APPLICATIONS OF A FINITE-ELEMENT MODE-MATCHING METHOD TO ACOUSTIC DESIGN OF THE BYPASS DUCT OF TURBOFAN ENGINES

摘要

A finite-element mode-matching approach for duct acoustics with flow and circumferentially varying liners is presented. This general prediction method comprises two stages. The first stage is a fully numerical procedure to determine the acoustic modes in ducts of arbitrary cross-section and mean flow profiles. The second stage is a numerical mode-matching method using a modified matching technique in order to deal more accurately with liner discontinuity with flow. By matching modal expansions at the interface between different uniform duct segments, the effect of axial variations of impedance can be modeled with far fewer parameters than would be required for a 3-D numerical transmission analysis. An analytic radiation model can also be integrated with the mode-matching procedure in order to obtain far-field directivity for tones and broadband multi-mode noise. Validation results of the method using analytical results are presented. An application of particular interest to the acoustic design of turbofan engines is the effect of hard patches in lined bypass ducts. These hard patches are introduced to meet other mechanical or aerothermal design requirements but deteriorate the performance of acoustic treatments by reducing the liner area and by introducing significant modal scattering inside the duct. A parametric study of the geometry of a rectangular hard patch in a straight annular duct was carried out using the finite-element mode-matching method. The effect of the hard patch is discussed both in terms of modal scattering inside the duct and in terms of noise radiated to the far field.