Effect of a semi-annular thermal acoustic shield on jet exhaust noise

Previous experimental studies have shown that reductions in jet exhaust noise were obtained by the use of an annular thermal acoustic shield consisting of a high temperature, low velocity gas stream surrounding the high velocity central jet exhaust. The reduction is believed to be caused partly by reflection of the central jet exhaust noise by the shield. However, with an annular shield, multiple reflections are possible which help contribute to the total noise level of the nozzle. The present work consisted of an exploratory study to investigate the effect of a semi-annular shield on jet exhaust noise with the rationale that such a configuration would eliminate or reduce the multiple reflection mechanism. Noise measurements for a 10-cm conical nozzle with a semi-annular acoustic shield are presented in terms of lossless free field spectral data at various angular locations with respect to the nozzle. Measurements were made on both the shielded and unshielded sides of the nozzle. The results are presented parametrically, showing the effects of various shield and central stream velocities and temperatures.