1 January 1988 Investigation Of Aeroacoustic Mechanisms By Remote Thermal Imaging
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A hush house is a hangar-like structure designed to isolate, from the surrounding environment, the noise produced by extended aircraft engine operations during diagnostic testing. While hush houses meet this intended need by suppressing audible noise, they do emit significant subaudible acoustic energy which has caused structural vibrations in nearby facilities. As a first step in mitigating the problems associated with hush house induced vibrations, it is necessary to identify the mechanism responsible for the low frequency acoustic emissions. It was hypothesized that the low frequency acoustic waves are a result of acoustic Cherenkov radiation. This radiation is in the form of a coherent wave produced by the engine exhaust gas flow. The speed of sound in the exhaust gas is quite high as a result of its elevated temperature. Therefore, the gas flow is sonic or subsonic relative to its own sound speed, but is supersonic relative to sound speed in the surrounding cooler air and, as a result, produces acoustic Cherenkov radiation. To confirm this hypothesis, thermographic surveys were conducted to image the thermal structure of the engine exhaust gas within the hush house. In the near-field, these images revealed that the exhaust gases did not behave like a high Reynolds number turbulent jet, but rather, the transition to turbulence is delayed by a suppression in growth of the self-excited instability wave as a result of acoustic Cherenkov radiation.
© (1988) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Alan J. Witten and George E. Courville "Investigation Of Aeroacoustic Mechanisms By Remote Thermal Imaging", Proc. SPIE 0934, Thermosense X: Thermal Infrared Sensing for Diagnostics and Control, (1 January 1988); doi: 10.1117/12.968507; https://doi.org/10.1117/12.968507

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