Propagation of Acoustic Modes in the Discharge Section of a Low-Speed Fan

In this work, the propagation of acoustic modes in a low-speed fan test rig – named CRAFT and representative for Urban Air Mobility vehicles – is investigated based on numerical simulations. The focus is put on the acoustic mode reflections occurring downstream of the rotor-stator stage. These are due to three thick pylons supporting the rotational shaft. The simulations complement acoustic measurement data. They help the interpretation of the experimental data and the understanding of the physical phenomena. The in-house CFD solver and framework TRACE is used to predict the tonal component of rotor–stator interaction noise by using the Harmonic Balance method. Three operating points of the reference fan stage are simulated, including the interaction of a downstream propagating Tyler–Sofrin (TS) mode, which is excited at the fundamental blade passing frequency, with the pylons. Mode scattering and reflections due to the pylons are analyzed by means of modal decomposition. The simulation results are compared to the acoustic measurement data. To separate relevant physical effects, the isolated fan stage and isolated pylons are simulated and compared to the installed fan-stage simulations. The results evidence the fact that the upstream propagating TS-mode evaluated in the inlet can be masked by a scattered and reflected acoustic mode due to the interaction with the pylons.