Impedance measurements of acoustic duct liners with grazing flow

The impedance properties of acoustic duct liners are affected by airflow in the duct. Measurements have shown that interactions between the duct flow and the acoustically induced flow at and within the acoustic liners can cause significant changes in the effective liner impedance. Acoustic measurements in a flow environment are used to develop acoustic impedance mathematical models for use in lining design. These impedance measurements are made in a 2 × 2-in. cross-section flow duct using waveguide principles. The design of the duct and the data analysis technique are based on solution of the convected wave equation in an infinite waveguide. The sheared boundary layer velocity profiles in the duct are measured and used to calculate the acoustic impedance of the liner. Examples of measured attenuation and phase rate data are presented which show effects of duct flow velocity and wall impedance. A comparison with standing-wave impedance tube measurements in the limiting case at zero flow is also presented.