Acoustic liner optimization for fan noise reduction

Aviation has become increasingly integral to modern society over the years, addressing daily the needs of millions of travelers worldwide. However, this comes with a significant environmental cost, including pollutants and greenhouse gas emissions, as well as noise emissions. High noise emissions have important implications for the health, leading to issues such as insomnia and consequently contributing to cardiovascular diseases. This has led institutions to develop increasingly stringent regulations for noise pollution control in the vicinity of airports. This, coupled with the trend of modern engines toward high bypass ratio turbofans, has given rise to new challenges in ensuring compliance with acoustic reduction standards. Shifting towards ultra-high bypass ratio turbofan allows for increased efficiency by lowering fuel consumption. However, this has an impact on the acoustic emissions of the engine now primarily attributed to the fan noise rather than the jet noise: an increase in the by-pass ratio leads to a reduction in the exhaust gas velocities, making it easier to comply with regulations. On the other side, a higher BPR results in a lower fan rotation speed, leading to a lower-frequency and broader acoustic emission spectrum. This shift makes it more challenging to mitigate the noise produced by the fan, which now plays a primary role. Acoustic liners are an important ally in noise reduction, allowing to comply with the regulatory limit. The changes mentioned above, however, give rise to several new challenges: lower attenuation frequencies require greater liner depths, this conflicting with the need for slimmer nacelles due to the significant increase in their diameter to achieve ever higher Bypass Ratios. Additionally, broadband attenuation must be ensured. Hence, the need to develop new liner geometries and models emerged, along with the necessity for methods that enable determining the geometric parameters of the liner that best allow compliance with regulations at certification points. This thesis aims to define an optimization code in Python that, coupled with the aeroacoustic calculation software Actran, allows for the definition of geometric parameters of the acoustic liner, such as cavity depth and diameter of the orifices, ensuring minimal acoustic emission in the far field. In doing so, the most suitable semi-empirical models have been chosen, where the liner's response depends precisely on the geometric parameters to be optimized. In particular, the use of both single-degree-of-freedom and two-degree-of-freedom semi-empirical liner models allowed for evaluating how much the noise was attenuated depending on the type of liner used compared to the case without acoustic treatment. Subsequently, the attenuation achieved was also compared for cases of absence of flow, uniform flow, and non-uniform flow. To determine the best optimization approach, the results obtained were compared using both local optimization algorithms and global optimization algorithms, before leaning towards a sequential use of the two. This optimization approach has therefore allowed for obtaining an improvement of several dB compared to the case of a non-optimized liner.