Experimental Fluid Mechanic Evaluation of Liner Drag and Acoustic-Drag Interaction

The purpose of this dissertation is to establish the current state of the art in acoustic liner drag research, describe recently designed and built facilities at the University of Notre Dame for conducting such research, discuss the coupling of aerodynamic drag and acoustic excitation with liners, and offer conclusions and recommendations for future work in the area.

Conventional acoustic liners for ducted turbofan engine nacelles have been proven to reliably reduce engine noise in commercial aircraft for a narrow frequency range. Traditional acoustic liners consist of a porous facesheet, honeycomb core, and solid backing. These liners are placed in the fore and aft bypass ducts of commercial jet engine nacelles. As technology has developed, new liner designs, including variable-depth cores, show promise of reducing noise in a broad frequency range. Additionally, the next generation of aircraft design may allow for more aircraft surface area to be covered by acoustic liners, further reducing aircraft noise perceived on the ground. Before these advanced acoustic technologies can make their way onto commercial aircraft fleets, the aerodynamic drag caused by the liners must be understood. Using the Mach 0.6 Wind Tunnel at the University of Notre Dame, a direct drag measurement system has been developed utilizing a linear force balance. In combination with indirect drag measurements taken at the Curved Duct Test Rig at the NASA Langley Research Center, measurement confidence has been established.

Using hotwire anemometry, mean velocity profiles were measured as well as the turbulence intensity, burst phenomena, and spectra for several liner types. The results suggest a strong correlation between the aerodynamic drag measured and the presence of burst events in the boundary layer. Lastly the coupling of acoustic excitation and liner drag will be explored as measured drag increases when in the presence of high levels of tonal noise.

This dissertation aims to provide a comprehensive report on a new method for evaluating liner drag, analyze a database of velocity profile measurements, and describe a proposal for a reduced drag concept utilizing known capabilities for smooth walls.