Impact of Bypass Duct Bifurcations on Fan Noise

Abstract

The present work is concerned with the impact of bifurcations, i.e., extensions of the pylon inside the bypass duct, on fan tonal noise radiated by civil turbofan engines. The investigation is carried out on a typical configuration composed of a rotor–stator stage, a row of struts, and one upper and one lower bifurcation. It relies on steady and unsteady Reynolds-averaged Navier–Stokes calculations for a thin streamtube at 60% rotor span. The comparison to the isolated fan stage is used to evidence the impact of the bifurcations and the struts. The strongly nonuniform mean flowfield created by the bifurcations downstream of the fan represents the main acoustic risk. As a first effect, tonal noise is generated by the rotor due to load fluctuations on the blades during the rotation. As a second effect, the rotor wakes impinging on two adjacent stator vanes feature different shapes according to the position of the vane relative to the bifurcations; as a consequence, different pressure fields are radiated by all the vanes. Instead of the discrete azimuthal mode spectrum typical of an isolated rotor–stator stage, as predicted by Tyler and Sofrin (“Axial Flow Compressor Noise Studies,” SAE Transactions, Vol. 70, 1962, pp. 309–332), a broad mode spectrum is obtained for the acoustic field with bifurcations. Clearly, cutoff design is jeopardized, with, as a consequence, a significant increase of the sound power level by 2–3 dB at the blade passing frequency (1xBPF) and its harmonics. In the inlet, the shocks formed on the blades do not propagate as the corresponding pressure patterns are cut off. However, one can observe that their position along the chord moves back and forth during one revolution, which is expected to influence the generation of multiple tones, should it occur.