Experimental investigation of factors influencing acoustic liner drag using direct measurement

Nacelle acoustic liners inevitably generate greater aerodynamic drag than smooth walls. It is necessary to study the factors influencing acoustic liner drag to reduce engine fuel consumption and emissions. In this study, experimental measurements were conducted in a grazing flow tube with bulk Mach numbers ranging from 0.1 to 0.6. A drag balance was designed to measure the liner drag directly. This balance can eliminate the interference of the vertical force and moment by arranging the sensors appropriately. A wedge-shaped sidewall was proposed that can eliminate the side effects of the differential pressure force effectively. The test samples involved Helmholtz resonator liners, including conventional acoustic liners and multislot acoustic liners, and a smooth wall set as a reference for comparison. The effects of geometric structures on the acoustic liner drag were studied, including perforation size, plate thickness, porosity, perforation, slot patterns, and fabric mesh. Experimental results show that Mach number (M) 0.4 is the critical point. At M < 0.4, the acoustic liner drag is proportional to the perforation diameter. At M > 0.4, the opposite trend is observed. Moreover, the acoustic liner drag is directly proportional to the porosity and perforated plate thickness at M = 0.1 to 0.6. Experimental results also indicate that some structural characteristics can effectively reduce the acoustic liner drag, including placing a fabric mesh under the perforated panel, adopting a staggered hole layout, and slotting perpendicular to the flow direction. The summarized influencing factors are helpful for avoiding high flow drag structures in the design of acoustic liners and subsequently, reducing the aircraft fuel consumption.