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Nonuniform Upstream Airfoil Spacing Effects on Rotor Blade Noise Generation and Forced Response

A mathematical model is developed to analyse the effect of altering the harmonic content of the inlet flow field to a rotor blade row on noise generation and forced response. This is accomplished by periodic nonuniform circumferential spacing of the airfoils in the upstream row. Thus, the blade row interactions are modeled by considering a cascade of rotor blades moving relative to a uniform circumferentially spaced upstream airfoil row. The rotor inlet flow field is determined by tracking a typical rotor blade through the wakes of the upstream airfoil row, with each velocity history decomposed into harmonic components. The potentially beneficial effects of nonuniform upstream row airfoil spacing are then demonstrated by applying this model to a modern compressor configuration and a counter-rotating turboprop geometry.