Acoustic characteristics of a Helmholtz-resonator liner with flexible structure elements

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Liners are frequently applied for the reduction of noise emissions of aero-engines. They usually consist of a flush-mounted perforation and an array of cavities underneath this face sheet. For future improvements in noise reduction, the liner technology needs to be improved with respect to the frequency range of significant attenuation and in particular also for the low-frequency range.In the course of the proposed project, an innovative approach for the improvement of the acoustic damping performance shall be investigated in detail and described by appropriate models: using a classical Helmholtz resonator base structure, part of the cell wall structures shall be replaced by a flexible material with intrinsic damping. It is assumed that - by the fluctuating acoustic pressure inside the cell - the flexible wall is subject to vibrations which extract energy from the acoustic excitation to maintain the vibration and additionally dissipates energy inside the material.The concept was proven successfully in a previous experimental study. However, the detailed mechanism, the quantitative share of individual effects, and the interaction of basic elements are not sufficiently understood.By an experimental investigation of a basic element (consisting of a resonator cell and an additional back cavity) and accompanying numerical simulations, the subsequent physical mechanisms shall be captured and described by an appropriate physics-based model.Therefore, the measurement of spatial distribution of deflection of the flexible wall and the determination of the pressure field in the resonator cell and the back cavity is of utmost importance.Subsequently, the investigation shall be extended to an array of basic elements taking into account also the interaction between neighboring elements via a common back cavity. This extension forms the basis for a practical application of the concept. Further acoustic measurements and corresponding numerical simulations will investigate the different combinations of placement of basic elements.For future applications of the new liner concept, an improved model is required providing an impedance description of the full arrangement of active cells and back cavities. This model will build on the basic element models and take the interactions into account.