Acoustic resonance in rotor cascades caused by inlet distortion

Acoustic resonance is a critical issue in turbomachinery that induces noise and structural vibrations. The resonant mechanism for stator blade rows was first revealed more than half a century ago, along with the well-known concept of stationary Parker modes. However, despite various efforts based on this mechanism, previous studies have failed to explain the experimental Parker-type resonance results observed in rotor blade rows. This study establishes a theoretical model to elucidate acoustic resonance phenomena in rotating annular cascades, with focus on the effects of inlet distortion. The results demonstrate that the present model captures the experimental trends for Parker-type rotor acoustic resonances, which also implies that the conventional stationary Parker modes no longer exist in rotor blade rows due to rotation and the frequency scattering effect. Meanwhile, theoretical predictions on inlet-distortion–rotor interaction reveal that the unsteady blade loading is significantly higher at resonance frequencies compared to the cut-on frequency in duct acoustics. Accordingly, a modified Campbell diagram incorporating acoustic natural frequencies is proposed to aid in avoiding resonance-induced blade vibrations during the design stage. It is shown that the acoustic resonance frequencies intersect with the synchronous excitation frequencies across a wide speed range. High-amplitude unsteady blade loading is induced at these frequency crossings, due to the Parker-type acoustic resonance eigenmodes being excited by inlet-distortion–rotor interactions.