Spinning mode sound propagation in ducts with acoustic treatment

Abstract

Recent acoustic data have shown large noise attenuations for acoustically treated aircraft engine inlets without splitter rings. These encouraging data have stimulated a more detailed theoretical study of the acoustic propagation of spinning modes in acoustically treated open circular ducts. In addition, the suppressor with splitter rings was modeled by using the rectangular approximation to the annular duct. The theoretical models were used to optimize the suppressor designs for several spinning lobe numbers from zero to 50. Some interesting results of the analysis are that for circular ducts the maximum possible attenuation and the optimum wall impedance are strong functions of the lobe number. For annular ducts, the attenuation and optimum wall impedance are insensitive to the spinning lobe number for well-cut-on modes. The above results help explain why suppressors with splitter rings have been quite effective in spite of the lack of detailed information on the noise-source modal structure. Conversely, effective use of wall treatment only will require expanded knowledge of the noise-source structure. Approximate solutions are presented to help in understanding the more exact theoretical results.