Laser based tomographic measurement of the local acoustic impedance of overflowed liners (TOMLIM)

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The increasing growth of air traffic demands highly efficient noise reduction measures especially addressing the noise emitted from aero-engines. Common applications for noise attenuation are acoustically lined surfaces, called liner, placed along the flow pass in the engine nacelle. Liners consist basically of perforated walls with a cavity structure behind. Up to now the design of these liners for an optimal performance under flow conditions is based on heuristic and empirical methods due to the lack of sufficiently describing, fully analytical models. The applicants could even show in previous projects that under certain circumstances lined surfaces can contribute to noise generation instead of attenuation. This demonstrates the need for improved liner model descriptions which allows a liner design optimization with robust prediction ability.A key quantity in describing the liner performance is the acoustic impedance, the ratio of the pressure fluctuations to the wall-normal velocity fluctuation of the sound field. This liner impedance is commonly determined globally, homogenously distributed over the line surface, by indirect methods. However, due to the lack of corresponding measurement techniques the local distribution of the impedance and its correlation with the global impedance is still unknown.This will be addressed in the present application. A novel non-intrusive impedance measurement technique, a combination of tomographic acoustic pressure measurements and Doppler Global Velocimetry based acoustic particle velocity measurements, will be developed, validated and applied on liner setups under realistic grazing flow conditions. Therefore, the optical setups as well as the post-processing algorithms need to be developed to facilitate the application with limited optical access to the liner section in a flow duct test rig.