Nonlinear Propagation of Supersonic Fan Tones in Turbofan Intake Ducts

Supersonic fan tones are a critical issue for large fans; it is well known that these tones are a key noise source at high-power operating conditions for modern turbofan aeroengines. Nonlinear propagation of the rotor-locked pressure field generated in a turbofan intake duct is calculated by implementing a nonlinear weak-shock theory numerically via a combined time-/frequency-domain algorithm. The aim of the prediction method is to model the nonlinear attenuation and liner absorption of the rotor-locked pressure field within the intake duct. In this hybrid method, the time-domain approach provides a robust and accurate prediction of the nonlinear attenuation, whereas the frequency domain is required to represent the liner attenuation. The focus of this work is to compare the time-/frequency-domain method with time-domain or frequency-domain methods that have been developed previously to calculate the nonlinear propagation of the rotor-locked pressure field in either a rigid or lined intake duct, respectively. The capability of the time-/frequency-domain method to accurately calculate the nonlinear propagation of the rotor-locked pressure field, which can provide an engineering method for buzzsaw noise prediction, is demonstrated.