Mechanisms and Prediction Methods for Fan Blade Stall Flutter

A e rst aim is to distinguish between two e utter regimes that are commonly referred to as stall e utter because of their occurrence between a raised working line and the stall boundary. The e rst one is of aeroacoustic origin, and it arises from a match between the acoustic impedance of the intake duct and the upstream pressure perturbation due to fan vibration. The second type is directly related to e ow separation effects and shock properties. Further objectives are to describe several numerical prediction methods, to compare their relative computational requirements, and to establish their bounds of applicability to various e utter types. The unsteady e ow is either a linearization about a viscous nonlinear steady-state e ow for a given mode of vibration, or it is fully nonlinear. The prediction methods are classie ed according to the way they treat the unsteady e ow. In all cases, the e uid mesh was moved during the unsteady e ow computations to follow the structural motion. The performance of the methods was ranked for a rig fan blade for which measurements were available. It was found that, near the stall boundary,e utterboundary couldonlybecaptured withan adequaterepresentationoftheunsteady viscouseffects. It was concluded that the shock had a stabilizing effect whereas the separation area behind it had a destabilizing effect.