Low-Drag Acoustic Liner Development

Interest in characterization of the aerodynamic drag of acoustic liners has increased in the past several years. This report details experiments in the NASA Langley Grazing Flow Impedance Tube to quantify the relative drag of several perforate-over-honeycomb liner configurations at flow speeds of M=0.3 and 0.5. Various perforate geometries and orientations are investigated to determine their resistance factors using a static pressure drop approach. Comparison of these resistance factors gives a relative measurement of liner drag. For these same flow conditions, acoustic measurements are performed simultaneously with the drag measurements for tonal excitation from 400 to 3000 Hz at source sound pressure levels of 140 and 150 dB. Educed impedance and attenuation spectra are used to determine the impact of variations in perforate geometry on acoustic performance. The goal is to identify a perforate that will reduce the drag penalty associated with conventional round-hole perforates by 60%. One perforate design, based on a slot geometry, is shown to reduce this penalty by 50%. Further reductions may be possible but require a reduction in measurement uncertainty to allow a statistically rigorous evaluation.