Passive control of fan noise by vane sweep and lean

Rotor wake/stator interaction is one of the primary sound sources of fan tonal noise. As the annular-cascade geometry has important impact on the solution for sound sources and sound propagation in the duct, the three-dimensional effect should be emphasized in the discussion of passive control of fan noise by vane sweep and lean. In this paper, the three-dimensional lifting surface method is employed as the vane response function to calculate the unsteady surface pressure and the radiated sound field. In comparison with the numerical solutions, there is no dispersion or dissipation in this method. With high efficiency, this method is more suitable for the early stage of the design, and can provide the validation for numerical solutions. The mechanism of fan noise reduction by vane sweep and lean is further discussed by a comparison with existing results. The calculation result of the swept-and-leaned vanes shows that, in some condition, the suitable combination of swept and leaned angles can gain very good control performance for fan tonal noise reduction. Because of the difference in axial wave-numbers, the sound power changing trends with swept and leaned vanes would be different for upstream and downstream.