A comparison between measured and predicted fan broadband noise due to rotor-stator interaction

Abstract

This paper presents a comparison between predictions and measurements of the broad-band noise generated in a low-speed fan rig. It is assumed here that the dominant generation mechanism is due to rotor-stator interaction. In the broadband noise model the stator is modelled as a cascade of flat plate airfoils on which turbulent wakes due to the rotor impinge. The turbulence transverse length scale and intensity of the rotor wake are determined by data from measurement and RANS calculations. The theoretical expression for the acoustic power spectra is shown to be in the form of the product of an aerodynamic response function and the spectrum of the velocity fluctuations normal to the stator blade direction. The response function is obtained by solving numerically an aerodynamic integral equation, while the turbulence velocity spectrum is expressed analytically in a form similar to an isotropic homogeneous turbulence model. The turbulence is also modelled by random velocity fluctuations superimposed on a statistic distribution of the rms velocity. The statistical distribution is composed of a constant term, modelling the background turbulence and a periodic Gaussian-profile to represent the rotor-wake turbulence. Extensive hot wire measurements of both the steady and unsteady wake velocity was measured. Comparisons between these measurements and RANS predictions are also presented.