Impact of bypass-duct bifurcations on fan noise

Kurzfassung

The present work deals with the impact of the potential field of bypass-duct bifurcations on the turbofan tonal noise. The investigation is carried out at take-off condition on a typical rotor-stator fan stage extended with struts, upper and lower bifurcations. the results of quasi-3D steady and unsteady RANS calculations at 60% rotor span are presented and compared to the isolated configuration. Two sorts of impact on noise are observed: on the one hand a direct impact on rotor noise due to the periodic loading of the rotor interacting with the circumferentially non-uniform pressure field imposed backwards by the bifurcations; on the other hand an indirect impact on the noise generation at the stator due to the deformation of the rotor wakes under the influence of the potential field of the bifurcations. In the present study, tonal noise due to the rotor-wake interaction with the stator vanes appears to be dominant in the bypass duct. Under the effect of the bypass components a dramatic change in the azimuthal structure of the acoustic field is observed at the blade passing frequency and ist harmonics with a significant increase of the sound power level. It is expected that the bypass components will jeopardise the cut-off design of the blade passing frequency in the full-3D configuration. In the inlet, the shocks built on the blades do not propagate as the corresponding pressure patterns are cut-off. However one can observe that the shock position moves back and forth during one revolution which certainly should influence the generation and propagation of buzz-saw noise going to the full-3D configuration.