About Helmholtz resonators with flexible walls for acoustic damping

Aircraft engines are a dominant contributor to aircraft noise, which needs to be reduced because it impacts the health and quality of life of dwellers in the vicinity of airports. To reduce fuel consumption, aircraft engines are expected to become larger with a more low-frequency and broadband noise signature. Conventional damping structures (liners) in aircraft engines cannot effectively attenuate such noise. Therefore, novel liners need to be developed that achieve higher and broader peak noise attenuation at lower frequencies. This thesis investigates a novel liner concept in which a conventional Helmholtz resonator is combined with flexible walls. These flexible walls are expected to alter the resonance behaviour and provide additional damping through mechanical losses. For a successful future application, this liner concept needs to be acoustically beneficial and predictable. Therefore, it is investigated whether the flexible walls provide additional damping and how the behaviour of this multi-degree-of-freedom damping system can be understood and modelled. To answer these questions, an analytical and numerical model of the liner concept was developed to predict the acoustic properties of the coupled resonator system. The coupled resonator system consists of a face sheet and a cavity, which is subdivided by a flexible wall. The analytical model combines the effects of individual elements using waveguide theory. Face sheet effects are accounted for by semi-analytical models from the literature. The flexible wall is modelled as an equivalent impedance of a thin clamped rectangular or circular plate that takes into account higher-order plate modes. Mechanical losses are represented by a complex Young's modulus. The analytical model offers a physical understanding of the concept and a fast approach to investigate the individual contributions towards the overall damping capabilities. A numerical finite element model was created to resolve the coupling effects between the Helmholtz resonator and the flexible plates in greater detail. Additionally, resolving the spatial sound fields and structural vibrations can be used to gain a deeper understanding of the underlying physics. The numerical simulations also allow the quantification of the contribution of the flexible wall to the overall noise absorption. A modular Helmholtz resonator with a flexible wall was designed and evaluated in a normal incidence tube. The modular approach allowed for an investigation of the sub-elements of the liner structure like the face sheet, flexible wall material, shape, and back cavity size. These experimental results provide a validation basis for the analytical and numerical models. Additionally, the plate behaviour was investigated in-situ using a laser vibrometer and cavity microphones. A multi-cell liner sample tested in a grazing flow test rig showed that additional dissipation is also achieved in more realistic conditions with high sound pressure levels and grazing flow. The experimental results demonstrate that the novel liner concept provides additional dissipation, both in an acoustic normal and grazing incidence setting with and without grazing flow. The validated analytical and numerical models provide a basis for future optimisation of the proposed concept to efficiently reduce future aircraft noise.