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 ...