Estimation of increased attenuation of turbofan inlet noise with optimized and active liners

Abstract

A modified version of the Eversman finite element code is used to estimate the maximum attenuation benefit obtainable in a typical aircraft nacelle lined with a perfect absorber, having the optimum impedance at each frequency and for each mode. The radiated acoustic wave pattern is computed from the fan face to the far field. Calculations are presented for the first 4 radial modes of the first cut-on circumferential mode at 2 BPF (blade passing frequency) for an Advanced Ducted Propeller model. For each mode the program performed a parametric variation of the nacelle wall liner impedance to determine the optimum impedance which minimized the peak sound pressure level at the far-field boundary. Results for an inlet having a hard wall, an optimal passive liner, a uniform (single element) active liner having an optimized impedance, a liner having linearly varying impedance from fan face to the inlet, and a fully optimized (30 liner segments) active liner are presented. Attenuation of the optimized liner having uniform impedance compared to the hard-wall case is 11.2 dB, 4.9 dB, 4.2 dB, and 7.4 dB for radial modes 0, 1, 2, and 3, respectively. Linearly varying the impedance increased the attenuation slightly. The ability of the liner to significantly alter the pressure pattern within the nacelle is shown quite clearly in the visualized pressure field.