Experimental investigations on the acoustical performance of open-celled metal foams under grazing flow

Open-celled metal foams offer a possible solution for implementing acoustic liners in non-traditional nacelle locations, such as directly over the turbofan rotor. Previously, we have shown that the normal incidence absorption performance of metal foams can be further improved by compressing them to reduce their effective pore size. In this paper, we focus on the acoustical performance of compressed metal foams under grazing flow conditions. Experimental tests were conducted using a grazing flow impedance tube set up within Spirit AeroSystems' acoustics lab. Metal foams with four different compression ratios were tested under various flow rates and incident sound pressure levels. The sound pressure measurements were then utilized to educe their effective impedance under grazing flow conditions. Our results show that a uniform through-thickness compression can improve the acoustic performance of open-celled metal foams under grazing flow conditions and can aid the further development of acoustic liners for over the rotor applications.