Experimental Investigations of Flexible Wall Effects in Helmholtz Resonators for Aircraft Engine Acoustic Liners

Abstract

Acoustic liners working like Helmholtz resonators offer an effective way to dampen tonal noise in aircraft engines. In order to broaden the bandwidth of a Helmholtz resonator, we added flexible walls that result in an additional low frequency dissipation. We investigated different parameters of the concept with a modular setup. In this paper, the results of measurements for a resonator with a flexible wall - representing the basic element of an advanced liner concept - and its potential to dampen lower frequencies are shown. The measurements are conducted at an aero-acoustic wind tunnel for multiple combinations of up to two resonators and up to four flexible walls with different parameters such as material, thickness, distance, position, orientation and shape. In this paper we show which parameter has an influence on the dissipation caused by the flexible wall. An additional dissipation occurs at specific material parameter whereas the geometric parameters alter the frequency of the additional dissipation.