Broadband Noise of a Hovering Rotor: Measurement, Prediction, and Reduction

The rotor broadband noise is generated by the interactions of blades and the surrounding turbulence. For a hovering rotor, the turbulence is mainly located in the boundary layer on the blade surface. In this work, we conduct a systematic investigation on a benchmark rotor to characterize the noise features, develop prediction models, and explore the effectiveness of the noise reduction methods. The two-bladed rotor has a radius of 0.5 m, and the tests were conducted in an anechoic chamber. Measurements show that the amplitude of rotor broadband noise can be scaled by the fifth power law of tip Mach number at St=2-100, with the frequency f scaled by the St=fR/U-tip, where R is the blade radius and U-tip is the tip velocity. The BPM model for airfoil self-noise computation is extended to a rotation frame for the rotor noise prediction. Prediction results show reasonable agreement with the measurements, suggesting that the broadband rotor noise is dominated by the trailing-edge noise source, mainly contributed by the suction side boundary layer and the effect of the nonzero angles of attack. Additionally, attempts for broadband reduction are conducted by applying particulate roughness on the blade surface, which can achieve a reduction of 3 dB above 5000 Hz. Moreover, results suggest that applying surface treatments on the blade leading edge can achieve nearly identical reductions as the full-span treatments.