Evaluation of Optimized Multisectioned Acoustic Liners

Abstract

A critical examination is presented of the use of optimized axially segmented acoustic liners to increase the attenuation of a liner. New calculations show that segmenting is most efficient at high frequencies with relatively long duct lengths where the attenuation is low for both uniform and segmented liners. Statistical considerations indicate little advantage in using optimized liners with more than two segments, while the bandwidth of an optimized two-segment liner is shown to be nearly equal to that of a uniform liner. Multielement liner calculations show a large degradation in performance, due to changes in assumed input model structure. Finally, in order to substantiate previous and future analytical results, in-house (finite difference) and contractor (mode matching) programs are used to generate theoretical attenuations for a number of liner configurations for liners in a rectangular duct with no mean flow. Overall, the use of optimized multisectioned liners (sometimes called phased liners) fails to offer sufficient advantage over a uniform liner to warrant their use, except in lowfrequency, single-mode application. AdB F f* H* / M m p pA x x* y y* 6 Nomenclature speed of sound sound attenuation function frequency channel height length of duct Mach number transverse mode number dimensionless Fourier coefficient of pressure, p(x,y),p*/pA amplitude of pressure fluctuation or p JcJ 2 dimensionless axial coordinate, x* /H* axial coordiante dimensionless transverse coordinate, y*/H* transverse coordinate fractional impedance variation specific acoustic impedance dimensionless frequency/*//*/cJ or co*//*/27rcJ