Abstract:

The development of broadband acoustic liners requires taking the three-dimensional acoustic field into consideration in impedance eduction. However， in existing three-dimensional impedance eduction methods， the grazing flow is generally assumed to be uniform， which is inconsistent with the actual situation of shear flow in flow ducts. To simultaneously solve the problems of three-dimensional acoustic field and shear grazing flow in acoustic liner impedance eduction， a three-dimensional straightforward method considering a one-dimensional equivalent shear flow is developed. Using sound pressure data from a rectangular array of measuring points arranged on the rigid wall opposite to the liner surface， spanwise modal decomposition is firstly conducted to obtain intermediate variables， which can then be solved through Runge-Kutta iteration to compute the axial wavenumber. Finally， impedance is educed according to the impedance boundary conditions. The accuracy of this method is verified through numerical simulation experiments based on the finite element method for both plane-wave and plane-wave dominant incidence. By comparing the educed impedances with and without considering the shear grazing flow， it can be found that although shear flow has a relatively small impact on impedance eduction in the mid to low frequency range below the cutoff frequency， it may significantly affect the accuracy of the eduction result in the high frequency range. Moreover， as the flow Mach number increases， the influence of shear flow becomes more important.

Key words: aviation acoustic liner, broadband, impedance eduction, three-dimensional sound field, shear flow