1 inch wide x 2·84 inch long) in shape—immersed in both “clean” and turbulent flows 4·8 inches upstream of the nozzle exit are described. Variations of overall sound pressure levels (OASPL's) and power Watt levels (PWL's) with jet exit velocity (Vj) and the directivities of OASPL's are considered in detail. Considerable care was taken to ensure that any comparison of the obstruction generated noise with the “clean” jet noise is for the same mean jet exit velocity and thrust. The mean jet exit velocity was derived from the measured velocity profile at the nozzle exit in each case. For the same mean exit velocity and nozzle diameter, the noise levels with the circular obstruction were measured to be as much as 40 dB higher than the “clean” jet noise. The rectangular obstruction increased the noise by up to 10 dB. Flow over these obstructions generated discrete tones as well. The velocity index for the obstruction generated noise were between 8 and 9 between Vj=350 ft/s and 1000 ft/s. This contradicts the traditional V6 law. The main reason for the discrepancy has been attributed to the variation of the coefficient of fluctuating lift, C L 2 ¯ , with Reynolds number. The angle of peak overall sound pressure levels (OASPL's) was between 45° and 60°, as compared to between 0° and 20° for the clean jet noise. When the obstructions were immersed in turbulent flows most of the discrete tones which were obtained from the obstructions in smooth flows disappeared. The high frequency noise levels, however, increased by as much as 20 dB.