Experimental and Numerical Analysis of the Vibro-Acoustic Behavior of a Helmholtz Resonator with a Flexible Wall

Abstract

Modern aircraft engines have large fans with a low-frequency noise signature, which conventional liners struggle to damp efficiently. New liner concepts are therefore needed to address low-frequency aircraft noise. In this paper, a Helmholtz resonator is combined with a flexible plate dividing the cavity into two. The flexible plate extends the conventional single-degree of freedom system to a multi-degree of freedom system, with resonances below the conventional Helmholtz resonance. The plate's motion is investigated experimentally at the aero-acoustic wind tunnel DUCT-R with a vibrometer and compared to numerical simulations based on FEM, where good agreement is found. We showed that the face sheet and plate size is a crucial parameter of the concept which alters the number of resonance frequencies, their absorption amplitudes and their resonance frequencies. The flexible wall vibrates strongly and adds multiple damping peaks when the plate resonances lie in the vicinity of the Helmholtz resonance. Furthermore, we resolved the deflection shapes and the absorption of the Helmholtz resonator with a flexible wall of variable size numerically. The results help to fortify confidence in the proposed liner concept for future optimization and application.