Statistical Analysis in Transient Rotor Noise Strength

Turbulent flow in the blade boundary layer causes unsteady surface pressure and produces rotor noise in broadband frequencies. Turbulent intermittency affects surface pressure and creates sporadic high-strength noise sources. In this study, we demonstrate experimentally that the strength of transient rotor noise follows a log-normal distribution. A wavelet-based beamforming method is used to capture transient noise sources from 2 kHz to 10 kHz on a rotor with rotating speeds ranging from 1800 rpm to 7200 rpm. We suggest that noise sources at different frequencies are generated by the same noise generation process, and a frequency-independent distribution function exists to unify the strength distributions of noise sources at different frequencies. Results show that the variance of the transient noise source strength at different frequencies is controlled by the surface pressure spectrum, which takes the form of σ^2∝(C_σ-50log_10⁡ω)^2, where C_σ is a constant independent of rotating speed and ω is angular frequency. A dimensionless statistical variable is proposed to value the effect of turbulent intermittency on noise generation process. Our statistical analysis is validated by experimental results and provides new insights into rotor noise.