Predictions of Fan Broadband Noise Using Lifting Surface Method

This paper adopts the three-dimensional lifting surface method as a new cascade-response function to predict fan broadband interaction noise. The stator is modeled as an annular cascade, and the vanes are assumed to be zero-thickness plates with no stagger angle. The unsteady loading spectrum on the stator vanes is considered under a fully three-dimensional condition rather than calculated by the strip theories that are widely used in the existing analytical broadband fan noise models. The sound power spectrum is calculated in an annular duct using numerical integration weighted by statistical turbulence spectrum. The present theory is verified by calculating the category 4 benchmark problem of the Third Computational Aeroacoustics Workshop for the response function and by comparing the predictions of a benchmark test for the broadband formulations of the sound power spectra. Then, the predictions of the inlet and exhaust sound power spectra for the NASA source diagnostic test are presented, which both show good agreement with the experimental results. The discrepancies indicate the importance of further corrections for stagger and swirling mean flow effects in realistic predictions. In the end, the variation trend of the sound power with different vane numbers is well captured, which indicates that this effect mainly refers to the propagation of cut-on acoustic modes in an annular duct. This model has the capability for further studies on the vane swept or lean effects.