Acoustic excitation impact on aerodynamic drag measured in aeroacoustic liners

Abstract

Research interest has steadily grown for understanding the aerodynamic drag produced by acoustic liners for commercial turbofan engines. This is driven by an aim to understand the phenomena fundamentally as well as for application in flight. Stringent government regulations on aircraft noise and next generation aircraft designs that may include liners on more surfaces are key drivers for industry involvement. While the conventional perforate-over-honeycomb liner has proven effective acoustically for decades, liner drag production has not been fully understood. When an acoustic liner sample is excited with sound pressure levels above 140dB re: 20 micropascals, a measurable drag increase is observed at flight velocity. Recent measurements have shown that tonal noise at the same level can produce more than a 50 percent increase in drag coefficient for a liner sample at lower test speeds. By testing liner samples at low speed in the Notre Dame Hessert Laboratory, detailed hotwire probe measurements near the wall have been made and drag coefficient comparisons have been made with the use of a linear air-bearing force balance. The development of the measurement setup, the results produced, and a discussion of implications will be included in this paper.