Modelling Broadband fan Noise and Comparison with Experiments

The interaction of upstream turbulence with an annular cascade is investigated. The model uses a spectral representation of the impinging upstream turbulence and a multiple scale analysis for the evolution of turbulence in a nonuniform swirling flow. The aerodynamic‐acoustic response of the annular cascade in a nonuniform swirling flow is calculated for upstream Fourier disturbances. The broadband sectional lift and the noise power radiated are then calculated by summing the spectral density weighted contributions of all Fourier components of a turbulence realization and averaging the turbulence stochastic variables. Broadband noise spectra are calculated for four dierent cases to examine the eects of swirl, blade loading and nonuniform inflow angle distribution. Comparison of the results indicate that the eects of these parameters on the acoustic power density spectra is about 3 to 4 dB and that these eects strongly vary with the frequency, being stronger at low frequencies and moderately high frequencies. At very high frequency the results tend toward the two-dimensional model results with 1 to 2 dB dierence. The eect of dierent turbulence spectral representation is examined since they have dierent turbulent energy distribution. A high frequency asymptotic analysis shows that the acoustic response of the cascade will be dominated by the local blade geometry. This suggests that a simplified two-dimensional cascade model may be adequate for calculating the fan acoustic response at high frequency. This conclusion is confirmed for unloaded blades by a comparison of the numerical results of the radiated noise spectra of a 3D annular cascade and a 2D linear cascade. The model is validated by comparison with BFAN and NASA 22-in fan Source Diagnostic Test data.