3D multimodal inverse method for liner impedance eduction

In aeroacoustic engineering, precise measurements of liner impedance with grazing flow are crucial for optimizing noise reduction strategies. This paper introduces a novel 3D multimodal inverse method designed to educe the acoustic impedance of an acoustic liner in a large duct where many modes can propagate. The cost function is built as the difference between experimental and computed scattering matrices. Without flow, the educed impedance is shown to be in excellent agreement with the impedance measured for similar samples in a small duct where only plane waves propagate. Moreover, the multimodal scattering matrix offers more constraints on the cost function, which improves the method's robustness at high frequencies. This 3D multimodal inverse method is also shown to provide relatively converged results in the presence of flows with mean Mach numbers up to 0.2, holding great promises for improving the design and the optimization of ducted systems in various engineering applications, such as aircraft engines and heating, ventilation, and air-conditioning (HVAC) systems.