Mode Analysis for the Rotating Aerodynamic Disturbance of a Transonic Fan

Abstract

The non-synchronous blade vibration induced by a rotating aerodynamic disturbance is encountered for the fan of a turbofan engine working at 65% speedline. To analyze the unsteady characteristics, two methods based on the phase analysis and the compressive sensing (CS) for spatially-undersampled data are proposed and applied to obtain the circumferential mode order of the aerodynamic disturbance, and their performance has been carefully examined at various parameters. It was shown that when there are sufficient probes, i.e., 7–8 in the present study, both the two methods can give a correct prediction to the dominant mode order (the highest is 26 in this work) for all non-engine-order (EO) frequencies. However, if less probes are used, the deterioration of accuracy can be observed for both the two methods and a better performance can be observed for the CS method. In order to suppress the blade vibration, the fan blade is trimmed to alter its solid eigenfrequencies and avoid the resonance, which is proved to be an effective anti-vibration design. The existence of the aerodynamic disturbance after the blade is trimmed also suggests that the vibration is not self-excited but induced by the resonance of the flow excitation and the structure. For the cases with and without vibration, the origin of these non-EO frequencies can be well explained by the spinning mode theory and the interaction between the blade vibration and a high-order blade passing frequency has been observed in this study.