Fan Buzz-Saw Noise Under Intake Flow Distortion: A Computational Study

Steady intake flow distortion due to non-axisymmetric intake geometry and angle of attack of the flight stream can affect various fan tone noise, especially fan buzz-saw noise. In this paper, unsteady CFD simulations have been performed on fully coupled fan and intake models to study the effect of intake flow distortion on fan buzz-saw noise. Three cases have been investigated, namely a baseline case, an axisymmetric distorted case, and a drooped distorted case, and comparisons have been made on the intake noise data extracted from the CFD calculations. The distortion fields generated in the last two cases have been compared and characterised by Fourier-Bessel harmonics. The results show that the distortion generated in the drooped case is more significant and has resulted in stronger circumferential mode scattering appearing initially in the fan domain. The main effect of the distortions in both cases is to cause scattering of the rotor-locked modes into adjacent circumferential harmonics, as the shock waves propagate through the intake ducts.