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Parasitic noise from probes and struts in flows Hoglund, Leif Erik

Abstract

When a fast-responding static pressure probe is inserted into a flow, there are several possible mechanisms for the generation of extraneous noise. If the probe signal is cross-correlated with the far field sound, then the "probe noise" may contribute a dominant fraction of the total correlatable noise in the source region. For a standard probe, there is likely to be contamination from the tip due to large fluctuating side forces, and from the stem, due to drag fluctuations. A theoretical model is suggested for predicting the distortion of "causality" correlation signatures (obtained when in-flow probes are cross-correlated with the far field sound), due to the probe tip contamination. The predicted shapes agree well with experiment. In the experimental investigations, the contaminated portion of the causality correlation signature is displaced in time from the "true" jet pressure correlation. The unexpected result is the absence of any significant jet pressure correlation. This leads to the conclusion that the extent of jet noise sources may be very small, so that probing devices inserted into the flow will generally produce a large portion of the total correlation.

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