Parametric numerical study on the impact of steady cosine inflow distortions on tonal rotor-stator interaction noise

Kurzfassung

The effect of steady cosine inflow distortion on the generation of tonal Rotor-Stator- Interaction (RSI) noise by a low-speed fan is investigated in this work using Computational Fluid Dynamics (CFD). Unsteady flow simulations are performed with the DLR in-house solver TRACE using the hybrid frequency-domain Harmonic Balance (HB) method to predict periodic inflow distortion–rotor and rotor–stator interactions and the generated acoustic modes from the stator at the blade passing frequency and multiples. The simulations are based on the fan test rig CRAFT at the DLR Engine Acoustics Department. However, this study does not aim at reproducing the observed experimental results but rather at investigating the effects of the modal content of inflow distortions on tonal RSI-noise. For this, quasi-3D simulations are carried out with cosine inflow distortions of increasing azimuthal mode order. We demonstrate that additional acoustic RSI-spinning modes are triggered by additional unsteady force components acting on the stator vanes. The modulation of rotor wakes by the inflow distortion is a result of their interaction and therefore, rotor wakes contain a broader spatial spectrum compared to clean inflow. Resulting from their interaction with stator vanes, a broader acoustic mode spectrum is observed. It is observed, that especially for high mode orders of inflow distortions, these additional modes can dominate the acoustic spectrum, especially when TS-modes radiate inefficiently.