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

Applicant Professor Dr.-Ing. Ennes Sarradj, since 9/2023
Subject Area Acoustics
Lightweight Construction, Textile Technology
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 416728553
 
Final Report Year 2024

Final Report Abstract

Helmholtz resonators can be combined with flexible plates to effectively dampen noise of modern aircraft engines. Within the scope of this research project, the acoustic damping mechanisms of this resonator concept were investigated and an analytical model was created to describe and predict the damping properties of a Helmholtz resonator liner with additional structural elements. For this purpose, a modular single-chamber system of this novel silencer concept was designed, manufactured and analysed experimentally, numerically and analytically. It was shown that different plate materials with low bending stiffness broaden the damping behaviour and supplement it at low frequencies. The Helmholtz resonance is shifted towards lower frequencies if the dominant structural resonance is slightly above the Helmholtz resonance and vice versa. The position and orientation of the flexible plate are irrelevant; a larger back cavity shifts the structural resonance to lower frequencies and the number of flexible walls increases the dissipative effect and also shifts it to lower frequencies. Using a vibrometer setup, it could be shown that in the circular case only the radial mode shapes are excited and the plate vibrates strongly, especially at low operating mode shapes close to the Helmholtz resonance. For effective use, the structural resonances must therefore have the lowest possible order and be close to the Helmholtz resonance. Using the analytical model developed as part of this project, the contribution of the various damping sources, i.e. the cavity walls, the flexible wall and the perforate, can be quantified. It was shown that the flexible wall can improve the resonance behaviour and, depending on the material damping, also provides substantial additional damping. It could be shown that in a resonator array the damping effect of both the flexible panel and the Helmholtz resonance are enhanced. At high sound pressures and background flow, the damping effect (vortex or flow separation) of the perforate dominates and the system behaves similarly to a conventional Helmholtz resonator liner. This behaviour is particularly dependent on the porosity of the perforated plate. The project results enable the concept to be further developed for use in future aircraft engines or other flow channels where low-frequency damping is required in a small space.

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