Acoustic Impedance Eduction of MDOF Liners With Numerical and Analytical Approaches Under Grazing Flow Conditions

Despite the ongoing COVID-19 pandemic, the latest forecasts about air traffic in Europe are predicting a recovery to the 2019 levels by the next two years. Since noise pollution of urban areas close to airports is a serious issue, international regulations are more and more oriented to restrict the maximum allowable aircraft noise. A viable strategy to pursue the low-noise design of the aircraft engines is the installation of acoustic liners on the intake duct and exhaust nozzles. However, the narrow-band absorption of the traditional single degree of freedom (SDOF) liners restricts the noise suppression just around the resonant frequency of the liner resonators. This means that SDOF liners are designed to absorb a well-defined tonal excitation, becoming ineffective when a change in the acoustic frequency occurs. To overcome this limitation, an effective way to broaden the absorption range of acoustic liners is to design a multi degrees of freedom (MDOF) arrangement of the liner cells. This is achieved by adding multiple honeycomb layers of whatever depth in between the rigid back-plate and the perforated face-sheet. Doing so, more than a single resonant frequency is deployed in the audible range leading to multiple absorption peaks in the frequency response of the liner. In this work, the authors present high fidelity LES simulations aimed at validating an in-house extension to MDOF liners of the semi-analytical model of Hersh. The acoustic impedance of a three degrees of freedom (TDOF) liner cell is calculated from the LES through a three-microphone insitu method. Then, the numerical results are compared with the outcomes of the semi-analytical model to assess the reliability of the in-house formulation. Both the effects of the excitation frequency and the grazing flow Mach number on the acoustic impedance of the TDOF resonators are considered for the validation procedure. The close matching between the numerical and the analytical results demonstrates the robustness of the in-house extension of the Hersh model. Moreover, it also suggests that the extended Hersh model is an attractive tool to have quick and accurate predictions of the acoustic performance of whatever MDOF liner geometry.