Aerodynamic Drag Reduction of Acoustic Liners Without Compromising Sound Absorption Performance

Conventional acoustic liners impose significant drag penalties due to their permeable surfaces, thereby increasing aircraft fuel consumption. This study investigates low-drag acoustic liner designs that maintain effective sound absorption performance. Aerodynamic and acoustic measurements were conducted in a grazing flow impedance tube across bulk Reynolds numbers ranging from Formula: see text to Formula: see text (corresponding to Mach numbers of 0.1–0.6) and sound pressure levels of up to 150 dB. Results confirm that the spanwise-slot liner produces lower drag than the streamwise-slot configuration, while exhibiting negligible differences in transmission loss and acoustic impedance. Analysis reveals that shortening the streamwise slot and enlarging the spanwise slot are key to achieving drag reduction without altering porosity and the orifice area. Inspired by this, elliptical- and crescent-hole liners were designed and experimentally validated, demonstrating superior drag reduction compared with the conventional circular-hole liner, particularly under nonlinear acoustic excitation (up to 16.5%), while maintaining nearly unchanged acoustic impedance. The proposed configurations offer practical pathways for reducing nacelle drag while preserving noise-suppression capability.