Long-Range and Nonlinear Propagation of Helicopter High-Speed Impulsive Noise

This paper presents a new approach to predict the nonlinear propagation of helicopter high-speed impulsive noise. The local nonlinear source terms and cumulative nonlinear propagation effects are considered separately. The local nonlinear sources are captured in Euler computational fluid dynamics (CFD), and the cumulative nonlinear steepening is predicted by the frequency-domain Burgers equation. The geometric transformation of sound propagation between two different coordinates is explained. In the Burgers equation, the atmospheric absorption effect is also included. The Burgers equation model is validated against an analytic solution for a sinusoidal wave and then applied to a model-scale rotor noise. The Euler CFD data are used to extract the starting signal and to validate the prediction obtained by the Burgers equation. The sensitivities to the starting signal location and the propagation path that are used in the Burgers equation are studied. The nonlinear propagation is considered for a full-scale UH-1H helicopter. It is shown that the nonlinear propagation significantly distorts the waveform, resulting in a reduction of the pressure amplitude and a broadening of the waveform. However, the atmospheric absorption becomes equally important for large distances as it attenuates high-frequency energy and makes the waveform smoother. The nonlinear propagation and atmospheric attenuation are balanced and checked over long-range propagation. Finally, a nonlinearity indicator that is suitable for rotor noise is proposed to identify the energy transfer due to nonlinear propagation in the frequency domain. It is found that the energy transfer occurs in low frequency and generates scalloped sound pressure level distributions for nonlinear propagation, which is an interesting feature of helicopter nonlinear sound propagation.