Abstract
A thermal acoustic shield on a scale-model, high-radius-ratio plug nozzle has been static tested at nozzle pressure ratios up to 4.5, with shield velocity ratios of 0.4-0.7, at two nominal thicknesses. Shield flow rates and acoustic attenuation were less than expected due to the interaction of the primary nozzle flow, which effectively reduced the shield thickness. Noise reduction due to shielding improves up to a pressure ratio of 3.0 and diminishes at greater ratios for the configuration tested. There appears to be some reduction in shock-cell noise up to 3.5 pressure ratio, with amplification at greater pressure ratios. Absolute jet temperature (as opposed to temperature ratio) appears to be significant to noise reduction at the higher jet velocities. It is concluded that a thermal acoustic shield has great potential, but that the acoustic benefit is implementation-dependent and, consequently, that obtaining the greatest benefits of the shield will be a challenge to designers.
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