Design of Acoustic Linings for Ducts with Flow

Abstract

A procedure for the design of acoustic linings for the attenuation of fan generated noise in turbo fan engines is described. This procedure is based on an interaction between an extensive analytical lining performance pre- diction capability and flow duct testing. The theoretical basis for lining analysis in rectangular, circular, and annular ducts with either uniform or sheared flow is formulated and the results of the development of a lining mathematical model are presented. The predictive capability is employed in conjunction with a least attenuated mode lining optimization scheme to determine the lining impedance which will provide the required attenuation spectrum. A flow duct test facility is briefly described and results of the testing of lining designs and the com- parison with theoretical predictions is presented. N response to recently enacted regulations,1 powerplant and airframe manufacturers have begun a program to sub- stantially reduce the noise of conventional and turbo-fan engines. Success in jet noise suppression has lowered the noise floor sufficiently that the reduction of fan and com- pressor noise has become a major consideration. The technique for achieving fan noise reduction can be broken down into two distinct approaches. One method consists of reduction of the noise at the source. This is to be accomplished by the development of a detailed under- standing of the noise generation mechanism in the fan stages and the use of this knowledge to design fans and compressors which operate at reduced noise levels. The second method accepts the presence of the noise source and seeks to attenuate the sound in the fan inlet and exhaust ducts by acoustic treat- ment of the duct walls. The present paper deals with the second approach and details the elements of a theoretical and experimental program for duct lining design and optimization. The theoretical analysis and prediction of the performance of acoustic lining materials in an actual engine application is made difficult and costly by the complicated duct geometries and operating conditions which are encountered. Exact theoretical analysis is not practical and experimental work under full-scale operating conditions, although necessary for final design verification, is too costly for developmental work aimed at optimization of lining configurations. For these reasons, the study of lining materials is generally based on simplified, but representative, mathematical models and by experimental investigations in flow ducts which roughly match the conditions found in actual fan inlet and exhaust ducts. Theoretical and experimental investigations of the trans- mission and attenuation of sound in ducts has been a field of fruitful research for a number of years. Original interest was