Abstract
Jet noise diagnostics via cross‐correlation of the postulated source and radiated noise are most easily implemented when the source terms are proportional to certain readily measurable scalars. One such description is the Proudman form of Lighthill's theory. However, attempts to measure the source term by means of a cross‐correlation technique have been only partially successful. The major difficulty is suspected to be spurious “probe noise” generated by turbulence/hot wire interaction. Theoretical considerations show that for low Mach numbers, at which the cross‐correlation experiments are typically performed, the noise radiated by the probe is comparable with that part of the jet noise radiated from the correlation volume (eddy) surrounding the probe. This assertion is supported by comparison of jet noise‐jet flow cross‐correlations measured with a hot wire and with a nonintrusive device: a Laser Doppler Velocimeter (LDV). The hot wire is found to generate a spurious but well‐correlated probe noise that leads to major error in the jet noise.
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