Abstract
A major problem in the measurement of aircraft engine fan noise is the difficulty in simulating in a ground-based facility the noise that occurs during flight. Flight-type noise as contrasted to the usual ground-static noise exhibits substantial reductions in (1) the time unsteadiness of tone noise and (2) the mean level of tones calculated to be nonpropagating or cutoff. A model fan designed to have a nonpropagating tone was acoustically tested in the anechoic wind tunnel under both static and tunnel flow conditions. The properties that characterize flight-type noise were progressively simulated with increasing tunnel flow. The distinctive lobed directivity patterns of propagating duct modes were also observed. The acoustical data with tunnel flow appear uncontaminated by the excess noise attributed to the ingestion of flow disturbances that prevail near most static test facilities. This anechoic wind tunnel appears to be a useful facility for flight applied acoustic research on aircraft engine fans.
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