Non-Synchronous Vibration: Characterisation of the Aerodynamic Disturbance and its Dependency on Local Tip Clearance

Abstract Non-synchronous vibrations (NSV) occurring near the stall boundary pose a major challenge to the design of safe and efficient compressors and fans. NSV occurs when aerodynamic disturbances which develop close to the aerodynamic stability limit couple with blade vibration. Despite its importance and a recent rise in research efforts, the exact nature of the aerodynamic disturbance is unknown and there are no robust modelling and prediction methods available. A characteristic parameter crucial to the successful prediction of NSV is the circumferential propagation velocity of the aerodynamic disturbance, which determines whether lock-in with a structural vibration mode is possible. This paper uses detailed aeroelastic measurements and coupled time-accurate simulations on the open-test-case composite fan ECL5, which was experimentally tested at Ecole Centrale de Lyon in project CATANA, to characterize the aerodynamic disturbance and determine the mechanisms responsible for its circumferential propagation. As part of this, the influence of blade-to-blade variations in tip clearance size is investigated. The disturbance is identified as a vortical structure which originates in the tip clearance flow close to the leading edge and is convected across the passage to the trailing blade where it triggers the release of a new vortical structure. The tip clearance plays a crucial role in setting the convection speed across the passage and also influences the transfer mechanism across the blade. Overall, a larger tip clearance is found to increase the speed and strength of the disturbance in the subsequent passage. The implications of tip clearance variations for vibration are modelled and the results confirmed in the experimental analysis. It is shown that blade-to-blade variations in the tip clearance act as a form of aerodynamic mistuning, leading to vibrations in multiple nodal diameters.