Discrete frequency noise reduction modeling for application to fanjet engines

Abstract

A model for the generation of discrete frequency noise due to rotor–stator interactions in a fanjet engine is examined to ascertain its relevance to noise reduction studies. Predictions are obtained for the noise reduction between a base fan stage and one redesigned to achieve reduced discrete frequency noise utilizing response functions corresponding to incompressible flow isolated airfoil transverse and longitudinal gust analyses as well as incompressible and compressible flat plate airfoil cascade transverse gust analyses. The correlation of the various predictions with the actual fan test results demonstrated that the viscous wake-generated noise model utilizing the compressible flow cascade response function was the one most appropriate for noise reduction studies of fanjet engines. The significant parameters and their relative importance were then identified by means of a parametric study of the base fan based on the compressible flow cascade response analysis. This study showed that the dominant parameter relevant to discrete frequency noise generation is the interblade phase angle, equal to the ratio of the number of upstream rotor blades to downstream stator vanes. Other parameters of importance included the Mach number, reduced frequency, and vane solidity.