AN EXPERIMENTAL INVESTIGATION OF THE SOURCES OF PROPELLER NOISE DUE TO THE INGESTION OF TURBULENCE AT LOW-SPEEDS

Noise radiation from a four bladed, to in. diameter propeller operatin g in air at a rotational speed of 3000 RPM and a freestream velocity o f 33 ft/s was experimentally analyzed using hot-wire and microphone me asurements in an anechoic wind tunnel. Turbulence levels from 0.2 to 5 .5% at the propeller location were generated by square-mesh grids upst ream of the propeller. Autobicoherence measurements behind the blade t railing edges near the hub and tip showed regions of high phase-cohere nce between the blade-passage harmonics and the broadband frequencies. Inflow turbulence reduced this coherence. By relating the fluctuation velocities in the propeller wake to the unsteady blade forces, the pr imary regions of tonal noise generation have been identified as the hu b and tip regions, while the midspan has been identified as a region r esponsible for broadband noise generation. These measurements were com plimented by cross-spectra between the propeller wake-flow and the mea sured sound. The effect of turbulence on the radiated noise level show ed an overall increase of 2 dB in the broadband levels for every 1% in crease in turbulence. This effect varied for different frequency bands in the acoustic spectrum.