Evaluation of the Impedance Variability Effect of Acoustic Liner on Aircraft Engine Fan Noise in Calculation of Far Field Sound Modes Propagation

The influence of the impedance variability of the acoustic liner of an air intake on aircraft engine fan noise in the far field has been studied. The calculations were carried out in an axisymmetric statement for single modes based on the finite element solution to the Blokhintsev equation for the acoustic potential. The acoustic liner impedance was determined from a well-developed semiempirical model that takes into account the influence of a high sound pressure level (SPL) and grazing flow. A procedure has been developed for calculating the propagation of single sound modes through the air intake of an aircraft engine with a variable liner impedance depending on the flow velocity and SPL at a local point. The calculation procedure has been validated. Calculations were made for single mode propagation into the forward hemisphere for an air intake on the wall of which the following conditions were set: rigid wall, constant impedance (the impedance does not change along the liner) and variable impedance (the impedance changes along the liner depending on the changes in the velocity of the grazing flow and SPL). According to the calculation results, it was found that constant and variable impedances yield different SPL values in the far field; as well, taking into account the impedance variability changes not only the SPL values in the far field, but also the direction of the maximum mode radiation.