Experimental and Numerical Parameter Study of a Helmholtz Resonator with a Flexible Wall

Abstract

Modern aircraft engines have a large bypass ratio for fuel efficiency but consequently emit more low-frequency noise, which cannot be damped efficiently with conventional perforate-over-honeycomb liners. In this paper, a Helmholtz resonator is combined with a flexible plate subdividing the cavity to allow for resonances below the conventional Helmholtz resonance. This concept is investigated experimentally and numerically in a normal incidence tube to determine the impacts of the face sheet, the cavity dimensions and the flexible wall materials. Additional microphones are implemented inside the resonator to successfully separate Helmholtz and plate resonances. A numerical model was developed, validated with experimental results and used to resolve the impact of key parameters. We found that the face sheet and main cavity mainly alter the Helmholtz resonance and that the plate related resonances show great sensitivity towards the back cavity size and the material and size parameters. These results give valuable insight into the working principles of the concept and show the great adaptivity for future optimization and application.