Evaluation of Three Impedance Eduction Methods for Acoustic Liners Under Grazing Flow

Regular paper

Julio Apolinário Cordioli

Federal University of Santa Catarina

Wednesday 3 june, 2015, 16:20 - 16:40

0.3 Copenhagen (49)

The growth of the aircraft transportation sector coupled with the urbanization of airport surrounding areas brought with them stricter regulations regarding aircraft noise, mainly during take-off and landing, where engine noise dominates. This paper focuses on the comparison of three different indirect methods to measure the impedance of acoustic liners used in the treatment of aircraft engines. Data obtained from Finite Element simulations of a test setup considering different flow velocities were fed as input to the three methods, which attempted le to find the correct impedance imposed in the numerical model for each flow speed. An analysis was made in order to evaluate the capacity of the methods to handle higher order modes in the test duct and the effect of the position of the flush-mounted microphones on the duct side- walls. It was found that, even though the methods assume plane-wave propagation in the hard-wall portions of the duct, they are able to converge to correct results if the microphones are positioned in nodal positions of the higher order modes cut-on on the frequency range under analysis. The robustness of each method to errors in the input data was also evaluated.

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