Numerical determination of transmission losses of a turbofan inlet duct lined with porous materials

Acoustic performances of turbofan inlet duct lined with a metal foam are numerically investigated. The computational method, based on a mode matching technique, allows to tackle realistic turbofan inlet ducts specifications, which includes all cut-on modes. It permits to deliver very accurate results at high frequencies at a modest computational cost, and provide an adequate tool for optimization purposes. By choosing appropriate design variables, a methodology is proposed to determine the nearly-optimal liner consisting of a perforated plate backed by a porous material. Results are given for two different Mach numbers corresponding with sideline and cutback flight conditions. The optimized liner solution is compared to traditional double degree-of-freedom liners designed to provide the best acoustic attenuation. While similar acoustic performances are observed over most part of the spectrum, noticeable improvements can be found in the low frequency regime, i.e. below 500 Hz. These preliminary results demonstrate the interest for this new type of liners.