Micro-Perforated materials for the reduction of flow-induced noise

Turbulent Boundary Layer (TBL) induced noise is one of the dominant noise sources in modern aircraft, where classical materials are progressively being substituted by fiber-reinforced polymers. Although passive methods continue to be developed due to their straight realization in practical flight applications, they add additional weight and are also accompanied by some degradation in high-lift performance. We propose in this work to study the behaviour of insulating partitions composed of Micro-Perforated Panels (MPPs) subject to a TBL excitation, for replacing or complementing passive or active flow noise control solutions. We have carried out a set of simulations to establish a comparison between the performance of the MPP control devices when varying the physical configurations of the partition and the nature of the primary noise excitation. It can be shown that when exciting the partition with less correlated random loads, the corresponding TL increases progressively. For the model validation, experiments have been performed in the low-speed wind tunnel of the IRPHE Fluid Dynamics Laboratory in order to determine the acoustic and aerodynamic TL performance of a number of MPP multilayer partitions (double and triple layers) when subject to a TBL of free stream velocity 30.7 m/s. The effect of inserting a micro-perforated panel within the cavity at unequal distances from the front and back panels dampens more efficiently the Mass-Air-Mass controlled resonances of the Panel-Cavity-Panel system with respect to those of the MPP-Cavity-Panel system, already damped by the front MPP. This results in a higher TL difference between the triple and double partitions with a plain front panel, which is about 8 dB, with respect to the same configurations with a microperforated front panel, which is about 4 dB.