Impedance Eduction of Acoustic Liners with High Grazing Flow and High Frequency

The acoustic liner which installed at the internal part of the nacelle is applied to reduce the fan noise, which is the main noise source of the aircraft. Acoustic liner is submitted to high flow velocity over 0.6Ma and high frequency over 6000Hz. In order to research the sound absorption characteristics of the acoustic liner in the high frequency and high flow velocity range, the two-microphones method and the straightforward method were used for impedance eduction at high frequency and high flow velocity. In the straightforward method, the microphones distributed along the axial direction are located on the mid-length of the width of the upper wall opposite to the test liner to increase its frequency range. The applicability of a semi-empirical impedance model in the high frequency and high flow velocity range was evaluated. In the frequency range below 400Hz, the two-microphones method has higher test accuracy. The method based on the arrangement of the microphones located on the mid-length of the width of the upper wall opposite to the test liner can increase the upper limit of the impedance extraction frequency of the straightforward method. The two-microphones method is suitable for higher frequency and flow velocity ranges. For the classical perforated plate acoustic liner, the acoustic resistance increases with the increase of the flow velocity, and the acoustic impedance is hardly affected by the flow velocity. In the high flow velocity range, the impedance model predicted acoustic resistance slightly larger than the impedance measurement.