Investigation of Mistuned Oscillating Cascade Using Fully-Coupled Method

The uncoupled phase-shifted single-passage simulation is commonly used for turbomachinery aeroelastic problems. However, it has difficulties in dealing with unconventional phenomena such as strong fluid-structure interaction effects as well as blade mistuning effects. Regarding mistuning effects, structural mistuning has been studied extensively while aerodynamic mistuning has received far less attention. There seems to be a lack of clear and systematic understanding of physical behaviour and mechanisms of mistuned bladerows, particularly in the context of the aerodynamic mistuning versus structural one. In the present work, direct fully-coupled method is adopted to investigate the dynamics mechanism of a mistuned oscillating cascade. Both structurally and aerodynamically mistuned cascades show that the blades would couple and oscillate at a unique frequency and a constant inter-blade phase angle regardless of the individual blade’s eigen-frequency. The vibration amplitudes of blades of a mistuned row are different when excited. For structural mistuning, the mode localization effect is seen to be responsible for a monotonic increase of cascade aeroelastic stability with mistuning. On the other hand, the aerodynamically mistuned cascade shows a stabilizing effect at small amount of mistuning but exhibits a destabilizing effect at large mistuning. Such non-monotonic tendency could be explained using the aero-damping decomposition by the influence coefficient approach. At low reduced frequency, there is a striking difference between the tuned and aero-mistuned cascade. Although the tuned cascade is stable, the aero-mistuned cascade may experience flutter. A close inspection of the aero-mistuned cascade flutter reveals that there are two oscillating waves forming a beating signal.

Active Suppression of Cascade Flutter With Piezoelectric Device

Possibility of active control on cascade flutter with piezoelectric device was both experimentally and numerically studied under a subsonic condition. A blade on which a piezoelectric device was glued was installed in the test cascade. When AC voltage was provided on the device, the blade could be actively oscillated. The unsteady aerodynamic work induced by the active oscillation was measured on the cascade blades. From the results, the active oscillation of the piezo-blade was found to generate sufficiently large unsteady aerodynamic work for changing instability of blade vibration. The active control was effective for flutter suppression if the phase difference between the unstable blade vibration and the active oscillation was adequately selected. The numerical results also showed effectiveness of the piezoelectric device, and the active oscillation was observed to change the unsteady aerodynamic work distribution on blade surfaces, which change should cause the stabilization effect. By a developed numerical method with flow-structure coupling, a suppression method for instability with a control rule was tested numerically to confirm its effectiveness.