Localized Disturbance Evolution Under Circumferential Distortion: A Perspective on the Critical Angle

Abstract This article aims to explain the formation mechanism and variation patterns of critical distortion angles through disturbance evolution on a low-speed fan. Based on time-domain analysis and wavelet transform, the pattern of the circumferential pre-stall disturbance evolution is revealed. An identical irreversible zone is observed by first-order spatial modal analysis within one revolution prior to stall, regardless of distortion angles. The foam metal casing treatment (FMCT) is used to generate modal and spike stall precursors separately in the same fan by adjusting the axial location. It is found that the critical distortion angle exists with “spike” but vanishes with “modal.” Averaged wavelet spectra of pre-stall disturbance under different distortions prove that disturbance evolution rates depend on local loading, not distortion area. At last, a simple qualitative model on the critical distortion angle is proposed, establishing a direct correlation between the development of short length-scale disturbance and stability limits.