Experimental Investigation of Inflow-Distortion-Induced Tonal Noise in a Sub-Sonic Fan Test Rig

Experimental investigation on the influence of inflow distortions on fan noise was performed at the low-speed Co-/Counter Rotating Acoustic Fan Test rig (CRAFT). The investigated fan stage is designed such that the dominant interaction modes at the first and second BPF are excited weakly in upstream direction, but strongly in downstream direction, due to the mode propagation angle. Two major tonal source mechanisms are described in the literature and addressed in this work. The first mechanism is the inflow-distortion-rotor interaction, which can generate high sound power levels at the blade passing frequencies at high blade tip mach numbers or low rotor blade counts. For the measurements performed this source seems to be relevant only for extreme inflow distortion cases. The second mechanism is linked to the modulation of the rotor-stator interaction (RSI), which is caused by the periodically fluctuating incidence angle and the resulting fluctuation of the rotor wakes shape. The sources distorted phase and amplitude lead to a less effective constructive interference of the conventional interaction modes and a less a destructive interference of the adjacent modes. As consequence, a broader mode spectrum is propagating to the measurement sections. The overall sound power level for our specific fan stage can be more than 7 dB higher or 1 dB lower than the baseline, depending on the inflow distortion profile and BPF harmonic