Sound transmission in ducts with sheared mean flow

Abstract

In this article the prediction of sound transmission in ducts with sheared mean ∞ow is examined. The application of this work is the acoustics of aircraft engine duct systems, speciflcally sound propagation in a turbofan bypass duct. The bypass is modelled as a circular-section annular duct with acoustic lining on the outer and inner duct walls, and contains a sheared mean ∞ow. Owing to the circular geometry, it is convenient to represent the acoustic fleld in terms of modes. Two difierent numerical methods are used to flnd the modes. Then, a mode-matching method is utilized, to determine the sound power transmission loss in a lined annular duct with sheared mean ∞ow. Several difierent mean ∞ow proflles are examined. In each case, the sound transmission is calculated for an annular duct with lining on both walls, and also lining on the outer or inner wall only. Results are presented for examples which make use of realistic duct dimensions, mass ∞owrate, sound frequency and acoustic liner impedance, although here only axisymmetric modes with azimuthal mode order m = 0 are examined. The objective is to examine how difierent sheared ∞ows afiect sound transmission in a lined annular duct, compared with a uniform mean ∞ow.